QUIC                                                     J. Iyengar, Ed.
Internet-Draft                                                    Fastly
Intended status: Standards Track                         M. Thomson, Ed.
Expires: April 6, 26, 2019                                          Mozilla
                                                        October 03, 23, 2018

           QUIC: A UDP-Based Multiplexed and Secure Transport
                      draft-ietf-quic-transport-15
                      draft-ietf-quic-transport-16

Abstract

   This document defines the core of the QUIC transport protocol.  This
   document describes connection establishment, packet format,
   multiplexing, and reliability.  Accompanying documents describe the
   cryptographic handshake and loss detection.

Note to Readers

   Discussion of this draft takes place on the QUIC working group
   mailing list (quic@ietf.org), which is archived at
   <https://mailarchive.ietf.org/arch/search/?email_list=quic>.

   Working Group information can be found at <https://github.com/
   quicwg>; source code and issues list for this draft can be found at
   <https://github.com/quicwg/base-drafts/labels/-transport>.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Conventions and Definitions
     1.1.  Document Structure  . . . . . . . . . . . . . . . . .   6
     2.1.  Notational Conventions . .   6
     1.2.  Conventions and Definitions . . . . . . . . . . . . . . .   7
   3.  Versions
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   8
   2.  Streams . . . . . . . . .   7
   4.  Packet Types and Formats . . . . . . . . . . . . . . . . . .   8
     4.1.  Long Header . . . . . . . . . . . . . . . . . .
     2.1.  Stream Identifiers  . . . . .   8
     4.2.  Short Header . . . . . . . . . . . . . .   9
     2.2.  Stream Concurrency  . . . . . . . .  11
     4.3.  Version Negotiation Packet . . . . . . . . . . .  10
     2.3.  Sending and Receiving Data  . . . .  12
     4.4.  Retry Packet . . . . . . . . . . .  11
     2.4.  Stream Prioritization . . . . . . . . . . .  14
     4.5.  Cryptographic Handshake Packets . . . . . . .  11
   3.  Stream States: Life of a Stream . . . . . .  16
     4.6.  Initial Packet . . . . . . . . .  12
     3.1.  Send Stream States  . . . . . . . . . . . .  17
       4.6.1.  Connection IDs . . . . . . .  13
     3.2.  Receive Stream States . . . . . . . . . . . .  18
       4.6.2.  Tokens . . . . . .  15
     3.3.  Permitted Frame Types . . . . . . . . . . . . . . . . .  19
       4.6.3.  Starting Packet Numbers .  18
     3.4.  Bidirectional Stream States . . . . . . . . . . . . . .  20
       4.6.4.  0-RTT Packet Numbers .  18
     3.5.  Solicited State Transitions . . . . . . . . . . . . . . .  20
       4.6.5.  Minimum Packet Size  19
   4.  Flow Control  . . . . . . . . . . . . . . . . .  21
     4.7.  Handshake Packet . . . . . . .  20
     4.1.  Handling of Stream Cancellation . . . . . . . . . . . . .  21
     4.8.  Protected Packets . .
     4.2.  Data Limit Increments . . . . . . . . . . . . . . . . . .  22
     4.9.  Coalescing Packets  . . . . . . . . . . . . . . . . .
     4.3.  Stream Final Offset . .  22
     4.10. Connection ID Encoding . . . . . . . . . . . . . . . . .  23
     4.11. Packet Numbers  . . . . . . . . . . . . .
     4.4.  Flow Control for Cryptographic Handshake  . . . . . . . .  24
   5.  Frames and Frame Types  . . . . . . . . . . . . . . . . .
     4.5.  Stream Limit Increment  . .  27
     5.1.  Extension Frames . . . . . . . . . . . . . . .  24
   5.  Connections . . . . .  30
   6.  Life of a Connection . . . . . . . . . . . . . . . . . . . .  30
     6.1.  24
     5.1.  Connection ID . . . . . . . . . . . . . . . . . . . . . .  31
       6.1.1.  24
       5.1.1.  Issuing Connection IDs  . . . . . . . . . . . . . . .  31
       6.1.2.  25
       5.1.2.  Consuming and Retiring Connection IDs . . . . . . . .  32
     6.2.  26
     5.2.  Matching Packets to Connections . . . . . . . . . . . . .  32
       6.2.1.  27
       5.2.1.  Client Packet Handling  . . . . . . . . . . . . . . .  33
       6.2.2.  27
       5.2.2.  Server Packet Handling  . . . . . . . . . . . . . . .  33
     6.3.  27
     5.3.  Life of a QUIC Connection . . . . . . . . . . . . . . . .  28
   6.  Version Negotiation . . . . . . . . . . . . . . . . . . .  34
       6.3.1. . .  28
     6.1.  Sending Version Negotiation Packets . . . . . . . . .  34
       6.3.2. . .  29
     6.2.  Handling Version Negotiation Packets  . . . . . . . .  35
       6.3.3. . .  29
     6.3.  Using Reserved Versions . . . . . . . . . . . . . . .  35
     6.4. . .  30
   7.  Cryptographic and Transport Handshake . . . . . . . . . .  36
     6.5. . .  31
     7.1.  Example Handshake Flows . . . . . . . . . . . . . . . . .  37
     6.6.  Transport Parameters  32
     7.2.  Negotiating Connection IDs  . . . . . . . . . . . . . . .  33
     7.3.  Transport Parameters  . . .  38
       6.6.1.  Transport Parameter Definitions . . . . . . . . . . .  41
       6.6.2. . . . .  34
       7.3.1.  Values of Transport Parameters for 0-RTT  . . . . . .  43
       6.6.3.  35
       7.3.2.  New Transport Parameters  . . . . . . . . . . . . . .  44
       6.6.4.  36
       7.3.3.  Version Negotiation Validation  . . . . . . . . . . .  45
     6.7.  Stateless Retries . . . . . . . .  36
   8.  Address Validation  . . . . . . . . . . . .  46
     6.8.  Using Explicit Congestion Notification . . . . . . . . .  46
     6.9.  Proof of Source  37
     8.1.  Address Ownership . . . . . . . . . Validation During Connection Establishment  . . .  48
       6.9.1.  Client  38
       8.1.1.  Address Validation Procedure . . using Retry Packets  . . . . . . .  49
       6.9.2.  38
       8.1.2.  Address Validation for Future Connections . . . . . .  50
       6.9.3.  39
       8.1.3.  Address Validation Token Integrity  . . . . . . . . .  50
     6.10.  41
     8.2.  Path Validation . . . . . . . . . . . . . . . . . . . . .  51
       6.10.1.  Initiation .  41
     8.3.  Initiating Path Validation  . . . . . . . . . . . . . . .  42
     8.4.  Path Validation Responses . . . . .  51
       6.10.2.  Response . . . . . . . . . . .  42
     8.5.  Successful Path Validation  . . . . . . . . . . .  52
       6.10.3.  Completion . . . .  42
     8.6.  Failed Path Validation  . . . . . . . . . . . . . . . . .  52
       6.10.4.  Abandonment  43
   9.  Connection Migration  . . . . . . . . . . . . . . . . . . . .  53
     6.11. Connection Migration  43
     9.1.  Probing a New Path  . . . . . . . . . . . . . . . . . .  53
       6.11.1.  Probing a New Path .  44
     9.2.  Initiating Connection Migration . . . . . . . . . . . . .  45
     9.3.  Responding to Connection Migration  . . .  54
       6.11.2.  Initiating Connection Migration . . . . . . . .  45
       9.3.1.  Handling Address Spoofing by a Peer . .  54
       6.11.3.  Responding to Connection Migration . . . . . . .  46
       9.3.2.  Handling Address Spoofing by an On-path Attacker  . .  55
       6.11.4.  46
     9.4.  Loss Detection and Congestion Control . . . . . . .  56
       6.11.5. . . .  47
     9.5.  Privacy Implications of Connection Migration  . . . .  57
     6.12. . .  48
     9.6.  Server's Preferred Address  . . . . . . . . . . . . . . .  58
       6.12.1.  49
       9.6.1.  Communicating A Preferred Address . . . . . . . . .  59
       6.12.2. .  49
       9.6.2.  Responding to Connection Migration  . . . . . . . . .  59
       6.12.3.  49
       9.6.3.  Interaction of Client Migration and Preferred Address  50
   10. Connection Termination  . . . . . . . . . . . . . . . . . . .  50
     10.1.  Closing and Draining Connection States . . .  59
     6.13. Connection Termination . . . . . .  51
     10.2.  Idle Timeout . . . . . . . . . . .  60
       6.13.1.  Closing and Draining Connection States . . . . . . .  60
       6.13.2.  Idle Timeout . . . .  52
     10.3.  Immediate Close  . . . . . . . . . . . . . . . .  61
       6.13.3.  Immediate Close . . . .  52
     10.4.  Stateless Reset  . . . . . . . . . . . . . .  62
       6.13.4.  Stateless Reset . . . . . .  53
       10.4.1.  Detecting a Stateless Reset  . . . . . . . . . . . .  63
   7.  Frame Types and Formats  56
       10.4.2.  Calculating a Stateless Reset Token  . . . . . . . .  56
       10.4.3.  Looping  . . . . . . . . . . .  67
     7.1.  Variable-Length Integer Encoding . . . . . . . . . . .  57
   11. Error Handling  .  67
     7.2.  PADDING Frame . . . . . . . . . . . . . . . . . . . . . .  68
     7.3.  RST_STREAM Frame  58
     11.1.  Connection Errors  . . . . . . . . . . . . . . . . . . .  58
     11.2.  Stream Errors  .  68
     7.4.  CONNECTION_CLOSE frame . . . . . . . . . . . . . . . . .  69
     7.5.  APPLICATION_CLOSE frame . . .  59
   12. Packets and Frames  . . . . . . . . . . . . . .  70
     7.6.  MAX_DATA Frame . . . . . . .  59
     12.1.  Protected Packets  . . . . . . . . . . . . . .  71
     7.7.  MAX_STREAM_DATA Frame . . . . .  59
     12.2.  Coalescing Packets . . . . . . . . . . . . .  72
     7.8.  MAX_STREAM_ID Frame . . . . . .  60
     12.3.  Packet Numbers . . . . . . . . . . . . . . .  73
     7.9.  PING Frame . . . . . .  61
     12.4.  Frames and Frame Types . . . . . . . . . . . . . . . . .  73
     7.10. BLOCKED Frame  62
   13. Packetization and Reliability . . . . . . . . . . . . . . . .  65
     13.1.  Packet Processing and Acknowledgment . . . . . .  74
     7.11. STREAM_BLOCKED Frame . . . .  66
       13.1.1.  Sending ACK Frames . . . . . . . . . . . . . .  74
     7.12. STREAM_ID_BLOCKED Frame . . .  66
       13.1.2.  ACK Frames and Packet Protection . . . . . . . . . .  67
     13.2.  Retransmission of Information  . . . . .  75
     7.13. NEW_CONNECTION_ID Frame . . . . . . . .  67
     13.3.  Explicit Congestion Notification . . . . . . . . .  75
     7.14. RETIRE_CONNECTION_ID Frame . . .  69
       13.3.1.  ECN Counters . . . . . . . . . . . .  77
     7.15. STOP_SENDING Frame . . . . . . . .  70
       13.3.2.  ECN Verification . . . . . . . . . . .  77
     7.16. ACK Frame . . . . . . .  70
   14. Packet Size . . . . . . . . . . . . . . . . .  78
       7.16.1.  ACK Block Section . . . . . . . .  71
     14.1.  Path Maximum Transmission Unit . . . . . . . . .  79
       7.16.2.  ECN section . . . .  72
       14.1.1.  IPv4 PMTU Discovery  . . . . . . . . . . . . . . . .  81
       7.16.3.  Sending ACK Frames  73
     14.2.  Special Considerations for Packetization Layer PMTU
            Discovery  . . . . . . . . . . . . . . . . .  82
       7.16.4.  ACK Frames and Packet Protection . . . . . .  73
   15. Versions  . . . .  83
     7.17. PATH_CHALLENGE Frame . . . . . . . . . . . . . . . . . .  83
     7.18. PATH_RESPONSE Frame . . . .  74
   16. Variable-Length Integer Encoding  . . . . . . . . . . . . . .  75
   17. Packet Formats  .  84
     7.19. NEW_TOKEN frame . . . . . . . . . . . . . . . . . . . . .  84
     7.20. STREAM Frames .  75
     17.1.  Packet Number Encoding and Decoding  . . . . . . . . . .  76
     17.2.  Long Header Packet . . . . . . . . . . .  84
     7.21. CRYPTO Frame . . . . . . . .  77
     17.3.  Short Header Packet  . . . . . . . . . . . . . .  86
   8.  Packetization and Reliability . . . .  79
     17.4.  Version Negotiation Packet . . . . . . . . . . . .  87
     8.1.  Packet Processing and Acknowledgment . . .  81
     17.5.  Initial Packet . . . . . . .  87
     8.2.  Retransmission of Information . . . . . . . . . . . . . .  88
     8.3.  82
       17.5.1.  Starting Packet Size Numbers  . . . . . . . . . . . . . .  84
       17.5.2.  0-RTT Packet Numbers . . . . . . . . .  90
     8.4.  Path Maximum Transmission Unit . . . . . . .  84
     17.6.  Handshake Packet . . . . . .  90
       8.4.1.  IPv4 PMTU Discovery . . . . . . . . . . . . . .  85
     17.7.  Retry Packet . . .  91
       8.4.2.  Special Considerations for Packetization Layer PMTU
               Discovery . . . . . . . . . . . . . . . . . . .  85
   18. Transport Parameter Encoding  . . .  92
   9.  Streams: QUIC's Data Structuring Abstraction . . . . . . . .  92
     9.1.  Stream Identifiers . . . . .  88
     18.1.  Transport Parameter Definitions  . . . . . . . . . . . .  90
   19. Frame Types and Formats . .  93
     9.2.  Stream States . . . . . . . . . . . . . . . . .  92
     19.1.  PADDING Frame  . . . . .  94
       9.2.1.  Send Stream States . . . . . . . . . . . . . . . .  93
     19.2.  RST_STREAM Frame .  95
       9.2.2.  Receive Stream States . . . . . . . . . . . . . . . .  97
       9.2.3.  Permitted Frame Types . . .  93
     19.3.  CONNECTION_CLOSE frame . . . . . . . . . . . . .  99
       9.2.4.  Bidirectional Stream States . . . .  94
     19.4.  APPLICATION_CLOSE frame  . . . . . . . . .  99
     9.3.  Solicited State Transitions . . . . . . .  95
     19.5.  MAX_DATA Frame . . . . . . . . 101
     9.4.  Stream Concurrency . . . . . . . . . . . . .  95
     19.6.  MAX_STREAM_DATA Frame  . . . . . . 101
     9.5.  Sending and Receiving Data . . . . . . . . . . .  96
     19.7.  MAX_STREAM_ID Frame  . . . . 102
     9.6.  Stream Prioritization . . . . . . . . . . . . . .  97
     19.8.  PING Frame . . . . 102
   10. Flow Control . . . . . . . . . . . . . . . . . . .  98
     19.9.  BLOCKED Frame  . . . . . 103
     10.1.  Edge Cases and Other Considerations . . . . . . . . . . 105
       10.1.1.  Response to a RST_STREAM . . . . . .  98
     19.10. STREAM_BLOCKED Frame . . . . . . . . 105
       10.1.2.  Data Limit Increments . . . . . . . . . .  99
     19.11. STREAM_ID_BLOCKED Frame  . . . . . 105
     10.2.  Stream Limit Increment . . . . . . . . . . .  99
     19.12. NEW_CONNECTION_ID Frame  . . . . . . 106
       10.2.1.  Blocking on Flow Control . . . . . . . . . . 100
     19.13. RETIRE_CONNECTION_ID Frame . . . . 106
     10.3.  Stream Final Offset . . . . . . . . . . . 101
     19.14. STOP_SENDING Frame . . . . . . . 107
     10.4.  Flow Control for Cryptographic Handshake . . . . . . . . 107
   11. Error Handling . . . . 102
     19.15. ACK Frame  . . . . . . . . . . . . . . . . . . . 107
     11.1.  Connection Errors . . . . 102
       19.15.1.  ACK Block Section . . . . . . . . . . . . . . . 108
     11.2.  Stream Errors . . 104
       19.15.2.  ECN section . . . . . . . . . . . . . . . . . . . 108
     11.3.  Transport Error Codes . 105
     19.16. PATH_CHALLENGE Frame . . . . . . . . . . . . . . . . 109
     11.4.  Application Protocol Error Codes . . 106
     19.17. PATH_RESPONSE Frame  . . . . . . . . . . 110
   12. Security Considerations . . . . . . . . 107
     19.18. NEW_TOKEN frame  . . . . . . . . . . . 110
     12.1.  Handshake Denial of Service . . . . . . . . . 107
     19.19. STREAM Frames  . . . . . 110
     12.2.  Spoofed ACK Attack . . . . . . . . . . . . . . . . 107
     19.20. CRYPTO Frame . . . 111
     12.3.  Optimistic ACK Attack . . . . . . . . . . . . . . . . . 112
     12.4.  Slowloris Attacks . . 109
     19.21. Extension Frames . . . . . . . . . . . . . . . . . 112
     12.5.  Stream Fragmentation and Reassembly Attacks . . . 110
   20. Transport Error Codes . . . 113
     12.6.  Stream Commitment Attack . . . . . . . . . . . . . . . . 113
     12.7.  Explicit Congestion Notification Attacks . 110
     20.1.  Application Protocol Error Codes . . . . . . . 114
     12.8.  Stateless Reset Oracle . . . . . 111
   21. Security Considerations . . . . . . . . . . . . 114
   13. IANA Considerations . . . . . . . 112
     21.1.  Handshake Denial of Service  . . . . . . . . . . . . . . 114
     13.1.  QUIC Transport Parameter Registry 112
     21.2.  Spoofed ACK Attack . . . . . . . . . . . 114
     13.2.  QUIC . . . . . . . . 113
     21.3.  Optimistic ACK Attack  . . . . . . . . . . . . . . . . . 113
     21.4.  Slowloris Attacks  . . . . . . . . . . . . . . . . . . . 114
     21.5.  Stream Fragmentation and Reassembly Attacks  . . . . . . 114
     21.6.  Stream Commitment Attack . . . . . . . . . . . . . . . . 114
     21.7.  Explicit Congestion Notification Attacks . . . . . . . . 115
     21.8.  Stateless Reset Oracle . . . . . . . . . . . . . . . . . 115
   22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 116
     22.1.  QUIC Transport Parameter Registry  . . . . . . . . . . . 116
     22.2.  QUIC Frame Type Registry . . . . . . . . . . . . . . . . 116
     13.3. 117
     22.3.  QUIC Transport Error Codes Registry  . . . . . . . . . . 117
   14. 118
   23. References  . . . . . . . . . . . . . . . . . . . . . . . . . 120
     14.1. 121
     23.1.  Normative References . . . . . . . . . . . . . . . . . . 120
     14.2. 121
     23.2.  Informative References . . . . . . . . . . . . . . . . . 121 122
   Appendix A.  Sample Packet Number Decoding Algorithm  . . . . . . 122 123
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 123 124
     B.1.  Since draft-ietf-quic-transport-14 draft-ietf-quic-transport-15  . . . . . . . . . . . 123 124
     B.2.  Since draft-ietf-quic-transport-13 draft-ietf-quic-transport-14  . . . . . . . . . . . 124
     B.3.  Since draft-ietf-quic-transport-12 draft-ietf-quic-transport-13  . . . . . . . . . . . 124 125
     B.4.  Since draft-ietf-quic-transport-11 draft-ietf-quic-transport-12  . . . . . . . . . . . 125 126
     B.5.  Since draft-ietf-quic-transport-10 draft-ietf-quic-transport-11  . . . . . . . . . . . 126
     B.6.  Since draft-ietf-quic-transport-09 draft-ietf-quic-transport-10  . . . . . . . . . . . 126 127
     B.7.  Since draft-ietf-quic-transport-08 draft-ietf-quic-transport-09  . . . . . . . . . . . 127
     B.8.  Since draft-ietf-quic-transport-07 draft-ietf-quic-transport-08  . . . . . . . . . . . 127 128
     B.9.  Since draft-ietf-quic-transport-06 draft-ietf-quic-transport-07  . . . . . . . . . . . 128 129
     B.10. Since draft-ietf-quic-transport-05 draft-ietf-quic-transport-06  . . . . . . . . . . . 129 130
     B.11. Since draft-ietf-quic-transport-04 draft-ietf-quic-transport-05  . . . . . . . . . . . 129 130
     B.12. Since draft-ietf-quic-transport-03 draft-ietf-quic-transport-04  . . . . . . . . . . . 130
     B.13. Since draft-ietf-quic-transport-02 draft-ietf-quic-transport-03  . . . . . . . . . . . 130 131
     B.14. Since draft-ietf-quic-transport-01 draft-ietf-quic-transport-02  . . . . . . . . . . . 131
     B.15. Since draft-ietf-quic-transport-00 draft-ietf-quic-transport-01  . . . . . . . . . . . 133 132
     B.16. Since draft-ietf-quic-transport-00  . . . . . . . . . . . 134
     B.17. Since draft-hamilton-quic-transport-protocol-01 . . . . . 133 134
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 133 134
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . . 134 135
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 134 135

1.  Introduction

   QUIC is a multiplexed and secure transport protocol that runs on top
   of UDP.  QUIC aims to provide a flexible set of features that allow
   it to be a general-purpose secure transport for multiple
   applications.

   o  Version negotiation

   o  Low-latency connection establishment

   o  Authenticated and encrypted header and payload

   o  Stream multiplexing

   o  Stream and connection-level flow control

   o  Connection migration and resilience to NAT rebinding

   QUIC uses UDP as a substrate to avoid requiring changes in legacy
   client operating systems and middleboxes.  QUIC authenticates all of
   its headers and encrypts most of the data it exchanges, including its
   signaling.  This allows the protocol to evolve without incurring a
   dependency on upgrades to middleboxes.

1.1.  Document Structure

   This document describes the core QUIC protocol, including the
   conceptual design, wire format, and mechanisms of is structured as
   follows:

   o  Streams are the basic service abstraction that QUIC protocol provides.

      *  Section 2 describes core concepts related to streams,

      *  Section 3 provides a reference model for connection establishment, stream multiplexing, stream states, and
   connection-level

      *  Section 4 outlines the operation of flow control, control.

   o  Connections are the context in which QUIC endpoints communicate.

      *  Section 5 describes core concepts related to connections,

      *  Section 6 describes version negotiation,

      *  Section 7 details the process for establishing connections,

      *  Section 8 specifies critical denial of service mitigation
         mechanisms,

      *  Section 9 describes how endpoints migrate a connection migration, to use a
         new network paths, and data
   reliability.

      *  Section 10 lists the options for terminating an open
         connection.

   o  Packets and frames are the basic unit used by QUIC to communicate.

      *  Section 12 describes concepts related to packets and frames,

      *  Section 13 defines models for the transmission, retransmission,
         and acknowledgement of information, and

      *  Section 14 contains a rules for managing the size of packets.

   o  Details of encoding of QUIC protocol elements is described in:

      *  Section 15 (Versions),

      *  Section 17 (Packet Headers),

      *  Section 18 (Transport Parameters),

      *  Section 19 (Frames), and

      *  Section 20 (Errors).

   Accompanying documents describe QUIC's loss detection and congestion
   control [QUIC-RECOVERY], and the use of TLS 1.3 for key negotiation
   [QUIC-TLS].

   QUIC version 1 conforms to the protocol invariants in
   [QUIC-INVARIANTS].

2.

1.2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Definitions of terms that are used in this document:

   Client:  The endpoint initiating a QUIC connection.

   Server:  The endpoint accepting incoming QUIC connections.

   Endpoint:  The client or server end of a connection.

   Stream:  A logical, bi-directional logical unidirectional or bidirectional channel of ordered
      bytes within a QUIC connection.

   Connection:  A conversation between two QUIC endpoints with a single
      encryption context that multiplexes streams within it.

   Connection ID:  An opaque identifier that is used to identify a QUIC
      connection at an endpoint.  Each endpoint sets a value that its
      peer includes in packets.

   QUIC packet:  The smallest unit of data that can be exchanged by QUIC
      endpoints.

   QUIC is a name, not an acronym.

2.1.

1.3.  Notational Conventions

   Packet and frame diagrams use the format described in Section 3.1 of
   [RFC2360], with the following additional conventions:

   [x]  Indicates that x is optional

   x (A)  Indicates that x is A bits long

   x (A/B/C) ...  Indicates that x is one of A, B, or C bits long

   x (i) ...  Indicates that x uses the variable-length encoding in
      Section 7.1 16

   x (*) ...  Indicates that x is variable-length

3.  Versions

2.  Streams

   Streams in QUIC versions are identified using provide a 32-bit unsigned number.

   The version 0x00000000 is reserved to represent version negotiation.
   This version lightweight, ordered byte-stream
   abstraction.

   There are two basic types of stream in QUIC.  Unidirectional streams
   carry data in one direction: from the specification is identified by the number
   0x00000001.

   Other versions initiator of QUIC might have different properties the stream to this
   version.  The properties of QUIC that are guaranteed its
   peer; bidirectional streams allow for data to be consistent
   across all versions of the protocol are described sent in
   [QUIC-INVARIANTS].

   Version 0x00000001 of QUIC uses TLS both
   directions.  Different stream identifiers are used to distinguish
   between unidirectional and bidirectional streams, as a cryptographic handshake
   protocol, well as described in [QUIC-TLS].

   Versions with the most significant 16 bits of the version number
   cleared are reserved for use in future IETF consensus documents.

   Versions to
   create a separation between streams that follow the pattern 0x?a?a?a?a are reserved for use in
   forcing version negotiation to be exercised.  That is, any version
   number where initiated by the low four bits of all octets is 1010 (in binary).  A client or
   and server MAY advertise support for any (see Section 2.1).

   Either type of these reserved
   versions.

   Reserved version numbers will probably never represent stream can be created by either endpoint, can
   concurrently send data interleaved with other streams, and can be
   cancelled.

   Streams can be created by sending data.  Other processes associated
   with stream management - ending, cancelling, and managing flow
   control - are all designed to impose minimal overheads.  For
   instance, a real
   protocol; single STREAM frame (Section 19.19) can open, carry data
   for, and close a client MAY use one of these version numbers with the
   expectation that stream.  Streams can also be long-lived and can last
   the server will initiate version negotiation; entire duration of a
   server MAY advertise support connection.

   Stream offsets allow for one of these versions and can expect
   that clients ignore the value.

   [[RFC editor: please remove octets on a stream to be placed in
   order.  An endpoint MUST be capable of delivering data received on a
   stream in order.  Implementations MAY choose to offer the remainder ability to
   deliver data out of this section before
   publication.]] order.  There is no means of ensuring ordering
   between octets on different streams.

   Streams are individually flow controlled, allowing an endpoint to
   limit memory commitment and to apply back pressure.  The version number for the final version creation of this specification
   (0x00000001),
   streams is reserved for also flow controlled, with each peer declaring the version maximum
   stream ID it is willing to accept at a given time.

   An alternative view of the protocol that QUIC streams is
   published as an RFC.

   Version numbers used elastic "message"
   abstraction, similar to identify IETF drafts are created by adding the draft number to 0xff000000.  For example, draft-ietf-quic-
   transport-13 would way ephemeral streams are used in SST
   [SST], which may be a more appealing description for some
   applications.

2.1.  Stream Identifiers

   Streams are identified by an unsigned 62-bit integer, referred to as 0xff00000D.

   Implementors
   the Stream ID.  Stream IDs are encouraged to register version numbers encoded as a variable-length integer
   (see Section 16).  The least significant two bits of QUIC that
   they are using for private experimentation on the GitHub wiki at
   <https://github.com/quicwg/base-drafts/wiki/QUIC-Versions>.

4.  Packet Types and Formats

   We first describe QUIC's packet types and their formats, since some
   are referenced in subsequent mechanisms.

   All numeric values Stream ID
   are encoded in network byte order (that is, big-
   endian) used to identify the type of stream (unidirectional or
   bidirectional) and all field sizes are in bits.  When discussing individual
   bits the initiator of fields, the stream.

   The least significant bit is referred to as bit 0.
   Hexadecimal notation is used for describing (0x1) of the value Stream ID identifies the
   initiator of fields.

   Any QUIC packet has either a long or a short header, as indicated by the Header Form bit.  Long headers are expected to be used early in stream.  Clients initiate even-numbered streams
   (those with the connection before version negotiation and establishment of 1-RTT
   keys.  Short headers are minimal version-specific headers, which are
   used after version negotiation and 1-RTT keys are established.

4.1.  Long Header
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|   Type (7)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Length (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packet Number (8/16/32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Payload (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1: Long Header Packet Format

   Long headers are used for packets that are sent prior least significant bit set to 0); servers initiate
   odd-numbered streams (with the
   completion bit set to 1).  Separation of version negotiation the
   stream identifiers ensures that client and establishment of 1-RTT keys.
   Once both conditions server are met, a sender switches able to sending packets
   using open
   streams without the short header (Section 4.2).  The long form allows latency imposed by negotiating for
   special packets - such as the Version Negotiation packet - to be
   represented in this uniform fixed-length packet format.  Packets an identifier.

   If an endpoint receives a frame for a stream that
   use it expects to
   initiate (i.e., odd-numbered for the long header contain client or even-numbered for the following fields:

   Header Form:
   server), but which it has not yet opened, it MUST close the
   connection with error code STREAM_STATE_ERROR.

   The most second least significant bit (0x80) (0x2) of octet 0 (the first
      octet) is the Stream ID
   differentiates between unidirectional streams and bidirectional
   streams.  Unidirectional streams always have this bit set to 1 for long headers.

   Long Packet Type: and
   bidirectional streams have this bit set to 0.

   The remaining seven two type bits of octet 0 contain the
      packet type.  This field can indicate one of 128 packet types.
      The types specified for this version are listed from a Stream ID therefore identify streams as
   summarized in Table 1.

   Version:

              +----------+----------------------------------+
              | Low Bits | Stream Type                      |
              +----------+----------------------------------+
              | 0x0      | Client-Initiated, Bidirectional  |
              |          |                                  |
              | 0x1      | Server-Initiated, Bidirectional  |
              |          |                                  |
              | 0x2      | Client-Initiated, Unidirectional |
              |          |                                  |
              | 0x3      | Server-Initiated, Unidirectional |
              +----------+----------------------------------+

                         Table 1: Stream ID Types

   The QUIC Version first bidirectional stream opened by the client is stream 0.

   A QUIC endpoint MUST NOT reuse a 32-bit field Stream ID.  Streams of each type are
   created in numeric order.  Streams that follows the Type.
      This field indicates which version are used out of QUIC is order result
   in use and
      determines how the rest opening all lower-numbered streams of the protocol fields are interpreted.

   DCIL and SCIL:  The octet following same type in the version contains same
   direction.

2.2.  Stream Concurrency

   QUIC allows for an arbitrary number of streams to operate
   concurrently.  An endpoint limits the lengths number of concurrently active
   incoming streams by limiting the two connection maximum stream ID fields that follow it.  These lengths are
      encoded as two 4-bit unsigned integers. (see Section 4.5).

   The Destination
      Connection maximum stream ID Length (DCIL) field occupies the 4 high bits of is specific to each endpoint and applies only
   to the
      octet peer that receives the setting.  That is, clients specify the
   maximum stream ID the server can initiate, and servers specify the Source Connection
   maximum stream ID Length (SCIL) field occupies the 4 low bits client can initiate.  Each endpoint may respond
   on streams initiated by the other peer, regardless of whether it is
   permitted to initiate new streams.

   Endpoints MUST NOT exceed the octet. limit set by their peer.  An encoded length of 0 indicates endpoint
   that the connection receives a STREAM frame with an ID is also 0 octets in length.  Non-zero
      encoded lengths are increased by 3 to get greater than the full length limit it has
   sent MUST treat this as a stream error of the
      connection ID, producing type STREAM_ID_ERROR
   (Section 11), unless this is a length between 4 and 18 octets
      inclusive.  For example, an octet with result of a change in the value 0x50 describes initial
   limits (see Section 7.3.1).

   A receiver cannot renege on an
      8-octet Destination Connection advertisement; that is, once a
   receiver advertises a stream ID and via a zero-length Source
      Connection ID.

   Destination Connection ID:  The Destination Connection MAX_STREAM_ID frame,
   advertising a smaller maximum ID field
      follows has no effect.  A receiver MUST
   ignore any MAX_STREAM_ID frame that does not increase the connection ID lengths maximum
   stream ID.

2.3.  Sending and is either 0 octets in length
      or between 4 Receiving Data

   Endpoints uses streams to send and 18 octets.  Section 4.10 describes receive data.  Endpoints send
   STREAM frames, which encapsulate data for a stream.  STREAM frames
   carry a flag that can be used to signal the use end of
      this field in more detail.

   Source Connection ID:  The Source Connection ID field follows the
      Destination Connection ID and a stream.

   Streams are an ordered byte-stream abstraction with no other
   structure that is either 0 octets in length visible to QUIC.  STREAM frame boundaries are not
   expected to preserved when data is transmitted, when data is
   retransmitted after packet loss, or
      between 4 and 18 octets.  Section 4.10 describes when data is delivered to the use of this
   application at the receiver.

   When new data is to be sent on a stream, a sender MUST set the
   encapsulating STREAM frame's offset field in more detail.

   Length:  The length of to the remainder stream offset of the packet (that is, the
      Packet Number and Payload fields) in octets, encoded as a
      variable-length integer (Section 7.1).

   Packet Number:  The packet number field is 1, 2, or 4 octets long.
   first octet of this new data.  The packet number first octet of data on a stream
   has confidentiality protection separate from
      packet protection, as described in Section 5.3 an offset of [QUIC-TLS]. 0.  An endpoint is expected to send every stream
   octet.  The
      length largest offset delivered on a stream MUST be less than
   2^62.

   QUIC makes no specific allowances for partial reliability or delivery
   of the packet number field stream data out of order.  Endpoints MUST be able to deliver
   stream data to an application as an ordered byte-stream.  Delivering
   an ordered byte-stream requires that an endpoint buffer any data that
   is encoded in the plaintext
      packet number.  See Section 4.11 for details.

   Payload:  The payload received out of order, up to the packet.

   The following packet types are defined:

                 +------+-----------------+-------------+
                 | Type | Name            | Section     |
                 +------+-----------------+-------------+
                 | 0x7F | Initial         | Section 4.6 |
                 |      |                 |             |
                 | 0x7E | Retry           | Section 4.4 |
                 |      |                 |             |
                 | 0x7D | Handshake       | Section 4.7 |
                 |      |                 |             |
                 | 0x7C | 0-RTT Protected | Section 4.8 |
                 +------+-----------------+-------------+

                     Table 1: Long Header Packet Types advertised flow control limit.

   An endpoint could receive the same octets multiple times; octets that
   have already been received can be discarded.  The header form, type, connection ID lengths octet, destination and
   source connection IDs, and version fields value for a given
   octet MUST NOT change if it is sent multiple times; an endpoint MAY
   treat receipt of a long header packet are
   version-independent.  The packet number and values for packet types
   defined changed octet as a connection error of type
   PROTOCOL_VIOLATION.

   An endpoint MUST NOT send data on any stream without ensuring that it
   is within the data limits set by its peer.  Flow control is described
   in Table 1 detail in Section 4.

2.4.  Stream Prioritization

   Stream multiplexing has a significant effect on application
   performance if resources allocated to streams are version-specific.  See [QUIC-INVARIANTS] correctly
   prioritized.  Experience with other multiplexed protocols, such as
   HTTP/2 [HTTP2], shows that effective prioritization strategies have a
   significant positive impact on performance.

   QUIC does not provide frames for
   details exchanging prioritization
   information.  Instead it relies on how packets receiving priority information
   from different versions of the application that uses QUIC.  Protocols that use QUIC are
   interpreted.

   The interpretation
   able to define any prioritization scheme that suits their application
   semantics.  A protocol might define explicit messages for signaling
   priority, such as those defined in HTTP/2; it could define rules that
   allow an endpoint to determine priority based on context; or it could
   leave the determination to the application.

   A QUIC implementation SHOULD provide ways in which an application can
   indicate the relative priority of streams.  When deciding which
   streams to dedicate resources to, QUIC SHOULD use the fields and information
   provided by the payload are specific application.  Failure to a
   version and packet type.  Type-specific semantics account for this version
   are described priority of
   streams can result in the following sections.

   The end suboptimal performance.

   Stream priority is most relevant when deciding which stream data will
   be transmitted.  Often, there will be limits on what can be
   transmitted as a result of connection flow control or the packet is determined by current
   congestion controller state.

   Giving preference to the Length field.  The Length
   field covers both transmission of its own management frames
   ensures that the Packet Number protocol functions efficiently.  That is,
   prioritizing frames other than STREAM frames ensures that loss
   recovery, congestion control, and Payload fields, both flow control operate effectively.

   CRYPTO frames SHOULD be prioritized over other streams prior to the
   completion of which
   are confidentiality protected and initially the cryptographic handshake.  This includes the
   retransmission of unknown length.  The
   size the second flight of client handshake messages,
   that is, the Payload field is learned once TLS Finished and any client authentication messages.

   STREAM data in frames determined to be lost SHOULD be retransmitted
   before sending new data, unless application priorities indicate
   otherwise.  Retransmitting lost stream data can fill in gaps, which
   allows the packet number
   protection is removed.

   Senders can sometimes coalesce multiple packets into one UDP
   datagram.  See Section 4.9 for more details.

4.2.  Short Header

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |0|K|1|1|0|R R R|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Destination Connection ID (0..144)           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Packet Number (8/16/32)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Protected Payload (*)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: Short Header Packet Format

   The short header can be used after the version peer to consume already received data and 1-RTT keys are
   negotiated.  Packets that use the short header contain free up the following
   fields:

   Header Form:  The most significant bit (0x80) flow
   control window.

3.  Stream States: Life of octet 0 is set to 0
      for a Stream

   This section describes the short header.

   Key Phase Bit:  The second bit (0x40) two types of octet 0 indicates the key
      phase, which allows a recipient QUIC stream in terms of a packet to identify the packet
      protection keys that
   states of their send or receive components.  Two state machines are used to protect the packet.  See
      [QUIC-TLS]
   described: one for details.

   [[Editor's Note: this section should be removed and streams on which an endpoint transmits data
   (Section 3.1); another for streams from which an endpoint receives
   data (Section 3.2).

   Unidirectional streams use the bit
   definitions changed before this draft goes to applicable state machine directly.
   Bidirectional streams use both state machines.  For the IESG.]]

   Third Bit:  The third bit (0x20) most part,
   the use of octet 0 these state machines is set to 1.

   [[Editor's Note: this section should be removed and the bit
   definitions changed before this draft goes to same whether the IESG.]]

   Fourth Bit: stream is
   unidirectional or bidirectional.  The fourth bit (0x10) conditions for opening a stream
   are slightly more complex for a bidirectional stream because the
   opening of octet 0 is set to 1.

   [[Editor's Note: this section should be removed and either send or receive sides causes the bit
   definitions changed before this draft goes stream to open in
   both directions.

   An endpoint can open streams up to its maximum stream limit in any
   order, however endpoints SHOULD open the IESG.]]

   Google QUIC Demultiplexing Bit:  The fifth bit (0x8) send side of octet 0 is
      set to 0. streams for
   each type in order.

   Note:  These states are largely informative.  This allows implementations of Google QUIC to
      distinguish Google QUIC packets from short header packets sent by
      a client because Google QUIC servers expect the connection ID document uses
      stream states to
      always be present.  The special interpretation describe rules for when and how different types
      of this bit SHOULD frames can be removed from this specification when Google QUIC has finished
      transitioning to the new header format.

   Reserved:  The sixth, seventh, sent and eighth bits (0x7) the reactions that are expected when
      different types of octet 0 frames are
      reserved for experimentation.  Endpoints MUST ignore received.  Though these bits on
      packets they receive unless they state
      machines are participating intended to be useful in an
      experiment that uses implementing QUIC, these bits.
      states aren't intended to constrain implementations.  An endpoint not actively using
      these bits SHOULD set the value randomly on packets they send to
      protect against unwanted inference about particular values.

   Destination Connection ID:  The Destination Connection ID is
      implementation can define a
      connection ID that is chosen by the intended recipient of the
      packet.  See Section 6.1 for more details.

   Packet Number:  The packet number field is 1, 2, or 4 octets long.
      The packet number has confidentiality protection separate from
      packet protection, different state machine as described in Section 5.3 of [QUIC-TLS].  The
      length of the packet number field long as its
      behavior is encoded in the plaintext
      packet number.  See Section 4.11 for details.

   Protected Payload:  Packets consistent with a short header always include a
      1-RTT protected payload.

   The header form and connection ID field of a short header packet are
   version-independent.  The remaining fields are specific to an implementation that implements
      these states.

3.1.  Send Stream States

   Figure 1 shows the
   selected QUIC version.  See [QUIC-INVARIANTS] states for details on how
   packets from different versions of QUIC are interpreted.

4.3.  Version Negotiation Packet

   A Version Negotiation packet is inherently not version-specific, and
   does not use the long packet header (see Section 4.1.  Upon receipt
   by a client, it will appear to be a packet using the long header, but
   will be identified as a Version Negotiation packet based on the
   Version field having a value part of 0.

   The Version Negotiation packet is a response to a client packet that
   contains a version stream that is not supported by the server, and is only
   sent by servers.

   The layout of sends data to
   a Version Negotiation packet is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|  Unused (7) peer.

          o
          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Create Stream (Sending)
          |                          Version (32) Create Bidirectional Stream (Receiving)
          v
      +-------+
      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Ready |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Send RST_STREAM
      |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |-----------------------.
      +-------+                       |                    Supported Version 1 (32)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                   [Supported Version 2 (32)]                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                           |                   [Supported Version N (32)]                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | Send STREAM /             |
          |      STREAM_BLOCKED       |
          |                           |
          | Create Bidirectional      |
          |      Stream (Receiving)   |
          v                           |
      +-------+                       |
      | Send  | Send RST_STREAM       |
      |       |---------------------->|
      +-------+                       |
          |                           |
          | Send STREAM + FIN         |
          v                           v
      +-------+                   +-------+
      | Data  | Send RST_STREAM   | Reset |
      | Sent  |------------------>| Sent  |
      +-------+                   +-------+
          |                           |
          | Recv All ACKs             | Recv ACK
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Recvd |                   | Recvd |
      +-------+                   +-------+

                     Figure 3: Version Negotiation Packet 1: States for Send Streams

   The value in sending part of stream that the Unused field endpoint initiates (types 0 and 2
   for clients, 1 and 3 for servers) is selected randomly opened by the server. application or
   application protocol.  The Version field of "Ready" state represents a Version Negotiation packet MUST be set newly created
   stream that is able to
   0x00000000.

   The server MUST include the value accept data from the Source Connection ID field
   of the packet it receives in the Destination Connection ID field.
   The value for Source Connection ID MUST application.  Stream data
   might be copied from buffered in this state in preparation for sending.

   Sending the
   Destination Connection first STREAM or STREAM_BLOCKED frame causes a send stream
   to enter the "Send" state.  An implementation might choose to defer
   allocating a Stream ID of to a send stream until it sends the received packet, first
   frame and enters this state, which is initially
   randomly selected can allow for better stream
   prioritization.

   The sending part of a bidirectional stream initiated by a client.  Echoing both connection IDs gives
   clients some assurance that peer (type
   0 for a server, type 1 for a client) enters the server received "Ready" state then
   immediately transitions to the packet and that "Send" state if the Version Negotiation packet was not generated by receiving part
   enters the "Recv" state.

   In the "Send" state, an off-path
   attacker. endpoint transmits - and retransmits as
   necessary - data in STREAM frames.  The remainder endpoint respects the flow
   control limits of its peer, accepting MAX_STREAM_DATA frames.  An
   endpoint in the Version Negotiation packet "Send" state generates STREAM_BLOCKED frames if it
   encounters flow control limits.

   After the application indicates that stream data is complete and a list of 32-bit
   versions which
   STREAM frame containing the server supports.

   A Version Negotiation packet cannot be explicitly acknowledged FIN bit is sent, the send stream enters
   the "Data Sent" state.  From this state, the endpoint only
   retransmits stream data as necessary.  The endpoint no longer needs
   to track flow control limits or send STREAM_BLOCKED frames for a send
   stream in this state.  The endpoint can ignore any MAX_STREAM_DATA
   frames it receives from its peer in this state; MAX_STREAM_DATA
   frames might be received until the peer receives the final stream
   offset.

   Once all stream data has been successfully acknowledged, the send
   stream enters the "Data Recvd" state, which is a terminal state.

   From any of the "Ready", "Send", or "Data Sent" states, an
   ACK frame by
   application can signal that it wishes to abandon transmission of
   stream data.  Similarly, the endpoint might receive a client.  Receiving another Initial packet implicitly
   acknowledges STOP_SENDING
   frame from its peer.  In either case, the endpoint sends a Version Negotiation packet.

   The Version Negotiation packet does not include RST_STREAM
   frame, which causes the Packet Number and
   Length fields present in other packets that use stream to enter the long header form.
   Consequently, "Reset Sent" state.

   An endpoint MAY send a Version Negotiation packet consumes an entire UDP
   datagram.

   See Section 6.3 for RST_STREAM as the first frame on a description of send
   stream; this causes the version negotiation process.

4.4.  Retry Packet

   A Retry packet uses send stream to open and then immediately
   transition to the "Reset Sent" state.

   Once a long packet header with containing a type value of 0x7E.
   It carries an address validation token created by RST_STREAM has been acknowledged, the server.  It send
   stream enters the "Reset Recvd" state, which is
   used by a server terminal state.

3.2.  Receive Stream States

   Figure 2 shows the states for the part of a stream that wishes to perform receives data
   from a stateless retry (see
   Section 6.7).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|    0x7e     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    ODCIL(8)   |      Original Destination Connection ID (*)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Retry Token (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 4: Retry Packet

   A Retry packet (shown in Figure 4) peer.  The states for a receive stream mirror only uses the invariant portion some of the long packet header [QUIC-INVARIANTS]; that is,
   states of the fields up to
   and including send stream at the Destination and Source Connection ID fields. peer.  A
   Retry packet does not contain any protected fields.  Like Version
   Negotiation, a Retry packet contains receive stream doesn't
   track states on the long header including send stream that cannot be observed, such as the
   connection IDs, but omits
   "Ready" state; instead, receive streams track the Length, Packet Number, and Payload
   fields.  These are replaced with:

   ODCIL:  The length delivery of data to
   the Original Destination Connection ID field.
      The length is encoded in the least significant 4 bits application or application protocol some of which cannot be
   observed by the
      octet, using the same encoding as the DCIL and SCIL fields. sender.

          o
          | Recv STREAM / STREAM_BLOCKED / RST_STREAM
          | Create Bidirectional Stream (Sending)
          | Recv MAX_STREAM_DATA
          | Create Higher-Numbered Stream
          v
      +-------+
      | Recv  | Recv RST_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Recv STREAM + FIN         |
          v                           |
      +-------+                       |
      | Size  | Recv RST_STREAM       |
      | Known |---------------------->|
      +-------+                       |
          |                           |
          | Recv All Data             |
          v                           v
      +-------+  Recv RST_STREAM  +-------+
      | Data  |--- (optional) --->| Reset |
      | Recvd |  Recv All Data    | Recvd |
      +-------+<-- (optional) ----+-------+
          |                           |
          | App Read All Data         | App Read RST
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Read  |                   | Read  |
      +-------+                   +-------+

                   Figure 2: States for Receive Streams

   The
      most significant 4 bits receiving part of this octet are reserved.  Unless a use stream initiated by a peer (types 1 and 3 for these bits has been negotiated, endpoints SHOULD send
      randomized values
   a client, or 0 and MUST ignore any value that it receives.

   Original Destination Connection ID:  The Original Destination
      Connection ID contains the value of the Destination Connection ID
      from 2 for a server) are created when the Initial packet that this Retry first STREAM,
   STREAM_BLOCKED, RST_STREAM, or MAX_STREAM_DATA (bidirectional only,
   see below) is in response to. received for that stream.  The
      length of this field initial state for a
   receive stream is given in ODCIL.

   Retry Token:  An opaque token that "Recv".  Receiving a RST_STREAM frame causes the server can use
   receive stream to immediately transition to validate the
      client's address. "Reset Recvd".

   The server populates the Destination Connection ID with the
   connection ID that receive stream enters the client included in "Recv" state when the Source Connection ID sending part of
   the Initial packet.

   The server includes a connection ID of its choice in the Source
   Connection ID field.  This value MUST not be equal to the Destination
   Connection ID field of the packet sent
   bidirectional stream initiated by the client.  The client
   MUST use this connection ID in endpoint (type 0 for a client,
   type 1 for a server) enters the Destination Connection ID of
   subsequent packets that it sends.

   A server MAY send Retry packets in response to Initial and 0-RTT
   packets. "Ready" state.

   A server can either discard or buffer 0-RTT packets bidirectional stream also opens when a MAX_STREAM_DATA frame is
   received.  Receiving a MAX_STREAM_DATA frame implies that it
   receives.  A server can send multiple Retry packets as it receives
   Initial or 0-RTT packets.

   A client MUST accept and process at most one Retry packet for each
   connection attempt.  After the client remote
   peer has received opened the stream and processed an
   Initial is providing flow control credit.  A
   MAX_STREAM_DATA frame might arrive before a STREAM or Retry packet from the server, it MUST discard any
   subsequent Retry STREAM_BLOCKED
   frame if packets that it receives.

   Clients are lost or reordered.

   Before creating a stream, all lower-numbered streams of the same type
   MUST discard Retry packets that contain an Original
   Destination Connection ID field be created.  That means that does not match the Destination
   Connection ID from its Initial packet.  This prevents an off-path
   attacker from injecting a Retry packet.

   The client responds to receipt of a Retry packet with an Initial packet frame that
   includes would open
   a stream causes all lower-numbered streams of the provided Retry Token same type to continue connection
   establishment.

   A client sets be
   opened in numeric order.  This ensures that the Destination Connection ID field of this Initial
   packet to creation order for
   streams is consistent on both endpoints.

   In the value from "Recv" state, the Source Connection ID in endpoint receives STREAM and STREAM_BLOCKED
   frames.  Incoming data is buffered and can be reassembled into the Retry
   packet.  Changing Destination Connection ID also results in a change
   correct order for delivery to the keys used to protect application.  As data is consumed
   by the Initial packet.  It also sets application and buffer space becomes available, the
   Token field endpoint
   sends MAX_STREAM_DATA frames to allow the token provided in peer to send more data.

   When a STREAM frame with a FIN bit is received, the Retry. final offset (see
   Section 4.3) is known.  The client MUST NOT
   change the Source Connection ID because receive stream enters the server could include "Size Known"
   state.  In this state, the
   connection ID as part endpoint no longer needs to send
   MAX_STREAM_DATA frames, it only receives any retransmissions of its token validation logic (see
   Section 4.6.2).

   All subsequent Initial packets from
   stream data.

   Once all data for the client MUST use stream has been received, the
   connection ID and token values from receive stream
   enters the Retry packet.  Aside from
   this, "Data Recvd" state.  This might happen as a result of
   receiving the Initial packet sent by same STREAM frame that causes the client is subject transition to "Size
   Known".  In this state, the same
   restrictions as endpoint has all stream data.  Any STREAM
   or STREAM_BLOCKED frames it receives for the first Initial packet.  A client stream can either reuse be discarded.

   The "Data Recvd" state persists until stream data has been delivered
   to the cryptographic handshake message application or construct application protocol.  Once stream data has
   been delivered, the stream enters the "Data Read" state, which is a new one at its
   discretion.

   A client MAY attempt 0-RTT after receiving
   terminal state.

   Receiving a Retry packet by sending
   0-RTT packets to the connection ID provided by RST_STREAM frame in the server.  A client
   that sends additional 0-RTT packets without constructing a new
   cryptographic handshake message MUST NOT reset "Recv" or "Size Known" states
   causes the packet number stream to 0
   after a Retry packet, see Section 4.6.4.

   A server acknowledges the use of a Retry packet for a connection
   using the original_connection_id transport parameter (see
   Section 6.6.1).  If the server sends a Retry packet, it MUST include enter the value of "Reset Recvd" state.  This might cause
   the Original Destination Connection ID field delivery of stream data to the
   Retry packet application to be interrupted.

   It is possible that all stream data is received when a RST_STREAM is
   received (that is, the Destination Connection ID field from the
   client's first Initial packet) in the transport parameter.

   If the client received and processed a Retry packet, it validates
   that the original_connection_id transport parameter is present and
   correct; otherwise, "Data Recvd" state).  Similarly, it validates that the transport parameter is
   absent.  A client MUST treat
   possible for remaining stream data to arrive after receiving a failed validation
   RST_STREAM frame (the "Reset Recvd" state).  An implementation is
   able to manage this situation as a connection
   error they choose.  Sending RST_STREAM
   means that an endpoint cannot guarantee delivery of type TRANSPORT_PARAMETER_ERROR.

   A Retry packet does stream data;
   however there is no requirement that stream data not include a packet number and cannot be
   explicitly acknowledged by delivered if
   a client.

4.5.  Cryptographic Handshake Packets

   Once version negotiation is complete, the cryptographic handshake is
   used to agree on cryptographic keys.  The cryptographic handshake RST_STREAM is
   carried in Initial (Section 4.6) received.  An implementation MAY interrupt delivery
   of stream data, discard any data that was not consumed, and Handshake (Section 4.7) packets.

   All these packets use signal
   the long header and contain existence of the current QUIC
   version in RST_STREAM immediately.  Alternatively, the version field.

   In order to prevent tampering by version-unaware middleboxes, Initial
   packets are protected with connection- and version-specific keys
   (Initial keys) as described in [QUIC-TLS].  This protection does not
   provide confidentiality
   RST_STREAM signal might be suppressed or integrity against on-path attackers, but
   provides some level of protection against off-path attackers.

4.6.  Initial Packet

   The Initial packet uses long headers with a type value of 0x7F.  It
   carries withheld if stream data is
   completely received.  In the first CRYPTO frames sent by latter case, the client and server receive stream
   effectively transitions to
   perform key exchange, and carries ACKs in either direction.  The
   Initial packet is protected by Initial keys as described in
   [QUIC-TLS].

   The Initial packet (shown in Figure 5) "Data Recvd" from "Reset Recvd".

   Once the application has two additional header
   fields been delivered the signal indicating that are added to
   the Long Header before receive stream was reset, the Length field.

   +-+-+-+-+-+-+-+-+
   |1|    0x7f     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Token Length (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Token (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Length (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packet Number (8/16/32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Payload (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 5: Initial Packet

   These fields include receive stream transitions to the token that was previously provided in a
   Retry packet or NEW_TOKEN frame:

   Token Length:  A variable-length integer specifying the length of the
      Token field, in bytes.  This value is zero if no token is present.
      Initial packets sent by the server MUST set the Token Length field
      to zero; clients that receive an Initial packet with a non-zero
      Token Length field MUST either discard the packet or generate a
      connection error of type PROTOCOL_VIOLATION.

   Token:  The value of the token.

   The client and server use the Initial packet type for any packet that
   contains an initial cryptographic handshake message.  This includes
   all cases where a new packet containing the initial cryptographic
   message needs to be created, such as the packets sent after receiving
   a Version Negotiation (Section 4.3) or Retry packet (Section 4.4).

   A server sends its first Initial packet in response to a client
   Initial.  A server may send multiple Initial packets.  The
   cryptographic key exchange could require multiple round trips or
   retransmissions of this data.

   The payload of an Initial packet includes a CRYPTO frame (or frames)
   containing a cryptographic handshake message, ACK frames, or both.
   PADDING and CONNECTION_CLOSE frames are also permitted.  An endpoint
   that receives an Initial packet containing other frames can either
   discard the packet as spurious or treat it as a connection error.

   The first packet sent by a client always includes a CRYPTO frame that
   contains the entirety of the first cryptographic handshake message.
   This packet, and the cryptographic handshake message, MUST fit in a
   single UDP datagram (see Section 6.4).  The first CRYPTO frame sent
   always begins at an offset of 0 (see Section 6.4).

   Note that if the server sends a HelloRetryRequest, the client will
   send a second Initial packet.  This Initial packet will continue the
   cryptographic handshake and will contain a CRYPTO frame with an
   offset matching the size of the CRYPTO frame sent in the first
   Initial packet.  Cryptographic handshake messages subsequent to the
   first do not need to fit within a single UDP datagram.

4.6.1.  Connection IDs

   When an Initial packet is sent by a client
   "Reset Read" state, which has not previously
   received a Retry packet from the server, it populates the Destination
   Connection ID field with an unpredictable value.  This MUST be at
   least 8 octets in length.  Until a packet is received from the
   server, the client MUST use the same value unless it abandons the
   connection attempt and starts a new one.  The initial Destination
   Connection ID is used to determine packet protection keys for Initial
   packets.

   The client populates the Source Connection ID field with a value of
   its choosing and sets the SCIL field to match.

   The Destination Connection ID field in the server's Initial packet
   contains a connection ID that is chosen by the recipient of the
   packet (i.e., the client); the Source Connection ID includes the
   connection ID that the sender of the packet wishes to use (see
   Section 6.1).  The server MUST use consistent Source Connection IDs
   during the handshake.

   On first receiving an Initial or Retry packet from the server, the
   client uses the Source Connection ID supplied by the server as the
   Destination Connection ID for subsequent packets.  That means that a
   client might change the Destination Connection ID twice during
   connection establishment.  Once a client has received an Initial
   packet from the server, it MUST discard any packet it receives with a
   different Source Connection ID.

4.6.2.  Tokens

   If the client has a token received in a NEW_TOKEN frame on a previous
   connection to what it believes to be the same server, it can include
   that value in the Token field of its Initial packet.

   A token allows a server to correlate activity between connections.
   Specifically, the connection where the token was issued, and any
   connection where it is used.  Clients that want to break continuity
   of identity with a server MAY discard tokens provided using the
   NEW_TOKEN frame.  Tokens obtained in Retry packets MUST NOT be
   discarded.

   A client SHOULD NOT reuse a token.  Reusing a token allows
   connections to be linked by entities on the network path (see
   Section 6.11.5).  A client MUST NOT reuse a token if it believes that
   its point of network attachment has changed since the token was last
   used; that is, if there is a change in its local IP address or
   network interface.  A client needs to start the connection process
   over if it migrates prior to completing the handshake.

   When a server receives an Initial packet with an address validation
   token, it SHOULD attempt to validate it.  If the token is invalid
   then the server SHOULD proceed as if the client did not have a
   validated address, including potentially sending a Retry.  If the
   validation succeeds, the server SHOULD then allow the handshake to
   proceed (see Section 6.7).

   Note:  The rationale for treating the client as unvalidated rather
      than discarding the packet is that the client might have received
      the token in a previous connection using the NEW_TOKEN frame, and
      if the server has lost state, it might be unable to validate the
      token at all, leading to connection failure if the packet is
      discarded.  A server MAY encode tokens provided with NEW_TOKEN
      frames and Retry packets differently, and validate the latter more
      strictly.

   In a stateless design, a server can use encrypted and authenticated
   tokens to pass information to clients that the server can later
   recover and use to validate a client address.  Tokens are not
   integrated into the cryptographic handshake and so they are not
   authenticated.  For instance, a client might be able to reuse a
   token.  To avoid attacks that exploit this property, a server can
   limit its use of tokens to only the information needed validate
   client addresses.

4.6.3.  Starting Packet Numbers

   The first Initial packet sent by either endpoint contains a packet
   number of 0.  The packet number MUST increase monotonically
   thereafter.  Initial packets are in a different packet number space
   to other packets (see Section 4.11).

4.6.4.  0-RTT Packet Numbers

   Packet numbers for 0-RTT protected packets use the same space as
   1-RTT protected packets.

   After a client receives a Retry or Version Negotiation packet, 0-RTT
   packets are likely to have been lost or discarded by the server.  A
   client MAY attempt to resend data in 0-RTT packets after it sends a
   new Initial packet.

   A client MUST NOT reset the packet number it uses for 0-RTT packets.
   The keys used to protect 0-RTT packets will not change as a result of
   responding to a Retry or Version Negotiation packet unless the client
   also regenerates the cryptographic handshake message.  Sending
   packets with the same packet number in that case is likely to
   compromise the packet protection for all 0-RTT packets because the
   same key and nonce could be used to protect different content.

   Receiving a Retry or Version Negotiation packet, especially a Retry
   that changes the connection ID used for subsequent packets, indicates
   a strong possibility that 0-RTT packets could be lost.  A client only
   receives acknowledgments for its 0-RTT packets once the handshake is
   complete.  Consequently, a server might expect 0-RTT packets to start
   with a packet number of 0.  Therefore, in determining the length of
   the packet number encoding for 0-RTT packets, a client MUST assume
   that all packets up to the current packet number are in flight,
   starting from a packet number of 0.  Thus, 0-RTT packets could need
   to use a longer packet number encoding.

   A client SHOULD instead generate a fresh cryptographic handshake
   message and start packet numbers from 0.  This ensures that new 0-RTT
   packets will not use the same keys, avoiding any risk of key and
   nonce reuse; this also prevents 0-RTT packets from previous handshake
   attempts from being accepted as part of the connection.

4.6.5.  Minimum Packet Size

   The payload of a UDP datagram carrying the Initial packet MUST be
   expanded to at least 1200 octets (see Section 8), by adding PADDING
   frames to the Initial packet and/or by combining the Initial packet
   with a 0-RTT packet (see Section 4.9).

4.7.  Handshake Packet

   A Handshake packet uses long headers with a type value of 0x7D.  It
   is used to carry acknowledgments and cryptographic handshake messages
   from the server and client.

   A server sends its cryptographic handshake in one or more Handshake
   packets in response to an Initial packet if it does not send a Retry
   packet.  Once a client has received a Handshake packet from a server,
   it uses Handshake packets to send subsequent cryptographic handshake
   messages and acknowledgments to the server.

   The Destination Connection ID field in a Handshake packet contains a
   connection ID that is chosen by the recipient of the packet; the
   Source Connection ID includes the connection ID that the sender of
   the packet wishes to use (see Section 4.10).

   The first Handshake packet sent by a server contains a packet number
   of 0.  Handshake packets are their own packet number space.  Packet
   numbers are incremented normally for other Handshake packets.

   Servers MUST NOT send more than three times as many bytes as the
   number of bytes received prior to verifying the client's address.
   Source addresses can be verified through an address validation token
   (delivered via a Retry packet or a NEW_TOKEN frame) or by processing
   any message from the client encrypted using the Handshake keys.  This
   limit exists to mitigate amplification attacks.

   In order to prevent this limit causing a handshake deadlock, the
   client SHOULD always send a packet upon a handshake timeout, as
   described in [QUIC-RECOVERY].  If the client has no data to
   retransmit and does not have Handshake keys, it SHOULD send an
   Initial packet in a UDP datagram of at least 1200 octets.  If the
   client has Handshake keys, it SHOULD send a Handshake packet.

   The payload of this packet contains CRYPTO frames and could contain
   PADDING, or ACK frames.  Handshake packets MAY contain
   CONNECTION_CLOSE or APPLICATION_CLOSE frames.  Endpoints MUST treat
   receipt of Handshake packets with other frames as a connection error.

4.8.  Protected Packets

   All QUIC packets use packet protection.  Packets that are protected
   with the static handshake keys or the 0-RTT keys are sent with long
   headers; all packets protected with 1-RTT keys are sent with short
   headers.  The different packet types explicitly indicate the
   encryption level and therefore the keys that are used to remove
   packet protection.  0-RTT and 1-RTT protected packets share a single
   packet number space.

   Packets protected with handshake keys only use packet protection to
   ensure that the sender of the packet is on the network path.  This
   packet protection is not effective confidentiality protection; any
   entity that receives the Initial packet from a client can recover the
   keys necessary to remove packet protection or to generate packets
   that will be successfully authenticated.

   Packets protected with 0-RTT and 1-RTT keys are expected to have
   confidentiality and data origin authentication; the cryptographic
   handshake ensures that only the communicating endpoints receive the
   corresponding keys.

   Packets protected with 0-RTT keys use a type value of 0x7C.  The
   connection ID fields for a 0-RTT packet MUST match the values used in
   the Initial packet (Section 4.6).

   The version field for protected packets is the current QUIC version.

   The packet number field contains a packet number, which has
   additional confidentiality protection that is applied after packet
   protection is applied (see [QUIC-TLS] for details).  The underlying
   packet number increases with each packet sent, see Section 4.11 for
   details.

   The payload is protected using authenticated encryption.  [QUIC-TLS]
   describes packet protection in detail.  After decryption, the
   plaintext consists of a sequence of frames, as described in
   Section 5.

4.9.  Coalescing Packets

   A sender can coalesce multiple QUIC packets (typically a
   Cryptographic Handshake packet and a Protected packet) into one UDP
   datagram.  This can reduce the number of UDP datagrams needed to send
   application data during the handshake and immediately afterwards.  It
   is not necessary for senders to coalesce packets, though failing to
   do so will require sending a significantly larger number of datagrams
   during the handshake.  Receivers MUST be able to process coalesced
   packets.

   Coalescing packets in order of increasing encryption levels (Initial,
   0-RTT, Handshake, 1-RTT) makes it more likely the receiver will be
   able to process all the packets in a single pass.  A packet with a
   short header does not include a length, so it will always be the last
   packet included in a UDP datagram.

   Senders MUST NOT coalesce QUIC packets with different Destination
   Connection IDs into a single UDP datagram.  Receivers SHOULD ignore
   any subsequent packets with a different Destination Connection ID
   than the first packet in the datagram.

   Every QUIC packet that is coalesced into a single UDP datagram is
   separate and complete.  Though the values of some fields in the
   packet header might be redundant, no fields are omitted.  The
   receiver of coalesced QUIC packets MUST individually process each
   QUIC packet and separately acknowledge them, as if they were received
   as the payload of different UDP datagrams.  If one or more packets in
   a datagram cannot be processed yet (because the keys are not yet
   available) or processing fails (decryption failure, unknown type,
   etc.), the receiver MUST still attempt to process the remaining
   packets.  The skipped packets MAY either be discarded or buffered for
   later processing, just as if the packets were received out-of-order
   in separate datagrams.

   Retry (Section 4.4) and Version Negotiation (Section 4.3) packets
   cannot be coalesced.

4.10.  Connection ID Encoding

   A connection ID is used to ensure consistent routing of packets, as
   described in Section 6.1.  The long header contains two connection
   IDs: the Destination Connection ID is chosen by the recipient of the
   packet and is used to provide consistent routing; the Source
   Connection ID is used to set the Destination Connection ID used by
   the peer.

   During the handshake, packets with the long header are used to
   establish the connection ID that each endpoint uses.  Each endpoint
   uses the Source Connection ID field to specify the connection ID that
   is used in the Destination Connection ID field of packets being sent
   to them.  Upon receiving a packet, each endpoint sets the Destination
   Connection ID it sends to match the value of the Source Connection ID
   that they receive.

   During the handshake, a client can receive both a Retry and an
   Initial packet, and thus be given two opportunities to update the
   Destination Connection ID it sends.  A client MUST only change the
   value it sends in the Destination Connection ID in response to the
   first packet of each type it receives from the server (Retry or
   Initial); a server MUST set its value based on the Initial packet.
   Any additional changes are not permitted; if subsequent packets of
   those types include a different Source Connection ID, they MUST be
   discarded.  This avoids problems that might arise from stateless
   processing of multiple Initial packets producing different connection
   IDs.

   Short headers only include the Destination Connection ID and omit the
   explicit length.  The length of the Destination Connection ID field
   is expected to be known to endpoints.

   Endpoints using a connection-ID based load balancer could agree with
   the load balancer on a fixed or minimum length and on an encoding for
   connection IDs.  This fixed portion could encode an explicit length,
   which allows the entire connection ID to vary in length and still be
   used by the load balancer.

   The very first packet sent by a client includes a random value for
   Destination Connection ID.  The same value MUST be used for all 0-RTT
   packets sent on that connection (Section 4.8).  This randomized value
   is used to determine the packet protection keys for Initial packets
   (see Section 5.2 of [QUIC-TLS]).

   A Version Negotiation (Section 4.3) packet MUST use both connection
   IDs selected by the client, swapped to ensure correct routing toward
   the client.

   The connection ID can change over the lifetime of a connection,
   especially in response to connection migration (Section 6.11).
   NEW_CONNECTION_ID frames (Section 7.13) are used to provide new
   connection ID values.

4.11.  Packet Numbers

   The packet number is an integer in the range 0 to 2^62-1.  The value
   is used in determining the cryptographic nonce for packet protection.
   Each endpoint maintains a separate packet number for sending and
   receiving.

   Packet numbers are divided into 3 spaces in QUIC:

   o  Initial space: All Initial packets Section 4.6 are in this space.

   o  Handshake space: All Handshake packets Section 4.7 are in this
      space.

   o  Application data space: All 0-RTT and 1-RTT encrypted packets
      Section 4.8 are in this space.

   As described in [QUIC-TLS], each packet type uses different
   protection keys.

   Conceptually, a packet number space is the context in which a packet
   can be processed and acknowledged.  Initial packets can only be sent
   with Initial packet protection keys and acknowledged in packets which
   are also Initial packets.  Similarly, Handshake packets are sent at
   the Handshake encryption level and can only be acknowledged in
   Handshake packets.

   This enforces cryptographic separation between the data sent in the
   different packet sequence number spaces.  Each packet number space
   starts at packet number 0.  Subsequent packets sent in the same
   packet number space MUST increase the packet number by at least one.

   0-RTT and 1-RTT data exist in the same packet number space to make
   loss recovery algorithms easier to implement between the two packet
   types.

   A QUIC endpoint MUST NOT reuse a packet number within the same packet
   number space in one connection (that is, under the same cryptographic
   keys).  If the packet number for sending reaches 2^62 - 1, the sender
   MUST close the connection without sending a CONNECTION_CLOSE frame or
   any further packets; an endpoint MAY send a Stateless Reset
   (Section 6.13.4) in response to further packets that it receives.

   In the QUIC long and short packet headers, the number of bits
   required to represent the packet number is reduced by including only
   a variable number of the least significant bits of the packet number.
   One or two of the most significant bits of the first octet determine
   how many bits of the packet number are provided, as shown in Table 2.

          +---------------------+----------------+--------------+
          | First octet pattern | Encoded Length | Bits Present |
          +---------------------+----------------+--------------+
          | 0b0xxxxxxx          | 1 octet        | 7            |
          |                     |                |              |
          | 0b10xxxxxx          | 2              | 14           |
          |                     |                |              |
          | 0b11xxxxxx          | 4              | 30           |
          +---------------------+----------------+--------------+

            Table 2: Packet Number Encodings for Packet Headers

   Note that these encodings are similar to those in Section 7.1, but
   use different values.

   The encoded packet number is protected as described in Section 5.3
   [QUIC-TLS].  Protection of the packet number is removed prior to
   recovering the full packet number.  The full packet number is
   reconstructed at the receiver based on the number of significant bits
   present, the value of those bits, and the largest packet number
   received on a successfully authenticated packet.  Recovering the full
   packet number is necessary to successfully remove packet protection.

   Once packet number protection is removed, the packet number is
   decoded by finding the packet number value that is closest to the
   next expected packet.  The next expected packet is the highest
   received packet number plus one.  For example, if the highest
   successfully authenticated packet had a packet number of 0xaa82f30e,
   then a packet containing a 14-bit value of 0x9b3 will be decoded as
   0xaa8309b3.  Example pseudo-code for packet number decoding can be
   found in Appendix A.

   The sender MUST use a packet number size able to represent more than
   twice as large a range than the difference between the largest
   acknowledged packet and packet number being sent.  A peer receiving
   the packet will then correctly decode the packet number, unless the
   packet is delayed in transit such that it arrives after many higher-
   numbered packets have been received.  An endpoint SHOULD use a large
   enough packet number encoding to allow the packet number to be
   recovered even if the packet arrives after packets that are sent
   afterwards.

   As a result, the size of the packet number encoding is at least one
   more than the base 2 logarithm of the number of contiguous
   unacknowledged packet numbers, including the new packet.

   For example, if an endpoint has received an acknowledgment for packet
   0x6afa2f, sending a packet with a number of 0x6b2d79 requires a
   packet number encoding with 14 bits or more; whereas the 30-bit
   packet number encoding is needed to send a packet with a number of
   0x6bc107.

   A receiver MUST discard a newly unprotected packet unless it is
   certain that it has not processed another packet with the same packet
   number from the same packet number space.  Duplicate suppression MUST
   happen after removing packet protection for the reasons described in
   Section 9.3 of [QUIC-TLS].  An efficient algorithm for duplicate
   suppression can be found in Section 3.4.3 of [RFC2406].

   A Version Negotiation packet (Section 4.3) does not include a packet
   number.  The Retry packet (Section 4.4) has special rules for
   populating the packet number field.

5.  Frames and Frame Types

   The payload of all packets, after removing packet protection,
   consists of a sequence of frames, as shown in Figure 6.  Version
   Negotiation and Stateless Reset do not contain frames.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame 1 (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame 2 (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame N (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 6: QUIC Payload

   QUIC payloads MUST contain at least one frame, and MAY contain
   multiple frames and multiple frame types.

   Frames MUST fit within a single QUIC packet and MUST NOT span a QUIC
   packet boundary.  Each frame begins with a Frame Type, indicating its
   type, followed by additional type-dependent fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Frame Type (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Type-Dependent Fields (*)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 7: Generic terminal state.

3.3.  Permitted Frame Layout Types

   The sender of a stream sends just three frame types defined that affect the
   state of a stream at either sender or receiver: STREAM
   (Section 19.19), STREAM_BLOCKED (Section 19.10), and RST_STREAM
   (Section 19.2).

   A sender MUST NOT send any of these frames from a terminal state
   ("Data Recvd" or "Reset Recvd").  A sender MUST NOT send STREAM or
   STREAM_BLOCKED after sending a RST_STREAM; that is, in this specification are listed the "Reset
   Sent" state in Table 3.
   The Frame Type addition to the terminal states.  A receiver could
   receive any of these frames in STREAM any state, but only due to the
   possibility of delayed delivery of packets carrying them.

   The receiver of a stream sends MAX_STREAM_DATA (Section 19.6) and
   STOP_SENDING frames (Section 19.14).

   The receiver only sends MAX_STREAM_DATA in the "Recv" state.  A
   receiver can send STOP_SENDING in any state where it has not received
   a RST_STREAM frame; that is used to carry states other frame-specific
   flags.  For than "Reset Recvd" or "Reset
   Read".  However there is little value in sending a STOP_SENDING frame
   after all other frames, the Frame Type field simply identifies stream data has been received in the frame.  These "Data Recvd" state.  A
   sender could receive these frames are explained in more detail any state as they a result of delayed
   delivery of packets.

3.4.  Bidirectional Stream States

   A bidirectional stream is composed of a send stream and a receive
   stream.  Implementations may represent states of the bidirectional
   stream as composites of send and receive stream states.  The simplest
   model presents the stream as "open" when either send or receive
   stream is in a non-terminal state and "closed" when both send and
   receive streams are
   referenced later in a terminal state.

   Table 2 shows a more complex mapping of bidirectional stream states
   that loosely correspond to the document.

           +-------------+----------------------+--------------+
           | Type Value  | Frame Type Name      | Definition   |
           +-------------+----------------------+--------------+
           | 0x00        | PADDING              | Section 7.2  |
           |             |                      |              |
           | 0x01        | RST_STREAM           | Section 7.3  |
           |             |                      |              |
           | 0x02        | CONNECTION_CLOSE     | Section 7.4  |
           |             |                      |              |
           | 0x03        | APPLICATION_CLOSE    | Section 7.5  |
           |             |                      |              |
           | 0x04        | MAX_DATA             | Section 7.6  |
           |             |                      |              | stream states in HTTP/2 [HTTP2].  This
   shows that multiple states on send or receive streams are mapped to
   the same composite state.  Note that this is just one possibility for
   such a mapping; this mapping requires that data is acknowledged
   before the transition to a "closed" or "half-closed" state.

   +-----------------------+---------------------+---------------------+
   | 0x05 Send Stream           | MAX_STREAM_DATA Receive Stream      | Section 7.7 Composite State     |
   +-----------------------+---------------------+---------------------+
   | No Stream/Ready       | No Stream/Recv *1   | idle                |
   | 0x06                       | MAX_STREAM_ID                     | Section 7.8                     |
   | Ready/Send/Data Sent  | Recv/Size Known     | open                |
   | 0x07                       | PING                     | Section 7.9                     |
   | Ready/Send/Data Sent  | Data Recvd/Data     | half-closed         |
   | 0x08                       | BLOCKED Read                | Section 7.10 (remote)            |
   |                       |                     |                     |
   | 0x09 Ready/Send/Data Sent  | STREAM_BLOCKED Reset Recvd/Reset   | Section 7.11 half-closed         |
   |                       | Read                | (remote)            |
   | 0x0a                       | STREAM_ID_BLOCKED                     | Section 7.12                     |
   | Data Recvd            | Recv/Size Known     | half-closed (local) |
   | 0x0b                       | NEW_CONNECTION_ID                     | Section 7.13                     |
   | Reset Sent/Reset      | Recv/Size Known     | half-closed (local) |
   | 0x0c Recvd                 | STOP_SENDING                     | Section 7.15                     |
   |                       |                     |                     |
   | 0x0d Data Recvd            | RETIRE_CONNECTION_ID Recv/Size Known     | Section 7.14 half-closed (local) |
   |                       |                     |                     |
   | 0x0e Reset Sent/Reset      | PATH_CHALLENGE Data Recvd/Data     | Section 7.17 closed              |
   | Recvd                 | Read                |                     |
   | 0x0f                       | PATH_RESPONSE                     | Section 7.18                     |
   | Reset Sent/Reset      | Reset Recvd/Reset   | closed              |
   | 0x10 - 0x17 Recvd                 | STREAM Read                | Section 7.20                     |
   |                       |                     |                     |
   | 0x18 Data Recvd            | CRYPTO Data Recvd/Data     | Section 7.21 closed              |
   |                       | Read                |                     |
   | 0x19                       | NEW_TOKEN                     | Section 7.19                     |
   | Data Recvd            | Reset Recvd/Reset   | closed              |
   | 0x1a - 0x1b                       | ACK Read                | Section 7.16                     |
           +-------------+----------------------+--------------+
   +-----------------------+---------------------+---------------------+

           Table 3: Frame Types

   All QUIC frames are idempotent.  That is, a valid frame does 2: Possible Mapping of Stream States to HTTP/2

   Note (*1):  A stream is considered "idle" if it has not
   cause undesirable side effects yet been
      created, or errors when if the receive stream is in the "Recv" state without
      yet having received more than
   once.

   The Frame Type field uses any frames.

3.5.  Solicited State Transitions

   If an endpoint is no longer interested in the data it is receiving on
   a variable length integer encoding (see
   Section 7.1) with one exception.  To ensure simple and efficient
   implementations stream, it MAY send a STOP_SENDING frame identifying that stream to
   prompt closure of the stream in the opposite direction.  This
   typically indicates that the receiving application is no longer
   reading data it receives from the stream, but is not a guarantee that
   incoming data will be ignored.

   STREAM frames received after sending STOP_SENDING are still counted
   toward the connection and stream flow-control windows, even though
   these frames will be discarded upon receipt.  This avoids potential
   ambiguity about which STREAM frames count toward flow control.

   A STOP_SENDING frame parsing, requests that the receiving endpoint send a
   RST_STREAM frame.  An endpoint that receives a STOP_SENDING frame type
   MUST send a RST_STREAM frame for that stream, and can use an error
   code of STOPPING.  If the shortest
   possible encoding.  Though STOP_SENDING frame is received on a two-, four- or eight-octet encoding of send
   stream that is already in the "Data Sent" state, a RST_STREAM frame types defined
   MAY still be sent in this document order to cancel retransmission of previously-
   sent STREAM frames.

   STOP_SENDING SHOULD only be sent for a receive stream that has not
   been reset.  STOP_SENDING is possible, the Frame Type
   field most useful for these frames streams in the "Recv" or
   "Size Known" states.

   An endpoint is encoded on expected to send another STOP_SENDING frame if a single octet.  For instance,
   though 0x4007
   packet containing a previous STOP_SENDING is lost.  However, once
   either all stream data or a legitimate two-octet encoding RST_STREAM frame has been received for
   the stream - that is, the stream is in any state other than "Recv" or
   "Size Known" - sending a variable-
   length integer with STOP_SENDING frame is unnecessary.

4.  Flow Control

   It is necessary to limit the amount of data that a value sender may have
   outstanding at any time, so as to prevent a fast sender from
   overwhelming a slow receiver, or to prevent a malicious sender from
   consuming significant resources at a receiver.  To this end, QUIC
   employs a credit-based flow-control scheme similar to that in HTTP/2
   [HTTP2].  A receiver advertises the number of 7, PING octets it is prepared
   to receive on a given stream and for the entire connection.  This
   leads to two levels of flow control in QUIC:

   o  Stream flow control, which prevents a single stream from consuming
      the entire receive buffer for a connection.

   o  Connection flow control, which prevents senders from exceeding a
      receiver's buffer capacity for the connection, and

   A data receiver sets initial credits for all streams by sending
   transport parameters during the handshake (Section 7.3).

   A data receiver sends MAX_STREAM_DATA or MAX_DATA frames are always encoded as a
   single octet with to the value 0x07.  An endpoint MUST treat
   sender to advertise additional credit.  MAX_STREAM_DATA frames send
   the receipt maximum absolute byte offset of a frame type that uses a longer encoding than necessary as a
   connection error of type PROTOCOL_VIOLATION.

5.1.  Extension Frames

   QUIC stream, while MAX_DATA frames do not use a self-describing encoding.  An endpoint
   therefore needs to understand
   send the syntax maximum of all frames before it can
   successfully process a packet.  This allows for efficient encoding the sum of
   frames, but it means that an endpoint cannot send a frame the absolute byte offsets of all
   streams.

   A receiver advertises credit for a type
   that is unknown to its peer.

   An extension to QUIC that wishes to use stream by sending a new type of
   MAX_STREAM_DATA frame MUST
   first ensure that a peer is able to understand with the frame.  An
   endpoint can Stream ID set appropriately.  A
   receiver could use a transport parameter to signal its willingness the current offset of data consumed to
   receive one or more extension frame types with determine
   the one transport
   parameter.

   Extension frames MUST be congestion controlled and MUST cause an ACK
   frame flow control offset to be sent.  The exception is extension frames that replace or
   supplement the ACK frame.  Extension advertised.  A receiver MAY send
   MAX_STREAM_DATA frames are not included in flow
   control unless specified multiple packets in the extension.

   An IANA registry is used order to manage make sure that
   the assignment sender receives an update before running out of frame types, see
   Section 13.2.

6.  Life flow control
   credit, even if one of a the packets is lost.

   Connection

   A QUIC connection flow control is a single conversation between two QUIC
   endpoints.  QUIC's connection establishment intertwines version
   negotiation with the cryptographic and transport handshakes to reduce
   connection establishment latency, as described in Section 6.4.  Once
   established, a connection may migrate to a different IP or port at
   either endpoint, due limit to NAT rebinding or mobility, as described the total bytes of stream data
   sent in
   Section 6.11.  Finally, STREAM frames on all streams.  A receiver advertises credit
   for a connection may be terminated by either
   endpoint, as described in Section 6.13.

6.1.  Connection ID

   Each connection possesses sending a set of identifiers, any MAX_DATA frame.  A receiver maintains a
   cumulative sum of bytes received on all contributing streams, which could be
   used to distinguish it from other connections.  Connection IDs
   are
   selected independently in each direction.  Each Connection ID has an
   associated sequence number to assist in deduplicating messages.

   The primary function of a connection ID is used to ensure that changes in
   addressing at lower protocol layers (UDP, IP, and below) don't cause
   packets check for flow control violations.  A receiver might use
   a QUIC connection to be delivered sum of bytes consumed on all streams to determine the maximum data
   limit to be advertised.

   A receiver MAY advertise a larger offset at any point by sending
   MAX_STREAM_DATA or MAX_DATA frames.  A receiver cannot renege on an
   advertisement; that is, once a receiver advertises an offset,
   advertising a smaller offset has no effect.  A sender MUST therefore
   ignore any MAX_STREAM_DATA or MAX_DATA frames that do not increase
   flow control limits.

   A receiver MUST close the wrong endpoint.
   Each endpoint selects connection IDs using an implementation-specific
   (and perhaps deployment-specific) method which will allow packets with that a FLOW_CONTROL_ERROR error
   (Section 11) if the peer violates the advertised connection ID or stream
   data limits.

   A sender SHOULD send STREAM_BLOCKED or BLOCKED frames to be routed back indicate it
   has data to the endpoint and
   identified write but is blocked by the endpoint upon receipt.

   Connection IDs MUST NOT contain any information that can flow control limits.  These
   frames are expected to be sent infrequently in common cases, but they
   are considered useful for debugging and monitoring purposes.

   A similar method is used to
   correlate them control the number of open streams (see
   Section 4.5 for details).

4.1.  Handling of Stream Cancellation

   There are some edge cases which must be considered when dealing with other
   stream and connection IDs for level flow control.  Given enough time, both
   endpoints must agree on flow control state.  If one end believes it
   can send more than the same connection.  As
   a trivial example, this means other end is willing to receive, the same
   connection ID MUST NOT will be
   issued torn down when too much data arrives.  Conversely
   if a sender believes it is blocked, while endpoint B expects more than once on
   data can be received, then the same connection.

   A zero-length connection ID MAY can be used when in a deadlock, with
   the connection ID is not
   needed sender waiting for routing and the address/port tuple a MAX_STREAM_DATA or MAX_DATA frame which will
   never come.

   On receipt of packets is
   sufficient to identify a connection.  An RST_STREAM frame, an endpoint whose peer has
   selected a zero-length connection ID MUST continue to use a zero-
   length connection ID will tear down state
   for the lifetime matching stream and ignore further data arriving on that
   stream.  This could result in the endpoints getting out of sync,
   since the connection RST_STREAM frame may have arrived out of order and MUST NOT
   send packets from any other local address.

   When an endpoint has requested a non-zero-length there
   may be further bytes in flight.  The data sender would have counted
   the data against its connection ID, it
   needs to ensure level flow control budget, but a
   receiver that the peer has a supply of connection IDs from
   which not received these bytes would not know to choose for packets include
   them as well.  The receiver must learn the number of bytes that were
   sent on the stream to make the endpoint.  These same adjustment in its connection
   IDs are supplied by the endpoint using flow
   controller.

   To ensure that endpoints maintain a consistent connection-level flow
   control state, the NEW_CONNECTION_ID RST_STREAM frame (Section 7.13).

6.1.1.  Issuing Connection IDs

   The initial connection ID issued by an endpoint is the Source
   Connection ID during 19.2) includes the handshake.  The sequence number
   largest offset of data sent on the
   initial connection ID is 0.  If stream.  On receiving a RST_STREAM
   frame, a receiver definitively knows how many bytes were sent on that
   stream before the preferred_address transport
   parameter is sent, RST_STREAM frame, and the sequence number of receiver MUST use the supplied connection ID
   is 1.  Subsequent connection IDs are communicated
   final offset to account for all bytes sent on the peer using
   NEW_CONNECTION_ID frames (Section 7.13), and the sequence number stream in its
   connection level flow controller.

   RST_STREAM terminates one direction of a stream abruptly.  Whether
   any action or response can or should be taken on
   each newly-issued connection ID MUST increase by 1.  The connection
   ID randomly selected by the client data already
   received is application specific.

   For a bidirectional stream, RST_STREAM has no effect on data flow in
   the Initial packet and any
   connection ID provided by a Reset packet are not assigned sequence
   numbers unless opposite direction.  The RST_STREAM sender can send a server opts
   STOP_SENDING frame to retain them as its initial connection
   ID.

   When an endpoint issues a connection ID, it encourage prompt termination.  Both endpoints
   MUST accept packets that
   carry this connection ID maintain state for the duration of stream in the connection or unterminated direction
   until
   its peer invalidates the connection ID via a RETIRE_CONNECTION_ID
   frame (Section 7.14).

   An endpoint SHOULD ensure that its peer has direction enters a sufficient number of
   available terminal state, or either side sends
   CONNECTION_CLOSE or APPLICATION_CLOSE.

4.2.  Data Limit Increments

   This document leaves when and unused connection IDs.  While each endpoint
   independently chooses how many connection IDs bytes to issue, endpoints
   SHOULD provide and maintain at least eight connection IDs.  The
   endpoint can do this by always supplying a new connection ID when advertise in a
   connection ID is retired by its peer
   MAX_DATA or when the endpoint receives a
   packet with MAX_STREAM_DATA to implementations, but offers a previously unused connection ID.  Endpoints that
   initiate migration and require non-zero-length few
   considerations.  These frames contribute to connection IDs SHOULD
   provide their peers overhead.
   Therefore frequently sending frames with new connection IDs before migration, or risk small changes is
   undesirable.  At the peer closing same time, larger increments to limits are
   necessary to avoid blocking if updates are less frequent, requiring
   larger resource commitments at the connection.

6.1.2.  Consuming receiver.  Thus there is a trade-
   off between resource commitment and Retiring Connection IDs

   An endpoint can change overhead when determining how
   large a limit is advertised.

   A receiver MAY use an autotuning mechanism to tune the connection ID frequency and
   amount that it uses for increases data limits based on a peer to
   another available one at any round-trip time during
   estimate and the connection.  An endpoint
   consumes connection IDs in response to a migrating peer, see
   Section 6.11.5 for more.

   An endpoint maintains a set of connection IDs received from its peer,
   any of rate at which it can use when sending packets.  When the endpoint
   wishes receiving application consumes
   data, similar to remove common TCP implementations.

   If a connection ID from use, sender runs out of flow control credit, it sends a
   RETIRE_CONNECTION_ID frame will be unable to its peer, indicating that the peer
   might bring a
   send new connection ID into circulation using the
   NEW_CONNECTION_ID frame.

   An endpoint that retires a connection ID can retain knowledge of that
   connection ID for a period of time after sending data.  That is, the
   RETIRE_CONNECTION_ID frame, or until that frame sender is acknowledged.

   As discussed in Section 6.11.5, each connection ID MUST be used on
   packets sent from only one local address.  An endpoint that migrates
   away from a local address blocked.  A blocked sender
   SHOULD retire all connection IDs used on
   that address once it no longer plans to use that address.

6.2.  Matching Packets to Connections

   Incoming packets are classified on receipt.  Packets can either be
   associated with an existing connection, send a STREAM_BLOCKED or - BLOCKED frame.  A receiver uses these
   frames for debugging purposes.  A receiver MUST NOT wait for servers -
   potentially create a new connection.

   Hosts try to associate
   STREAM_BLOCKED or BLOCKED frame before sending MAX_STREAM_DATA or
   MAX_DATA, since doing so will mean that a packet with sender will be blocked for
   an existing connection.  If entire round trip and the
   packet has peer may never send a Destination Connection ID corresponding STREAM_BLOCKED or
   BLOCKED frame.

   It is generally considered best to an existing
   connection, QUIC processes that packet accordingly.  Note that more
   than one connection ID can be associated with a connection; see
   Section 6.1.

   If not let the Destination Connection ID is zero length sender go into
   quiescence if avoidable.  To avoid blocking a sender, and to
   reasonably account for the packet
   matches the address/port tuple possibility of loss, a connection where the host did not
   require connection IDs, QUIC processes receiver should
   send a MAX_DATA or MAX_STREAM_DATA frame at least two round trips
   before it expects the packet as part of that
   connection.  Endpoints MUST drop packets with zero-length Destination
   Connection ID fields if they do not correspond sender to get blocked.

   A sender sends a single
   connection.

   Endpoints BLOCKED or STREAM_BLOCKED frame only once
   when it reaches a data limit.  A sender SHOULD NOT send a Stateless Reset (Section 6.13.4) multiple
   BLOCKED or STREAM_BLOCKED frames for any
   packets that cannot be attributed to an existing connection.

6.2.1.  Client Packet Handling

   Valid packets sent to clients always include a Destination Connection
   ID that matches the value same data limit, unless the client selects.  Clients that choose
   original frame is determined to
   receive zero-length connection IDs be lost.  Another BLOCKED or
   STREAM_BLOCKED frame can use be sent after the address/port tuple to
   identify a connection.  Packets data limit is increased.

4.3.  Stream Final Offset

   The final offset is the count of the number of octets that don't match an existing
   connection are discarded.

   Due to packet reordering or loss, clients might receive packets for
   transmitted on a
   connection stream.  For a stream that are encrypted is reset, the final
   offset is carried explicitly in a RST_STREAM frame.  Otherwise, the
   final offset is the offset of the end of the data carried in a STREAM
   frame marked with a key it has not yet computed.
   Clients MAY drop these packets, FIN flag, or MAY buffer them 0 in anticipation the case of
   later packets that allow it to compute incoming
   unidirectional streams.

   An endpoint will know the key.

   If final offset for a client receives stream when the receive
   stream enters the "Size Known" or "Reset Recvd" state.

   An endpoint MUST NOT send data on a packet that has an unsupported version, stream at or beyond the final
   offset.

   Once a final offset for a stream is known, it
   MUST discard that packet.

6.2.2.  Server Packet Handling cannot change.  If a server receives
   RST_STREAM or STREAM frame causes the final offset to change for a packet that has
   stream, an unsupported version, but endpoint SHOULD respond with a FINAL_OFFSET_ERROR error
   (see Section 11).  A receiver SHOULD treat receipt of data at or
   beyond the packet final offset as a FINAL_OFFSET_ERROR error, even after a
   stream is sufficiently large closed.  Generating these errors is not mandatory, but only
   because requiring that an endpoint generate these errors also means
   that the endpoint needs to initiate maintain the final offset state for closed
   streams, which could mean a new connection significant state commitment.

4.4.  Flow Control for any
   version supported by Cryptographic Handshake

   Data sent in CRYPTO frames is not flow controlled in the server, it SHOULD send a Version Negotiation
   packet same way as described in Section 6.3.1.  Servers MAY rate control these
   packets
   STREAM frames.  QUIC relies on the cryptographic protocol
   implementation to avoid storms excessive buffering of Version Negotiation packets. data, see [QUIC-TLS].
   The first packet for implementation SHOULD provide an unsupported version can use different
   semantics and encodings for any version-specific field.  In
   particular, different packet protection keys might be used for
   different versions.  Servers that do not support a particular version
   are unlikely interface to be able QUIC to decrypt tell it
   about its buffering limits so that there is not excessive buffering
   at multiple layers.

4.5.  Stream Limit Increment

   An endpoint limits the payload number of concurrently active incoming streams
   by limiting the packet.
   Servers SHOULD NOT attempt maximum stream ID.  An initial value is set in the
   transport parameters (see Section 18.1) and is subsequently increased
   by MAX_STREAM_ID frames (see Section 19.7).

   As with stream and connection flow control, this document leaves when
   and how many streams to make available to decode or decrypt a packet from an
   unknown version, peer via MAX_STREAM_ID to
   implementations, but instead send offers a Version Negotiation packet,
   provided that few considerations.  MAX_STREAM_ID
   frames constitute minimal overhead, while withholding MAX_STREAM_ID
   frames can prevent the packet peer from using the available parallelism.

   The STREAM_ID_BLOCKED frame (Section 19.11) can be used to signal a
   shortage of available streams.  Implementations will likely want to
   increase the maximum stream ID as peer-initiated streams close.

5.  Connections

   A QUIC connection is sufficiently long.

   Servers MUST drop other packets that contain unsupported versions.

   Packets with a supported version, or no single conversation between two QUIC
   endpoints.  QUIC's connection establishment combines version field, are matched
   negotiation with the cryptographic and transport handshakes to
   a reduce
   connection establishment latency, as described in Section 6.2.  If not matched, the server
   continues below.

   If the packet is an Initial packet fully conforming with the
   specification, the server proceeds with the handshake (Section 6.4).
   This commits the server to the version that the client selected.

   If 7.  Once
   established, a server isn't currently accepting any new connections, it SHOULD
   send an Initial packet containing connection may migrate to a CONNECTION_CLOSE frame with error
   code SERVER_BUSY.

   If the packet is different IP or port at
   either endpoint as described in Section 9.  Finally, a 0-RTT packet, the server MAY buffer connection may
   be terminated by either endpoint, as described in Section 10.

5.1.  Connection ID

   Each connection possesses a limited
   number set of these packets in anticipation connection identifiers, or
   connection IDs, each of a late-arriving Initial
   Packet.  Clients which can be identify the connection.
   Connection IDs are forbidden from sending Handshake packets prior
   to receiving independently selected by endpoints; each endpoint
   selects the connection IDs that its peer uses.

   The primary function of a server response, so servers SHOULD ignore any such
   packets.

   Servers MUST drop incoming packets under all other circumstances.

6.3.  Version Negotiation

   Version negotiation ensures connection ID is to ensure that client changes in
   addressing at lower protocol layers (UDP, IP, and server agree to below) don't cause
   packets for a QUIC
   version connection to be delivered to the wrong endpoint.
   Each endpoint selects connection IDs using an implementation-specific
   (and perhaps deployment-specific) method which will allow packets
   with that is mutually supported.  A server sends a Version
   Negotiation packet in response connection ID to each packet be routed back to the endpoint and
   identified by the endpoint upon receipt.

   Connection IDs MUST NOT contain any information that might initiate can be used to
   correlate them with other connection IDs for the same connection.  As
   a trivial example, this means the same connection ID MUST NOT be
   issued more than once on the same connection.

   Packets with long headers include Source Connection ID and
   Destination Connection ID fields.  These fields are used to set the
   connection IDs for new connection, connections, see Section 6.2 7.2 for details.

   Packets with short headers (Section 17.3) only include the
   Destination Connection ID and omit the explicit length.  The size length
   of the first packet sent by a client will determine whether
   a server sends Destination Connection ID field is expected to be known to
   endpoints.  Endpoints using a Version Negotiation packet.  Clients load balancer that support
   multiple QUIC versions SHOULD pad the first packet they send to routes based on
   connection ID could agree with the
   largest of load balancer on a fixed length
   for connection IDs, or agree on an encoding scheme.  A fixed portion
   could encode an explicit length, which allows the minimum packet sizes across all versions they support.
   This ensures that entire connection
   ID to vary in length and still be used by the server responds if there is a mutually
   supported version.

6.3.1.  Sending load balancer.

   A Version Negotiation Packets

   If (Section 17.4) packet echoes the version connection IDs
   selected by the client is not acceptable client, both to ensure correct routing toward the
   server, the server responds with a Version Negotiation packet (see
   Section 4.3).  This includes a list of versions that the server will
   accept.

   This system allows a server
   client and to process packets with unsupported
   versions without retaining state.  Though either allow the Initial packet
   or client to validate that the Version Negotiation packet that is sent in
   response could to an Initial packet.

   A zero-length connection ID MAY be
   lost, the client will send new packets until it successfully receives
   a response or it abandons used when the connection attempt.

6.3.2.  Handling Version Negotiation Packets

   When the client receives a Version Negotiation packet, it first
   checks that the Destination and Source Connection ID fields match the
   Source is not
   needed for routing and Destination Connection the address/port tuple of packets is
   sufficient to identify a connection.  An endpoint whose peer has
   selected a zero-length connection ID fields in MUST continue to use a packet that zero-
   length connection ID for the
   client sent.  If this check fails, lifetime of the packet connection and MUST be discarded.

   Once the Version Negotiation packet is determined NOT
   send packets from any other local address.

   When an endpoint has requested a non-zero-length connection ID, it
   needs to be valid, ensure that the
   client then selects an acceptable protocol version peer has a supply of connection IDs from
   which to choose for packets sent to the list
   provided endpoint.  These connection
   IDs are supplied by the server. endpoint using the NEW_CONNECTION_ID frame
   (Section 19.12).

5.1.1.  Issuing Connection IDs

   Each Connection ID has an associated sequence number to assist in
   deduplicating messages.  The client then attempts to create a initial connection using that version.  Though ID issued by an
   endpoint is sent in the content Source Connection ID field of the Initial long packet
   header (Section 17.2) during the client sends might not change in response handshake.  The sequence number of
   the initial connection ID is 0.  If the preferred_address transport
   parameter is sent, the sequence number of the supplied connection ID
   is 1.

   Additional connection IDs are communicated to version
   negotiation, a client MUST increase the packet peer using
   NEW_CONNECTION_ID frames (Section 19.12).  The sequence number it uses on
   every packet it sends.  Packets MUST continue to use long headers and
   each newly-issued connection ID MUST include the new negotiated protocol version. increase by 1.  The connection
   ID randomly selected by the client MUST use in the long header format and include its selected
   version on all packets until it has 1-RTT keys Initial packet and it has received any
   connection ID provided by a Reset packet from the server which is are not a Version Negotiation packet.

   A client MUST NOT change the version it uses assigned sequence
   numbers unless it is in response
   to a Version Negotiation packet from the server.  Once a client
   receives a packet from the server which is not opts to retain them as its initial connection
   ID.

   When an endpoint issues a Version Negotiation
   packet, connection ID, it MUST discard other Version Negotiation accept packets on the same
   connection.  Similarly, a client MUST ignore a Version Negotiation
   packet if it has already received and acted on a Version Negotiation
   packet.

   A client MUST ignore a Version Negotiation packet that lists the
   client's chosen version.

   A client MAY attempt 0-RTT after receiving a Version Negotiation
   packet.  A client that sends additional 0-RTT packets MUST NOT reset
   carry this connection ID for the packet number to 0 as a result, see Section 4.6.4.

   Version negotiation packets have no cryptographic protection.  The
   result duration of the negotiation MUST be revalidated as part of connection or until
   its peer invalidates the
   cryptographic handshake (see Section 6.6.4).

6.3.3.  Using Reserved Versions

   For a server to use connection ID via a new version in the future, clients must
   correctly handle unsupported versions.  To help RETIRE_CONNECTION_ID
   frame (Section 19.13).

   An endpoint SHOULD ensure this, that its peer has a server sufficient number of
   available and unused connection IDs.  While each endpoint
   independently chooses how many connection IDs to issue, endpoints
   SHOULD include provide and maintain at least eight connection IDs.  The
   endpoint can do this by always supplying a reserved version (see Section 3) while generating new connection ID when a
   Version Negotiation packet.

   The design of version negotiation permits
   connection ID is retired by its peer or when the endpoint receives a server to avoid
   maintaining state for packets
   packet with a previously unused connection ID.  Endpoints that it rejects in this fashion.  The
   validation of version negotiation (see Section 6.6.4) only validates
   initiate migration and require non-zero-length connection IDs SHOULD
   provide their peers with new connection IDs before migration, or risk
   the result of version negotiation, which is peer closing the same no matter which
   reserved version was sent.  A server MAY therefore send different
   reserved version numbers in connection.

5.1.2.  Consuming and Retiring Connection IDs

   An endpoint can change the connection ID it uses for a peer to
   another available one at any time during the Version Negotiation Packet and connection.  An endpoint
   consumes connection IDs in its
   transport parameters.

   A client MAY send response to a packet using migrating peer, see
   Section 9.5 for more.

   An endpoint maintains a reserved version number.  This set of connection IDs received from its peer,
   any of which it can
   be used use when sending packets.  When the endpoint
   wishes to solicit remove a list of supported versions connection ID from use, it sends a server.

6.4.  Cryptographic and Transport Handshake

   QUIC relies on a combined cryptographic and transport handshake
   RETIRE_CONNECTION_ID frame to
   minimize its peer, indicating that the peer
   might bring a new connection establishment latency.  QUIC uses ID into circulation using the CRYPTO
   NEW_CONNECTION_ID frame.

   An endpoint that retires a connection ID can retain knowledge of that
   connection ID for a period of time after sending the
   RETIRE_CONNECTION_ID frame, or until that frame is acknowledged.

   As discussed in Section 7.21 9.5, each connection ID MUST be used on
   packets sent from only one local address.  An endpoint that migrates
   away from a local address SHOULD retire all connection IDs used on
   that address once it no longer plans to transmit use that address.

5.2.  Matching Packets to Connections

   Incoming packets are classified on receipt.  Packets can either be
   associated with an existing connection, or - for servers -
   potentially create a new connection.

   Hosts try to associate a packet with an existing connection.  If the cryptographic handshake.  Version
   0x00000001 of QUIC uses TLS 1.3 as described in [QUIC-TLS];
   packet has a
   different Destination Connection ID corresponding to an existing
   connection, QUIC version number could indicate processes that packet accordingly.  Note that more
   than one connection ID can be associated with a different
   cryptographic handshake protocol connection; see
   Section 5.1.

   If the Destination Connection ID is in use.

   QUIC provides reliable, ordered delivery of zero length and the cryptographic
   handshake data.  QUIC packet protection ensures confidentiality and
   integrity protection that meets
   matches the requirements address/port tuple of the cryptographic
   handshake protocol:

   o  authenticated key exchange, where

      *  a server is always authenticated,

      * a client is optionally authenticated,

      *  every connection produces distinct and unrelated keys,

      *  keying material is usable for packet protection for both 0-RTT
         and 1-RTT packets, and

      *  1-RTT keys have forward secrecy

   o  authenticated values for where the transport parameters of host did not
   require connection IDs, QUIC processes the peer (see
      Section 6.6)

   o  authenticated confirmation of version negotiation (see
      Section 6.6.4)

   o  authenticated negotiation packet as part of an application protocol (TLS uses
      ALPN [RFC7301] that
   connection.  Endpoints MUST drop packets with zero-length Destination
   Connection ID fields if they do not correspond to a single
   connection.

   Endpoints SHOULD send a Stateless Reset (Section 10.4) for this purpose)

   o any
   packets that cannot be attributed to an existing connection.

   Packets that are matched to an existing connection, but for which the server, the ability
   endpoint cannot remove packet protection, are discarded.

5.2.1.  Client Packet Handling

   Valid packets sent to carry data that provides assurance clients always include a Destination Connection
   ID that matches a value the client selects.  Clients that choose to
   receive zero-length connection IDs can use the address/port tuple to
   identify a connection.  Packets that don't match an existing
   connection are discarded.

   Due to packet reordering or loss, clients might receive packets for a
   connection that are addressed encrypted with the
      transport address a key it has not yet computed.
   Clients MAY drop these packets, or MAY buffer them in anticipation of
   later packets that is claimed by allow it to compute the key.

   If a client (see Section 6.9)

   The first CRYPTO frame MUST be sent in receives a single packet.  Any second
   attempt packet that is triggered by address validation has an unsupported version, it
   MUST also be sent
   within a single discard that packet.  This avoids having to reassemble

5.2.2.  Server Packet Handling

   If a message
   from multiple packets.

   The first client packet of the cryptographic handshake protocol MUST
   fit within server receives a 1232 octet QUIC packet payload.  This includes overheads that reduce has an unsupported version, but
   the space available packet is sufficiently large to initiate a new connection for any
   version supported by the cryptographic handshake
   protocol. server, it SHOULD send a Version Negotiation
   packet as described in Section 6.1.  Servers MAY rate control these
   packets to avoid storms of Version Negotiation packets.

   The CRYPTO frame first packet for an unsupported version can be sent in use different
   semantics and encodings for any version-specific field.  In
   particular, different packet number spaces.
   CRYPTO frames in each packet number space carry protection keys might be used for
   different versions.  Servers that do not support a separate sequence particular version
   are unlikely to be able to decrypt the payload of handshake data starting the packet.
   Servers SHOULD NOT attempt to decode or decrypt a packet from an offset of 0.

6.5.  Example Handshake Flows

   Details of how TLS is integrated with QUIC are provided in
   [QUIC-TLS],
   unknown version, but some examples are instead send a Version Negotiation packet,
   provided here.

   Figure 8 provides an overview of that the 1-RTT handshake.  Each line
   shows a QUIC packet is sufficiently long.

   Servers MUST drop other packets that contain unsupported versions.

   Packets with a supported version, or no version field, are matched to
   a connection using the connection ID or - for packets with zero-
   length connection IDs - the address tuple.  If the packet type and packet number shown
   first, followed by doesn't
   match an existing connection, the frames that are typically contained in those
   packets.  So, for instance server continues below.

   If the first packet is of type Initial, an Initial packet fully conforming with the
   specification, the server proceeds with the handshake (Section 7).
   This commits the server to the version that the client selected.

   If a server isn't currently accepting any new connections, it SHOULD
   send an Initial packet number 0, and contains containing a CRYPTO CONNECTION_CLOSE frame carrying with error
   code SERVER_BUSY.

   If the
   ClientHello.

   Note that multiple QUIC packet is a 0-RTT packet, the server MAY buffer a limited
   number of these packets - even in anticipation of different encryption levels
   - may be coalesced into a single UDP datagram (see Section 4.9), and late-arriving Initial
   Packet.  Clients are forbidden from sending Handshake packets prior
   to receiving a server response, so this handshake may consist of as few as 4 UDP datagrams, or servers SHOULD ignore any
   number more.  For instance, the server's such
   packets.

   Servers MUST drop incoming packets under all other circumstances.

5.3.  Life of a QUIC Connection

   TBD.

6.  Version Negotiation

   Version negotiation ensures that client and server agree to a QUIC
   version that is mutually supported.  A server sends a Version
   Negotiation packet in response to each packet that might initiate a
   new connection, see Section 5.2 for details.

   The first flight contains
   packets from the Initial encryption level (obfuscation), the
   Handshake level, and "0.5-RTT data" from the few messages of an exchange between a client attempting to
   create a new connection with server at the 1-RTT
   encryption level. is shown in Figure 3.  After
   version negotiation completes, connection establishment can proceed,
   for example as shown in Section 7.1.

   Client                                                  Server

   Initial[0]: CRYPTO[CH]

   Packet (v=X) ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                          Handshake[0]: CRYPTO[EE, CERT, CV, FIN]

                           <- 1-RTT[0]: STREAM[1, "..."]

   Initial[1]: ACK[0]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[0]: STREAM[0, "..."], ACK[0] Version Negotiation (supported=Y,Z)

   Packet (v=Y) ->

                              1-RTT[1]: STREAM[55, "..."], ACK[0]

                                               <- Handshake[1]: ACK[0] Packet(s) (v=Y)

              Figure 8: 3: Example 1-RTT Handshake

   Figure 9 shows an example Version Negotiation Exchange

   The size of the first packet sent by a connection with client will determine whether
   a 0-RTT handshake and server sends a single Version Negotiation packet.  Clients that support
   multiple QUIC versions SHOULD pad the first packet they send to the
   largest of 0-RTT data.  Note the minimum packet sizes across all versions they support.
   This ensures that as described in
   Section 4.11, the server ACKs responds if there is a mutually
   supported version.

6.1.  Sending Version Negotiation Packets

   If the 0-RTT data at version selected by the 1-RTT encryption
   level, and client is not acceptable to the client's sequence numbers at
   server, the 1-RTT encryption
   level continue to increment from its 0-RTT packets.

   Client                                                  Server

   Initial[0]: CRYPTO[CH]
   0-RTT[0]: STREAM[0, "..."] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                           Handshake[0] CRYPTO[EE, CERT, CV, FIN]
                             <- 1-RTT[0]: STREAM[1, "..."] ACK[0]

   Initial[1]: ACK[0]
   0-RTT[1]: CRYPTO[EOED]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[2]: STREAM[0, "..."] ACK[0] ->

                            1-RTT[1]: STREAM[55, "..."], ACK[1,2]
                                          <- Handshake[1]: ACK[0]

                     Figure 9: Example 0-RTT Handshake

6.6.  Transport Parameters

   During connection establishment, both endpoints make authenticated
   declarations server responds with a Version Negotiation packet (see
   Section 17.4).  This includes a list of their transport parameters.  These declarations are
   made unilaterally by each endpoint.  Endpoints are required versions that the server will
   accept.

   This system allows a server to comply process packets with unsupported
   versions without retaining state.  Though either the restrictions implied by these parameters; the description of
   each parameter includes rules for its handling.

   The format of Initial packet
   or the transport parameters Version Negotiation packet that is sent in response could be
   lost, the TransportParameters
   struct from Figure 10.  This is described using client will send new packets until it successfully receives
   a response or it abandons the presentation
   language from Section 3 of [TLS13].

      uint32 QuicVersion;

      enum {
         initial_max_stream_data_bidi_local(0),
         initial_max_data(1),
         initial_max_bidi_streams(2),
         idle_timeout(3),
         preferred_address(4),
         max_packet_size(5),
         stateless_reset_token(6),
         ack_delay_exponent(7),
         initial_max_uni_streams(8),
         disable_migration(9),
         initial_max_stream_data_bidi_remote(10),
         initial_max_stream_data_uni(11),
         max_ack_delay(12),
         original_connection_id(13),
         (65535)
      } TransportParameterId;

      struct {
         TransportParameterId parameter;
         opaque value<0..2^16-1>;
      } TransportParameter;

      struct {
         select (Handshake.msg_type) {
            case client_hello:
               QuicVersion initial_version;

            case encrypted_extensions:
               QuicVersion negotiated_version;
               QuicVersion supported_versions<4..2^8-4>;
         };
         TransportParameter parameters<22..2^16-1>;
      } TransportParameters;

      struct {
        enum { IPv4(4), IPv6(6), (15) } ipVersion;
        opaque ipAddress<4..2^8-1>;
        uint16 port;
        opaque connectionId<0..18>;
        opaque statelessResetToken[16];
      } PreferredAddress;

               Figure 10: Definition of TransportParameters

   The "extension_data" field connection attempt.

   A server MAY limit the number of Version Negotiation packets it
   sends.  For instance, a server that is able to recognize packets as
   0-RTT might choose not to send Version Negotiation packets in
   response to 0-RTT packets with the expectation that it will
   eventually receive an Initial packet.

6.2.  Handling Version Negotiation Packets

   When the client receives a Version Negotiation packet, it first
   checks that the quic_transport_parameters extension
   defined Destination and Source Connection ID fields match the
   Source and Destination Connection ID fields in [QUIC-TLS] contains a TransportParameters value.  TLS
   encoding rules are therefore used packet that the
   client sent.  If this check fails, the packet MUST be discarded.

   Once the Version Negotiation packet is determined to encode be valid, the transport parameters.

   QUIC encodes transport parameters into
   client then selects an acceptable protocol version from the list
   provided by the server.  The client then attempts to create a sequence
   connection using that version.  Though the content of octets, which
   are then included in the cryptographic handshake.  Once Initial
   packet the handshake
   completes, client sends might not change in response to version
   negotiation, a client MUST increase the transport parameters declared by packet number it uses on
   every packet it sends.  Packets MUST continue to use long headers
   (Section 17.2) and MUST include the peer are
   available.  Each endpoint validates new negotiated protocol version.

   The client MUST use the value provided by long header format and include its peer.
   In particular, selected
   version negotiation MUST be validated (see
   Section 6.6.4) before on all packets until it has 1-RTT keys and it has received a
   packet from the connection establishment server which is considered
   properly complete.

   Definitions for each of not a Version Negotiation packet.

   A client MUST NOT change the defined transport parameters are included version it uses unless it is in Section 6.6.1.  Any given parameter response
   to a Version Negotiation packet from the server.  Once a client
   receives a packet from the server which is not a Version Negotiation
   packet, it MUST appear at most once in discard other Version Negotiation packets on the same
   connection.  Similarly, a
   given transport parameters extension.  An endpoint client MUST treat receipt
   of duplicate transport parameters as ignore a connection error of type
   TRANSPORT_PARAMETER_ERROR.

6.6.1.  Transport Parameter Definitions

   An endpoint Version Negotiation
   packet if it has already received and acted on a Version Negotiation
   packet.

   A client MUST ignore a Version Negotiation packet that lists the
   client's chosen version.

   A client MAY use attempt 0-RTT after receiving a Version Negotiation
   packet.  A client that sends additional 0-RTT packets MUST NOT reset
   the following transport parameters:

   initial_max_data (0x0001): packet number to 0 as a result, see Section 17.5.2.

   Version negotiation packets have no cryptographic protection.  The initial maximum data parameter
      contains
   result of the initial value for negotiation MUST be revalidated as part of the maximum amount
   cryptographic handshake (see Section 7.3.3).

6.3.  Using Reserved Versions

   For a server to use a new version in the future, clients must
   correctly handle unsupported versions.  To help ensure this, a server
   SHOULD include a reserved version (see Section 15) while generating a
   Version Negotiation packet.

   The design of data version negotiation permits a server to avoid
   maintaining state for packets that can
      be sent on it rejects in this fashion.  The
   validation of version negotiation (see Section 7.3.3) only validates
   the connection.  This parameter result of version negotiation, which is encoded as an
      unsigned 32-bit integer the same no matter which
   reserved version was sent.  A server MAY therefore send different
   reserved version numbers in units of octets. the Version Negotiation Packet and in its
   transport parameters.

   A client MAY send a packet using a reserved version number.  This is equivalent can
   be used to
      sending solicit a MAX_DATA (Section 7.6) for the connection immediately
      after completing the handshake.  If the list of supported versions from a server.

7.  Cryptographic and Transport Handshake

   QUIC relies on a combined cryptographic and transport parameter is
      absent, the handshake to
   minimize connection starts with a flow control limit of 0.

   initial_max_bidi_streams (0x0002):  The initial maximum bidirectional
      streams parameter contains establishment latency.  QUIC uses the initial maximum number of
      bidirectional streams CRYPTO
   frame Section 19.20 to transmit the peer may initiate, encoded cryptographic handshake.  Version
   0x00000001 of QUIC uses TLS 1.3 as an
      unsigned 16-bit integer.  If this parameter is absent or zero,
      bidirectional streams cannot be created until described in [QUIC-TLS]; a MAX_STREAM_ID
      frame is sent.  Setting this parameter is equivalent to sending
   different QUIC version number could indicate that a
      MAX_STREAM_ID (Section 7.8) immediately after completing different
   cryptographic handshake protocol is in use.

   QUIC provides reliable, ordered delivery of the cryptographic
   handshake containing data.  QUIC packet protection ensures confidentiality and
   integrity protection that meets the corresponding Stream ID.  For example, a
      value requirements of 0x05 would be equivalent to receiving the cryptographic
   handshake protocol:

   o  authenticated key exchange, where

      *  a MAX_STREAM_ID
      containing 16 when received by server is always authenticated,

      *  a client or 17 when received by a
      server.

   initial_max_uni_streams (0x0008):  The initial maximum unidirectional
      streams parameter contains is optionally authenticated,

      *  every connection produces distinct and unrelated keys,

      *  keying material is usable for packet protection for both 0-RTT
         and 1-RTT packets, and

      *  1-RTT keys have forward secrecy

   o  authenticated values for the initial maximum number transport parameters of
      unidirectional streams the peer may initiate, encoded as (see
      Section 7.3)

   o  authenticated confirmation of version negotiation (see
      Section 7.3.3)

   o  authenticated negotiation of an
      unsigned 16-bit integer.  If application protocol (TLS uses
      ALPN [RFC7301] for this parameter is absent or zero,
      unidirectional streams cannot be created until a MAX_STREAM_ID purpose)

   The first CRYPTO frame is sent.  Setting this parameter is equivalent to sending a
      MAX_STREAM_ID (Section 7.8) immediately after completing the
      handshake containing the corresponding Stream ID.  For example, from a
      value of 0x05 would client MUST be equivalent to receiving a MAX_STREAM_ID
      containing 18 when received by sent in a client or 19 when received single packet.
   Any second attempt that is triggered by address validation (see
   Section 8.1) MUST also be sent within a
      server.

   idle_timeout (0x0003):  The idle timeout is single packet.  This avoids
   having to reassemble a value in seconds that
      is encoded as an unsigned 16-bit integer.  If this parameter is
      absent or zero then the idle timeout is disabled.

   max_packet_size (0x0005): message from multiple packets.

   The maximum first client packet size parameter places a
      limit on the size of packets that the endpoint is willing to
      receive, encoded as an unsigned 16-bit integer. cryptographic handshake protocol MUST
   fit within a 1232 octet QUIC packet payload.  This indicates includes overheads
   that packets larger than this limit will reduce the space available to the cryptographic handshake
   protocol.

   The CRYPTO frame can be dropped. sent in different packet number spaces.  The default
      for this parameter is the maximum permitted UDP payload
   sequence numbers used by CRYPTO frames to ensure ordered delivery of
   cryptographic handshake data start from zero in each packet number
   space.

7.1.  Example Handshake Flows

   Details of 65527.
      Values below 1200 how TLS is integrated with QUIC are invalid.  This limit only applies to
      protected packets (Section 4.8).

   ack_delay_exponent (0x0007): provided in
   [QUIC-TLS], but some examples are provided here.  An 8-bit unsigned integer value
      indicating an exponent used extension of
   this exchange to decode the ACK Delay field support client address validation is shown in the
      ACK frame, see
   Section 7.16.  If this value is absent, a default
      value of 3 8.1.1.

   Once any version negotiation and address validation exchanges are
   complete, the cryptographic handshake is assumed (indicating a multiplier of 8). used to agree on
   cryptographic keys.  The default
      value cryptographic handshake is also used for ACK frames that are sent carried in
   Initial (Section 17.5) and Handshake packets.  Values above 20 are invalid.

   disable_migration (0x0009):  The endpoint does not support connection
      migration (Section 6.11).  Peers MUST NOT send any packets,
      including probing packets (Section 6.11.1), from 17.6) packets.

   Figure 4 provides an overview of the 1-RTT handshake.  Each line
   shows a local address
      other than QUIC packet with the packet type and packet number shown
   first, followed by the frames that used to perform are typically contained in those
   packets.  So, for instance the handshake.  This parameter first packet is of type Initial, with
   packet number 0, and contains a zero-length value.

   max_ack_delay (0x000c):  An 8 bit unsigned integer value indicating CRYPTO frame carrying the maximum amount of time in milliseconds by which it will delay
      sending
   ClientHello.

   Note that multiple QUIC packets - even of acknowledgments.  If this value is absent, different encryption levels
   - may be coalesced into a default single UDP datagram (see Section 12.2), and
   so this handshake may consist of
      25 milliseconds is assumed.

   Either peer MAY advertise an initial value for as few as 4 UDP datagrams, or any
   number more.  For instance, the flow control on
   each type server's first flight contains
   packets from the Initial encryption level (obfuscation), the
   Handshake level, and "0.5-RTT data" from the server at the 1-RTT
   encryption level.

   Client                                                  Server

   Initial[0]: CRYPTO[CH] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                          Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                    <- 1-RTT[0]: STREAM[1, "..."]

   Initial[1]: ACK[0]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[0]: STREAM[0, "..."], ACK[0] ->

                              1-RTT[1]: STREAM[55, "..."], ACK[0]
                                          <- Handshake[1]: ACK[0]

                     Figure 4: Example 1-RTT Handshake

   Figure 5 shows an example of stream on which they might receive data.  Each a connection with a 0-RTT handshake and
   a single packet of the
   following transport parameters is encoded 0-RTT data.  Note that as an unsigned 32-bit
   integer described in units of octets:

   initial_max_stream_data_bidi_local (0x0000):  The initial stream
      maximum data for bidirectional, locally-initiated streams
      parameter contains
   Section 12.3, the initial flow control limit for newly
      created bidirectional streams opened by server acknowledges 0-RTT data at the endpoint that sets 1-RTT
   encryption level, and the
      transport parameter.  In client transport parameters, this applies
      to streams with an identifier ending in 0x0; sends 1-RTT packets in server transport
      parameters, this applies the same
   packet number space.

   Client                                                  Server

   Initial[0]: CRYPTO[CH]
   0-RTT[0]: STREAM[0, "..."] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                           Handshake[0] CRYPTO[EE, CERT, CV, FIN]
                             <- 1-RTT[0]: STREAM[1, "..."] ACK[0]

   Initial[1]: ACK[0]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[2]: STREAM[0, "..."] ACK[0] ->

                            1-RTT[1]: STREAM[55, "..."], ACK[1,2]
                                          <- Handshake[1]: ACK[0]

                     Figure 5: Example 0-RTT Handshake

7.2.  Negotiating Connection IDs

   A connection ID is used to streams ending ensure consistent routing of packets, as
   described in 0x1.

   initial_max_stream_data_bidi_remote (0x000a): Section 5.1.  The initial stream
      maximum data for bidirectional, peer-initiated streams parameter long header contains two connection
   IDs: the initial flow control limit for newly created
      bidirectional streams opened Destination Connection ID is chosen by the endpoint that receives recipient of the
      transport parameter.  In client transport parameters, this applies
   packet and is used to streams with an identifier ending in 0x1; in server transport
      parameters, this applies provide consistent routing; the Source
   Connection ID is used to streams ending in 0x0.

   initial_max_stream_data_uni (0x000b):  The initial stream maximum
      data for unidirectional streams parameter contains set the initial
      flow control limit for newly created unidirectional streams opened Destination Connection ID used by
   the endpoint that receives peer.

   During the transport parameter.  In client
      transport parameters, this applies to streams handshake, packets with an identifier
      ending in 0x3; in server transport parameters, this applies to
      streams ending in 0x2.

   If present, transport parameters that set initial stream flow control
   limits the long header (Section 17.2) are equivalent
   used to sending a MAX_STREAM_DATA frame
   (Section 7.7) on every stream of establish the corresponding type immediately
   after opening.  If connection ID that each endpoint uses.  Each
   endpoint uses the transport parameter Source Connection ID field to specify the
   connection ID that is absent, streams used in the Destination Connection ID field of that
   type start with
   packets being sent to them.  Upon receiving a flow control limit of 0.

   A server MUST include packet, each endpoint
   sets the original_connection_id transport parameter
   if Destination Connection ID it sent a Retry packet:

   original_connection_id (0x000d):  The sends to match the value of the Destination
   Source Connection ID field from the first that they receive.

   When an Initial packet sent by the
      client.  This transport parameter is only sent by a client which has not previously
   received a Retry packet from the server.

   A server MAY include server, it populates the following transport parameters:

   stateless_reset_token (0x0006):  The Stateless Reset Token is used in
      verifying a stateless reset, see Section 6.13.4. Destination
   Connection ID field with an unpredictable value.  This parameter
      is MUST be at
   least 8 octets in length.  Until a sequence of 16 octets.

   preferred_address (0x0004):  The server's Preferred Address packet is used
      to effect a change in server address at received from the end of
   server, the handshake,
      as described in Section 6.12.

   A client MUST NOT include a stateless reset token or a preferred
   address.  A server MUST treat receipt of either transport parameter
   as a use the same value unless it abandons the
   connection error of type TRANSPORT_PARAMETER_ERROR.

6.6.2.  Values of Transport Parameters attempt and starts a new one.  The initial Destination
   Connection ID is used to determine packet protection keys for 0-RTT

   A Initial
   packets.

   The client that attempts to send 0-RTT data MUST remember populates the transport
   parameters used by Source Connection ID field with a value of
   its choosing and sets the server. SCIL field to match.

   The transport parameters that Destination Connection ID field in the
   server advertises during server's Initial packet
   contains a connection establishment apply to all
   connections that are resumed using the keying material established
   during ID that handshake.  Remembered transport parameters apply to the
   new connection until the handshake completes and new transport
   parameters from the server can be provided.

   A server can remember is chosen by the transport parameters that it advertised, or
   store an integrity-protected copy recipient of the values in
   packet (i.e., the client); the ticket and
   recover Source Connection ID includes the information when accepting 0-RTT data.  A server uses
   connection ID that the
   transport parameters in determining whether sender of the packet wishes to accept 0-RTT data.

   A use (see
   Section 5.1).  The server MAY accept 0-RTT and subsequently provide different values
   for transport parameters for MUST use in consistent Source Connection IDs
   during the new connection.  If 0-RTT
   data is accepted by handshake.

   On first receiving an Initial or Retry packet from the server, the server MUST NOT reduce any limits
   or alter any values that might be violated
   client uses the Source Connection ID supplied by the client with its
   0-RTT data.  In particular, a server that accepts 0-RTT data MUST NOT
   set values as the
   Destination Connection ID for initial_max_data, initial_max_stream_data_bidi_local,
   initial_max_stream_data_bidi_remote, and initial_max_stream_data_uni subsequent packets.  That means that are smaller than the remembered value of those parameters.
   Similarly, a server MUST NOT reduce
   client might change the value of
   initial_max_bidi_streams or initial_max_uni_streams.

   Omitting or setting Destination Connection ID twice during
   connection establishment.  Once a zero value for certain transport parameters can
   result in 0-RTT data being enabled, but not usable.  The applicable
   subset of transport parameters that permit sending of application
   data SHOULD be set to non-zero values for 0-RTT.  This includes
   initial_max_data and either initial_max_bidi_streams and
   initial_max_stream_data_bidi_remote, or initial_max_uni_streams and
   initial_max_stream_data_uni.

   The value of client has received an Initial
   packet from the server's previous preferred_address server, it MUST NOT be used
   when establishing discard any packet it receives with a new connection; rather, the
   different Source Connection ID.

   A client should wait to
   observe MUST only change the server's new preferred_address value it sends in the handshake.

   A Destination
   Connection ID in response to the first packet of each type it
   receives from the server MUST reject 0-RTT data (Retry or even abort Initial); a handshake if the
   implied values for transport parameters cannot be supported.

6.6.3.  New Transport Parameters

   New transport parameters can be used to negotiate new protocol
   behavior.  An endpoint server MUST ignore transport parameters that it does set its
   value based on the Initial packet.  Any additional changes are not support.  Absence
   permitted; if subsequent packets of those types include a transport parameter therefore disables any
   optional protocol feature different
   Source Connection ID, they MUST be discarded.  This avoids problems
   that is negotiated using the parameter.

   New transport parameters might arise from stateless processing of multiple Initial
   packets producing different connection IDs.

   The connection ID can be registered according to change over the rules lifetime of a connection,
   especially in response to connection migration (Section 9), see
   Section 13.1.

6.6.4.  Version Negotiation Validation

   Though the cryptographic handshake has integrity protection, two
   forms 5.1.1 for details.

7.3.  Transport Parameters

   During connection establishment, both endpoints make authenticated
   declarations of QUIC version downgrade are possible.  In the first, an
   attacker replaces the QUIC version in the Initial packet.  In the
   second, a fake Version Negotiation packet is sent by an attacker.  To
   protect against these attacks, the their transport parameters include three
   fields that encode version information. parameters.  These parameters declarations are used
   made unilaterally by each endpoint.  Endpoints are required to
   retroactively authenticate comply
   with the choice restrictions implied by these parameters; the description of version (see Section 6.3).

   The cryptographic handshake provides integrity protection
   each parameter includes rules for the
   negotiated version as part its handling.

   The encoding of the transport parameters (see is detailed in Section 6.6).  As a result, attacks on version negotiation by an
   attacker can be detected.

   The client 18.

   QUIC includes the initial_version field in its encoded transport
   parameters.  The initial_version is parameters in the version that cryptographic
   handshake.  Once the client
   initially attempted to use.  If handshake completes, the server did not send a Version
   Negotiation packet Section 4.3, this will be identical to transport parameters
   declared by the
   negotiated_version field in peer are available.  Each endpoint validates the server transport parameters.

   A server that processes all packets in a stateful fashion can
   remember how
   value provided by its peer.  In particular, version negotiation was performed and validate MUST
   be validated (see Section 7.3.3) before the
   initial_version value.

   A server that does not maintain state connection establishment
   is considered properly complete.

   Definitions for every packet it receives
   (i.e., a stateless server) uses a different process.  If the
   initial_version matches the version each of QUIC that is the defined transport parameters are included
   in Section 18.1.  Any given parameter MUST appear at most once in use, a
   stateless server can accept the value.

   If the initial_version is different from the version
   given transport parameters extension.  An endpoint MUST treat receipt
   of QUIC that is
   in use, duplicate transport parameters as a stateless connection error of type
   TRANSPORT_PARAMETER_ERROR.

   A server MUST check that it would have sent a
   Version Negotiation packet include the original_connection_id transport parameter
   (Section 18.1) if it had received a packet with the
   indicated initial_version.  If sent a server would have accepted the
   version included in the initial_version and Retry packet.

7.3.1.  Values of Transport Parameters for 0-RTT

   A client that attempts to send 0-RTT data MUST remember the value differs from transport
   parameters used by the QUIC version server.  The transport parameters that is in use, the
   server MUST terminate the advertises during connection with a VERSION_NEGOTIATION_ERROR error.

   The server includes both the version of QUIC establishment apply to all
   connections that is in use and a
   list of are resumed using the QUIC versions keying material established
   during that handshake.  Remembered transport parameters apply to the server supports.

   The negotiated_version field is
   new connection until the version that is in use.  This
   MUST be set by handshake completes and new transport
   parameters from the server to the value that is on can be provided.

   A server can remember the Initial packet transport parameters that it accepts (not an Initial packet that triggers a Retry advertised, or
   Version Negotiation packet).  A client that receives a
   negotiated_version that does not match the version of QUIC that is in
   use MUST terminate the connection with a VERSION_NEGOTIATION_ERROR
   error code.

   The server includes a list
   store an integrity-protected copy of versions that it would send in any
   version negotiation packet (Section 4.3) in the supported_versions
   field.  The server populates this field even if it did not send a
   version negotiation packet.

   The client validates that the negotiated_version is included values in the
   supported_versions list ticket and - if version negotiation was performed -
   that it would have selected
   recover the negotiated version. information when accepting 0-RTT data.  A client MUST
   terminate server uses the connection with a VERSION_NEGOTIATION_ERROR error code
   if
   transport parameters in determining whether to accept 0-RTT data.

   A server MAY accept 0-RTT and subsequently provide different values
   for transport parameters for use in the current QUIC version new connection.  If 0-RTT
   data is not listed in accepted by the supported_versions
   list.  A client server, the server MUST terminate NOT reduce any limits
   or alter any values that might be violated by the client with its
   0-RTT data.  In particular, a VERSION_NEGOTIATION_ERROR error
   code if version negotiation occurred but it would have selected a
   different version based on server that accepts 0-RTT data MUST NOT
   set values for initial_max_data, initial_max_stream_data_bidi_local,
   initial_max_stream_data_bidi_remote, initial_max_stream_data_uni,
   initial_max_bidi_streams, or initial_max_uni_streams (Section 18.1)
   that are smaller than the remembered value of the supported_versions list.

   When an endpoint accepts multiple QUIC versions, it can potentially
   interpret those parameters.

   Omitting or setting a zero value for certain transport parameters as they are defined by any of the QUIC
   versions it supports.  The version field can
   result in the QUIC packet header is
   authenticated using transport parameters. 0-RTT data being enabled, but not usable.  The position and the
   format applicable
   subset of the version fields in transport parameters MUST either that permit sending of application
   data SHOULD be
   identical across different QUIC versions, set to non-zero values for 0-RTT.  This includes
   initial_max_data and either initial_max_bidi_streams and
   initial_max_stream_data_bidi_remote, or initial_max_uni_streams and
   initial_max_stream_data_uni.

   The value of the server's previous preferred_address MUST NOT be unambiguously
   different to ensure no confusion about their interpretation.  One way
   that used
   when establishing a new format could be introduced is to define a TLS extension
   with a different codepoint.

6.7.  Stateless Retries

   A server can process an Initial packet from a connection; rather, the client without
   committing any state.  This allows a server to perform address
   validation (Section 6.9), or should wait to defer connection establishment costs.
   observe the server's new preferred_address value in the handshake.

   A server that generates MUST reject 0-RTT data or even abort a response handshake if the
   implied values for transport parameters cannot be supported.

7.3.2.  New Transport Parameters

   New transport parameters can be used to an Initial packet without
   retaining connection state negotiate new protocol
   behavior.  An endpoint MUST use the Retry packet (Section 4.4).
   This packet causes ignore transport parameters that it does
   not support.  Absence of a client to restart transport parameter therefore disables any
   optional protocol feature that is negotiated using the connection attempt and
   includes parameter.

   New transport parameters can be registered according to the token rules in
   Section 22.1.

7.3.3.  Version Negotiation Validation

   Though the new Initial packet (Section 4.6) to prove
   source address ownership.

6.8.  Using Explicit Congestion Notification cryptographic handshake has integrity protection, two
   forms of QUIC endpoints use Explicit Congestion Notification (ECN) [RFC3168]
   to detect and respond to network congestion.  ECN allows a network
   node to indicate congestion in version downgrade are possible.  In the network by setting a codepoint first, an
   attacker replaces the QUIC version in the IP header of Initial packet.  In the
   second, a fake Version Negotiation packet instead of dropping it.  Endpoints react to
   congestion is sent by reducing their sending rate in response, as described
   in [QUIC-RECOVERY]. an attacker.  To use ECN, QUIC endpoints first determine whether a path supports
   ECN marking and
   protect against these attacks, the peer is able transport parameters include three
   fields that encode version information.  These parameters are used to access the ECN codepoint in
   retroactively authenticate the
   IP header.  A network path does not support ECN if ECN marked packets
   get dropped or ECN markings are rewritten on choice of version (see Section 6).

   The cryptographic handshake provides integrity protection for the path.  An endpoint
   verifies
   negotiated version as part of the path, both during connection establishment and when
   migrating to a new path transport parameters (see
   Section 6.11).

   Each endpoint independently verifies and enables use of ECN 18.1).  As a result, attacks on version negotiation by
   setting the IP header ECN codepoint to ECN Capable Transport (ECT)
   for the path from it to an
   attacker can be detected.

   The client includes the other peer.  Even if ECN initial_version field in its transport
   parameters.  The initial_version is not used on the path to version that the peer, client
   initially attempted to use.  If the endpoint MUST provide feedback about ECN
   markings received (if accessible).

   To verify both that server did not send a path supports ECN and the peer can provide ECN
   feedback, an endpoint MUST set Version
   Negotiation packet Section 17.4, this will be identical to the ECT(0) codepoint
   negotiated_version field in the IP header
   of server transport parameters.

   A server that processes all outgoing packets [RFC8311].

   If an ECT codepoint set in a stateful fashion can
   remember how version negotiation was performed and validate the IP header is
   initial_version value.

   A server that does not corrupted by maintain state for every packet it receives
   (i.e., a
   network device, then stateless server) uses a received packet contains either the codepoint
   sent by different process.  If the peer or
   initial_version matches the Congestion Experienced (CE) codepoint set by
   a network device version of QUIC that is experiencing congestion.

   On receiving in use, a packet with an ECT or CE codepoint, an endpoint that
   stateless server can access accept the IP ECN codepoints increases value.

   If the corresponding ECT(0),
   ECT(1), or CE count, and includes these counters in subsequent (see
   Section 8.1) ACK frames (see Section 7.16).

   A packet detected by a receiver as a duplicate does not affect initial_version is different from the
   receiver's local ECN codepoint counts; see (Section 12.7) for
   relevant security concerns.

   If an endpoint receives version of QUIC that is
   in use, a packet without an ECT or CE codepoint, stateless server MUST check that it
   responds per Section 8.1 with an ACK frame.

   If an endpoint does not would have access to received ECN codepoints, sent a
   Version Negotiation packet if it
   acknowledges had received packets per Section 8.1 with an ACK frame.

   If a packet sent with an ECT codepoint is newly acknowledged by the
   peer in an ACK frame,
   indicated initial_version.  If a server would have accepted the endpoint stops setting ECT codepoints
   version included in
   subsequent packets, with the expectation that either the network or the peer no longer supports ECN.

   To protect initial_version and the connection value differs from arbitrary corruption of ECN codepoints
   by the network, an endpoint verifies
   the following when an ACK frame QUIC version that is received:

   o  The increase in ECT(0) and ECT(1) counters use, the server MUST be at least terminate the
      number of packets newly acknowledged that were sent
   connection with the
      corresponding codepoint.

   o a VERSION_NEGOTIATION_ERROR error.

   The total increase in ECT(0), ECT(1), and CE counters reported in
      the ACK frame MUST be at least server includes both the total number version of packets newly
      acknowledged in this ACK frame.

   An endpoint could miss acknowledgements for a packet when ACK frames
   are lost.  It QUIC that is therefore possible for the total increase in ECT(0),
   ECT(1), use and CE counters to be greater than a
   list of the QUIC versions that the number of packets
   acknowledged in an ACK frame.  When this happens, server supports (see
   Section 18.1).

   The negotiated_version field is the local reference
   counts version that is in use.  This
   MUST be increased set by the server to match the counters in value that is on the ACK frame.

   Upon successful verification, Initial packet
   that it accepts (not an endpoint continues to set ECT
   codepoints Initial packet that triggers a Retry or
   Version Negotiation packet).  A client that receives a
   negotiated_version that does not match the version of QUIC that is in subsequent packets
   use MUST terminate the connection with a VERSION_NEGOTIATION_ERROR
   error code.

   The server includes a list of versions that it would send in any
   version negotiation packet (Section 17.4) in the expectation supported_versions
   field.  The server populates this field even if it did not send a
   version negotiation packet.

   The client validates that the path negotiated_version is ECN-capable.

   If verification fails, then the endpoint ceases setting ECT
   codepoints included in subsequent packets with the expectation
   supported_versions list and - if version negotiation was performed -
   that either it would have selected the
   network or negotiated version.  A client MUST
   terminate the peer does connection with a VERSION_NEGOTIATION_ERROR error code
   if the current QUIC version is not support ECN.

   If an endpoint sets ECT codepoints on outgoing packets and encounters listed in the supported_versions
   list.  A client MUST terminate with a retransmission timeout due to VERSION_NEGOTIATION_ERROR error
   code if version negotiation occurred but it would have selected a
   different version based on the absence value of acknowledgments from the peer (see [QUIC-RECOVERY]), or if supported_versions list.

   When an endpoint has reason to
   believe that a network element might be corrupting ECN codepoints, accepts multiple QUIC versions, it can potentially
   interpret transport parameters as they are defined by any of the endpoint MAY cease setting ECT codepoints QUIC
   versions it supports.  The version field in subsequent packets.
   Doing so allows the connection to traverse network elements that drop
   or corrupt ECN codepoints in QUIC packet header is
   authenticated using transport parameters.  The position and the IP header.

6.9.  Proof
   format of Source Address Ownership

   Transport protocols commonly spend a round trip checking that a
   client owns the version fields in transport address (IP and port) that it claims.
   Verifying parameters MUST either be
   identical across different QUIC versions, or be unambiguously
   different to ensure no confusion about their interpretation.  One way
   that a client can receive packets sent new format could be introduced is to its claimed
   transport address protects against spoofing of this information by
   malicious clients.

   This technique define a TLS extension
   with a different codepoint.

8.  Address Validation

   Address validation is used primarily by QUIC to avoid QUIC from being used for a traffic
   amplification attack.  In such an attack, a packet is sent to a
   server with spoofed source address information that identifies a
   victim.  If a server generates more or larger packets in response to
   that packet, the attacker can use use the server to send more data toward
   the victim than it would be able to send on its own.

   The primary defense against amplification attack is verifying that an
   endpoint is able to receive packets at the transport address that it
   claims.  Address validation is performed both during connection
   establishment (see Section 8.1) and during connection migration (see
   Section 8.2).

8.1.  Address Validation During Connection Establishment

   Connection establishment implicitly provides address validation for
   both endpoints.  In particular, receipt of a packet protected with
   Handshake keys confirms that the client received the Initial packet
   from the server.  Once the server has successfully processed a
   Handshake packet from the client, it can consider the server client address
   to have been validated.

   Prior to validating the client address, servers MUST NOT send more data toward
   the victim
   than it would three times as many bytes as the number of bytes they have
   received.  This limits the magnitude of any amplification attack that
   can be able to send on its own.

   Several methods are used in QUIC to mitigate this attack.  Firstly, mounted using spoofed source addresses.

   To ensure that the initial handshake packet server is sent in a not overly constrained by this
   restriction, clients MUST send UDP datagram that contains datagrams with at least 1200
   octets of UDP payload.  This allows a payload until the server has completed address validation,
   see Section 14.

   In order to send prevent a
   similar amount of data without risking causing an amplification
   attack toward an unproven remote address.

   A server eventually confirms that handshake deadlock as a client has received its messages
   when the first Handshake-level message is received.  This might be
   insufficient, either because the server wishes to avoid the
   computational cost of completing the handshake, or it might be that
   the size result of the packets that are sent during the handshake is too
   large.  This is especially important for 0-RTT, where the server
   might wish
   being unable to provide application data traffic - such as send, clients SHOULD send a response
   to packet upon a request - handshake
   timeout, as described in response to [QUIC-RECOVERY].  If the client has no data carried
   to retransmit and does not have Handshake keys, it SHOULD send an
   Initial packet in a UDP datagram of at least 1200 octets.  If the early data from
   the client.

   To
   client has Handshake keys, it SHOULD send additional data prior to completing the cryptographic
   handshake, the a Handshake packet.

   A server then needs might wish to validate that the client owns the address that it claims.

   Source before starting
   the cryptographic handshake.  Client addresses can be verified using
   an address validation token.  This token is therefore performed by the core
   transport protocol delivered during the
   connection establishment of with a connection.

   A different type of source address validation is performed after Retry packet (see Section 8.1.1) or
   in a previous connection migration, see using the NEW_TOKEN frame (see
   Section 6.10.

6.9.1.  Client 8.1.2).

8.1.1.  Address Validation Procedure using Retry Packets

   QUIC uses token-based address validation. validation during connection
   establishment.  Any time the server wishes to validate a client
   address, it provides the client with a token.  As long as the token's authenticity can be checked it is not
   possible for an attacker to generate a valid token for its own
   address (see Section 6.9.3) 8.1.3) and the client is able to return that
   token, it proves to the server that it received the token.

   Upon receiving the client's Initial packet, the server can request
   address validation by sending a Retry packet (Section 17.7)
   containing a token.  This token is repeated in the client's next Initial packet.  Because
   the token is consumed by the server that generates it, there is no
   need for a single well-defined format.  A token could include
   information about the claimed client address (IP and port), a
   timestamp, and any other supplementary information the server will
   need to validate the token in the future.

   The Retry packet is sent to the client and a legitimate client will
   respond with an
   Initial packet containing the token from the Retry
   packet when after it continues receives the handshake. Retry packet.  In response to
   receiving the
   token, a server can either abort the connection or permit it to
   proceed.

   A connection MAY be accepted without address validation - or with
   only limited validation - but a server SHOULD limit the data it sends
   toward an unvalidated address.  Successful completion of the
   cryptographic handshake implicitly provides proof that the client has
   received packets from the server.

   The client should allow for additional Retry packets being sent in
   response to Initial packets sent containing a token.  There are
   several situations token in which the an Initial packet, a server might not be able to use can either abort the
   previously generated token
   connection or permit it to validate the client's address and must
   send a new Retry. proceed.

   A reasonable limit server can also use a Retry packet to defer the number state and
   processing costs of tries the
   client allows for, before connection establishment.  By giving up, is 3.  That is, the client MUST
   echo a
   different connection ID to use, a server can cause the address validation token from connection to
   be routed to a server instance with more resources available for new
   connections.

   A flow showing the use of a Retry packet up to 3
   times.  After that, it MAY give up on the connection attempt.

6.9.2. is shown in Figure 6.

   Client                                                  Server

   Initial[0]: CRYPTO[CH] ->

                                                   <- Retry+Token

   Initial+Token[0]: CRYPTO[CH] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                          Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                    <- 1-RTT[0]: STREAM[1, "..."]

                  Figure 6: Example Handshake with Retry

8.1.2.  Address Validation for Future Connections

   A server MAY provide clients with an address validation token during
   one connection that can be used on a subsequent connection.  Address
   validation is especially important with 0-RTT because a server
   potentially sends a significant amount of data to a client in
   response to 0-RTT data.

   The server uses uses the NEW_TOKEN frame Section 19.18 to provide the
   client with an address validation token that can be used to validate
   future connections.  The client may then use this token to validate
   future connections by including it in the Initial packet's header.
   The client MUST NOT use the token provided in a Retry for future
   connections.

   Unlike the token that is created for a Retry packet, there might be
   some time between when the token is created and when the token is
   subsequently used.  Thus, a resumption token SHOULD include an
   expiration time.  The server MAY include either an explicit
   expiration time or an issued timestamp and dynamically calculate the
   expiration time.  It is also unlikely that the client port number is
   the same on two different connections; validating the port is
   therefore unlikely to be successful.

   A resumption token SHOULD be constructed to be easily distinguishable
   from tokens that are sent in Retry packets as they are carried in the
   same field.

   If the client has a token received in a NEW_TOKEN frame on a previous
   connection to what it believes to be the same server, it can include
   that value in the Token field of its Initial packet.

   A token allows a server to correlate activity between the connection
   where the token was issued and any connection where it is used.
   Clients that want to break continuity of identity with a server MAY
   discard tokens provided using the NEW_TOKEN frame.  Tokens obtained
   in Retry packets MUST NOT be discarded.

   A client SHOULD NOT reuse a token.  Reusing a token allows
   connections to be linked by entities on the network path (see
   Section 9.5).  A client MUST NOT reuse a token if it believes that
   its point of network attachment has changed since the token was last
   used; that is, if there is a change in its local IP address or
   network interface.  A client needs to start the connection process
   over if it migrates prior to completing the handshake.

   When a server receives an Initial packet with an address validation
   token, it SHOULD attempt to validate it.  If the token is invalid
   then the server SHOULD proceed as if the client did not have a
   validated address, including potentially sending a Retry.  If the
   validation succeeds, the server SHOULD then allow the NEW_TOKEN frame Section 7.19 handshake to provide
   proceed.

   Note:  The rationale for treating the client with an address validation token as unvalidated rather
      than discarding the packet is that can be used to validate
   future connections.  The client may then use this token to validate
   future connections by including it in the Initial packet's header.
   The client MUST NOT use might have received
      the token provided in a Retry for future
   connections.

   Unlike previous connection using the token that is created for a Retry packet, there NEW_TOKEN frame, and
      if the server has lost state, it might be
   some time between when unable to validate the
      token is created and when at all, leading to connection failure if the token packet is
   subsequently used.  Thus, a resumption token SHOULD include an
   expiration time.  The
      discarded.  A server MAY include either an explicit
   expiration time or an issued timestamp encode tokens provided with NEW_TOKEN
      frames and dynamically calculate Retry packets differently, and validate the
   expiration time.  It is also unlikely latter more
      strictly.

   In a stateless design, a server can use encrypted and authenticated
   tokens to pass information to clients that the server can later
   recover and use to validate a client port number is
   the same on two different connections; validating address.  Tokens are not
   integrated into the port is
   therefore unlikely to cryptographic handshake and so they are not
   authenticated.  For instance, a client might be successful.

6.9.3. able to reuse a
   token.  To avoid attacks that exploit this property, a server can
   limit its use of tokens to only the information needed validate
   client addresses.

8.1.3.  Address Validation Token Integrity

   An address validation token MUST be difficult to guess.  Including a
   large enough random value in the token would be sufficient, but this
   depends on the server remembering the value it sends to clients.

   A token-based scheme allows the server to offload any state
   associated with validation to the client.  For this design to work,
   the token MUST be covered by integrity protection against
   modification or falsification by clients.  Without integrity
   protection, malicious clients could generate or guess values for
   tokens that would be accepted by the server.  Only the server
   requires access to the integrity protection key for tokens.

6.10.

   There is no need for a single well-defined format for the token
   because the server that generates the token also consumes it.  A
   token could include information about the claimed client address (IP
   and port), a timestamp, and any other supplementary information the
   server will need to validate the token in the future.

8.2.  Path Validation

   Path validation is used during connection migration (see Section 9
   and Section 9.6) by an the migrating endpoint to verify reachability of
   a peer over from a specific path.  That is, it tests new local address.  In path validation, endpoints test
   reachability between a specific local address and a specific peer
   address, where an address is the two-tuple of IP address and port.

   Path validation tests that packets can be both sent to and received
   from a peer.

   Path validation is used during connection migration (see Section 6.11
   and Section 6.12) by the migrating endpoint to verify reachability of
   a peer from a new local address.  Path validation is also used by the peer to verify that the migrating endpoint is able to receive packets
   sent to on the its new address.  That is, path.  Importantly, it validates that the packets
   received from the migrating endpoint do not carry a spoofed source
   address.

   Path validation can be used at any time by either endpoint.  For
   instance, an endpoint might check that a peer is still in possession
   of its address after a period of quiescence.

   Path validation is not designed as a NAT traversal mechanism.  Though
   the mechanism described here might be effective for the creation of
   NAT bindings that support NAT traversal, the expectation is that one
   or other peer is able to receive packets without first having sent a
   packet on that path.  Effective NAT traversal needs additional
   synchronization mechanisms that are not provided here.

   An endpoint MAY bundle PATH_CHALLENGE and PATH_RESPONSE frames that
   are used for path validation with other frames.  For instance,  In particular, an
   endpoint may pad a packet carrying a PATH_CHALLENGE for PMTU
   discovery, or an endpoint may bundle a PATH_RESPONSE with its own
   PATH_CHALLENGE.

   When probing a new path, an endpoint might want to ensure that its
   peer has an unused connection ID available for responses.  The
   endpoint can send NEW_CONNECTION_ID and PATH_CHALLENGE frames in the
   same packet.  This ensures that an unused connection ID will be
   available to the peer when sending a response.

6.10.1.  Initiation

8.3.  Initiating Path Validation

   To initiate path validation, an endpoint sends a PATH_CHALLENGE frame
   containing a random payload on the path to be validated.

   An endpoint MAY send additional multiple PATH_CHALLENGE frames to handle guard against
   packet loss.  An endpoint SHOULD NOT send a PATH_CHALLENGE more
   frequently than it would an Initial packet, ensuring that connection
   migration is no more load on a new path than establishing a new
   connection.

   The endpoint MUST use fresh random data in every PATH_CHALLENGE frame
   so that it can associate the peer's response with the causative
   PATH_CHALLENGE.

6.10.2.  Response

8.4.  Path Validation Responses

   On receiving a PATH_CHALLENGE frame, an endpoint MUST respond
   immediately by echoing the data contained in the PATH_CHALLENGE frame
   in a PATH_RESPONSE frame, with the following stipulation.  Since frame.  However, because a PATH_CHALLENGE might be
   sent from a spoofed address, an endpoint MAY MUST limit the rate at which
   it sends PATH_RESPONSE frames and MAY silently discard PATH_CHALLENGE
   frames that would cause it to respond at a higher rate.

   To ensure that packets can be both sent to and received from the
   peer, the PATH_RESPONSE MUST be sent on the same path as the
   triggering PATH_CHALLENGE: PATH_CHALLENGE.  That is, from the same local address on
   which the PATH_CHALLENGE was received, to the same remote address
   from which the PATH_CHALLENGE was received.

6.10.3.  Completion

8.5.  Successful Path Validation

   A new address is considered valid when a PATH_RESPONSE frame is
   received containing data that was sent in a previous PATH_CHALLENGE.
   Receipt of an acknowledgment for a packet containing a PATH_CHALLENGE
   frame is not adequate validation, since the acknowledgment can be
   spoofed by a malicious peer.

   For path validation to be successful, a PATH_RESPONSE frame MUST be
   received from the same remote address to which the corresponding
   PATH_CHALLENGE was sent.  If a PATH_RESPONSE frame is received from a
   different remote address than the one to which the PATH_CHALLENGE was
   sent, path validation is considered to have failed, even if the data
   matches that sent in the PATH_CHALLENGE.

   Additionally, the PATH_RESPONSE frame MUST be received on the same
   local address from which the corresponding PATH_CHALLENGE was sent.
   If a PATH_RESPONSE frame is received on a different local address
   than the one from which the PATH_CHALLENGE was sent, path validation
   is considered to have failed, even if the data matches that sent in
   the PATH_CHALLENGE.  Thus, the endpoint considers the path to be
   valid when a PATH_RESPONSE frame is received on the same path with
   the same payload as the PATH_CHALLENGE frame.

6.10.4.  Abandonment

   An

8.6.  Failed Path Validation

   Path validation only fails when the endpoint attempting to validate
   the path abandons its attempt to validate the path.

   Endpoints SHOULD abandon path validation after sending some number
   of PATH_CHALLENGE frames or after some time has passed. based on a timer.  When
   setting this timer, implementations are cautioned that the new path
   could have a longer round-trip time than the original.

   Note that the endpoint might receive packets containing other frames
   on the new path, but a PATH_RESPONSE frame with appropriate data is
   required for path validation to succeed.

   If

   When an endpoint abandons path validation fails, validation, it determines that the
   path is deemed unusable.  This does not necessarily imply a failure of the
   connection - endpoints can continue sending packets over other paths
   as appropriate.  If no paths are available, an endpoint can wait for
   a new path to become available or close the connection.

   A path validation might be abandoned for other reasons besides
   failure.  Primarily, this happens if a connection migration to a new
   path is initiated while a path validation on the old path is in
   progress.

6.11.

9.  Connection Migration

   QUIC

   The use of a connection ID allows connections to survive changes to
   endpoint addresses (that is, IP address and/or port), such as those
   caused by an endpoint migrating to a new network.  This section
   describes the process by which an endpoint migrates to a new address.

   An endpoint MUST NOT initiate connection migration before the
   handshake is finished and the endpoint has 1-RTT keys.  The design of
   QUIC relies on endpoints retaining a stable address for the duration
   of the handshake.

   An endpoint also MUST NOT initiate connection migration if the peer
   sent the "disable_migration" transport parameter during the
   handshake.  An endpoint which has sent this transport parameter, but
   detects that a peer has nonetheless migrated to a different network
   MAY treat this as a connection error of type INVALID_MIGRATION.

   Not all changes of peer address are intentional migrations.  The peer
   could experience NAT rebinding: a change of address due to a
   middlebox, usually a NAT, allocating a new outgoing port or even a
   new outgoing IP address for a flow.  Endpoints  NAT rebinding is not connection
   migration as defined in this section, though an endpoint SHOULD
   perform path validation (Section 6.10) 8.2) if it detects a NAT rebinding does not cause change in the
   connection to fail.
   IP address of its peer.

   This document limits migration of connections to new client
   addresses, except as described in Section 6.12. 9.6.  Clients are
   responsible for initiating all migrations.  Servers do not send non-
   probing packets (see Section 6.11.1) 9.1) toward a client address until they
   see a non-probing packet from that address.  If a client receives
   packets from an unknown server address, the client MAY discard these
   packets.

6.11.1.

9.1.  Probing a New Path

   An endpoint MAY probe for peer reachability from a new local address
   using path validation Section 6.10 8.2 prior to migrating the connection
   to the new local address.  Failure of path validation simply means
   that the new path is not usable for this connection.  Failure to
   validate a path does not cause the connection to end unless there are
   no valid alternative paths available.

   An endpoint uses a new connection ID for probes sent from a new local
   address, see Section 6.11.5 9.5 for further discussion.  An endpoint that
   uses a new local address needs to ensure that at least one new
   connection ID is available at the peer.  That can be achieved by
   including a NEW_CONNECTION_ID frame in the probe.

   Receiving a PATH_CHALLENGE frame from a peer indicates that the peer
   is probing for reachability on a path.  An endpoint sends a
   PATH_RESPONSE in response as per Section 6.10. 8.2.

   PATH_CHALLENGE, PATH_RESPONSE, NEW_CONNECTION_ID, and PADDING frames
   are "probing frames", and all other frames are "non-probing frames".
   A packet containing only probing frames is a "probing packet", and a
   packet containing any other frame is a "non-probing packet".

6.11.2.

9.2.  Initiating Connection Migration

   An endpoint can migrate a connection to a new local address by
   sending packets containing frames other than probing non-probing frames from that address.

   Each endpoint validates its peer's address during connection
   establishment.  Therefore, a migrating endpoint can send to its peer
   knowing that the peer is willing to receive at the peer's current
   address.  Thus an endpoint can migrate to a new local address without
   first validating the peer's address.

   When migrating, the new path might not support the endpoint's current
   sending rate.  Therefore, the endpoint resets its congestion
   controller, as described in Section 6.11.4. 9.4.

   The new path might not have the same ECN capability.  Therefore, the
   endpoint verifies ECN capability as described in Section 6.8. 13.3.

   Receiving acknowledgments for data sent on the new path serves as
   proof of the peer's reachability from the new address.  Note that
   since acknowledgments may be received on any path, return
   reachability on the new path is not established.  To establish return
   reachability on the new path, an endpoint MAY concurrently initiate
   path validation Section 6.10 8.2 on the new path.

6.11.3.

9.3.  Responding to Connection Migration

   Receiving a packet from a new peer address containing a non-probing
   frame indicates that the peer has migrated to that address.

   In response to such a packet, an endpoint MUST start sending
   subsequent packets to the new peer address and MUST initiate path
   validation (Section 6.10) 8.2) to verify the peer's ownership of the
   unvalidated address.

   An endpoint MAY send data to an unvalidated peer address, but it MUST
   protect against potential attacks as described in Section 6.11.3.1 9.3.1 and
   Section 6.11.3.2. 9.3.2.  An endpoint MAY skip validation of a peer address if
   that address has been seen recently.

   An endpoint only changes the address that it sends packets to in
   response to the highest-numbered non-probing packet.  This ensures
   that an endpoint does not send packets to an old peer address in the
   case that it receives reordered packets.

   After changing the address to which it sends non-probing packets, an
   endpoint could abandon any path validation for other addresses.

   Receiving a packet from a new peer address might be the result of a
   NAT rebinding at the peer.

   After verifying a new client address, the server SHOULD send new
   address validation tokens (Section 6.9) 8) to the client.

6.11.3.1.

9.3.1.  Handling Address Spoofing by a Peer

   It is possible that a peer is spoofing its source address to cause an
   endpoint to send excessive amounts of data to an unwilling host.  If
   the endpoint sends significantly more data than the spoofing peer,
   connection migration might be used to amplify the volume of data that
   an attacker can generate toward a victim.

   As described in Section 6.11.3, 9.3, an endpoint is required to validate a
   peer's new address to confirm the peer's possession of the new
   address.  Until a peer's address is deemed valid, an endpoint MUST
   limit the rate at which it sends data to this address.  The endpoint
   MUST NOT send more than a minimum congestion window's worth of data
   per estimated round-trip time (kMinimumWindow, as defined in
   [QUIC-RECOVERY]).  In the absence of this limit, an endpoint risks
   being used for a denial of service attack against an unsuspecting
   victim.  Note that since the endpoint will not have any round-trip
   time measurements to this address, the estimate SHOULD be the default
   initial value (see [QUIC-RECOVERY]).

   If an endpoint skips validation of a peer address as described in
   Section 6.11.3, 9.3, it does not need to limit its sending rate.

6.11.3.2.

9.3.2.  Handling Address Spoofing by an On-path Attacker

   An on-path attacker could cause a spurious connection migration by
   copying and forwarding a packet with a spoofed address such that it
   arrives before the original packet.  The packet with the spoofed
   address will be seen to come from a migrating connection, and the
   original packet will be seen as a duplicate and dropped.  After a
   spurious migration, validation of the source address will fail
   because the entity at the source address does not have the necessary
   cryptographic keys to read or respond to the PATH_CHALLENGE frame
   that is sent to it even if it wanted to.

   To protect the connection from failing due to such a spurious
   migration, an endpoint MUST revert to using the last validated peer
   address when validation of a new peer address fails.

   If an endpoint has no state about the last validated peer address, it
   MUST close the connection silently by discarding all connection
   state.  This results in new packets on the connection being handled
   generically.  For instance, an endpoint MAY send a stateless reset in
   response to any further incoming packets.

   Note that receipt of packets with higher packet numbers from the
   legitimate peer address will trigger another connection migration.
   This will cause the validation of the address of the spurious
   migration to be abandoned.

6.11.4.

9.4.  Loss Detection and Congestion Control

   The capacity available on the new path might not be the same as the
   old path.  Packets sent on the old path SHOULD NOT contribute to
   congestion control or RTT estimation for the new path.

   On confirming a peer's ownership of its new address, an endpoint
   SHOULD immediately reset the congestion controller and round-trip
   time estimator for the new path.

   An endpoint MUST NOT return to the send rate used for the previous
   path unless it is reasonably sure that the previous send rate is
   valid for the new path.  For instance, a change in the client's port
   number is likely indicative of a rebinding in a middlebox and not a
   complete change in path.  This determination likely depends on
   heuristics, which could be imperfect; if the new path capacity is
   significantly reduced, ultimately this relies on the congestion
   controller responding to congestion signals and reducing send rates
   appropriately.

   There may be apparent reordering at the receiver when an endpoint
   sends data and probes from/to multiple addresses during the migration
   period, since the two resulting paths may have different round-trip
   times.  A receiver of packets on multiple paths will still send ACK
   frames covering all received packets.

   While multiple paths might be used during connection migration, a
   single congestion control context and a single loss recovery context
   (as described in [QUIC-RECOVERY]) may be adequate.  A sender can make
   exceptions for probe packets so that their loss detection is
   independent and does not unduly cause the congestion controller to
   reduce its sending rate.  An endpoint might set a separate timer when
   a PATH_CHALLENGE is sent, which is cancelled when the corresponding
   PATH_RESPONSE is received.  If the timer fires before the
   PATH_RESPONSE is received, the endpoint might send a new
   PATH_CHALLENGE, and restart the timer for a longer period of time.

6.11.5.

9.5.  Privacy Implications of Connection Migration

   Using a stable connection ID on multiple network paths allows a
   passive observer to correlate activity between those paths.  An
   endpoint that moves between networks might not wish to have their
   activity correlated by any entity other than their peer, so different
   connection IDs are used when sending from different local addresses,
   as discussed in Section 6.1. 5.1.  For this to be effective endpoints need
   to ensure that connections IDs they provide cannot be linked by any
   other entity.

   This eliminates the use of the connection ID for linking activity
   from the same connection on different networks.  Protection of packet
   numbers ensures that packet numbers cannot be used to correlate
   activity.  This does not prevent other properties of packets, such as
   timing and size, from being used to correlate activity.

   Clients MAY move to a new connection ID at any time based on
   implementation-specific concerns.  For example, after a period of
   network inactivity NAT rebinding might occur when the client begins
   sending data again.

   A client might wish to reduce linkability by employing a new
   connection ID and source UDP port when sending traffic after a period
   of inactivity.  Changing the UDP port from which it sends packets at
   the same time might cause the packet to appear as a connection
   migration.  This ensures that the mechanisms that support migration
   are exercised even for clients that don't experience NAT rebindings
   or genuine migrations.  Changing port number can cause a peer to
   reset its congestion state (see Section 6.11.4), 9.4), so the port SHOULD only
   be changed infrequently.

   Endpoints that use connection IDs with length greater than zero could
   have their activity correlated if their peers keep using the same
   destination connection ID after migration.  Endpoints that receive
   packets with a previously unused Destination Connection ID SHOULD
   change to sending packets with a connection ID that has not been used
   on any other network path.  The goal here is to ensure that packets
   sent on different paths cannot be correlated.  To fulfill this
   privacy requirement, endpoints that initiate migration and use
   connection IDs with length greater than zero SHOULD provide their
   peers with new connection IDs before migration.

   Caution:  If both endpoints change connection ID in response to
      seeing a change in connection ID from their peer, then this can
      trigger an infinite sequence of changes.

6.12.

9.6.  Server's Preferred Address

   QUIC allows servers to accept connections on one IP address and
   attempt to transfer these connections to a more preferred address
   shortly after the handshake.  This is particularly useful when
   clients initially connect to an address shared by multiple servers
   but would prefer to use a unicast address to ensure connection
   stability.  This section describes the protocol for migrating a
   connection to a preferred server address.

   Migrating a connection to a new server address mid-connection is left
   for future work.  If a client receives packets from a new server
   address not indicated by the preferred_address transport parameter,
   the client SHOULD discard these packets.

6.12.1.

9.6.1.  Communicating A Preferred Address

   A server conveys a preferred address by including the
   preferred_address transport parameter in the TLS handshake.

   Once the handshake is finished, the client SHOULD initiate path
   validation (see Section 6.10) 8.2) of the server's preferred address using
   the connection ID provided in the preferred_address transport
   parameter.

   If path validation succeeds, the client SHOULD immediately begin
   sending all future packets to the new server address using the new new server address using the new
   connection ID and discontinue use of the old server address.  If path
   validation fails, the client MUST continue sending all future packets
   to the server's original IP address.

9.6.2.  Responding to Connection Migration

   A server might receive a packet addressed to its preferred IP address
   at any time after it accepts a connection.  If this packet contains a
   PATH_CHALLENGE frame, the server sends a PATH_RESPONSE frame as per
   Section 8.2.  The server MAY send other non-probing frames from its
   preferred address, but MUST continue sending all probing packets from
   its original IP address.

   The server SHOULD also initiate path validation of the client using
   its preferred address and the address from which it received the
   client probe.  This helps to guard against spurious migration
   initiated by an attacker.

   Once the server has completed its path validation and has received a
   non-probing packet with a new largest packet number on its preferred
   address, the server begins sending non-probing packets to the client
   exclusively from its preferred IP address.  It SHOULD drop packets
   for this connection received on the old IP address, but MAY continue
   to process delayed packets.

9.6.3.  Interaction of Client Migration and Preferred Address

   A client might need to perform a connection migration before it has
   migrated to the server's preferred address.  In this case, the client
   SHOULD perform path validation to both the original and preferred
   server address from the client's new address concurrently.

   If path validation of the server's preferred address succeeds, the
   client MUST abandon validation of the original address and migrate to
   using the server's preferred address.  If path validation of the
   server's preferred address fails but validation of the server's
   original address succeeds, the client MAY migrate to its new address
   and continue sending to the server's original address.

   If the connection to the server's preferred address is not from the
   same client address, the server MUST protect against potential
   attacks as described in Section 9.3.1 and Section 9.3.2.  In addition
   to intentional simultaneous migration, this might also occur because
   the client's access network used a different NAT binding for the
   server's preferred address.

   Servers SHOULD initiate path validation to the client's new address
   upon receiving a probe packet from a different address.  Servers MUST
   NOT send more than a minimum congestion window's worth of non-probing
   packets to the new address before path validation is complete.

10.  Connection Termination

   Connections should remain open until they become idle for a pre-
   negotiated period of time.  A QUIC connection, once established, can
   be terminated in one of three ways:

   o  idle timeout (Section 10.2)

   o  immediate close (Section 10.3)

   o  stateless reset (Section 10.4)

10.1.  Closing and Draining Connection States

   The closing and draining connection states exist to ensure that
   connections close cleanly and that delayed or reordered packets are
   properly discarded.  These states SHOULD persist for three times the
   current Retransmission Timeout (RTO) interval as defined in
   [QUIC-RECOVERY].

   An endpoint enters a closing period after initiating an immediate
   close (Section 10.3).  While closing, an endpoint MUST NOT send
   packets unless they contain a CONNECTION_CLOSE or APPLICATION_CLOSE
   frame (see Section 10.3 for details).  An endpoint retains only
   enough information to generate a packet containing a closing frame
   and to identify packets as belonging to the connection.  The
   connection ID and discontinue use of the old server address.  If path
   validation fails, the client MUST continue sending QUIC version is sufficient information to identify
   packets for a closing connection; an endpoint can discard all future other
   connection state.  An endpoint MAY retain packet protection keys for
   incoming packets to the server's original IP address.

6.12.2.  Responding allow it to Connection Migration

   A server might receive read and process a packet addressed to closing frame.

   The draining state is entered once an endpoint receives a signal that
   its preferred IP address
   at any time after the handshake peer is completed.  If this closing or draining.  While otherwise identical to the
   closing state, an endpoint in the draining state MUST NOT send any
   packets.  Retaining packet
   contains protection keys is unnecessary once a PATH_CHALLENGE frame,
   connection is in the server sends draining state.

   An endpoint MAY transition from the closing period to the draining
   period if it can confirm that its peer is also closing or draining.
   Receiving a PATH_RESPONSE closing frame is sufficient confirmation, as per Section 6.10, but the server MUST continue sending all
   other packets from its original IP address. is receiving
   a stateless reset.  The server draining period SHOULD also initiate path validation of end when the client using
   its preferred address and closing
   period would have ended.  In other words, the address from which it received endpoint can use the
   client probe.  This helps
   same end time, but cease retransmission of the closing packet.

   Disposing of connection state prior to guard against spurious migration
   initiated by an attacker.

   Once the server has completed its path validation and has received a
   non-probing packet with a new largest packet number end of the closing or
   draining period could cause delayed or reordered packets to be
   handled poorly.  Endpoints that have some alternative means to ensure
   that late-arriving packets on its preferred
   address, the server begins sending connection do not create QUIC
   state, such as those that are able to close the client exclusively from its
   preferred IP address.  It SHOULD drop packets UDP socket, MAY use
   an abbreviated draining period which can allow for this faster resource
   recovery.  Servers that retain an open socket for accepting new
   connections SHOULD NOT exit the closing or draining period early.

   Once the closing or draining period has ended, an endpoint SHOULD
   discard all connection
   received state.  This results in new packets on the old IP address, but
   connection being handled generically.  For instance, an endpoint MAY continue
   send a stateless reset in response to process delayed any further incoming packets.

6.12.3.  Interaction of Client Migration

   The draining and Preferred Address

   A client might need to perform closing periods do not apply when a connection migration before it has
   migrated stateless reset
   (Section 10.4) is sent.

   An endpoint is not expected to handle key updates when it is closing
   or draining.  A key update might prevent the server's preferred address.  In this case, endpoint from moving
   from the client
   SHOULD perform path validation closing state to both the original and preferred
   server address draining, but it otherwise has no impact.

   An endpoint could receive packets from the client's a new address concurrently.

   If path validation of source address,
   indicating a client connection migration (Section 9), while in the server's preferred address succeeds,
   closing period.  An endpoint in the
   client closing state MUST abandon validation of the original address and migrate to
   using strictly limit
   the server's preferred address.  If path validation number of the
   server's preferred packets it sends to this new address fails, but validation of until the server's
   original address succeeds,
   is validated (see Section 8.2).  A server in the client closing state MAY migrate
   instead choose to using the
   original address discard packets received from the client's a new source address.

10.2.  Idle Timeout

   If the connection to the server's preferred address idle timeout is not from the
   same client address, enabled, a connection that remains idle for
   longer than the server MUST protect against potential
   attacks as described in Section 6.11.3.1 and advertised idle timeout (see Section 6.11.3.2.  In
   addition to intentional simultaneous migration, this might also occur
   because 18.1) is closed.
   A connection enters the client's access network used a different NAT binding for draining state when the server's preferred address.

   Servers SHOULD initiate path validation idle timeout expires.

   Each endpoint advertises its own idle timeout to its peer.  The idle
   timeout starts from the client's new address
   upon receiving a probe last packet from a different address.  Servers MUST
   NOT send more than a minimum congestion window's worth of non-probing
   packets received.  In order to the ensure
   that initiating new address before path validation is complete.

6.13.  Connection Termination

   Connections should remain open until they become activity postpones an idle for timeout, an endpoint
   restarts this timer when sending a pre-
   negotiated period of time.  A QUIC connection, once established, can
   be terminated in one of three ways:

   o packet.  An endpoint does not
   postpone the idle timeout (Section 6.13.2)

   o  immediate close (Section 6.13.3)

   o  stateless reset (Section 6.13.4)

6.13.1.  Closing and Draining Connection States

   The closing if another packet has been sent containing
   frames other than ACK or PADDING, and draining connection states exist to ensure that
   connections close cleanly and other packet has not been
   acknowledged or declared lost.  Packets that delayed contain only ACK or reordered packets
   PADDING frames are
   properly discarded.  These states SHOULD persist not acknowledged until an endpoint has other
   frames to send, so they could prevent the timeout from being
   refreshed.

   The value for three times an idle timeout can be asymmetric.  The value
   advertised by an endpoint is only used to determine whether the
   current Retransmission Timeout (RTO) interval as defined in
   [QUIC-RECOVERY].
   connection is live at that endpoint.  An endpoint that sends packets
   near the end of the idle timeout period of a peer risks having those
   packets discarded if its peer enters the draining state before the
   packets arrive.  If a closing period after initiating an immediate
   close (Section 6.13.3).  While closing, peer could timeout within an RTO (see
   Section 4.3.3 of [QUIC-RECOVERY]), it is advisable to test for
   liveness before sending any data that cannot be retried safely.

10.3.  Immediate Close

   An endpoint MUST NOT send
   packets unless they contain sends a CONNECTION_CLOSE closing frame (CONNECTION_CLOSE or APPLICATION_CLOSE
   APPLICATION_CLOSE) to terminate the connection immediately.  Any
   closing frame (see Section 6.13.3 for details).

   In causes all streams to immediately become closed; open
   streams can be assumed to be implicitly reset.

   After sending a closing frame, endpoints immediately enter the
   closing state, only a packet containing state.  During the closing period, an endpoint that sends a
   closing frame can be
   sent.  An endpoint retains only enough information SHOULD respond to generate a any packet that it receives with
   another packet containing a closing frame and to identify packets as
   belonging to frame.  To minimize the connection.  The connection ID and QUIC version is
   sufficient information to identify packets state
   that an endpoint maintains for a closing connection;
   an endpoint can discard all other connection state.  An endpoint connection, endpoints MAY
   retain packet protection keys for incoming
   send the exact same packet.  However, endpoints SHOULD limit the
   number of packets to allow it to
   read and process they generate containing a closing frame.

   The draining state is entered once  For
   instance, an endpoint receives a signal could progressively increase the number of
   packets that
   its peer is closing it receives before sending additional packets or draining.  While otherwise identical to
   increase the
   closing state, an endpoint time between packets.

   Note:  Allowing retransmission of a packet contradicts other advice
      in this document that recommends the draining state MUST NOT send any
   packets.  Retaining creation of new packet protection keys
      numbers for every packet.  Sending new packet numbers is unnecessary once primarily
      of advantage to loss recovery and congestion control, which are
      not expected to be relevant for a
   connection is in closed connection.
      Retransmitting the draining final packet requires less state.

   An endpoint MAY transition from the

   After receiving a closing period to frame, endpoints enter the draining
   period if it can confirm state.
   An endpoint that its peer is also closing or draining.
   Receiving receives a closing frame is sufficient confirmation, as is receiving MAY send a single packet
   containing a stateless reset.  The draining period SHOULD end when the closing
   period would have ended.  In other words, frame before entering the draining state, using
   a CONNECTION_CLOSE frame and a NO_ERROR code if appropriate.  An
   endpoint can use the
   same end time, but cease retransmission MUST NOT send further packets, which could result in a
   constant exchange of the closing packet.

   Disposing of connection state prior to the end of frames until the closing or
   draining period could cause delayed or reordered packets to on
   either peer ended.

   An immediate close can be
   handled poorly.  Endpoints that have some alternative means used after an application protocol has
   arranged to ensure
   that late-arriving packets on close a connection.  This might be after the connection do not create QUIC
   state, such as those application
   protocols negotiates a graceful shutdown.  The application protocol
   exchanges whatever messages that are able needed to cause both endpoints
   to agree to close the UDP socket, MAY use
   an abbreviated draining period connection, after which can allow for faster resource
   recovery.  Servers the application
   requests that retain an open socket for accepting new
   connections SHOULD NOT exit the closing or draining period early.

   Once connection be closed.  The application protocol can
   use an APPLICATION_CLOSE message with an appropriate error code to
   signal closure.

   If the closing or draining period connection has ended, an endpoint SHOULD
   discard all been successfully established, endpoints MUST
   send any closing frames in a 1-RTT packet.  Prior to connection state.  This results
   establishment a peer might not have 1-RTT keys, so endpoints SHOULD
   send closing frames in new packets on a Handshake packet.  If the
   connection being handled generically.  For instance, an endpoint does not
   have Handshake keys, or it is not certain that the peer has Handshake
   keys, it MAY send a stateless reset closing frames in response an Initial packet.  If multiple
   packets are sent, they can be coalesced (see Section 12.2) to any further incoming packets.

   The draining and closing periods do not apply when a
   facilitate retransmission.

10.4.  Stateless Reset

   A stateless reset
   (Section 6.13.4) is sent.

   An provided as an option of last resort for an
   endpoint is that does not expected have access to handle key updates when it is closing
   or draining. the state of a connection.  A key update
   crash or outage might prevent the result in peers continuing to send data to an
   endpoint from moving
   from the closing state that is unable to draining, but it otherwise has no impact. properly continue the connection.  An
   endpoint could receive packets from that wishes to communicate a new source address,
   indicating fatal connection error MUST use
   a closing frame if it has sufficient state to do so.

   To support this process, a token is sent by endpoints.  The token is
   carried in the NEW_CONNECTION_ID frame sent by either peer, and
   servers can specify the stateless_reset_token transport parameter
   during the handshake (clients cannot because their transport
   parameters don't have confidentiality protection).  This value is
   protected by encryption, so only client and server know this value.
   Tokens sent via NEW_CONNECTION_ID frames are invalidated when their
   associated connection migration ID is retired via a RETIRE_CONNECTION_ID frame
   (Section 6.11), while in the
   closing period. 19.13).

   An endpoint in the closing state MUST strictly limit
   the number of that receives packets that it cannot process sends to this new address until the address
   is validated (see Section 6.10).  A server a
   packet in the closing state MAY
   instead choose following layout:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |0|K|1|1|0|0|0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Random Octets (160..)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 7: Stateless Reset Packet

   This design ensures that a stateless reset packet is - to discard packets received the extent
   possible - indistinguishable from a new source address.

6.13.2.  Idle Timeout

   If the idle timeout is enabled, regular packet with a connection that remains idle for
   longer than the advertised idle timeout (see Section 6.6.1) is
   closed. short
   header.

   The message consists of a header octet, followed by an arbitrary
   number of random octets, followed by a Stateless Reset Token.

   A connection enters stateless reset will be interpreted by a recipient as a packet with
   a short header.  For the draining state when packet to appear as valid, the idle timeout
   expires.

   Each endpoint advertises their own idle timeout Random Octets
   field needs to their peer.  The
   idle timeout starts from include at least 20 octets of random or unpredictable
   values.  This is intended to allow for a destination connection ID of
   the last maximum length permitted, a packet received.  In order number, and minimal payload.
   The Stateless Reset Token corresponds to
   ensure that initiating new activity postpones an idle timeout, an the minimum expansion of the
   packet protection AEAD.  More random octets might be necessary if the
   endpoint restarts this timer when sending could have negotiated a packet. packet protection scheme with a
   larger minimum AEAD expansion.

   An endpoint does
   not postpone SHOULD NOT send a stateless reset that is significantly
   larger than the idle timeout if another packet has been sent
   containing frames other than ACK or PADDING, and it receives.  Endpoints MUST discard packets
   that other are too small to be valid QUIC packets.  With the set of AEAD
   functions defined in [QUIC-TLS], packets less than 19 octets long are
   never valid.

   An endpoint MAY send a stateless reset in response to a packet
   has with a
   long header.  This would not been acknowledged or declared lost.  Packets that contain
   only ACK or PADDING frames be effective if the stateless reset
   token was not yet available to a peer.  In this QUIC version, packets
   with a long header are only used during connection establishment.
   Because the stateless reset token is not acknowledged available until connection
   establishment is complete or near completion, ignoring an unknown
   packet with a long header might be more effective.

   An endpoint has
   other frames to send, so they could prevent cannot determine the timeout Source Connection ID from being
   refreshed.

   The value for an idle timeout can be asymmetric. a packet
   with a short header, therefore it cannot set the Destination
   Connection ID in the stateless reset packet.  The Destination
   Connection ID will therefore differ from the value
   advertised by an endpoint is only used to determine whether in previous
   packets.  A random Destination Connection ID makes the connection is live at that endpoint.  An endpoint that sends packets
   near the end of ID
   appear to be the idle timeout period result of moving to a peer risks having those
   packets discarded if its peer enters the draining state before new connection ID that was
   provided using a NEW_CONNECTION_ID frame (Section 19.12).

   Using a randomized connection ID results in two problems:

   o  The packet might not reach the
   packets arrive. peer.  If a peer the Destination
      Connection ID is critical for routing toward the peer, then this
      packet could timeout within an RTO (see be incorrectly routed.  This might also trigger
      another Stateless Reset in response, see Section 4.3.3 of [QUIC-RECOVERY]), it 10.4.3.  A
      Stateless Reset that is advisable not correctly routed is ineffective in
      causing errors to test for
   liveness before sending any data that cannot be retried safely.

6.13.3.  Immediate Close

   An endpoint sends a closing frame (CONNECTION_CLOSE or
   APPLICATION_CLOSE) quickly detected and recovered.  In this
      case, endpoints will need to terminate rely on other methods - such as
      timers - to detect that the connection immediately.  Any
   closing frame causes all streams to immediately become closed; open
   streams has failed.

   o  The randomly generated connection ID can be assumed used by entities other
      than the peer to be implicitly reset.

   After sending identify this as a closing frame, endpoints immediately enter the
   closing state.  During the closing period, an potential stateless reset.  An
      endpoint that sends a
   closing frame SHOULD respond to any packet that it receives with
   another packet containing a closing frame.  To minimize occasionally uses different connection IDs might
      introduce some uncertainty about this.

   Finally, the state
   that an endpoint maintains for a closing connection, endpoints MAY
   send last 16 octets of the exact same packet.  However, endpoints SHOULD limit packet are set to the
   number value of packets they generate containing a closing frame.  For
   instance, an endpoint could progressively increase the number of
   packets
   Stateless Reset Token.

   A stateless reset is not appropriate for signaling error conditions.
   An endpoint that it receives before sending additional packets wishes to communicate a fatal connection error MUST
   use a CONNECTION_CLOSE or
   increase the time between packets.

   Note:  Allowing retransmission APPLICATION_CLOSE frame if it has
   sufficient state to do so.

   This stateless reset design is specific to QUIC version 1.  An
   endpoint that supports multiple versions of QUIC needs to generate a packet contradicts other advice
      in this document
   stateless reset that will be accepted by peers that support any
   version that recommends the creation endpoint might support (or might have supported
   prior to losing state).  Designers of new packet
      numbers for every packet.  Sending new packet numbers is primarily versions of advantage to loss recovery and congestion control, which are
      not expected QUIC need to be relevant for
   aware of this and either reuse this design, or use a closed connection.
      Retransmitting portion of the final
   packet requires less state.

   After receiving a closing frame, endpoints enter other than the draining state. last 16 octets for carrying data.

10.4.1.  Detecting a Stateless Reset

   An endpoint that receives detects a closing frame MAY send potential stateless reset when a single packet
   containing with a closing frame before entering
   short header either cannot be decrypted or is marked as a duplicate
   packet.  The endpoint then compares the draining state, using last 16 octets of the packet
   with the Stateless Reset Token provided by its peer, either in a CONNECTION_CLOSE
   NEW_CONNECTION_ID frame and a NO_ERROR code if appropriate.  An or the server's transport parameters.  If
   these values are identical, the endpoint MUST NOT send further packets, which could result in a
   constant exchange of closing frames until enter the closing draining
   period and not send any further packets on
   either peer ended.

   An immediate close this connection.  If the
   comparison fails, the packet can be used after an application protocol has
   arranged to close discarded.

10.4.2.  Calculating a connection.  This might Stateless Reset Token

   The stateless reset token MUST be after the application
   protocols negotiates difficult to guess.  In order to
   create a graceful shutdown.  The application protocol
   exchanges whatever messages Stateless Reset Token, an endpoint could randomly generate
   [RFC4086] a secret for every connection that it creates.  However,
   this presents a coordination problem when there are needed to cause both endpoints multiple
   instances in a cluster or a storage problem for an endpoint that
   might lose state.  Stateless reset specifically exists to agree handle the
   case where state is lost, so this approach is suboptimal.

   A single static key can be used across all connections to close the connection, after which same
   endpoint by generating the application
   requests proof using a second iteration of a
   preimage-resistant function that takes a static key and the
   connection be closed.  The application protocol can ID chosen by the endpoint (see Section 5.1) as input.  An
   endpoint could use an APPLICATION_CLOSE message HMAC [RFC2104] (for example, HMAC(static_key,
   connection_id)) or HKDF [RFC5869] (for example, using the static key
   as input keying material, with an appropriate error code to
   signal closure.

6.13.4.  Stateless Reset

   A stateless reset is provided the connection ID as an option salt).  The
   output of last resort this function is truncated to 16 octets to produce the
   Stateless Reset Token for an that connection.

   An endpoint that does not have access loses state can use the same method to generate a
   valid Stateless Reset Token.  The connection ID comes from the state of packet
   that the endpoint receives.

   This design relies on the peer always sending a connection.  A
   crash or outage might result connection ID in peers continuing to send data to an
   endpoint its
   packets so that is unable the endpoint can use the connection ID from a packet
   to properly continue reset the connection.  An endpoint that wishes to communicate a fatal connection error uses this design MUST
   either use
   a closing frame if the same connection ID length for all connections or
   encode the length of the connection ID such that it has sufficient state to do so.

   To support this process, can be recovered
   without state.  In addition, it cannot provide a token is sent by endpoints.  The token is
   carried in zero-length
   connection ID.

   Revealing the NEW_CONNECTION_ID frame sent by either peer, and
   servers Stateless Reset Token allows any entity to terminate
   the connection, so a value can specify only be used once.  This method for
   choosing the stateless_reset_token transport parameter
   during Stateless Reset Token means that the handshake (clients combination of
   connection ID and static key cannot because their transport
   parameters don't have confidentiality protection).  This value occur for another connection.  A
   denial of service attack is
   protected by encryption, so only client and server know this value.
   Tokens sent via NEW_CONNECTION_ID frames are invalidated when their
   associated possible if the same connection ID is retired via a RETIRE_CONNECTION_ID frame
   (Section 7.14).

   An endpoint that receives packets
   used by instances that it cannot process sends share a static key, or if an attacker can
   cause a packet in to be routed to an instance that has no state but the
   same static key (see Section 21.8).  A connection ID from a
   connection that is reset by revealing the following layout:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |0|K|1|1|0|0|0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Random Octets (160..)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   + Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 11:
   cannot be reused for new connections at nodes that share a static
   key.

   Note that Stateless Reset Packet

   This packets do not have any cryptographic
   protection.

10.4.3.  Looping

   The design ensures that of a stateless reset packet Stateless Reset is such that it is - to the extent
   possible - indistinguishable
   from a regular packet with valid packet.  This means that a short
   header.

   The message consists Stateless Reset might trigger
   the sending of a header octet, followed by an arbitrary Stateless Reset in response, which could lead to
   infinite exchanges.

   An endpoint MUST ensure that every Stateless Reset that it sends is
   smaller than the packet which triggered it, unless it maintains state
   sufficient to prevent looping.  In the event of a loop, this results
   in packets eventually being too small to trigger a response.

   An endpoint can remember the number of random octets, followed by Stateless Reset packets that
   it has sent and stop generating new Stateless Reset packets once a
   limit is reached.  Using separate limits for different remote
   addresses will ensure that Stateless Reset Token.

   A stateless reset will packets can be interpreted by a recipient as a packet with
   a short header.  For the packet to appear as valid, the Random Octets
   field needs used to include at least 20 octets of random
   close connections when other peers or unpredictable
   values.  This is intended to allow for a destination connection ID of connections have exhausted
   limits.

   Reducing the maximum length permitted, size of a packet number, and minimal payload.
   The Stateless Reset Token corresponds to below the recommended minimum expansion
   size of the
   packet protection AEAD.  More random 37 octets might be necessary if the
   endpoint could have negotiated a packet protection scheme with a
   larger minimum AEAD expansion.

   An endpoint SHOULD NOT send a stateless reset mean that is significantly
   larger than the packet it receives.  Endpoints MUST discard packets could reveal to an
   observer that are too small it is a Stateless Reset.  Conversely, refusing to be valid QUIC packets.  With the set of AEAD
   functions defined in [QUIC-TLS], packets less than 19 octets long are
   never valid.

   An endpoint MAY send
   a stateless reset Stateless Reset in response to a small packet with a
   long header.  This would not be effective if the stateless reset
   token was might result in
   Stateless Reset not yet available to a peer.  In this QUIC version, being useful in detecting cases of broken
   connections where only very small packets
   with a long header are only used during connection establishment.
   Because the stateless reset token is not available until connection
   establishment is complete or near completion, ignoring an unknown
   packet with a long header sent; such failures
   might only be more effective. detected by other means, such as timers.

   An endpoint cannot determine can increase the Source Connection ID from odds that a packet
   with will trigger a short header, therefore
   Stateless Reset if it cannot set the Destination
   Connection ID in the stateless reset packet.  The Destination
   Connection ID will therefore differ from the value used in previous
   packets.  A random Destination Connection ID makes the connection ID
   appear to be processed by padding it to at least
   38 octets.

11.  Error Handling

   An endpoint that detects an error SHOULD signal the result existence of moving that
   error to a new its peer.  Both transport-level and application-level errors
   can affect an entire connection ID that was
   provided using (see Section 11.1), while only
   application-level errors can be isolated to a NEW_CONNECTION_ID frame single stream (see
   Section 11.2).

   The most appropriate error code (Section 7.13).

   Using a randomized connection ID results 20) SHOULD be included in two problems:

   o  The packet might not reach the peer.  If
   the Destination
      Connection ID is critical for routing toward frame that signals the peer, then error.  Where this
      packet could be incorrectly routed.  This might specification
   identifies error conditions, it also trigger
      another Stateless Reset in response, see Section 6.13.4.3.  A
      Stateless Reset identifies the error code that
   is not correctly routed used.

   A stateless reset (Section 10.4) is ineffective in
      causing errors to be quickly detected and recovered.  In this
      case, endpoints will need to rely on other methods - such as
      timers - to detect not suitable for any error that the connection has failed.

   o  The randomly generated connection ID
   can be signaled with a CONNECTION_CLOSE, APPLICATION_CLOSE, or
   RST_STREAM frame.  A stateless reset MUST NOT be used by entities other
      than an endpoint
   that has the peer state necessary to identify this send a frame on the connection.

11.1.  Connection Errors

   Errors that result in the connection being unusable, such as an
   obvious violation of protocol semantics or corruption of state that
   affects an entire connection, MUST be signaled using a potential stateless reset.
   CONNECTION_CLOSE or APPLICATION_CLOSE frame (Section 19.3,
   Section 19.4).  An endpoint that occasionally uses different MAY close the connection IDs might
      introduce some uncertainty about this.

   Finally, in this manner
   even if the last 16 octets of error only affects a single stream.

   Application protocols can signal application-specific protocol errors
   using the packet APPLICATION_CLOSE frame.  Errors that are set specific to the value of
   transport, including all those described in this document, are
   carried in a CONNECTION_CLOSE frame.  Other than the
   Stateless Reset Token. type of error
   code they carry, these frames are identical in format and semantics.

   A stateless reset CONNECTION_CLOSE or APPLICATION_CLOSE frame could be sent in a
   packet that is not appropriate for signaling error conditions. lost.  An endpoint that wishes SHOULD be prepared to communicate retransmit a fatal connection error MUST
   use
   packet containing either frame type if it receives more packets on a
   terminated connection.  Limiting the number of retransmissions and
   the time over which this final packet is sent limits the effort
   expended on terminated connections.

   An endpoint that chooses not to retransmit packets containing
   CONNECTION_CLOSE or APPLICATION_CLOSE frame if it has
   sufficient state risks a peer missing the first
   such packet.  The only mechanism available to do so.

   This stateless reset design an endpoint that
   continues to receive data for a terminated connection is specific to QUIC version 1. use the
   stateless reset process (Section 10.4).

   An endpoint that supports multiple versions receives an invalid CONNECTION_CLOSE or
   APPLICATION_CLOSE frame MUST NOT signal the existence of QUIC needs the error to generate
   its peer.

11.2.  Stream Errors

   If an application-level error affects a
   stateless reset that will be accepted by peers that support any
   version that single stream, but otherwise
   leaves the connection in a recoverable state, the endpoint might support (or might have supported
   prior can send a
   RST_STREAM frame (Section 19.2) with an appropriate error code to losing state).  Designers of new versions
   terminate just the affected stream.

   Other than STOPPING (Section 3.5), RST_STREAM MUST be instigated by
   the application and MUST carry an application error code.  Resetting
   a stream without knowledge of QUIC need the application protocol could cause
   the protocol to enter an unrecoverable state.  Application protocols
   might require certain streams to be
   aware of this reliably delivered in order to
   guarantee consistent state between endpoints.

12.  Packets and either reuse this design, or use a portion Frames

   QUIC endpoints communicate by exchanging packets.  Packets are
   carried in UDP datagrams (see Section 12.2) and have confidentiality
   and integrity protection (see Section 12.1).

   This version of QUIC uses the long packet other than header (see Section 17.2)
   during connection establishment and the last 16 octets short header (see
   Section 17.3) once 1-RTT keys have been established.

   Packets that carry the long header are Initial Section 17.5, Retry
   Section 17.7, Handshake Section 17.6, and 0-RTT Protected packets
   Section 12.1.

   Packets with the short header are designed for carrying data.

6.13.4.1.  Detecting a Stateless Reset

   An endpoint detects minimal overhead and
   are used after a potential stateless reset when connection is established.

   Version negotiation uses a packet with a
   short header either cannot be decrypted or is marked as a duplicate
   packet.  The endpoint then compares the last 16 octets special format (see
   Section 17.4).

12.1.  Protected Packets

   All QUIC packets except Version Negotiation and Retry packets use
   authenticated encryption with additional data (AEAD) [RFC5119] to
   provide confidentiality and integrity protection.  Details of the packet
   with the Stateless Reset Token provided by its peer, either
   protection are found in a
   NEW_CONNECTION_ID frame or [QUIC-TLS]; this section includes an overview
   of the server's transport parameters.  If
   these values process.

   Initial packets are identical, protected using keys that are statically derived.
   This packet protection is not effective confidentiality protection,
   it only exists to ensure that the endpoint MUST enter sender of the draining
   period and not send any further packets packet is on this connection.  If the
   comparison fails,
   network path.  Any entity that receives the Initial packet can be discarded.

6.13.4.2.  Calculating from a Stateless Reset Token

   The stateless reset token MUST be difficult
   client can recover the keys necessary to guess.  In order remove packet protection or
   to
   create a Stateless Reset Token, an endpoint could randomly generate
   [RFC4086] a secret for every connection packets that it creates.  However,
   this presents a coordination problem when there will be successfully authenticated.

   All other packets are multiple
   instances in a cluster or a storage problem for an endpoint that
   might lose state.  Stateless reset specifically exists to handle protected with keys derived from the
   case where state is lost, so this approach is suboptimal.

   A single static
   cryptographic handshake.  The type of the packet from the long header
   or key can be phase from the short header are used across all connections to identify which
   encryption level - and therefore the same
   endpoint by generating the proof using a second iteration of a
   preimage-resistant function keys - that takes a static key are used.  Packets
   protected with 0-RTT and 1-RTT keys are expected to have
   confidentiality and data origin authentication; the
   connection ID chosen by cryptographic
   handshake ensures that only the endpoint (see Section 6.1) as input.  An
   endpoint could use HMAC [RFC2104] (for example, HMAC(static_key,
   connection_id)) or HKDF [RFC5869] (for example, using communicating endpoints receive the static key
   as input keying material,
   corresponding keys.

   The packet number field contains a packet number, which has
   additional confidentiality protection that is applied after packet
   protection is applied (see [QUIC-TLS] for details).  The underlying
   packet number increases with each packet sent, see Section 12.3 for
   details.

12.2.  Coalescing Packets

   A sender can coalesce multiple QUIC packets into one UDP datagram.
   This can reduce the connection ID as salt).  The
   output number of this function is truncated to 16 octets UDP datagrams needed to produce complete the
   Stateless Reset Token for that connection.

   An endpoint that loses state can use
   cryptographic handshake and starting sending data.  Receivers MUST be
   able to process coalesced packets.

   Coalescing packets in order of increasing encryption levels (Initial,
   0-RTT, Handshake, 1-RTT) makes it more likely the same method receiver will be
   able to generate process all the packets in a single pass.  A packet with a
   short header does not include a length, so it will always be the last
   packet included in a UDP datagram.

   Senders MUST NOT coalesce QUIC packets for different connections into
   a single UDP datagram.  Receivers SHOULD ignore any subsequent
   packets with a
   valid Stateless Reset Token.  The connection different Destination Connection ID comes from than the first
   packet
   that the endpoint receives.

   This design relies on the peer always sending a connection ID in its
   packets so that the endpoint can use the connection ID from a datagram.

   Every QUIC packet
   to reset the connection.  An endpoint that uses this design MUST
   either use the same connection ID length for all connections or
   encode is coalesced into a single UDP datagram is
   separate and complete.  Though the length values of some fields in the connection ID such that it can
   packet header might be recovered
   without state.  In addition, it redundant, no fields are omitted.  The
   receiver of coalesced QUIC packets MUST NOT provide a zero-length
   connection ID.

   Revealing individually process each
   QUIC packet and separately acknowledge them, as if they were received
   as the Stateless Reset Token allows payload of different UDP datagrams.  For example, if
   decryption fails (because the keys are not available or any entity to terminate other
   reason) or the connection, so a value can only be used once.  This method for
   choosing packet is of an unknown type, the Stateless Reset Token means that receiver MAY either
   discard or buffer the combination of
   connection ID packet for later processing and static key MUST attempt to
   process the remaining packets.

   Retry packets (Section 17.7), Version Negotiation packets
   (Section 17.4), and packets with a short header cannot occur for another connection.  A
   denial of service attack is possible if be followed by
   other packets in the same connection ID is
   used by instances that share a static key, or if an attacker can
   cause a UDP datagram.

12.3.  Packet Numbers

   The packet to be routed to number is an instance that has no state but integer in the
   same static key range 0 to 2^62-1.  Where
   present, packet numbers are encoded as a variable-length integer (see
   Section 12.8).  A connection ID from a
   connection that 16).  This number is reset by revealing used in determining the Stateless Reset Token
   cannot be reused cryptographic
   nonce for new connections at nodes that share packet protection.  Each endpoint maintains a static
   key.

   Note that Stateless Reset separate
   packet number for sending and receiving.

   Version Negotiation (Section 17.4) and Retry Section 17.7 packets do
   not have any cryptographic
   protection.

6.13.4.3.  Looping

   The design of include a Stateless Reset is such that it packet number.

   Packet numbers are divided into 3 spaces in QUIC:

   o  Initial space: All Initial packets Section 17.5 are in this space.

   o  Handshake space: All Handshake packets Section 17.6 are in this
      space.

   o  Application data space: All 0-RTT and 1-RTT encrypted packets
      Section 12.1 are in this space.

   As described in [QUIC-TLS], each packet type uses different
   protection keys.

   Conceptually, a packet number space is indistinguishable
   from the context in which a valid packet. packet
   can be processed and acknowledged.  Initial packets can only be sent
   with Initial packet protection keys and acknowledged in packets which
   are also Initial packets.  Similarly, Handshake packets are sent at
   the Handshake encryption level and can only be acknowledged in
   Handshake packets.

   This means that a Stateless Reset might trigger enforces cryptographic separation between the sending of a Stateless Reset data sent in the
   different packet sequence number spaces.  Each packet number space
   starts at packet number 0.  Subsequent packets sent in response, which could lead to
   infinite exchanges.

   An endpoint the same
   packet number space MUST ensure that every Stateless Reset that it sends is
   smaller than increase the packet which triggered it, unless it maintains state
   sufficient number by at least one.

   0-RTT and 1-RTT data exist in the same packet number space to prevent looping.  In make
   loss recovery algorithms easier to implement between the event of two packet
   types.

   A QUIC endpoint MUST NOT reuse a loop, this results packet number within the same packet
   number space in packets eventually being too small to trigger a response.

   An endpoint can remember one connection (that is, under the same cryptographic
   keys).  If the packet number of Stateless Reset packets that
   it has sent and stop generating new Stateless Reset packets once a
   limit is reached.  Using separate limits for different remote
   addresses will ensure that Stateless Reset packets can be used to sending reaches 2^62 - 1, the sender
   MUST close connections when other peers or connections have exhausted
   limits.

   Reducing the size of connection without sending a Stateless Reset below the recommended minimum
   size of 37 octets could mean that the packet could reveal to CONNECTION_CLOSE frame or
   any further packets; an
   observer that it is a Stateless Reset.  Conversely, refusing to endpoint MAY send a Stateless Reset
   (Section 10.4) in response to further packets that it receives.

   A receiver MUST discard a small newly unprotected packet might result in
   Stateless Reset unless it is
   certain that it has not being useful processed another packet with the same packet
   number from the same packet number space.  Duplicate suppression MUST
   happen after removing packet protection for the reasons described in detecting cases
   Section 9.3 of broken
   connections where only very small packets are sent; such failures
   might only be detected by other means, such as timers. [QUIC-TLS].  An endpoint efficient algorithm for duplicate
   suppression can increase the odds that be found in Section 3.4.3 of [RFC2406].

   Packet number encoding at a sender and decoding at a receiver are
   described in Section 17.1.

12.4.  Frames and Frame Types

   The payload of QUIC packets, after removing packet will trigger protection,
   commonly consists of a sequence of frames, as shown in Figure 8.
   Version Negotiation, Stateless Reset if it cannot be processed by padding it to Reset, and Retry packets do not
   contain frames.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame 1 (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame 2 (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame N (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 8: QUIC Payload

   QUIC payloads MUST contain at least
   38 octets.

7. one frame, and MAY contain
   multiple frames and multiple frame types.

   Frames MUST fit within a single QUIC packet and MUST NOT span a QUIC
   packet boundary.  Each frame begins with a Frame Type, indicating its
   type, followed by additional type-dependent fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Frame Type (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Type-Dependent Fields (*)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 9: Generic Frame Types and Formats

   As described in Section 5, packets contain one or more frames.  This
   section describes the format and semantics of the core QUIC Layout

   The frame
   types.

7.1.  Variable-Length Integer Encoding

   QUIC frames commonly use a variable-length encoding for non-negative
   integer values.  This encoding ensures that smaller integer values
   need fewer octets to encode. types defined in this specification are listed in Table 3.
   The QUIC variable-length integer encoding reserves the two most
   significant bits of the first octet Frame Type in STREAM frames is used to encode carry other frame-specific
   flags.  For all other frames, the base 2 logarithm of Frame Type field simply identifies
   the integer encoding length frame.  These frames are explained in octets.  The integer value is encoded
   on the remaining bits, more detail in network byte order.

   This means that integers are encoded on 1, 2, 4, or 8 octets and can
   encode 6, 14, 30, or 62 bit values respectively.  Table 4 summarizes
   the encoding properties.

          +------+--------+-------------+-----------------------+ Section 19.

          +-------------+----------------------+---------------+
          | 2Bit Type Value  | Length Frame Type Name      | Usable Bits Definition    | Range
          +-------------+----------------------+---------------+
          |
          +------+--------+-------------+-----------------------+ 0x00        | 00 PADDING              | 1 Section 19.1  | 6
          | 0-63             |                      |               |
          | 0x01        | RST_STREAM           | Section 19.2  |
          | 01             | 2                      | 14               | 0-16383
          | 0x02        | CONNECTION_CLOSE     | Section 19.3  |
          |             |                      | 10               | 4
          | 30 0x03        | 0-1073741823 APPLICATION_CLOSE    | Section 19.4  |
          |             |                      |               |
          | 11 0x04        | 8 MAX_DATA             | 62 Section 19.5  | 0-4611686018427387903
          |
          +------+--------+-------------+-----------------------+             |                      |               |
          | 0x05        | MAX_STREAM_DATA      | Section 19.6  |
          |             |                      |               |
          | 0x06        | MAX_STREAM_ID        | Section 19.7  |
          |             |                      |               |
          | 0x07        | PING                 | Section 19.8  |
          |             |                      |               |
          | 0x08        | BLOCKED              | Section 19.9  |
          |             |                      |               |
          | 0x09        | STREAM_BLOCKED       | Section 19.10 |
          |             |                      |               |
          | 0x0a        | STREAM_ID_BLOCKED    | Section 19.11 |
          |             |                      |               |
          | 0x0b        | NEW_CONNECTION_ID    | Section 19.12 |
          |             |                      |               |
          | 0x0c        | STOP_SENDING         | Section 19.14 |
          |             |                      |               |
          | 0x0d        | RETIRE_CONNECTION_ID | Section 19.13 |
          |             |                      |               |
          | 0x0e        | PATH_CHALLENGE       | Section 19.16 |
          |             |                      |               |
          | 0x0f        | PATH_RESPONSE        | Section 19.17 |
          |             |                      |               |
          | 0x10 - 0x17 | STREAM               | Section 19.19 |
          |             |                      |               |
          | 0x18        | CRYPTO               | Section 19.20 |
          |             |                      |               |
          | 0x19        | NEW_TOKEN            | Section 19.18 |
          |             |                      |               |
          | 0x1a - 0x1b | ACK                  | Section 19.15 |
          +-------------+----------------------+---------------+

                           Table 4: Summary of Integer Encodings

   For example, the eight octet sequence c2 19 7c 5e ff 14 e8 8c (in
   hexadecimal) decodes to the decimal value 151288809941952652; the
   four octet sequence 9d 7f 3e 7d decodes to 494878333; the two octet
   sequence 7b bd decodes to 15293; and the single octet 25 decodes to
   37 (as does the two octet sequence 40 25).

   Error codes (Section 11.3) 3: Frame Types

   All QUIC frames are described using integers, but do idempotent.  That is, a valid frame does not
   use this encoding.

7.2.  PADDING Frame
   cause undesirable side effects or errors when received more than
   once.

   The PADDING frame (type=0x00) has no semantic value.  PADDING frames
   can be used to increase the size Frame Type field uses a variable length integer encoding (see
   Section 16) with one exception.  To ensure simple and efficient
   implementations of frame parsing, a packet.  Padding can be used to
   increase an initial client packet to frame type MUST use the minimum required size, shortest
   possible encoding.  Though a two-, four- or to
   provide protection against traffic analysis for protected packets.

   A PADDING eight-octet encoding of
   the frame has no content.  That is, types defined in this document is possible, the Frame Type
   field for these frames is encoded on a PADDING frame consists single octet.  For instance,
   though 0x4007 is a legitimate two-octet encoding for a variable-
   length integer with a value of
   the 7, PING frames are always encoded as a
   single octet that identifies with the frame as a PADDING frame.

7.3.  RST_STREAM Frame value 0x07.  An endpoint may use MUST treat the receipt
   of a RST_STREAM frame (type=0x01) to abruptly
   terminate type that uses a stream.

   After sending longer encoding than necessary as a RST_STREAM, an endpoint ceases transmission and
   retransmission
   connection error of STREAM type PROTOCOL_VIOLATION.

13.  Packetization and Reliability

   A sender bundles one or more frames on the identified stream. in a QUIC packet (see
   Section 12.4).

   A receiver
   of RST_STREAM sender can discard any data that it already received on that
   stream.

   An endpoint that receives minimize per-packet bandwidth and computational costs by
   bundling as many frames as possible within a RST_STREAM frame QUIC packet.  A sender
   MAY wait for a send-only stream
   MUST terminate the connection with error PROTOCOL_VIOLATION.

   The RST_STREAM frame short period of time to bundle multiple frames before
   sending a packet that is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Application Error Code (16)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Final Offset (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are: not maximally packed, to avoid sending out
   large numbers of small packets.  An implementation may use knowledge
   about application sending behavior or heuristics to determine whether
   and for how long to wait.  This waiting period is an implementation
   decision, and an implementation should be careful to delay
   conservatively, since any delay is likely to increase application-
   visible latency.

   Stream ID: multiplexing is achieved by interleaving STREAM frames from
   multiple streams into one or more QUIC packets.  A variable-length integer encoding single QUIC packet
   can include multiple STREAM frames from one or more streams.

   One of the Stream ID benefits of
      the stream being terminated.

   Application Protocol Error Code:  A 16-bit application protocol error
      code (see Section 11.4) which indicates why the stream QUIC is being
      closed.

   Final Offset: avoidance of head-of-line blocking
   across multiple streams.  When a packet loss occurs, only streams
   with data in that packet are blocked waiting for a retransmission to
   be received, while other streams can continue making progress.  Note
   that when data from multiple streams is bundled into a single QUIC
   packet, loss of that packet blocks all those streams from making
   progress.  Implementations are advised to bundle as few streams as
   necessary in outgoing packets without losing transmission efficiency
   to underfilled packets.

13.1.  Packet Processing and Acknowledgment

   A variable-length integer indicating packet MUST NOT be acknowledged until packet protection has been
   successfully removed and all frames contained in the absolute byte
      offset of packet have been
   processed.  For STREAM frames, this means the end of data written on this stream has been enqueued
   in preparation to be received by the RST_STREAM
      sender.

7.4.  CONNECTION_CLOSE frame

   An endpoint sends a CONNECTION_CLOSE frame (type=0x02) to notify its
   peer application protocol, but it
   does not require that the connection is being closed.  CONNECTION_CLOSE data is used
   to signal errors at delivered and consumed.

   Once the QUIC layer, packet has been fully processed, a receiver acknowledges
   receipt by sending one or more ACK frames containing the absence packet
   number of errors (with the NO_ERROR code).

   If received packet.

13.1.1.  Sending ACK Frames

   To avoid creating an indefinite feedback loop, an endpoint MUST NOT
   send an ACK frame in response to a packet containing only ACK or
   PADDING frames, even if there are open streams that haven't been explicitly closed, they
   are implicitly closed when packet gaps which precede the connection is closed.
   received packet.  The CONNECTION_CLOSE endpoint MUST however acknowledge packets
   containing only ACK or PADDING frames when sending ACK frames in
   response to other packets.

   While PADDING frames do not elicit an ACK frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Error Code (16)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Frame Type (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Reason Phrase Length (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Reason Phrase (*)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of from a CONNECTION_CLOSE frame receiver, they
   are as follows:

   Error Code:  A 16-bit error code which indicates the reason considered to be in flight for
      closing this connection.  CONNECTION_CLOSE uses codes congestion control purposes
   [QUIC-RECOVERY].  Sending only PADDING frames might cause the sender
   to become limited by the congestion controller (as described in
   [QUIC-RECOVERY]) with no acknowledgments forthcoming from the
      space defined
   receiver.  Therefore, a sender should ensure that other frames are
   sent in Section 11.3.

   Frame Type:  A variable-length integer encoding addition to PADDING frames to elicit acknowledgments from the type of
   receiver.

   An endpoint MUST NOT send more than one packet containing only an ACK
   frame per received packet that triggered contains frames other than ACK and
   PADDING frames.

   The receiver's delayed acknowledgment timer SHOULD NOT exceed the error.  A value of 0 (equivalent to
   current RTT estimate or the mention
      of value it indicates in the PADDING frame) "max_ack_delay"
   transport parameter.  This ensures an acknowledgment is used sent at least
   once per RTT when packets needing acknowledgement are received.  The
   sender can use the frame type is unknown.

   Reason Phrase Length:  A variable-length integer specifying the
      length receiver's "max_ack_delay" value in determining
   timeouts for timer-based retransmission.

   Strategies and implications of the reason phrase frequency of generating
   acknowledgments are discussed in bytes.  Note that a
      CONNECTION_CLOSE frame cannot be split between packets, so more detail in
      practice any limits on packet size will also [QUIC-RECOVERY].

   To limit ACK Blocks to those that have not yet been received by the space
      available for
   sender, the receiver SHOULD track which ACK frames have been
   acknowledged by its peer.  Once an ACK frame has been acknowledged,
   the packets it acknowledges SHOULD NOT be acknowledged again.

   Because ACK frames are not sent in response to ACK-only packets, a reason phrase.

   Reason Phrase:  A human-readable explanation
   receiver that is only sending ACK frames will only receive
   acknowledgements for why the connection
      was closed.  This can be zero length its packets if the sender chooses to not
      give details beyond the Error Code.  This includes them in
   packets with non-ACK frames.  A sender SHOULD be a UTF-8
      encoded string [RFC3629].

7.5.  APPLICATION_CLOSE frame

   An APPLICATION_CLOSE frame (type=0x03) is used to signal an error bundle ACK frames with
   other frames when possible.

   To limit receiver state or the protocol that uses QUIC.

   The APPLICATION_CLOSE frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Error Code (16)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Reason Phrase Length (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Reason Phrase (*)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields size of an APPLICATION_CLOSE frame are as follows:

   Error Code: ACK frames, a receiver MAY
   limit the number of ACK Blocks it sends.  A 16-bit error code which indicates receiver can do this even
   without receiving acknowledgment of its ACK frames, with the reason for
      closing
   knowledge this connection.  APPLICATION_CLOSE uses codes from could cause the
      application protocol error code space, see Section 11.4.

   Reason Phrase Length:  This field is identical in format and
      semantics sender to unnecessarily retransmit
   some data.  Standard QUIC [QUIC-RECOVERY] algorithms declare packets
   lost after sufficiently newer packets are acknowledged.  Therefore,
   the Reason Phrase Length field from CONNECTION_CLOSE.

   Reason Phrase:  This field is identical receiver SHOULD repeatedly acknowledge newly received packets in format and semantics
   preference to packets received in the Reason Phrase field from CONNECTION_CLOSE.

   APPLICATION_CLOSE has similar format past.

13.1.2.  ACK Frames and semantics to Packet Protection

   ACK frames MUST only be carried in a packet that has the
   CONNECTION_CLOSE frame (Section 7.4).  Aside from same packet
   number space as the semantics of packet being ACKed (see Section 12.1).  For
   instance, packets that are protected with 1-RTT keys MUST be
   acknowledged in packets that are also protected with 1-RTT keys.

   Packets that a client sends with 0-RTT packet protection MUST be
   acknowledged by the Error Code field and server in packets protected by 1-RTT keys.  This
   can mean that the omission of client is unable to use these acknowledgments if
   the Frame Type field, both
   frames server cryptographic handshake messages are used to close delayed or lost.
   Note that the connection.

7.6.  MAX_DATA Frame

   The MAX_DATA frame (type=0x04) is used in flow control same limitation applies to inform other data sent by the
   peer of
   server protected by the maximum amount 1-RTT keys.

   Endpoints SHOULD send acknowledgments for packets containing CRYPTO
   frames with a reduced delay; see Section 4.3.1 of data [QUIC-RECOVERY].

13.2.  Retransmission of Information

   QUIC packets that can are determined to be sent on the connection
   as a lost are not retransmitted
   whole.  The frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Maximum Data (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields in same applies to the MAX_DATA frame frames that are as follows:

   Maximum Data:  A variable-length integer indicating contained within lost
   packets.  Instead, the maximum
      amount of data information that can might be carried in frames is
   sent on again in new frames as needed.

   New frames and packets are used to carry information that is
   determined to have been lost.  In general, information is sent again
   when a packet containing that information is determined to be lost
   and sending ceases when a packet containing that information is
   acknowledged.

   o  Data sent in CRYPTO frames is retransmitted according to the entire connection, rules
      in units
      of octets.

   All [QUIC-RECOVERY], until all data has been acknowledged.

   o  Application data sent in STREAM frames counts toward this limit.  The sum of
   the largest received offsets on all streams - including streams is retransmitted in
   terminal states - MUST NOT exceed new
      STREAM frames unless the value advertised by a receiver.
   An endpoint MUST terminate a connection with has sent a FLOW_CONTROL_ERROR
   error if it receives more data than the maximum data value RST_STREAM for that it
   has sent, unless this is
      stream.  Once an endpoint sends a result RST_STREAM frame, no further
      STREAM frames are needed.

   o  The most recent set of a change acknowledgments are sent in the initial limits
   (see Section 6.6.2).

7.7.  MAX_STREAM_DATA Frame

   The MAX_STREAM_DATA ACK frames.  An
      ACK frame (type=0x05) is used SHOULD contain all unacknowledged acknowledgments, as
      described in Section 13.1.1.

   o  Cancellation of stream transmission, as carried in flow control to
   inform a RST_STREAM
      frame, is sent until acknowledged or until all stream data is
      acknowledged by the peer of (that is, either the maximum amount of data that can be sent "Reset Recvd" or
      "Data Recvd" state is reached on a
   stream.

   An endpoint that receives a MAX_STREAM_DATA frame for a receive-only
   stream MUST terminate the connection with error PROTOCOL_VIOLATION.

   An endpoint that receives send stream).  The content of
      a MAX_STREAM_DATA RST_STREAM frame for MUST NOT change when it is sent again.

   o  Similarly, a send-only request to cancel stream it has not opened MUST terminate transmission, as encoded in
      a STOP_SENDING frame, is sent until the connection with error
   PROTOCOL_VIOLATION.

   Note that an endpoint may legally receive a MAX_STREAM_DATA frame on
   a bidirectional stream it has enters
      either a "Data Recvd" or "Reset Recvd" state, see Section 3.5.

   o  Connection close signals, including those that use
      CONNECTION_CLOSE and APPLICATION_CLOSE frames, are not opened.

   The frame sent again
      when packet loss is detected, but as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Maximum Stream Data (i)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ described in Section 10.

   o  The fields current connection maximum data is sent in MAX_DATA frames.
      An updated value is sent in a MAX_DATA frame if the MAX_STREAM_DATA packet
      containing the most recently sent MAX_DATA frame are as follows:

   Stream ID:  The stream ID of is declared lost,
      or when the stream that endpoint decides to update the limit.  Care is affected encoded
      necessary to avoid sending this frame too often as a
      variable-length integer.

   Maximum Stream Data:  A variable-length integer indicating the limit can
      increase frequently and cause an unnecessarily large number of
      MAX_DATA frames to be sent.

   o  The current maximum amount of stream data that can be offset is sent on the identified stream, in units of octets.

   When counting data toward this limit, MAX_STREAM_DATA
      frames.  Like MAX_DATA, an endpoint accounts for the
   largest received offset of data that updated value is sent or received on when the
   stream.  Loss or reordering can mean that packet
      containing the largest received offset
   on most recent MAX_STREAM_DATA frame for a stream can be greater than is
      lost or when the total size of data received on
   that stream.  Receiving STREAM frames might not increase limit is updated, with care taken to prevent the largest
   received offset.

   The data
      frame from being sent on a stream MUST NOT exceed the largest maximum stream
   data value advertised by the receiver. too often.  An endpoint MUST terminate a
   connection with a FLOW_CONTROL_ERROR error if it receives more data
   than SHOULD stop sending
      MAX_STREAM_DATA frames when the largest receive stream enters a "Size
      Known" state.

   o  The maximum stream data that it has sent ID for the
   affected stream, unless this is a result stream of a change given type is sent in the initial
   limits (see Section 6.6.2).

7.8.  MAX_STREAM_ID Frame

   The
      MAX_STREAM_ID frame (type=0x06) informs the peer of the maximum
   stream ID that they are permitted to open.

   The frame frames.  Like MAX_DATA, an updated value is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Maximum Stream ID (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields in sent
      when a packet containing the most recent MAX_STREAM_ID for a
      stream type frame are as follows:

   Maximum Stream ID:  ID of the maximum unidirectional is declared lost or bidirectional
      peer-initiated stream ID for when the connection encoded as a variable-
      length integer.  The limit applies is updated,
      with care taken to unidirectional steams if the
      second least signification bit of prevent the frame from being sent too often.

   o  Blocked signals are carried in BLOCKED, STREAM_BLOCKED, and
      STREAM_ID_BLOCKED frames.  BLOCKED streams have connection scope,
      STREAM_BLOCKED frames have stream ID is 1, scope, and applies STREAM_ID_BLOCKED
      frames are scoped to bidirectional streams if it is 0.

   Loss or reordering can mean that a MAX_STREAM_ID frame can be
   received which states a lower specific stream limit than the client has
   previously received.  MAX_STREAM_ID type.  New frames which do not increase are sent
      if packets containing the
   maximum stream ID MUST be ignored.

   A peer MUST NOT initiate a stream with most recent frame for a higher stream ID than scope is lost,
      but only while the
   greatest maximum stream ID it has received.  An endpoint MUST
   terminate a connection with a STREAM_ID_ERROR error if a peer
   initiates a stream with a higher stream ID than it has sent, unless
   this is a result of a change in blocked on the initial limits (see
   Section 6.6.2).

7.9.  PING Frame

   Endpoints can use PING corresponding limit.
      These frames always include the limit that is causing blocking at
      the time that they are transmitted.

   o  A liveness or path validation check using PATH_CHALLENGE frames is
      sent periodically until a matching PATH_RESPONSE frame is received
      or until there is no remaining need for liveness or path
      validation checking.  PATH_CHALLENGE frames (type=0x07) to verify that their peers include a different
      payload each time they are still alive or to check reachability sent.

   o  Responses to the peer.  The PING path validation using PATH_RESPONSE frames are sent
      just once.  A new PATH_CHALLENGE frame
   contains no additional fields.

   The receiver of a PING will be sent if another
      PATH_RESPONSE frame simply needs to acknowledge is needed.

   o  New connection IDs are sent in NEW_CONNECTION_ID frames and
      retransmitted if the packet containing them is lost.
      Retransmissions of this frame.

   The PING frame can be used to keep a connection alive when an
   application or application protocol wishes to prevent carry the same sequence number
      value.  Likewise, retired connection
   from timing out.  An application protocol SHOULD provide guidance
   about the conditions under which generating a PING is recommended.
   This guidance SHOULD indicate whether it is the client or IDs are sent in
      RETIRE_CONNECTION_ID frames and retransmitted if the server
   that packet
      containing them is expected to send the PING.  Having both endpoints send PING lost.

   o  PADDING frames without coordination can produce an excessive number contain no information, so lost PADDING frames do
      not require repair.

   Upon detecting losses, a sender MUST take appropriate congestion
   control action.  The details of
   packets loss detection and poor performance.

   A connection will time out if no packets congestion control
   are sent or received for described in [QUIC-RECOVERY].

13.3.  Explicit Congestion Notification

   QUIC endpoints use Explicit Congestion Notification (ECN) [RFC3168]
   to detect and respond to network congestion.  ECN allows a
   period longer than network
   node to indicate congestion in the time specified network by setting a codepoint in
   the idle_timeout transport
   parameter (see Section 6.13).  However, state IP header of a packet instead of dropping it.  Endpoints react to
   congestion by reducing their sending rate in middleboxes might
   time out earlier than that.  Though REQ-5 response, as described
   in [RFC4787] recommends [QUIC-RECOVERY].

   To use ECN, QUIC endpoints first determine whether a 2
   minute timeout interval, experience shows that sending packets every
   15 to 30 seconds path supports
   ECN marking and the peer is necessary able to prevent access the majority of middleboxes
   from losing state for UDP flows.

7.10.  BLOCKED Frame ECN codepoint in the
   IP header.  A sender SHOULD send a BLOCKED frame (type=0x08) network path does not support ECN if ECN marked packets
   get dropped or ECN markings are rewritten on the path.  An endpoint
   verifies the path, both during connection establishment and when it wishes to
   send data, but is unable to due to connection-level flow control (see
   Section 10.2.1).  BLOCKED frames can be used as input
   migrating to tuning of
   flow control algorithms a new path (see Section 10.1.2).

   The BLOCKED frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Offset (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The BLOCKED frame contains 9).

13.3.1.  ECN Counters

   On receiving a single field.

   Offset:  A variable-length integer indicating packet with an ECT or CE codepoint, an endpoint that
   can access the connection-level
      offset at which IP ECN codepoints increases the blocking occurred.

7.11.  STREAM_BLOCKED Frame corresponding ECT(0),
   ECT(1), or CE count, and includes these counters in subsequent (see
   Section 13.1) ACK frames (see Section 19.15).

   A sender SHOULD send packet detected by a STREAM_BLOCKED frame (type=0x09) when receiver as a duplicate does not affect the
   receiver's local ECN codepoint counts; see (Section 21.7) for
   relevant security concerns.

   If an endpoint receives a packet without an ECT or CE codepoint, it
   wishes
   responds per Section 13.1 with an ACK frame.  If an endpoint does not
   have access to send data, but is unable received ECN codepoints, it acknowledges received
   packets per Section 13.1 with an ACK frame.

13.3.2.  ECN Verification

   Each endpoint independently verifies and enables use of ECN by
   setting the IP header ECN codepoint to due ECN Capable Transport (ECT)
   for the path from it to stream-level flow
   control.  This frame the other peer.  Even if ECN is analogous not used on
   the path to BLOCKED (Section 7.10).

   An the peer, the endpoint MUST provide feedback about ECN
   markings received (if accessible).

   To verify both that receives a STREAM_BLOCKED frame for a send-only
   stream path supports ECN and the peer can provide ECN
   feedback, an endpoint MUST terminate set the connection with error PROTOCOL_VIOLATION.

   The STREAM_BLOCKED frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Offset (i)                          ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The STREAM_BLOCKED frame contains two fields:

   Stream ID:  A variable-length integer indicating ECT(0) codepoint in the stream which IP header
   of all outgoing packets [RFC8311].

   If an ECT codepoint set in the IP header is
      flow control blocked.

   Offset:  A variable-length integer indicating not corrupted by a
   network device, then a received packet contains either the offset of codepoint
   sent by the
      stream at which peer or the blocking occurred.

7.12.  STREAM_ID_BLOCKED Frame

   A sender MAY send Congestion Experienced (CE) codepoint set by
   a STREAM_ID_BLOCKED frame (type=0x0a) when it
   wishes to open network device that is experiencing congestion.

   If a stream, but packet sent with an ECT codepoint is unable to due to the maximum stream
   ID limit set newly acknowledged by its the
   peer (see Section 7.8).  This does not open in an ACK frame without ECN feedback, the
   stream, but informs endpoint stops setting
   ECT codepoints in subsequent packets, with the peer expectation that a new stream was needed, but
   either the
   stream limit prevented network or the creation peer no longer supports ECN.

   To protect the connection from arbitrary corruption of ECN codepoints
   by the stream.

   The STREAM_ID_BLOCKED network, an endpoint verifies the following when an ACK frame
   is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ received:

   o  The STREAM_ID_BLOCKED frame contains a single field.

   Stream ID:  A variable-length integer indicating increase in ECT(0) and ECT(1) counters MUST be at least the highest stream
      ID
      number of packets newly acknowledged that were sent with the sender was permitted to open.

7.13.  NEW_CONNECTION_ID Frame
      corresponding codepoint.

   o  The total increase in ECT(0), ECT(1), and CE counters reported in
      the ACK frame MUST be at least the total number of packets newly
      acknowledged in this ACK frame.

   An endpoint sends could miss acknowledgements for a NEW_CONNECTION_ID frame (type=0x0b) to provide
   its peer with alternative connection IDs that can be used to break
   linkability packet when migrating connections (see Section 6.11.5).

   The NEW_CONNECTION_ID frame ACK frames
   are lost.  It is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Length (8)  |            Sequence Number (i)              ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Connection ID (32..144)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Length:  An 8-bit unsigned integer containing the length of therefore possible for the
      connection ID.  Values less than 4 total increase in ECT(0),
   ECT(1), and CE counters to be greater than 18 are invalid
      and the number of packets
   acknowledged in an ACK frame.  When this happens, the local reference
   counts MUST be treated as a connection error of type
      PROTOCOL_VIOLATION.

   Sequence Number:  The sequence number assigned increased to match the connection ID
      by counters in the sender.  See Section 6.1.1.

   Connection ID:  A connection ID of ACK frame.

   Upon successful verification, an endpoint continues to set ECT
   codepoints in subsequent packets with the specified length.

   Stateless Reset Token:  A 128-bit value expectation that will be used for a
      stateless reset when the associated connection ID path
   is used (see
      Section 6.13.4).

   An ECN-capable.

   If verification fails, then the endpoint MUST NOT send this frame if it currently requires ceases setting ECT
   codepoints in subsequent packets with the expectation that
   its either the
   network or the peer send does not support ECN.

   If an endpoint sets ECT codepoints on outgoing packets with and encounters
   a zero-length Destination Connection ID.
   Changing retransmission timeout due to the length absence of acknowledgments from
   the peer (see [QUIC-RECOVERY]), or if an endpoint has reason to
   believe that a network element might be corrupting ECN codepoints,
   the endpoint MAY cease setting ECT codepoints in subsequent packets.
   Doing so allows the connection ID to traverse network elements that drop
   or from zero-length makes
   it difficult to identify when corrupt ECN codepoints in the value of IP header.

14.  Packet Size

   The QUIC packet size includes the connection ID changed.
   An endpoint QUIC header and integrity check,
   but not the UDP or IP header.

   Clients MUST ensure that the first Initial packet they send is sending packets with sent
   in a zero-length Destination
   Connection ID MUST treat receipt of UDP datagram that is at least 1200 octets.  Padding the Initial
   packet or including a NEW_CONNECTION_ID frame as 0-RTT packet in the same datagram are ways to
   meet this requirement.  Sending a
   connection error UDP datagram of type PROTOCOL_VIOLATION. this size ensures
   that the network path supports a reasonable Maximum Transmission errors, timeouts Unit
   (MTU), and retransmissions might cause helps reduce the
   same NEW_CONNECTION_ID frame to be received multiple times.  Receipt amplitude of amplification attacks caused
   by server responses toward an unverified client address, see
   Section 8.

   The payload of a UDP datagram carrying the same frame multiple times Initial packet MUST NOT be treated as a connection
   error.  A receiver can use
   expanded to at least 1200 octets, by adding PADDING frames to the sequence number supplied in
   Initial packet and/or by combining the
   NEW_CONNECTION_ID frame to identify new connection IDs from old ones.

   If an endpoint receives a NEW_CONNECTION_ID frame that repeats a
   previously issued connection ID Initial packet with a different Stateless Reset
   Token or a different sequence number, 0-RTT
   packet (see Section 12.2).

   The datagram containing the endpoint MAY treat that
   receipt as a connection error of type PROTOCOL_VIOLATION.

7.14.  RETIRE_CONNECTION_ID Frame

   An endpoint sends a RETIRE_CONNECTION_ID frame (type=0x1b) to
   indicate that it will no longer use first Initial packet from a connection ID client MAY
   exceed 1200 octets if the client believes that was issued
   by its peer.  This may include the connection ID provided during Path Maximum
   Transmission Unit (PMTU) supports the
   handshake.  Sending size that it chooses.

   A server MAY send a RETIRE_CONNECTION_ID CONNECTION_CLOSE frame also serves as a
   request with error code
   PROTOCOL_VIOLATION in response to the peer to send additional connection IDs for future use
   (see Section 6.1).  New connection IDs can be delivered to first Initial packet it
   receives from a peer
   using client if the NEW_CONNECTION_ID UDP datagram is smaller than 1200
   octets.  It MUST NOT send any other frame (Section 7.13).

   Retiring a connection ID invalidates type in response, or
   otherwise behave as if any part of the stateless reset token
   associated with that connection ID.

   The RETIRE_CONNECTION_ID frame is offending packet was processed
   as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sequence Number (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Sequence Number: valid.

   The sequence server MUST also limit the number of bytes it sends before
   validating the address of the connection ID being
      retired.  See client, see Section 6.1.2.

   Receipt 8.

14.1.  Path Maximum Transmission Unit

   The Path Maximum Transmission Unit (PMTU) is the maximum size of a RETIRE_CONNECTION_ID frame containing a sequence number
   greater than the
   entire IP header, UDP header, and UDP payload.  The UDP payload
   includes the QUIC packet header, protected payload, and any previously sent
   authentication fields.

   All QUIC packets SHOULD be sized to fit within the peer estimated PMTU to
   avoid IP fragmentation or packet drops.  To optimize bandwidth
   efficiency, endpoints SHOULD use Packetization Layer PMTU Discovery
   ([PLPMTUD]).  Endpoints MAY use PMTU Discovery ([PMTUDv4], [PMTUDv6])
   for detecting the PMTU, setting the PMTU appropriately, and storing
   the result of previous PMTU determinations.

   In the absence of these mechanisms, QUIC endpoints SHOULD NOT send IP
   packets larger than 1280 octets.  Assuming the minimum IP header
   size, this results in a QUIC packet size of 1232 octets for IPv6 and
   1252 octets for IPv4.  Some QUIC implementations MAY be treated as more
   conservative in computing allowed QUIC packet size given unknown
   tunneling overheads or IP header options.

   QUIC endpoints that implement any kind of PMTU discovery SHOULD
   maintain an estimate for each combination of local and remote IP
   addresses.  Each pairing of local and remote addresses could have a
   connection error
   different maximum MTU in the path.

   QUIC depends on the network path supporting an MTU of type PROTOCOL_VIOLATION. at least 1280
   octets.  This is the IPv6 minimum MTU and therefore also supported by
   most modern IPv4 networks.  An endpoint cannot send MUST NOT reduce its MTU below
   this frame number, even if it was provided with a zero-
   length connection ID by its peer.  An endpoint receives signals that provides a zero-
   length connection ID MUST treat receipt of indicate a RETIRE_CONNECTION_ID
   frame as smaller
   limit might exist.

   If a connection error of type PROTOCOL_VIOLATION.

7.15.  STOP_SENDING Frame

   An QUIC endpoint may use a STOP_SENDING frame (type=0x0c) to communicate determines that incoming data is being discarded the PMTU between any pair of local
   and remote IP addresses has fallen below 1280 octets, it MUST
   immediately cease sending QUIC packets on receipt at application
   request. the affected path.  This signals a peer to abruptly terminate transmission on a
   stream.

   Receipt
   could result in termination of a STOP_SENDING frame the connection if an alternative path
   cannot be found.

14.1.1.  IPv4 PMTU Discovery

   Traditional ICMP-based path MTU discovery in IPv4 [PMTUDv4] is only valid for a send stream
   potentially vulnerable to off-path attacks that
   exists successfully guess
   the IP/port 4-tuple and is not in reduce the "Ready" state (see Section 9.2.1).
   Receiving a STOP_SENDING frame for a send stream that is "Ready" or
   non-existent MUST be treated as MTU to a connection error bandwidth-inefficient
   value.  TCP connections mitigate this risk by using the (at minimum)
   8 bytes of type
   PROTOCOL_VIOLATION.  An endpoint that receives a STOP_SENDING frame transport header echoed in the ICMP message to validate
   the TCP sequence number as valid for a receive-only stream MUST terminate the connection with error
   PROTOCOL_VIOLATION.

   The STOP_SENDING frame is current connection.
   However, as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Application Error Code (16)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Stream ID:  A variable-length integer carrying QUIC operates over UDP, in IPv4 the Stream ID echoed information
   could consist only of the
      stream being ignored.

   Application Error Code:  A 16-bit, application-specified reason the
      sender is ignoring the stream (see Section 11.4).

7.16.  ACK Frame

   Receivers send ACK frames (types 0x1a IP and 0x1b) UDP headers, which usually has
   insufficient entropy to inform senders mitigate off-path attacks.

   As a result, endpoints that implement PMTUD in IPv4 SHOULD take steps
   to mitigate this risk.  For instance, an application could:

   o  Set the IPv4 Don't Fragment (DF) bit on a small proportion of
      packets, so that most invalid ICMP messages arrive when there are
      no DF packets they have received outstanding, and processed.  The ACK frame contains one can therefore be identified as
      spurious.

   o  Store additional information from the IP or more ACK Blocks.  ACK Blocks are ranges of acknowledged packets.
   If UDP headers from DF
      packets (for example, the frame type IP ID or UDP checksum) to further
      authenticate incoming Datagram Too Big messages.

   o  Any reduction in PMTU due to a report contained in an ICMP packet
      is 0x1b, ACK frames also contain the sum of ECN
   marks received on the connection up provisional until this point.

   QUIC acknowledgements are irrevocable.  Once acknowledged, QUIC's loss detection algorithm determines
      that the packet is actually lost.

14.2.  Special Considerations for Packetization Layer PMTU Discovery

   The PADDING frame provides a packet
   remains acknowledged, even if it does useful option for PMTU probe packets.
   PADDING frames generate acknowledgements, but they need not appear in a future ACK
   frame.  This is unlike TCP SACKs ([RFC2018]).

   It is expected that be
   delivered reliably.  As a sender will reuse result, the same packet number across
   different packet number spaces.  ACK loss of PADDING frames only acknowledge in probe
   packets does not require delay-inducing retransmission.  However,
   PADDING frames do consume congestion window, which may delay the
   packet numbers that were transmitted by
   transmission of subsequent application data.

   When implementing the sender algorithm in Section 7.2 of [PLPMTUD], the same packet
   number space
   initial value of search_low SHOULD be consistent with the IPv6
   minimum packet size.  Paths that the ACK was received in.

   Version Negotiation do not support this size cannot
   deliver Initial packets, and Retry therefore are not QUIC-compliant.

   Section 7.3 of [PLPMTUD] discusses trade-offs between small and large
   increases in the size of probe packets.  As QUIC probe packets cannot be acknowledged because
   they do need
   not contain application data, aggressive increases in probe size
   carry fewer consequences.

15.  Versions

   QUIC versions are identified using a packet 32-bit unsigned number.  Rather than relying on ACK
   frames, these packets are implicitly acknowledged by the next Initial
   packet sent by the client.

   An ACK frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Largest Acknowledged (i)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Delay (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ACK Block Count (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Blocks (*)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [ECN Section]                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 12: ACK Frame Format

   The fields in the ACK frame are as follows:

   Largest Acknowledged:  A variable-length integer representing the
      largest packet number the peer is acknowledging; this version 0x00000000 is usually
      the largest packet number that the peer has received prior reserved to
      generating represent version negotiation.
   This version of the ACK frame.  Unlike specification is identified by the packet number in the
   0x00000001.

   Other versions of QUIC
      long or short header, the value in an ACK frame is not truncated.

   ACK Delay:  A variable-length integer including the time in
      microseconds that the largest acknowledged packet, as indicated in
      the Largest Acknowledged field, was received by this peer might have different properties to when this ACK was sent.
   version.  The value properties of the ACK Delay field is scaled by
      multiplying the encoded value by 2 QUIC that are guaranteed to the power be consistent
   across all versions of the value protocol are described in
   [QUIC-INVARIANTS].

   Version 0x00000001 of QUIC uses TLS as a cryptographic handshake
   protocol, as described in [QUIC-TLS].

   Versions with the "ack_delay_exponent" transport parameter set by the sender most significant 16 bits of the ACK frame.  The "ack_delay_exponent" defaults version number
   cleared are reserved for use in future IETF consensus documents.

   Versions that follow the pattern 0x?a?a?a?a are reserved for use in
   forcing version negotiation to 3, be exercised.  That is, any version
   number where the low four bits of all octets is 1010 (in binary).  A
   client or server MAY advertise support for any of these reserved
   versions.

   Reserved version numbers will probably never represent a
      multiplier of 8 (see Section 6.6.1).  Scaling in this fashion
      allows for real
   protocol; a larger range client MAY use one of values these version numbers with a shorter encoding at the
      cost of lower resolution.

   ACK Block Count:  A variable-length integer specifying the number of
      Additional ACK Block (and Gap) fields after
   expectation that the First ACK Block.

   ACK Blocks:  Contains server will initiate version negotiation; a
   server MAY advertise support for one or more blocks of packet numbers which have
      been successfully received, see Section 7.16.1.

7.16.1.  ACK Block Section

   The ACK Block Section consists of alternating Gap these versions and ACK Block
   fields in descending packet number order.  A First Ack Block field is
   followed by a variable number can expect
   that clients ignore the value.

   [[RFC editor: please remove the remainder of alternating Gap and Additional ACK
   Blocks. this section before
   publication.]]

   The version number for the final version of Gap and Additional ACK Block fields this specification
   (0x00000001), is
   determined by the ACK Block Count field.

   Gap and ACK Block fields use a relative integer encoding reserved for
   efficiency.  Though each encoded value is positive, the values are
   subtracted, so version of the protocol that each ACK Block describes progressively lower-
   numbered packets.  As long is
   published as contiguous ranges an RFC.

   Version numbers used to identify IETF drafts are created by adding
   the draft number to 0xff000000.  For example, draft-ietf-quic-
   transport-13 would be identified as 0xff00000D.

   Implementors are encouraged to register version numbers of packets QUIC that
   they are small, using for private experimentation on the GitHub wiki at
   <https://github.com/quicwg/base-drafts/wiki/QUIC-Versions>.

16.  Variable-Length Integer Encoding

   QUIC packets and frames commonly use a variable-length encoding for
   non-negative integer values.  This encoding ensures that each range can be
   expressed in a small number of octets. smaller
   integer values need fewer octets to encode.

   The ACK frame uses QUIC variable-length integer encoding reserves the least two most
   significant bit(bit (that is, type 0x1b)
   to indicate ECN feedback and report receipt of packets with ECN
   codepoints bits of ECT(0), ECT(1), or CE in the packet's IP header.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First ACK Block (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 13: ACK Block Section

   Each ACK Block acknowledges a contiguous range of packets by
   indicating first octet to encode the number base 2 logarithm of acknowledged packets that precede
   the
   largest packet number integer encoding length in that block.  A octets.  The integer value of zero indicates that
   only the largest packet number is acknowledged.  Larger ACK Block
   values indicate a larger range, with corresponding lower values for encoded
   on the smallest packet number remaining bits, in network byte order.

   This means that integers are encoded on 1, 2, 4, or 8 octets and can
   encode 6, 14, 30, or 62 bit values respectively.  Table 4 summarizes
   the range.  Thus, given a largest
   packet number for the ACK, the smallest value is determined by the
   formula:

      smallest = largest - ack_block

   The range encoding properties.

          +------+--------+-------------+-----------------------+
          | 2Bit | Length | Usable Bits | Range                 |
          +------+--------+-------------+-----------------------+
          | 00   | 1      | 6           | 0-63                  |
          |      |        |             |                       |
          | 01   | 2      | 14          | 0-16383               |
          |      |        |             |                       |
          | 10   | 4      | 30          | 0-1073741823          |
          |      |        |             |                       |
          | 11   | 8      | 62          | 0-4611686018427387903 |
          +------+--------+-------------+-----------------------+

                   Table 4: Summary of packets that are acknowledged by the ACK Block include
   the range from Integer Encodings

   For example, the smallest packet number eight octet sequence c2 19 7c 5e ff 14 e8 8c (in
   hexadecimal) decodes to the largest, inclusive.

   The largest decimal value for 151288809941952652; the First ACK Block is determined by
   four octet sequence 9d 7f 3e 7d decodes to 494878333; the
   Largest Acknowledged field; two octet
   sequence 7b bd decodes to 15293; and the largest for Additional ACK Blocks is
   determined by cumulatively subtracting single octet 25 decodes to
   37 (as does the size of all preceding ACK
   Blocks two octet sequence 40 25).

   Error codes (Section 20) and Gaps.

   Each Gap indicates a range of packets that versions Section 15 are described using
   integers, but do not being
   acknowledged.  The number of packets use this encoding.

17.  Packet Formats

   All numeric values are encoded in the gap network byte order (that is, big-
   endian) and all field sizes are in bits.  Hexadecimal notation is one higher than
   the encoded value of
   used for describing the Gap Field.

   The value of the Gap field establishes the largest fields.

17.1.  Packet Number Encoding and Decoding

   Packet numbers in long and short packet headers are encoded as
   follows.  The number
   value for the ACK Block that follows the gap using the following
   formula:

     largest = previous_smallest - gap - 2

   If of bits required to represent the calculated value for largest or smallest packet number for any
   ACK Block
   is negative, an endpoint MUST generate first reduced by including only a connection error
   of type FRAME_ENCODING_ERROR indicating an error in an ACK frame.

   The fields in the ACK Block Section are:

   First ACK Block:  A variable-length integer indicating the variable number of
      contiguous packets preceding the Largest Acknowledged that are
      being acknowledged.

   Gap (repeated):  A variable-length integer indicating the number least
   significant bits of
      contiguous unacknowledged packets preceding the packet number one
      lower than the smallest in number.  One or two of the preceding ACK Block.

   Additional ACK Block (repeated):  A variable-length integer
      indicating most
   significant bits of the number first octet are then used to represent how
   many bits of contiguous acknowledged packets preceding the largest packet number, as determined by the preceding Gap.

7.16.2.  ECN section

   The ECN section should only be parsed when the ACK frame type byte is
   0x1b.  The ECN section consists of 3 ECN counters number are provided, as shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 in Table 5.

          +---------------------+----------------+--------------+
          | First octet pattern | Encoded Length | Bits Present |
          +---------------------+----------------+--------------+
          | 0b0xxxxxxx          | 1 2 3 4 5 6 octet        | 7 8 9 0 1            |
          |                     |                |              |
          | 0b10xxxxxx          | 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+              |                        ECT(0) Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 14           |
          |                     |                |              |                        ECT(1) Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                        ECN-CE Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ECT(0) Count:  A variable-length integer representing the total
      number packets received with the ECT(0) codepoint.

   ECT(1) Count:  A variable-length integer representing the total
      number packets received with the ECT(1) codepoint.

   CE Count:  A variable-length integer representing 0b11xxxxxx          | 4              | 30           |
          +---------------------+----------------+--------------+

            Table 5: Packet Number Encodings for Packet Headers

   Note that these encodings are similar to those in Section 16, but use
   different values.

   Finally, the total encoded packet number
      packets received with the CE codepoint.

7.16.3.  Sending ACK Frames

   Implementations MUST NOT generate packets that only contain ACK
   frames is protected as described in response to packets which only contain ACK and PADDING
   frames.  However, they
   Section 5.3 of [QUIC-TLS].

   The sender MUST acknowledge packets containing only ACK
   and PADDING frames when sending ACK frames in response use a packet number size able to other
   packets.  Implementations MUST NOT send represent more than one packet
   containing only an ACK frame per received packet that contains frames
   other
   twice as large a range than ACK and PADDING frames.  Packets containing frames besides
   ACK and PADDING MUST be the difference between the largest
   acknowledged immediately or when a delayed
   ack timer expires.

   The receiver's delayed acknowledgment timer SHOULD NOT exceed packet and packet number being sent.  A peer receiving
   the
   current RTT estimate or packet will then correctly decode the value it indicates in packet number, unless the "max_ack_delay"
   transport parameter.  This ensures an acknowledgment
   packet is sent at least
   once per RTT when delayed in transit such that it arrives after many higher-
   numbered packets needing acknowledgement are have been received.  The
   sender can use the receiver's "max_ack_delay" value in determining
   timeouts for timer-based retransmission.  An acknowledgment endpoint SHOULD use a large
   enough packet number encoding to allow the packet number to be sent immediately
   recovered even if the packet arrives after receiving 2 packets that require acknowledgement, unless multiple packets are
   received together.

   To limit ACK Blocks to those that have not yet been received by sent
   afterwards.

   As a result, the
   sender, size of the receiver SHOULD track which ACK frames have been
   acknowledged by its peer.  Once packet number encoding is at least one
   more than the base 2 logarithm of the number of contiguous
   unacknowledged packet numbers, including the new packet.

   For example, if an ACK frame endpoint has been acknowledged,
   the packets it acknowledges SHOULD NOT be acknowledged again.

   Because ACK frames are not sent in response to ACK-only packets, a
   receiver that is only sending ACK frames will only receive
   acknowledgements received an acknowledgment for its packets if the sender includes them in
   packets with non-ACK frames.  A sender SHOULD bundle ACK frames with
   other frames when possible.

   Endpoints can only acknowledge packets sent in packet
   0x6afa2f, sending a particular packet with a number space by sending ACK frames in packets from the same of 0x6b2d79 requires a
   packet number space.

   To limit receiver state encoding with 14 bits or more; whereas the size 30-bit
   packet number encoding is needed to send a packet with a number of ACK frames,
   0x6bc107.

   At a receiver MAY
   limit receiver, protection of the packet number is removed prior to
   recovering the full packet number.  The full packet number is then
   reconstructed based on the number of ACK Blocks it sends.  A receiver can do this even
   without receiving acknowledgment of its ACK frames, with the
   knowledge this could cause significant bits present, the sender to unnecessarily retransmit
   some data.  Standard QUIC [QUIC-RECOVERY] algorithms declare packets
   lost after sufficiently newer packets are acknowledged.  Therefore,
   value of those bits, and the receiver SHOULD repeatedly acknowledge newly received packets in
   preference to packets largest packet number received in the past.

7.16.4.  ACK Frames and Packet Protection

   ACK frames MUST only be carried in on a packet that has
   successfully authenticated packet.  Recovering the same full packet number space as the
   is necessary to successfully remove packet being ACKed (see Section 4.8).  For
   instance, packets that are protected with 1-RTT keys MUST be
   acknowledged in packets that are also protected with 1-RTT keys.

   Packets that a client sends with 0-RTT protection.

   Once packet number protection MUST be
   acknowledged by is removed, the server in packets protected packet number is
   decoded by 1-RTT keys.  This
   can mean that finding the client packet number value that is unable closest to use these acknowledgments if
   the server cryptographic handshake messages are delayed or lost.
   Note that the same limitation applies to other data sent by
   next expected packet.  The next expected packet is the
   server protected by highest
   received packet number plus one.  For example, if the 1-RTT keys.

   Endpoints SHOULD send acknowledgments for packets highest
   successfully authenticated packet had a packet number of 0xaa82f30e,
   then a packet containing CRYPTO
   frames with a reduced delay; see Section 4.3.1 14-bit value of [QUIC-RECOVERY].

7.17.  PATH_CHALLENGE Frame

   Endpoints can use PATH_CHALLENGE frames (type=0x0e) to check
   reachability to the peer and 0x9b3 will be decoded as
   0xaa8309b3.  Example pseudo-code for path validation during connection
   migration.

   PATH_CHALLENGE frames contain an 8-byte payload. packet number decoding can be
   found in Appendix A.

17.2.  Long Header Packet

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|   Type (7)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +                            Data (8)                           +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data:  This 8-byte field contains arbitrary data.

   A PATH_CHALLENGE frame containing 8 octets
   |                           Length (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packet Number (8/16/32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Payload (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 10: Long Header Packet Format

   Long headers are used for packets that are hard sent prior to guess is
   sufficient the
   completion of version negotiation and establishment of 1-RTT keys.
   Once both conditions are met, a sender switches to ensure sending packets
   using the short header (Section 17.3).  The long form allows for
   special packets - such as the Version Negotiation packet - to be
   represented in this uniform fixed-length packet format.  Packets that it
   use the long header contain the following fields:

   Header Form:  The most significant bit (0x80) of octet 0 (the first
      octet) is easier set to receive 1 for long headers.

   Long Packet Type:  The remaining seven bits of octet 0 contain the
      packet than it type.  This field can indicate one of 128 packet types.
      The types specified for this version are listed in Table 6.

   Version:  The QUIC Version is to guess a 32-bit field that follows the Type.
      This field indicates which version of QUIC is in use and
      determines how the rest of the protocol fields are interpreted.

   DCIL and SCIL:  The octet following the version contains the value correctly.

   The recipient lengths
      of this frame MUST generate a PATH_RESPONSE frame
   (Section 7.18) containing the same Data.

7.18.  PATH_RESPONSE Frame two connection ID fields that follow it.  These lengths are
      encoded as two 4-bit unsigned integers.  The PATH_RESPONSE frame (type=0x0f) is sent in response to a
   PATH_CHALLENGE frame.  Its format is identical to the PATH_CHALLENGE
   frame (Section 7.17).

   If Destination
      Connection ID Length (DCIL) field occupies the content 4 high bits of a PATH_RESPONSE frame does not match the content
      octet and the Source Connection ID Length (SCIL) field occupies
      the 4 low bits of
   a PATH_CHALLENGE frame previously sent by the endpoint, octet.  An encoded length of 0 indicates
      that the endpoint
   MAY generate a connection error of type PROTOCOL_VIOLATION.

7.19.  NEW_TOKEN frame

   A server sends a NEW_TOKEN frame (type=0x19) ID is also 0 octets in length.  Non-zero
      encoded lengths are increased by 3 to provide get the client full length of the
      connection ID, producing a
   token to send in length between 4 and 18 octets
      inclusive.  For example, an octet with the header of value 0x50 describes an Initial packet for
      8-octet Destination Connection ID and a future
   connection. zero-length Source
      Connection ID.

   Destination Connection ID:  The NEW_TOKEN frame Destination Connection ID field
      follows the connection ID lengths and is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 either 0 1 2 3 octets in length
      or between 4 5 6 7 8 9 and 18 octets.  Section 7.2 describes the use of this
      field in more detail.

   Source Connection ID:  The Source Connection ID field follows the
      Destination Connection ID and is either 0 1 2 3 octets in length or
      between 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Token Length (i)  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Token (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ and 18 octets.  Section 7.2 describes the use of this
      field in more detail.

   Length:  The fields length of a NEW_TOKEN frame are the remainder of the packet (that is, the
      Packet Number and Payload fields) in octets, encoded as follows:

   Token Length:  A a
      variable-length integer specifying the (Section 16).

   Packet Number:  The packet number field is 1, 2, or 4 octets long.
      The packet number has confidentiality protection separate from
      packet protection, as described in Section 5.3 of [QUIC-TLS].  The
      length of the
      token packet number field is encoded in bytes.

   Token:  An opaque blob that the client may use with a future Initial plaintext
      packet number.  See Section 17.1 for details.

   Payload:  The payload of the packet.

7.20.  STREAM Frames

   STREAM frames implicitly create a stream

   The following packet types are defined:

                 +------+-----------------+--------------+
                 | Type | Name            | Section      |
                 +------+-----------------+--------------+
                 | 0x7F | Initial         | Section 17.5 |
                 |      |                 |              |
                 | 0x7E | Retry           | Section 17.7 |
                 |      |                 |              |
                 | 0x7D | Handshake       | Section 17.6 |
                 |      |                 |              |
                 | 0x7C | 0-RTT Protected | Section 12.1 |
                 +------+-----------------+--------------+

                     Table 6: Long Header Packet Types

   The header form, type, connection ID lengths octet, destination and carry stream data.
   source connection IDs, and version fields of a long header packet are
   version-independent.  The
   STREAM frame takes the form 0b00010XXX (or the set packet number and values for packet types
   defined in Table 6 are version-specific.  See [QUIC-INVARIANTS] for
   details on how packets from different versions of values from
   0x10 to 0x17). QUIC are
   interpreted.

   The value of the three low-order bits interpretation of the frame
   type determine the fields that and the payload are present specific to a
   version and packet type.  Type-specific semantics for this version
   are described in the frame.

   o following sections.

   The OFF bit (0x04) in end of the frame type is set to indicate that there packet is an Offset field present.  When set to 1, determined by the Offset Length field.  The Length
   field is
      present; when set to 0, covers both the Offset field is absent Packet Number and the Stream
      Data starts at an offset of 0 (that is, the frame contains the
      first octets Payload fields, both of the stream, or the end which
   are confidentiality protected and initially of a stream that includes
      no data).

   o unknown length.  The LEN bit (0x02) in the frame type is set to indicate that there
      is a Length field present.  If this bit is set to 0,
   size of the Length Payload field is absent and the Stream Data field extends to the end of learned once the packet.  If this bit packet number
   protection is set to 1, the removed.  The Length field is present.

   o  The FIN bit (0x01) of the frame type is set only on frames that
      contain the final offset of the stream.  Setting this bit
      indicates that the frame marks the end of the stream.

   An endpoint that receives a STREAM frame for a send-only stream MUST
   terminate the connection with error PROTOCOL_VIOLATION.

   A STREAM frame is shown below. enables packet coalescing
   (Section 12.2).

17.3.  Short Header Packet

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |0|K|1|1|0|R R R|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Stream                Destination Connection ID (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [Offset (i)] (0..144)           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [Length (i)]                      Packet Number (8/16/32)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream Data                     Protected Payload (*)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 14: STREAM Frame 11: Short Header Packet Format

   The STREAM frame contains short header can be used after the version and 1-RTT keys are
   negotiated.  Packets that use the short header contain the following
   fields:

   Stream ID:  A variable-length integer indicating

   Header Form:  The most significant bit (0x80) of octet 0 is set to 0
      for the stream ID short header.

   Key Phase Bit:  The second bit (0x40) of octet 0 indicates the
      stream (see Section 9.1).

   Offset:  A variable-length integer specifying key
      phase, which allows a recipient of a packet to identify the byte offset in packet
      protection keys that are used to protect the
      stream packet.  See
      [QUIC-TLS] for details.

   [[Editor's Note: this section should be removed and the data in bit
   definitions changed before this STREAM frame.  This field is present
      when draft goes to the OFF IESG.]]

   Third Bit:  The third bit (0x20) of octet 0 is set to 1.  When the Offset field is absent,

   [[Editor's Note: this section should be removed and the offset is 0.

   Length:  A variable-length integer specifying bit
   definitions changed before this draft goes to the length IESG.]]

   Fourth Bit:  The fourth bit (0x10) of octet 0 is set to 1.

   [[Editor's Note: this section should be removed and the
      Stream Data field in bit
   definitions changed before this STREAM frame.  This field draft goes to the IESG.]]

   Google QUIC Demultiplexing Bit:  The fifth bit (0x8) of octet 0 is present
      when
      set to 0.  This allows implementations of Google QUIC to
      distinguish Google QUIC packets from short header packets sent by
      a client because Google QUIC servers expect the LEN bit is set connection ID to 1.  When the LEN
      always be present.  The special interpretation of this bit is set SHOULD
      be removed from this specification when Google QUIC has finished
      transitioning to 0, the
      Stream Data field consumes all the remaining octets in the packet.

   Stream Data: new header format.

   Reserved:  The bytes from the designated stream to be delivered.

   When a Stream Data field has a length sixth, seventh, and eighth bits (0x7) of 0, the offset octet 0 are
      reserved for experimentation.  Endpoints MUST ignore these bits on
      packets they receive unless they are participating in the STREAM
   frame is the offset of the next byte an
      experiment that would be sent.

   The first byte in uses these bits.  An endpoint not actively using
      these bits SHOULD set the stream has an offset of 0.  The largest offset
   delivered value randomly on packets they send to
      protect against unwanted inference about particular values.

   Destination Connection ID:  The Destination Connection ID is a stream -
      connection ID that is chosen by the sum intended recipient of the re-constructed offset and data
   length - MUST be less than 2^62.

   Stream multiplexing
      packet.  See Section 5.1 for more details.

   Packet Number:  The packet number field is achieved by interleaving STREAM frames from
   multiple streams into one 1, 2, or more QUIC packets.  A single QUIC 4 octets long.
      The packet
   can include multiple STREAM frames number has confidentiality protection separate from one or more streams.

   Implementation note: One of the benefits
      packet protection, as described in Section 5.3 of QUIC is avoidance [QUIC-TLS].  The
      length of
   head-of-line blocking across multiple streams.  When a the packet loss
   occurs, only streams with data number field is encoded in that the plaintext
      packet are blocked waiting number.  See Section 17.1 for details.

   Protected Payload:  Packets with a retransmission to be received, while other streams can continue
   making progress.  Note that when data from multiple streams is
   bundled into short header always include a single QUIC packet, loss
      1-RTT protected payload.

   The header form and connection ID field of that a short header packet blocks all
   those streams from making progress.  An implementation is therefore
   advised are
   version-independent.  The remaining fields are specific to bundle as few streams as necessary in outgoing the
   selected QUIC version.  See [QUIC-INVARIANTS] for details on how
   packets
   without losing transmission efficiency to underfilled packets.

7.21.  CRYPTO Frame

   The CRYPTO frame (type=0x18) from different versions of QUIC are interpreted.

17.4.  Version Negotiation Packet

   A Version Negotiation packet is used inherently not version-specific, and
   does not use the long packet header (see Section 17.2).  Upon receipt
   by a client, it will appear to transmit cryptographic
   handshake messages.  It can be sent in all a packet types.  The CRYPTO
   frame offers using the cryptographic protocol an in-order stream long header, but
   will be identified as a Version Negotiation packet based on the
   Version field having a value of bytes.
   CRYPTO frames are functionally identical 0.

   The Version Negotiation packet is a response to STREAM frames, except a client packet that they do not bear
   contains a stream identifier; they are not flow
   controlled; and they do version that is not carry markers for optional offset,
   optional length, and the end of supported by the stream.

   A CRYPTO frame server, and is shown below. only
   sent by servers.

   The layout of a Version Negotiation packet is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|  Unused (7) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Offset                          Version (32)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Supported Version 1 (32)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   [Supported Version 2 (32)]                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   [Supported Version N (32)]                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 12: Version Negotiation Packet

   The value in the Unused field is selected randomly by the server.

   The Version field of a Version Negotiation packet MUST be set to
   0x00000000.

   The server MUST include the value from the Source Connection ID field
   of the packet it receives in the Destination Connection ID field.
   The value for Source Connection ID MUST be copied from the
   Destination Connection ID of the received packet, which is initially
   randomly selected by a client.  Echoing both connection IDs gives
   clients some assurance that the server received the packet and that
   the Version Negotiation packet was not generated by an off-path
   attacker.

   The remainder of the Version Negotiation packet is a list of 32-bit
   versions which the server supports.

   A Version Negotiation packet cannot be explicitly acknowledged in an
   ACK frame by a client.  Receiving another Initial packet implicitly
   acknowledges a Version Negotiation packet.

   The Version Negotiation packet does not include the Packet Number and
   Length fields present in other packets that use the long header form.
   Consequently, a Version Negotiation packet consumes an entire UDP
   datagram.

   See Section 6 for a description of the version negotiation process.

17.5.  Initial Packet

   An Initial packet uses long headers with a type value of 0x7F.  It
   carries the first CRYPTO frames sent by the client and server to
   perform key exchange, and carries ACKs in either direction.

   In order to prevent tampering by version-unaware middleboxes, Initial
   packets are protected with connection- and version-specific keys
   (Initial keys) as described in [QUIC-TLS].  This protection does not
   provide confidentiality or integrity against on-path attackers, but
   provides some level of protection against off-path attackers.

   An Initial packet (shown in Figure 13) has two additional header
   fields that are added to the Long Header before the Length field.

   +-+-+-+-+-+-+-+-+
   |1|    0x7f     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Token Length (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Token (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Length (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Crypto Data                     Packet Number (8/16/32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Payload (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 15: CRYPTO Frame Format

   The CRYPTO frame contains 13: Initial Packet

   These fields include the following fields:

   Offset: token that was previously provided in a
   Retry packet or NEW_TOKEN frame:

   Token Length:  A variable-length integer specifying the byte offset length of the
      Token field, in bytes.  This value is zero if no token is present.
      Initial packets sent by the server MUST set the Token Length field
      to zero; clients that receive an Initial packet with a non-zero
      Token Length field MUST either discard the packet or generate a
      connection error of type PROTOCOL_VIOLATION.

   Token:  The value of the token.

   The client and server use the Initial packet type for any packet that
   contains an initial cryptographic handshake message.  This includes
   all cases where a new packet containing the
      stream for initial cryptographic
   message needs to be created, such as the data packets sent after receiving
   a Version Negotiation (Section 17.4) or Retry packet (Section 17.7).

   A server sends its first Initial packet in this CRYPTO frame.

   Length: response to a client
   Initial.  A variable-length integer specifying the length server may send multiple Initial packets.  The
   cryptographic key exchange could require multiple round trips or
   retransmissions of the
      Crypto Data field in this CRYPTO frame.

   Crypto Data: data.

   The payload of an Initial packet includes a CRYPTO frame (or frames)
   containing a cryptographic message data.

   There is handshake message, ACK frames, or both.
   PADDING and CONNECTION_CLOSE frames are also permitted.  An endpoint
   that receives an Initial packet containing other frames can either
   discard the packet as spurious or treat it as a separate flow connection error.

   The first packet sent by a client always includes a CRYPTO frame that
   contains the entirety of the first cryptographic handshake data message.
   This packet, and the cryptographic handshake message, MUST fit in each
   encryption level, each of which starts a
   single UDP datagram (see Section 7).  The first CRYPTO frame sent
   always begins at an offset of 0.  This
   implies 0 (see Section 7).

   Note that each encryption level is treated as a separate CRYPTO
   stream of data.

   Unlike STREAM frames, which include if the server sends a Stream ID indicating to which
   stream HelloRetryRequest, the data belongs, client will
   send a second Initial packet.  This Initial packet will continue the
   cryptographic handshake and will contain a CRYPTO frame carries data for a single
   stream per encryption level.  The stream does not have with an explicit
   end, so
   offset matching the size of the CRYPTO frames frame sent in the first
   Initial packet.  Cryptographic handshake messages subsequent to the
   first do not have a FIN bit.

8.  Packetization and Reliability

   A sender bundles one or more frames in need to fit within a QUIC single UDP datagram.

17.5.1.  Starting Packet Numbers

   The first Initial packet (see Section 5).

   A sender SHOULD minimize per-packet bandwidth and computational costs sent by bundling as many frames as possible within a QUIC packet.  A
   sender MAY wait for either endpoint contains a short period packet
   number of time to bundle multiple frames
   before sending 0.  The packet number MUST increase monotonically
   thereafter.  Initial packets are in a different packet that is not maximally packed, number space
   to avoid
   sending out large other packets (see Section 12.3).

17.5.2.  0-RTT Packet Numbers

   Packet numbers of small packets.  An implementation may for 0-RTT protected packets use knowledge about application sending behavior the same space as
   1-RTT protected packets.

   After a client receives a Retry or heuristics to
   determine whether and for how long to wait.  This waiting period is
   an implementation decision, and an implementation should be careful
   to delay conservatively, since any delay is Version Negotiation packet, 0-RTT
   packets are likely to increase
   application-visible latency.

8.1.  Packet Processing and Acknowledgment

   A packet MUST NOT be acknowledged until packet protection has been
   successfully removed and all frames contained in the packet have been
   processed.  For STREAM frames, this means the data has been enqueued
   in preparation to be received lost or discarded by the application protocol, but it
   does not require that server.  A
   client MAY attempt to resend data is delivered and consumed.

   Once the packet has been fully processed, in 0-RTT packets after it sends a receiver acknowledges
   receipt by sending one or more ACK frames containing
   new Initial packet.

   A client MUST NOT reset the packet number it uses for 0-RTT packets.
   The keys used to protect 0-RTT packets will not change as a result of the received packet.  To avoid creating an indefinite
   feedback loop, an endpoint MUST NOT send an ACK frame in response
   responding to a packet containing only ACK Retry or PADDING frames, even if there are Version Negotiation packet gaps which precede unless the received packet.  The endpoint MUST
   acknowledge client
   also regenerates the cryptographic handshake message.  Sending
   packets containing only ACK or PADDING frames in with the next
   ACK frame same packet number in that it sends.

   While PADDING frames do not elicit an ACK frame from a receiver, they
   are considered case is likely to be in flight
   compromise the packet protection for congestion control purposes
   [QUIC-RECOVERY].  Sending only PADDING frames might cause all 0-RTT packets because the sender
   same key and nonce could be used to become limited by the congestion controller (as described in
   [QUIC-RECOVERY]) with no acknowledgments forthcoming from protect different content.

   Receiving a Retry or Version Negotiation packet, especially a Retry
   that changes the
   receiver.  Therefore, connection ID used for subsequent packets, indicates
   a sender should ensure strong possibility that other frames are
   sent in addition to PADDING frames to elicit 0-RTT packets could be lost.  A client only
   receives acknowledgments from for its 0-RTT packets once the
   receiver.

   Strategies and implications handshake is
   complete.  Consequently, a server might expect 0-RTT packets to start
   with a packet number of 0.  Therefore, in determining the frequency length of generating
   acknowledgments
   the packet number encoding for 0-RTT packets, a client MUST assume
   that all packets up to the current packet number are discussed in more detail in [QUIC-RECOVERY].

8.2.  Retransmission flight,
   starting from a packet number of Information

   QUIC 0.  Thus, 0-RTT packets that are determined to be lost are not retransmitted
   whole.  The same applies could need
   to the frames that are contained within lost
   packets.  Instead, the information use a longer packet number encoding.

   A client SHOULD instead generate a fresh cryptographic handshake
   message and start packet numbers from 0.  This ensures that might be carried in frames is
   sent again in new frames as needed.

   New frames 0-RTT
   packets will not use the same keys, avoiding any risk of key and
   nonce reuse; this also prevents 0-RTT packets are from previous handshake
   attempts from being accepted as part of the connection.

17.6.  Handshake Packet

   A Handshake packet uses long headers with a type value of 0x7D.  It
   is used to carry information that is
   determined to have been lost.  In general, information is sent again
   when acknowledgments and cryptographic handshake messages
   from the server and client.

   Once a client has received a Handshake packet containing that information is determined from a server, it uses
   Handshake packets to be lost send subsequent cryptographic handshake messages
   and sending ceases when acknowledgments to the server.

   The Destination Connection ID field in a Handshake packet containing contains a
   connection ID that information is
   acknowledged.

   o  Data sent in CRYPTO frames is retransmitted according to chosen by the rules
      in [QUIC-RECOVERY], until either all data has been acknowledged or recipient of the crypto state machine implicitly knows that packet; the
   Source Connection ID includes the peer received connection ID that the data.

   o  Application data sent in STREAM frames is retransmitted in new
      STREAM frames unless sender of
   the endpoint has packet wishes to use (see Section 7.2).

   The first Handshake packet sent by a RST_STREAM for that
      stream.  Once an endpoint sends server contains a RST_STREAM frame, no further
      STREAM frames packet number
   of 0.  Handshake packets are needed.

   o their own packet number space.  Packet
   numbers are incremented normally for other Handshake packets.

   The most recent set payload of acknowledgments are sent in this packet contains CRYPTO frames and could contain
   PADDING, or ACK frames.  An
      ACK frame SHOULD  Handshake packets MAY contain all unacknowledged acknowledgments, as
      described in Section 7.16.3.

   o  Cancellation
   CONNECTION_CLOSE or APPLICATION_CLOSE frames.  Endpoints MUST treat
   receipt of stream transmission, Handshake packets with other frames as carried in a RST_STREAM
      frame, is sent until acknowledged or until all stream data is
      acknowledged connection error.

17.7.  Retry Packet

   A Retry packet uses a long packet header with a type value of 0x7E.
   It carries an address validation token created by the peer (that is, either the "Reset Recvd" or
      "Data Recvd" state is reached on the send stream).  The content of
      a RST_STREAM frame MUST NOT change when it server.  It is sent again.

   o  Similarly,
   used by a request server that wishes to cancel stream transmission, as encoded in
      a STOP_SENDING frame, is sent until the receive stream enters
      either perform a "Data Recvd" or "Reset Recvd" state, see stateless retry (see
   Section 9.3.

   o 8.1).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|    0x7e     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection close signals, including those that use
      CONNECTION_CLOSE and APPLICATION_CLOSE frames, are not sent again
      when ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    ODCIL(8)   |      Original Destination Connection ID (*)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Retry Token (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 14: Retry Packet

   A Retry packet loss is detected, but as described in Section 6.13.

   o  The current connection maximum data is sent in MAX_DATA frames.
      An updated value is sent (shown in a MAX_DATA frame if the packet
      containing the most recently sent MAX_DATA frame is declared lost,
      or when the endpoint decides to update the limit.  Care is
      necessary to avoid sending this frame too often as Figure 14) only uses the limit can
      increase frequently and cause an unnecessarily large number invariant portion
   of
      MAX_DATA frames to be sent.

   o  The current maximum stream data offset is sent in MAX_STREAM_DATA
      frames.  Like MAX_DATA, an updated value is sent when the long packet
      containing the most recent MAX_STREAM_DATA frame for a stream is
      lost or when header [QUIC-INVARIANTS]; that is, the limit is updated, with care taken fields up
   to prevent the
      frame from being sent too often.  An endpoint SHOULD stop sending
      MAX_STREAM_DATA frames when and including the receive stream enters a "Size
      Known" state.

   o  The maximum stream Destination and Source Connection ID for a stream of a given type is sent in
      MAX_STREAM_ID frames. fields.  A
   Retry packet does not contain any protected fields.  Like MAX_DATA, an updated value is sent
      when Version
   Negotiation, a Retry packet containing the most recent MAX_STREAM_ID for a
      stream type frame is declared lost or when contains the limit is updated,
      with care taken to prevent long header including the frame from being sent too often.

   o  Blocked signals are carried in BLOCKED, STREAM_BLOCKED, and
      STREAM_ID_BLOCKED frames.  BLOCKED streams have
   connection scope,
      STREAM_BLOCKED frames have stream scope, IDs, but omits the Length, Packet Number, and STREAM_ID_BLOCKED
      frames are scoped to a specific stream type.  New frames Payload
   fields.  These are sent
      if packets containing replaced with:

   ODCIL:  The length of the most recent frame for a scope Original Destination Connection ID field.
      The length is lost,
      but only while encoded in the endpoint is blocked on least significant 4 bits of the corresponding limit.
      These frames always include
      octet, using the limit that is causing blocking at same encoding as the time that they DCIL and SCIL fields.  The
      most significant 4 bits of this octet are transmitted.

   o  A liveness or path validation check using PATH_CHALLENGE frames is
      sent periodically until reserved.  Unless a matching PATH_RESPONSE frame is received
      or until there is no remaining need use
      for liveness or path
      validation checking.  PATH_CHALLENGE frames include a different
      payload each time they are sent.

   o  Responses to path validation using PATH_RESPONSE frames are sent
      just once.  A new PATH_CHALLENGE frame will be sent if another
      PATH_RESPONSE frame is needed.

   o  New connection IDs are sent in NEW_CONNECTION_ID frames these bits has been negotiated, endpoints SHOULD send
      randomized values and
      retransmitted if MUST ignore any value that it receives.

   Original Destination Connection ID:  The Original Destination
      Connection ID contains the value of the Destination Connection ID
      from the Initial packet containing them that this Retry is lost.
      Retransmissions in response to.  The
      length of this frame carry field is given in ODCIL.

   Retry Token:  An opaque token that the server can use to validate the
      client's address.

   The server populates the Destination Connection ID with the same sequence number
      value.  Likewise, retired
   connection IDs are sent ID that the client included in
      RETIRE_CONNECTION_ID frames and retransmitted if the packet
      containing them is lost.

   o  PADDING frames contain no information, so lost PADDING frames do
      not require repair.

   Upon detecting losses, Source Connection ID of
   the Initial packet.

   The server includes a sender connection ID of its choice in the Source
   Connection ID field.  This value MUST take appropriate congestion
   control action. not be equal to the Destination
   Connection ID field of the packet sent by the client.  The details client
   MUST use this connection ID in the Destination Connection ID of loss detection and congestion control
   are described
   subsequent packets that it sends.

   A server MAY send Retry packets in [QUIC-RECOVERY].

8.3.  Packet Size

   The QUIC response to Initial and 0-RTT
   packets.  A server can either discard or buffer 0-RTT packets that it
   receives.  A server can send multiple Retry packets as it receives
   Initial or 0-RTT packets.

   A client MUST accept and process at most one Retry packet size includes for each
   connection attempt.  After the QUIC header client has received and integrity check,
   but not the UDP processed an
   Initial or IP header. Retry packet from the server, it MUST discard any
   subsequent Retry packets that it receives.

   Clients MUST ensure discard Retry packets that the first Initial packet they send is sent
   in a UDP datagram contain an Original
   Destination Connection ID field that is at least 1200 octets.  Padding does not match the Destination
   Connection ID from its Initial
   packet or including packet.  This prevents an off-path
   attacker from injecting a 0-RTT packet in the same datagram are ways Retry packet.

   The client responds to
   meet this requirement.  Sending a UDP datagram of this size ensures Retry packet with an Initial packet that
   includes the network path supports a reasonable Maximum Transmission Unit
   (MTU), and helps reduce the amplitude of amplification attacks caused
   by server responses toward an unverified provided Retry Token to continue connection
   establishment.

   A client address.

   The datagram containing sets the first Destination Connection ID field of this Initial
   packet from a client MAY
   exceed 1200 octets if to the client believes that value from the Path Maximum
   Transmission Unit (PMTU) supports Source Connection ID in the size that it chooses.

   A server MAY send a CONNECTION_CLOSE frame with error code
   PROTOCOL_VIOLATION Retry
   packet.  Changing Destination Connection ID also results in response a change
   to the first Initial packet it
   receives from a client if keys used to protect the UDP datagram is smaller than 1200
   octets. Initial packet.  It MUST NOT send any other frame type also sets the
   Token field to the token provided in response, or
   otherwise behave as if any part of the offending packet was processed
   as valid.

8.4.  Path Maximum Transmission Unit Retry.  The Path Maximum Transmission Unit (PMTU) is client MUST NOT
   change the maximum size of Source Connection ID because the
   entire IP header, UDP header, and UDP payload.  The UDP payload
   includes server could include the QUIC packet header, protected payload, and any
   authentication fields.
   connection ID as part of its token validation logic (see
   Section 8.1.2).

   All QUIC subsequent Initial packets SHOULD be sized to fit within from the estimated PMTU to
   avoid IP fragmentation or packet drops.  To optimize bandwidth
   efficiency, endpoints SHOULD use Packetization Layer PMTU Discovery
   ([PLPMTUD]).  Endpoints MAY client MUST use PMTU Discovery ([PMTUDv4], [PMTUDv6])
   for detecting the PMTU, setting the PMTU appropriately,
   connection ID and storing
   the result of previous PMTU determinations.

   In token values from the absence of these mechanisms, QUIC endpoints SHOULD NOT send IP
   packets larger than 1280 octets.  Assuming Retry packet.  Aside from
   this, the minimum IP header
   size, this results in a QUIC packet size of 1232 octets for IPv6 and
   1252 octets for IPv4.  Some QUIC implementations MAY be more
   conservative in computing allowed QUIC Initial packet size given unknown
   tunneling overheads or IP header options.

   QUIC endpoints that implement any kind of PMTU discovery SHOULD
   maintain an estimate for each combination of local and remote IP
   addresses.  Each pairing of local and remote addresses could have a
   different maximum MTU in the path.

   QUIC depends on sent by the network path supporting an MTU of at least 1280
   octets.  This client is subject to the IPv6 minimum MTU and therefore also supported by
   most modern IPv4 networks.  An endpoint MUST NOT reduce its MTU below
   this number, even if it receives signals that indicate same
   restrictions as the first Initial packet.  A client can either reuse
   the cryptographic handshake message or construct a smaller
   limit might exist.

   If new one at its
   discretion.

   A client MAY attempt 0-RTT after receiving a QUIC endpoint determines that the PMTU between any pair of local
   and remote IP addresses has fallen below 1280 octets, it MUST
   immediately cease Retry packet by sending QUIC
   0-RTT packets on the affected path.  This
   could result in termination of to the connection if an alternative path
   cannot be found.

8.4.1.  IPv4 PMTU Discovery

   Traditional ICMP-based path MTU discovery in IPv4 [PMTUDv4] is
   potentially vulnerable to off-path attacks that successfully guess ID provided by the IP/port 4-tuple and reduce server.  A client
   that sends additional 0-RTT packets without constructing a new
   cryptographic handshake message MUST NOT reset the MTU packet number to 0
   after a bandwidth-inefficient
   value.  TCP connections mitigate this risk by using Retry packet, see Section 17.5.2.

   A server acknowledges the (at minimum)
   8 bytes use of a Retry packet for a connection
   using the original_connection_id transport header echoed in parameter (see
   Section 18.1).  If the ICMP message to validate server sends a Retry packet, it MUST include
   the TCP sequence number as valid for value of the current connection.
   However, as QUIC operates over UDP, Original Destination Connection ID field of the
   Retry packet (that is, the Destination Connection ID field from the
   client's first Initial packet) in IPv4 the echoed information
   could consist only of transport parameter.

   If the IP client received and UDP headers, which usually has
   insufficient entropy to mitigate off-path attacks.

   As processed a result, endpoints Retry packet, it validates
   that implement PMTUD in IPv4 SHOULD take steps
   to mitigate this risk.  For instance, an application could:

   o  Set the IPv4 Don't Fragment (DF) bit on original_connection_id transport parameter is present and
   correct; otherwise, it validates that the transport parameter is
   absent.  A client MUST treat a small proportion failed validation as a connection
   error of
      packets, so that most invalid ICMP messages arrive when there are
      no DF packets outstanding, type TRANSPORT_PARAMETER_ERROR.

   A Retry packet does not include a packet number and can therefore cannot be identified as
      spurious.

   o  Store additional information from the IP or UDP headers from DF
      packets (for example, the IP ID or UDP checksum) to further
      authenticate incoming Datagram Too Big messages.

   o  Any reduction in PMTU due to
   explicitly acknowledged by a report contained in an ICMP packet client.

18.  Transport Parameter Encoding

   The format of the transport parameters is provisional until QUIC's loss detection algorithm determines
      that the packet TransportParameters
   struct from Figure 15.  This is actually lost.

8.4.2.  Special Considerations for Packetization Layer PMTU Discovery described using the presentation
   language from Section 3 of [TLS13].

      uint32 QuicVersion;

      enum {
         initial_max_stream_data_bidi_local(0),
         initial_max_data(1),
         initial_max_bidi_streams(2),
         idle_timeout(3),
         preferred_address(4),
         max_packet_size(5),
         stateless_reset_token(6),
         ack_delay_exponent(7),
         initial_max_uni_streams(8),
         disable_migration(9),
         initial_max_stream_data_bidi_remote(10),
         initial_max_stream_data_uni(11),
         max_ack_delay(12),
         original_connection_id(13),
         (65535)
      } TransportParameterId;

      struct {
         TransportParameterId parameter;
         opaque value<0..2^16-1>;
      } TransportParameter;

      struct {
         select (Handshake.msg_type) {
            case client_hello:
               QuicVersion initial_version;

            case encrypted_extensions:
               QuicVersion negotiated_version;
               QuicVersion supported_versions<4..2^8-4>;
         };
         TransportParameter parameters<0..2^16-1>;
      } TransportParameters;

      struct {
        enum { IPv4(4), IPv6(6), (15) } ipVersion;
        opaque ipAddress<4..2^8-1>;
        uint16 port;
        opaque connectionId<0..18>;
        opaque statelessResetToken[16];
      } PreferredAddress;

               Figure 15: Definition of TransportParameters

   The PADDING frame provides a useful option for PMTU probe packets.
   PADDING frames generate acknowledgements, but they need not be
   delivered reliably.  As "extension_data" field of the quic_transport_parameters extension
   defined in [QUIC-TLS] contains a result, TransportParameters value.  TLS
   encoding rules are therefore used to describe the loss encoding of PADDING frames in probe
   packets does not require delay-inducing retransmission.  However,
   PADDING frames do consume congestion window, which may delay the
   transmission
   transport parameters.

   QUIC encodes transport parameters into a sequence of subsequent application data.

   When implementing the algorithm octets, which
   are then included in Section 7.2 of [PLPMTUD], the
   initial value of search_low SHOULD be consistent with cryptographic handshake.

18.1.  Transport Parameter Definitions

   An endpoint MAY use the IPv6
   minimum packet size.  Paths following transport parameters:

   idle_timeout (0x0003):  The idle timeout is a value in seconds that do not support
      is encoded as an unsigned 16-bit integer.  If this size cannot
   deliver Initial packets, and therefore are not QUIC-compliant.

   Section 7.3 of [PLPMTUD] discusses trade-offs between small and large
   increases in parameter is
      absent or zero then the idle timeout is disabled.

   max_packet_size (0x0005):  The maximum packet size of probe packets.  As QUIC probe packets need
   not contain application data, aggressive increases in probe size
   carry fewer consequences.

9.  Streams: QUIC's Data Structuring Abstraction

   Streams in QUIC provide parameter places a lightweight, ordered byte-stream
   abstraction.

   There are two basic types of stream in QUIC.  Unidirectional streams
   carry data in one direction: from
      limit on the initiator size of packets that the stream to its
   peer; bidirectional streams allow for data to be sent in both
   directions.  Different stream identifiers are used endpoint is willing to distinguish
   between unidirectional and bidirectional streams, as well
      receive, encoded as to
   create a separation between streams an unsigned 16-bit integer.  This indicates
      that are initiated by the client
   and server (see Section 9.1).

   Either type of stream can be created by either endpoint, can
   concurrently send data interleaved with other streams, and can packets larger than this limit will be
   cancelled.

   Stream offsets allow dropped.  The default
      for this parameter is the octets on a stream maximum permitted UDP payload of 65527.
      Values below 1200 are invalid.  This limit only applies to be placed in
   order.
      protected packets (Section 12.1).

   ack_delay_exponent (0x0007):  An endpoint MUST be capable of delivering data received on a
   stream in order.  Implementations MAY choose 8-bit unsigned integer value
      indicating an exponent used to offer decode the ability to
   deliver data out ACK Delay field in the
      ACK frame, see Section 19.15.  If this value is absent, a default
      value of order.  There 3 is no means assumed (indicating a multiplier of ensuring ordering
   between octets on different streams. 8).  The creation and destruction of streams default
      value is also used for ACK frames that are expected to have minimal
   bandwidth and computational cost.  A single STREAM frame may create,
   carry data for, sent in Initial and terminate a stream, or a stream may last the
   entire duration of a connection.

   Streams
      Handshake packets.  Values above 20 are individually flow controlled, allowing an invalid.

   disable_migration (0x0009):  The endpoint does not support connection
      migration (Section 9).  Peers MUST NOT send any packets, including
      probing packets (Section 9.1), from a local address other than
      that used to
   limit memory commitment and to apply back pressure.  The creation of
   streams perform the handshake.  This parameter is also flow controlled, with each peer declaring a zero-
      length value.

   max_ack_delay (0x000c):  An 8 bit unsigned integer value indicating
      the maximum
   stream ID it amount of time in milliseconds by which the endpoint
      will delay sending acknowledgments.  If this value is willing to accept at absent, a given time.

   An alternative view
      default of QUIC streams 25 milliseconds is as assumed.

   Either peer MAY advertise an elastic "message"
   abstraction, similar to the way ephemeral streams are used in SST
   [SST], which may be a more appealing description initial value for some
   applications.

9.1.  Stream Identifiers

   Streams are identified by an unsigned 62-bit integer, referred to as
   the Stream ID.  The least significant two bits flow control of the Stream ID are
   used to identify the each
   type of stream (unidirectional or bidirectional)
   and the initiator of the stream.

   The least significant bit (0x1) on which they might receive data.  Each of the Stream ID identifies the
   initiator
   following transport parameters is encoded as an unsigned 32-bit
   integer in units of the stream.  Clients initiate even-numbered octets:

   initial_max_stream_data_bidi_local (0x0000):  The initial stream
      maximum data for bidirectional, locally-initiated streams
   (those with
      parameter contains the least significant bit set to 0); servers initiate
   odd-numbered initial flow control limit for newly
      created bidirectional streams (with the bit set to 1).  Separation of opened by the
   stream identifiers ensures endpoint that sets the
      transport parameter.  In client and server are able transport parameters, this applies
      to open streams without the latency imposed by negotiating for an identifier.

   If with an endpoint receives a frame for a stream that it expects identifier ending in 0x0; in server transport
      parameters, this applies to
   initiate (i.e., odd-numbered streams ending in 0x1.

   initial_max_stream_data_bidi_remote (0x000a):  The initial stream
      maximum data for bidirectional, peer-initiated streams parameter
      contains the client or even-numbered initial flow control limit for the
   server), but which it has not yet opened, it MUST close the
   connection with error code STREAM_STATE_ERROR.

   The second least significant bit (0x2) of the Stream ID
   differentiates between unidirectional streams and newly created
      bidirectional
   streams.  Unidirectional streams always have opened by the endpoint that receives the
      transport parameter.  In client transport parameters, this bit set applies
      to 1 and
   bidirectional streams have with an identifier ending in 0x1; in server transport
      parameters, this bit set applies to 0.

   The two type bits from a Stream ID therefore identify streams as
   summarized ending in Table 5.

              +----------+----------------------------------+
              | Low Bits | Stream Type                      |
              +----------+----------------------------------+
              | 0x0      | Client-Initiated, Bidirectional  |
              |          |                                  |
              | 0x1      | Server-Initiated, Bidirectional  |
              |          |                                  |
              | 0x2      | Client-Initiated, Unidirectional |
              |          |                                  |
              | 0x3      | Server-Initiated, Unidirectional |
              +----------+----------------------------------+

                         Table 5: Stream ID Types 0x0.

   initial_max_stream_data_uni (0x000b):  The first bi-directional initial stream maximum
      data for unidirectional streams parameter contains the initial
      flow control limit for newly created unidirectional streams opened
      by the client is stream 0.

   A QUIC endpoint MUST NOT reuse a Stream ID.  Streams of each type are
   created in numeric order.  Streams that are used out of order result receives the transport parameter.  In client
      transport parameters, this applies to streams with an identifier
      ending in opening all lower-numbered 0x3; in server transport parameters, this applies to
      streams of the same type ending in the same
   direction.

   Stream IDs 0x2.

   If present, transport parameters that set initial flow control limits
   (initial_max_stream_data_bidi_local,
   initial_max_stream_data_bidi_remote, and initial_max_stream_data_uni)
   are encoded as equivalent to sending a variable-length integer (see
   Section 7.1).

9.2.  Stream States

   This section describes MAX_STREAM_DATA frame (Section 19.6) on
   every stream of the corresponding type immediately after opening.  If
   the two types transport parameter is absent, streams of QUIC stream in terms that type start with a
   flow control limit of 0.

   initial_max_data (0x0001):  The initial maximum data parameter
      contains the
   states of their send or receive components.  Two state machines are
   described: one initial value for streams the maximum amount of data that can
      be sent on which the connection.  This parameter is encoded as an endpoint transmits data
      unsigned 32-bit integer in units of octets.  This is equivalent to
      sending a MAX_DATA (Section 9.2.1); another 19.5) for streams from which an endpoint receives
   data (Section 9.2.2).

   Unidirectional streams use the applicable state machine directly.
   Bidirectional streams use both state machines.  For connection immediately
      after completing the most part, handshake.  If the use of these state machines transport parameter is
      absent, the same whether the stream is
   unidirectional or bidirectional.  The conditions for opening a stream
   are slightly more complex for connection starts with a flow control limit of 0.

   initial_max_bidi_streams (0x0002):  The initial maximum bidirectional stream because
      streams parameter contains the
   opening initial maximum number of either send or receive sides causes the stream to open in
   both directions.

   An endpoint can open
      bidirectional streams up to its maximum stream limit in any
   order, however endpoints SHOULD open the send side of peer may initiate, encoded as an
      unsigned 16-bit integer.  If this parameter is absent or zero,
      bidirectional streams for
   each type in order.

   Note:  These states are largely informative.  This document uses
      stream states to describe rules for when and how different types
      of frames can cannot be sent and created until a MAX_STREAM_ID
      frame is sent.  Setting this parameter is equivalent to sending a
      MAX_STREAM_ID (Section 19.7) immediately after completing the reactions that are expected when
      different types
      handshake containing the corresponding Stream ID.  For example, a
      value of frames are received.  Though these state
      machines are intended to 0x05 would be useful in implementing QUIC, these
      states aren't intended equivalent to constrain implementations.  An
      implementation can define receiving a different state machine as long as its
      behavior is consistent with an implementation that implements
      these states.

9.2.1.  Send Stream States

   Figure MAX_STREAM_ID
      containing 16 shows the states for the part of when received by a stream that sends data
   to client or 17 when received by a peer.

          o
          | Create Stream (Sending)
          | Create Bidirectional Stream (Receiving)
          v
      +-------+
      | Ready | Send RST_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Send STREAM /             |
          |      STREAM_BLOCKED       |
          |                           |
          | Create Bidirectional      |
          |      Stream (Receiving)   |
          v                           |
      +-------+                       |
      | Send  | Send RST_STREAM       |
      |       |---------------------->|
      +-------+                       |
          |                           |
          | Send STREAM + FIN         |
          v                           v
      +-------+                   +-------+
      | Data  | Send RST_STREAM   | Reset |
      | Sent  |------------------>| Sent  |
      +-------+                   +-------+
          |                           |
          | Recv All ACKs             | Recv ACK
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Recvd |                   | Recvd |
      +-------+                   +-------+

                    Figure 16: States for Send Streams
      server.

   initial_max_uni_streams (0x0008):  The sending part initial maximum unidirectional
      streams parameter contains the initial maximum number of stream that
      unidirectional streams the endpoint initiates (types 0 and 2
   for clients, 1 and 3 for servers) peer may initiate, encoded as an
      unsigned 16-bit integer.  If this parameter is opened by the application absent or
   application protocol.  The "Ready" state represents a newly zero,
      unidirectional streams cannot be created
   stream that until a MAX_STREAM_ID
      frame is able sent.  Setting this parameter is equivalent to accept data from sending a
      MAX_STREAM_ID (Section 19.7) immediately after completing the application.
      handshake containing the corresponding Stream data
   might ID.  For example, a
      value of 0x05 would be buffered in this state in preparation for sending.

   Sending the first STREAM equivalent to receiving a MAX_STREAM_ID
      containing 18 when received by a client or STREAM_BLOCKED frame causes 19 when received by a send stream
   to enter
      server.

   A server MUST include the "Send" state.  An implementation might choose to defer
   allocating following transport parameter if it sent a Stream
   Retry packet:

   original_connection_id (0x000d):  The value of the Destination
      Connection ID to a send stream until it sends field from the first
   frame and enters this state, which can allow for better stream
   prioritization. Initial packet sent by the
      client.  This transport parameter is only sent by the server.

   A server MAY include the following transport parameters:

   stateless_reset_token (0x0006):  The sending part Stateless Reset Token is used in
      verifying a stateless reset, see Section 10.4.  This parameter is
      a sequence of 16 octets.

   preferred_address (0x0004):  The server's Preferred Address is used
      to effect a bidirectional stream initiated by change in server address at the end of the handshake,
      as described in Section 9.6.

   A client MUST NOT include an original connection ID, a peer (type
   0 for stateless
   reset token, or a server, type 1 for preferred address.  A server MUST treat receipt of
   any of these transport parameters as a client) enters connection error of type
   TRANSPORT_PARAMETER_ERROR.

19.  Frame Types and Formats

   As described in Section 12.4, packets contain one or more frames.
   This section describes the "Ready" state then
   immediately transitions format and semantics of the core QUIC
   frame types.

19.1.  PADDING Frame

   The PADDING frame (type=0x00) has no semantic value.  PADDING frames
   can be used to increase the "Send" state if size of a packet.  Padding can be used to
   increase an initial client packet to the receiving part
   enters minimum required size, or to
   provide protection against traffic analysis for protected packets.

   A PADDING frame has no content.  That is, a PADDING frame consists of
   the "Recv" state.

   In single octet that identifies the "Send" state, frame as a PADDING frame.

19.2.  RST_STREAM Frame

   An endpoint may use a RST_STREAM frame (type=0x01) to abruptly
   terminate a stream.

   After sending a RST_STREAM, an endpoint transmits - ceases transmission and retransmits as
   necessary - data in
   retransmission of STREAM frames.  The frames on the identified stream.  A receiver
   of RST_STREAM can discard any data that it already received on that
   stream.

   An endpoint respects that receives a RST_STREAM frame for a send-only stream
   MUST terminate the flow
   control limits connection with error PROTOCOL_VIOLATION.

   The RST_STREAM frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Application Error Code (16)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Final Offset (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Stream ID:  A variable-length integer encoding of its peer, accepting MAX_STREAM_DATA frames.  An
   endpoint in the "Send" state generates STREAM_BLOCKED frames if it
   encounters flow control limits.

   After Stream ID of
      the stream being terminated.

   Application Protocol Error Code:  A 16-bit application protocol error
      code (see Section 20.1) which indicates that why the stream data is complete and being
      closed.

   Final Offset:  A variable-length integer indicating the absolute byte
      offset of the end of data written on this stream by the RST_STREAM
      sender.

19.3.  CONNECTION_CLOSE frame

   An endpoint sends a
   STREAM CONNECTION_CLOSE frame containing (type=0x02) to notify its
   peer that the FIN bit connection is sent, being closed.  CONNECTION_CLOSE is used
   to signal errors at the send stream enters QUIC layer, or the "Data Sent" state.  From this state, absence of errors (with
   the endpoint only
   retransmits stream data NO_ERROR code).

   If there are open streams that haven't been explicitly closed, they
   are implicitly closed when the connection is closed.

   The CONNECTION_CLOSE frame is as necessary. follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Error Code (16)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Frame Type (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Reason Phrase Length (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Reason Phrase (*)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The endpoint no longer needs
   to track flow control limits or send STREAM_BLOCKED frames for fields of a send
   stream in CONNECTION_CLOSE frame are as follows:

   Error Code:  A 16-bit error code which indicates the reason for
      closing this state.  The endpoint can ignore any MAX_STREAM_DATA
   frames it receives connection.  CONNECTION_CLOSE uses codes from its peer the
      space defined in this state; MAX_STREAM_DATA
   frames might be received until Section 20.

   Frame Type:  A variable-length integer encoding the peer receives type of frame
      that triggered the final stream
   offset.

   Once all stream data has been successfully acknowledged, error.  A value of 0 (equivalent to the send
   stream enters mention
      of the "Data Recvd" state, which PADDING frame) is a terminal state.

   From any of used when the "Ready", "Send", or "Data Sent" states, an
   application can signal that it wishes to abandon transmission frame type is unknown.

   Reason Phrase Length:  A variable-length integer specifying the
      length of
   stream data.  Similarly, the endpoint might receive reason phrase in bytes.  Note that a STOP_SENDING
      CONNECTION_CLOSE frame from its peer.  In either case, cannot be split between packets, so in
      practice any limits on packet size will also limit the endpoint sends space
      available for a RST_STREAM
   frame, which causes reason phrase.

   Reason Phrase:  A human-readable explanation for why the stream connection
      was closed.  This can be zero length if the sender chooses to enter not
      give details beyond the "Reset Sent" state.

   An endpoint MAY send Error Code.  This SHOULD be a RST_STREAM UTF-8
      encoded string [RFC3629].

19.4.  APPLICATION_CLOSE frame

   An APPLICATION_CLOSE frame (type=0x03) is used to signal an error
   with the protocol that uses QUIC.

   The APPLICATION_CLOSE frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Error Code (16)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Reason Phrase Length (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Reason Phrase (*)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of an APPLICATION_CLOSE frame are as follows:

   Error Code:  A 16-bit error code which indicates the first frame on a send
   stream; reason for
      closing this causes connection.  APPLICATION_CLOSE uses codes from the send stream
      application protocol error code space, see Section 20.1.

   Reason Phrase Length:  This field is identical in format and
      semantics to open the Reason Phrase Length field from CONNECTION_CLOSE.

   Reason Phrase:  This field is identical in format and then immediately
   transition semantics to
      the "Reset Sent" state.

   Once a packet containing a RST_STREAM Reason Phrase field from CONNECTION_CLOSE.

   APPLICATION_CLOSE has been acknowledged, similar format and semantics to the send
   stream enters
   CONNECTION_CLOSE frame (Section 19.3).  Aside from the "Reset Recvd" state, which is a terminal state.

9.2.2.  Receive Stream States

   Figure 17 shows semantics of
   the states for Error Code field and the part of a stream that receives
   data from a peer.  The states for a receive stream mirror only some omission of the states of Frame Type field, both
   frames are used to close the send stream at connection.

19.5.  MAX_DATA Frame

   The MAX_DATA frame (type=0x04) is used in flow control to inform the peer.  A receive stream
   doesn't track states on
   peer of the send stream maximum amount of data that cannot can be observed, such sent on the connection
   as a whole.

   The frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Maximum Data (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The fields in the "Ready" state; instead, receive streams track MAX_DATA frame are as follows:

   Maximum Data:  A variable-length integer indicating the delivery maximum
      amount of data to the application or application protocol some of which cannot that can be observed by sent on the sender.

          o
          | Recv STREAM / STREAM_BLOCKED / RST_STREAM
          | Create Bidirectional Stream (Sending)
          | Recv MAX_STREAM_DATA
          | Create Higher-Numbered Stream
          v
      +-------+
      | Recv  | Recv RST_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Recv STREAM + FIN         |
          v                           |
      +-------+                       |
      | Size  | Recv RST_STREAM       |
      | Known |---------------------->|
      +-------+                       |
          |                           |
          | Recv All Data             |
          v                           v
      +-------+  Recv RST_STREAM  +-------+
      | Data  |--- (optional) --->| Reset |
      | Recvd |  Recv All Data    | Recvd |
      +-------+<-- (optional) ----+-------+
          |                           |
          | App Read entire connection, in units
      of octets.

   All Data         | App Read RST
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Read  |                   | Read  |
      +-------+                   +-------+

                   Figure 17: States for Receive Streams data sent in STREAM frames counts toward this limit.  The receiving part sum of a stream initiated
   the largest received offsets on all streams - including streams in
   terminal states - MUST NOT exceed the value advertised by a peer (types 1 and 3 for receiver.
   An endpoint MUST terminate a client, or 0 and 2 for connection with a server) are created when FLOW_CONTROL_ERROR
   error if it receives more data than the first STREAM,
   STREAM_BLOCKED, RST_STREAM, or MAX_STREAM_DATA (bidirectional only,
   see below) is received for maximum data value that stream.  The initial state for a
   receive stream it
   has sent, unless this is "Recv".  Receiving a RST_STREAM frame causes the
   receive stream to immediately transition to result of a change in the "Reset Recvd". initial limits
   (see Section 7.3.1).

19.6.  MAX_STREAM_DATA Frame

   The receive stream enters the "Recv" state when MAX_STREAM_DATA frame (type=0x05) is used in flow control to
   inform a peer of the sending part maximum amount of data that can be sent on a
   bidirectional stream initiated by the
   stream.

   An endpoint (type 0 for that receives a client,
   type 1 MAX_STREAM_DATA frame for a server) enters the "Ready" state.

   A bidirectional receive-only
   stream also opens when MUST terminate the connection with error PROTOCOL_VIOLATION.

   An endpoint that receives a MAX_STREAM_DATA frame is
   received.  Receiving for a MAX_STREAM_DATA frame implies that the remote
   peer send-only
   stream it has not opened MUST terminate the stream and is providing flow control credit.  A connection with error
   PROTOCOL_VIOLATION.

   Note that an endpoint may legally receive a MAX_STREAM_DATA frame might arrive before on
   a STREAM or STREAM_BLOCKED bidirectional stream it has not opened.

   The frame if packets are lost or reordered.

   Before creating a stream, all lower-numbered streams of is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Maximum Stream Data (i)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields in the same type
   MUST be created.  That means that receipt of a MAX_STREAM_DATA frame that would open
   a are as follows:

   Stream ID:  The stream causes all lower-numbered streams ID of the same type to be
   opened in numeric order.  This ensures stream that the creation order for
   streams is consistent on both endpoints.

   In the "Recv" state, affected encoded as a
      variable-length integer.

   Maximum Stream Data:  A variable-length integer indicating the endpoint receives STREAM and STREAM_BLOCKED
   frames.  Incoming
      maximum amount of data is buffered and that can be reassembled into sent on the
   correct order identified stream,
      in units of octets.

   When counting data toward this limit, an endpoint accounts for delivery to the application.  As
   largest received offset of data that is consumed
   by the application and buffer space becomes available, the endpoint
   sends MAX_STREAM_DATA frames to allow sent or received on the peer to send more data.

   When a STREAM frame with a FIN bit is received,
   stream.  Loss or reordering can mean that the final largest received offset (see
   Section 10.3) is known.  The receive
   on a stream enters the "Size Known"
   state.  In this state, can be greater than the endpoint no longer needs to send
   MAX_STREAM_DATA frames, it only receives any retransmissions total size of
   stream data.

   Once all data for received on
   that stream.  Receiving STREAM frames might not increase the largest
   received offset.

   The data sent on a stream has been received, MUST NOT exceed the receive largest maximum stream
   enters the "Data Recvd" state.  This might happen as a result of
   receiving the same STREAM frame that causes the transition to "Size
   Known".  In this state,
   data value advertised by the receiver.  An endpoint has all stream data.  Any STREAM
   or STREAM_BLOCKED frames MUST terminate a
   connection with a FLOW_CONTROL_ERROR error if it receives for the stream can be discarded.

   The "Data Recvd" state persists until stream more data has been delivered
   to
   than the application or application protocol.  Once largest maximum stream data that it has
   been delivered, the stream enters sent for the "Data Read" state, which
   affected stream, unless this is a
   terminal state.

   Receiving result of a RST_STREAM frame change in the "Recv" or "Size Known" states
   causes initial
   limits (see Section 7.3.1).

19.7.  MAX_STREAM_ID Frame

   The MAX_STREAM_ID frame (type=0x06) informs the peer of the maximum
   stream ID that they are permitted to enter the "Reset Recvd" state.  This might cause open.

   The frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Maximum Stream ID (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields in the delivery MAX_STREAM_ID frame are as follows:

   Maximum Stream ID:  ID of stream data to the application to be interrupted.

   It is possible that all maximum unidirectional or bidirectional
      peer-initiated stream data is received when a RST_STREAM is
   received (that is, from the "Data Recvd" state).  Similarly, it is
   possible ID for remaining stream data to arrive after receiving the connection encoded as a
   RST_STREAM frame (the "Reset Recvd" state).  An implementation is
   able variable-
      length integer.  The limit applies to manage this situation as they choose.  Sending RST_STREAM
   means that an endpoint cannot guarantee delivery unidirectional steams if the
      second least signification bit of the stream data;
   however there ID is no requirement 1, and applies
      to bidirectional streams if it is 0.

   Loss or reordering can mean that stream data not a MAX_STREAM_ID frame can be delivered if
   received which states a RST_STREAM is received.  An implementation MAY interrupt delivery
   of lower stream data, discard any data that was not consumed, and signal
   the existence of limit than the RST_STREAM immediately.  Alternatively, client has
   previously received.  MAX_STREAM_ID frames which do not increase the
   RST_STREAM signal might
   maximum stream ID MUST be suppressed or withheld if ignored.

   A peer MUST NOT initiate a stream data is
   completely received.  In the latter case, the receive with a higher stream
   effectively transitions to "Data Recvd" from "Reset Recvd".

   Once ID than the application
   greatest maximum stream ID it has been delivered the signal indicating that
   the receive received.  An endpoint MUST
   terminate a connection with a STREAM_ID_ERROR error if a peer
   initiates a stream was reset, the receive with a higher stream transitions to the
   "Reset Read" state, which ID than it has sent, unless
   this is a terminal state.

9.2.3.  Permitted result of a change in the initial limits (see
   Section 7.3.1).

19.8.  PING Frame Types

   Endpoints can use PING frames (type=0x07) to verify that their peers
   are still alive or to check reachability to the peer.  The sender PING frame
   contains no additional fields.

   The receiver of a stream sends just three PING frame types that affect simply needs to acknowledge the
   state of packet
   containing this frame.

   The PING frame can be used to keep a stream at either sender connection alive when an
   application or receiver: STREAM
   (Section 7.20), STREAM_BLOCKED (Section 7.11), and RST_STREAM
   (Section 7.3).

   A sender MUST NOT application protocol wishes to prevent the connection
   from timing out.  An application protocol SHOULD provide guidance
   about the conditions under which generating a PING is recommended.
   This guidance SHOULD indicate whether it is the client or the server
   that is expected to send the PING.  Having both endpoints send any of these PING
   frames from a terminal state
   ("Data Recvd" or "Reset Recvd"). without coordination can produce an excessive number of
   packets and poor performance.

   A sender MUST NOT send STREAM connection will time out if no packets are sent or
   STREAM_BLOCKED after sending received for a RST_STREAM; that is,
   period longer than the time specified in the "Reset
   Sent" idle_timeout transport
   parameter (see Section 10).  However, state in addition middleboxes might time
   out earlier than that.  Though REQ-5 in [RFC4787] recommends a 2
   minute timeout interval, experience shows that sending packets every
   15 to 30 seconds is necessary to prevent the terminal states.  A receiver could
   receive any majority of these frames in any state, middleboxes
   from losing state for UDP flows.

19.9.  BLOCKED Frame

   A sender SHOULD send a BLOCKED frame (type=0x08) when it wishes to
   send data, but only is unable to due to the
   possibility of delayed delivery connection-level flow control (see
   Section 4).  BLOCKED frames can be used as input to tuning of packets carrying them. flow
   control algorithms (see Section 4.2).

   The receiver of a stream sends MAX_STREAM_DATA (Section 7.7) and
   STOP_SENDING frames (Section 7.15). BLOCKED frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Offset (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The receiver only sends MAX_STREAM_DATA in BLOCKED frame contains a single field.

   Offset:  A variable-length integer indicating the "Recv" state. connection-level
      offset at which the blocking occurred.

19.10.  STREAM_BLOCKED Frame

   A
   receiver can sender SHOULD send STOP_SENDING in any state where it has not received a RST_STREAM frame; that STREAM_BLOCKED frame (type=0x09) when it
   wishes to send data, but is states other than "Reset Recvd" or "Reset
   Read".  However there unable to due to stream-level flow
   control.  This frame is little value in sending analogous to BLOCKED (Section 19.9).

   An endpoint that receives a STOP_SENDING STREAM_BLOCKED frame
   after all for a send-only
   stream data has been received in MUST terminate the "Data Recvd" state.  A
   sender could receive these frames in any state connection with error PROTOCOL_VIOLATION.

   The STREAM_BLOCKED frame is as a result of delayed
   delivery of packets.

9.2.4.  Bidirectional follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream States ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Offset (i)                          ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The STREAM_BLOCKED frame contains two fields:

   Stream ID:  A bidirectional variable-length integer indicating the stream which is composed of a send stream and a receive
   stream.  Implementations may represent states of
      flow control blocked.

   Offset:  A variable-length integer indicating the bidirectional
   stream as composites offset of send and receive stream states.  The simplest
   model presents the
      stream as "open" when either at which the blocking occurred.

19.11.  STREAM_ID_BLOCKED Frame

   A sender SHOULD send or receive
   stream is in a non-terminal state and "closed" STREAM_ID_BLOCKED frame (type=0x0a) when both send and
   receive streams are in it
   wishes to open a terminal state.

   Table 6 shows stream, but is unable to due to the maximum stream
   ID limit set by its peer (see Section 19.7).  This does not open the
   stream, but informs the peer that a more complex mapping of bidirectional new stream states
   that loosely correspond to was needed, but the
   stream states in HTTP/2 [HTTP2].  This
   shows that multiple states on send or receive streams are mapped to limit prevented the same composite state.  Note that this creation of the stream.

   The STREAM_ID_BLOCKED frame is just one possibility for
   such as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The STREAM_ID_BLOCKED frame contains a mapping; this mapping requires single field.

   Stream ID:  A variable-length integer indicating the highest stream
      ID that data is acknowledged
   before the transition sender was permitted to open.

19.12.  NEW_CONNECTION_ID Frame

   An endpoint sends a "closed" or "half-closed" state.

   +-----------------------+---------------------+---------------------+
   | Send Stream           | Receive Stream      | Composite State     |
   +-----------------------+---------------------+---------------------+
   | No Stream/Ready       | No Stream/Recv *1   | idle                |
   |                       |                     |                     |
   | Ready/Send/Data Sent  | Recv/Size Known     | open                |
   |                       |                     |                     |
   | Ready/Send/Data Sent  | Data Recvd/Data     | half-closed         |
   |                       | Read                | (remote)            |
   |                       |                     |                     |
   | Ready/Send/Data Sent  | Reset Recvd/Reset   | half-closed         |
   |                       | Read                | (remote)            |
   |                       |                     |                     |
   | Data Recvd            | Recv/Size Known     | half-closed (local) |
   |                       |                     |                     |
   | Reset Sent/Reset      | Recv/Size Known     | half-closed (local) |
   | Recvd                 |                     |                     |
   |                       |                     |                     |
   | Data Recvd            | Recv/Size Known     | half-closed (local) |
   |                       |                     |                     |
   | Reset Sent/Reset      | Data Recvd/Data     | closed              |
   | Recvd                 | Read                |                     |
   |                       |                     |                     |
   | Reset Sent/Reset      | Reset Recvd/Reset   | closed              |
   | Recvd                 | Read                |                     |
   |                       |                     |                     |
   | Data Recvd            | Data Recvd/Data     | closed              |
   |                       | Read                | NEW_CONNECTION_ID frame (type=0x0b) to provide
   its peer with alternative connection IDs that can be used to break
   linkability when migrating connections (see Section 9.5).

   The NEW_CONNECTION_ID frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Length (8)  |            Sequence Number (i)              ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Connection ID (32..144)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   | Data Recvd                                                               |
   +                   Stateless Reset Recvd/Reset   | closed              | Token (128)                 +
   |                                                               | Read
   +                                                               +
   |                                                               |
   +-----------------------+---------------------+---------------------+

           Table 6: Possible Mapping of Stream States to HTTP/2

   Note (*1):  A stream is considered "idle" if it has not yet been
      created, or if the receive stream is in
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Length:  An 8-bit unsigned integer containing the "Recv" state without
      yet having received any frames.

9.3.  Solicited State Transitions

   If an endpoint is no longer interested in length of the data it is receiving on
   a stream, it MAY send
      connection ID.  Values less than 4 and greater than 18 are invalid
      and MUST be treated as a STOP_SENDING frame identifying that stream connection error of type
      PROTOCOL_VIOLATION.

   Sequence Number:  The sequence number assigned to
   prompt closure of the stream in the opposite direction.  This
   typically indicates that connection ID
      by the receiving application is no longer
   reading data it receives from sender.  See Section 5.1.1.

   Connection ID:  A connection ID of the stream, but is not a guarantee specified length.

   Stateless Reset Token:  A 128-bit value that
   incoming data will be ignored.

   STREAM frames received after sending STOP_SENDING are still counted
   toward used for a
      stateless reset when the associated connection and stream flow-control windows, even though
   these frames will be discarded upon receipt.  This avoids potential
   ambiguity about which STREAM frames count toward flow control.

   A STOP_SENDING ID is used (see
      Section 10.4).

   An endpoint MUST NOT send this frame requests if it currently requires that the receiving endpoint
   its peer send packets with a
   RST_STREAM frame. zero-length Destination Connection ID.
   Changing the length of a connection ID to or from zero-length makes
   it difficult to identify when the value of the connection ID changed.
   An endpoint that receives is sending packets with a STOP_SENDING frame zero-length Destination
   Connection ID MUST send treat receipt of a RST_STREAM NEW_CONNECTION_ID frame for that stream, and can use an as a
   connection error
   code of STOPPING.  If type PROTOCOL_VIOLATION.

   Transmission errors, timeouts and retransmissions might cause the STOP_SENDING
   same NEW_CONNECTION_ID frame is to be received on a send
   stream that is already in multiple times.  Receipt
   of the "Data Sent" state, a RST_STREAM same frame
   MAY still be sent in order to cancel retransmission of previously-
   sent STREAM frames.

   STOP_SENDING SHOULD only multiple times MUST NOT be sent for treated as a receive stream that has not
   been reset.  STOP_SENDING is most useful for streams connection
   error.  A receiver can use the sequence number supplied in the "Recv" or
   "Size Known" states.

   An endpoint is expected
   NEW_CONNECTION_ID frame to send another STOP_SENDING identify new connection IDs from old ones.

   If an endpoint receives a NEW_CONNECTION_ID frame if that repeats a
   packet containing
   previously issued connection ID with a previous STOP_SENDING is lost.  However, once
   either all stream data different Stateless Reset
   Token or a RST_STREAM frame has been received for different sequence number, the stream - endpoint MAY treat that is, the stream is in any state other than "Recv" or
   "Size Known" - sending
   receipt as a STOP_SENDING frame is unnecessary.

9.4.  Stream Concurrency connection error of type PROTOCOL_VIOLATION.

19.13.  RETIRE_CONNECTION_ID Frame

   An endpoint limits the number of concurrently active incoming streams sends a RETIRE_CONNECTION_ID frame (type=0x1b) to
   indicate that it will no longer use a connection ID that was issued
   by adjusting its peer.  This may include the maximum stream ID.  An initial value is set in connection ID provided during the
   transport parameters (see Section 6.6.1) and is subsequently
   increased by MAX_STREAM_ID frames
   handshake.  Sending a RETIRE_CONNECTION_ID frame also serves as a
   request to the peer to send additional connection IDs for future use
   (see Section 7.8).

   The maximum stream ID is specific to each endpoint and applies only 5.1).  New connection IDs can be delivered to the a peer that receives the setting.  That is, clients specify
   using the
   maximum stream NEW_CONNECTION_ID frame (Section 19.12).

   Retiring a connection ID invalidates the server can initiate, and servers specify stateless reset token
   associated with that connection ID.

   The RETIRE_CONNECTION_ID frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sequence Number (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Sequence Number:  The sequence number of the
   maximum stream connection ID the client can initiate.  Each endpoint may respond
   on streams initiated by the other peer, regardless being
      retired.  See Section 5.1.2.

   Receipt of whether it is
   permitted to initiate new streams.

   Endpoints MUST NOT exceed the limit set by their peer.  An endpoint
   that receives a STREAM RETIRE_CONNECTION_ID frame with an ID containing a sequence number
   greater than the limit it has any previously sent MUST treat this to the peer MAY be treated as a stream
   connection error of type STREAM_ID_ERROR
   (Section 11), unless this is a result of a change in the initial
   limits (see Section 6.6.2).

   A receiver PROTOCOL_VIOLATION.

   An endpoint cannot renege on an advertisement; that is, once a
   receiver advertises send this frame if it was provided with a stream zero-
   length connection ID via a MAX_STREAM_ID frame,
   advertising by its peer.  An endpoint that provides a smaller maximum zero-
   length connection ID has no effect.  A sender MUST ignore
   any MAX_STREAM_ID treat receipt of a RETIRE_CONNECTION_ID
   frame that does not increase the maximum stream ID.

9.5.  Sending and Receiving Data

   Once as a stream is created, endpoints may use the stream to send and
   receive data.  Each connection error of type PROTOCOL_VIOLATION.

19.14.  STOP_SENDING Frame

   An endpoint may send a series of STREAM frames
   encapsulating data on use a stream until the stream is terminated in that
   direction.  Streams are an ordered byte-stream abstraction, and they
   have no other structure within them.  STREAM STOP_SENDING frame boundaries are not
   expected to be preserved in retransmissions from the sender or during
   delivery (type=0x0c) to the application at the receiver.

   When new communicate
   that incoming data is to be sent being discarded on receipt at application
   request.  This signals a stream, a sender MUST set the
   encapsulating STREAM frame's offset field peer to the stream offset of the
   first byte of this new data.  The first octet of data abruptly terminate transmission on a stream has
   an offset
   stream.

   Receipt of 0.  An endpoint a STOP_SENDING frame is expected to only valid for a send every stream octet.
   The largest offset delivered on that
   exists and is not in the "Ready" state (see Section 3.1).  Receiving
   a stream MUST be less than 2^62.

   QUIC makes no specific allowances STOP_SENDING frame for partial reliability or delivery
   of a send stream data out of order.  Endpoints that is "Ready" or non-
   existent MUST be able to deliver
   stream data to an application treated as an ordered byte-stream.  Delivering
   an ordered byte-stream requires that an endpoint buffer any data that
   is received out a connection error of order, up to the advertised flow control limit. type
   PROTOCOL_VIOLATION.  An endpoint could receive the same octets multiple times; octets that
   have already been received can be discarded. receives a STOP_SENDING frame
   for a receive-only stream MUST terminate the connection with error
   PROTOCOL_VIOLATION.

   The STOP_SENDING frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Application Error Code (16)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value for a given
   octet MUST NOT change if it is sent multiple times; an endpoint MAY
   treat receipt of a changed octet as a connection error fields are:

   Stream ID:  A variable-length integer carrying the Stream ID of type
   PROTOCOL_VIOLATION.

   An endpoint MUST NOT send data on any the
      stream without ensuring that it
   is within being ignored.

   Application Error Code:  A 16-bit, application-specified reason the data limits set by its peer.

   Flow control
      sender is described in detail in ignoring the stream (see Section 10, 20.1).

19.15.  ACK Frame

   Receivers send ACK frames (types 0x1a and congestion
   control is described in the companion document [QUIC-RECOVERY].

9.6.  Stream Prioritization

   Stream multiplexing has a significant effect on application
   performance if resources allocated 0x1b) to streams are correctly
   prioritized.  Experience with other multiplexed protocols, such as
   HTTP/2 [HTTP2], shows that effective prioritization strategies inform senders of
   packets they have a
   significant positive impact on performance.

   QUIC does not provide received and processed.  The ACK frame contains one
   or more ACK Blocks.  ACK Blocks are ranges of acknowledged packets.
   If the frame type is 0x1b, ACK frames for exchanging prioritization
   information.  Instead it relies also contain the sum of ECN
   marks received on receiving priority information
   from the application that uses QUIC.  Protocols that use connection up until this point.

   QUIC acknowledgements are
   able to define any prioritization scheme that suits their application
   semantics.  A protocol might define explicit messages for signaling
   priority, such as those defined in HTTP/2; irrevocable.  Once acknowledged, a packet
   remains acknowledged, even if it could define rules does not appear in a future ACK
   frame.  This is unlike TCP SACKs ([RFC2018]).

   It is expected that
   allow an endpoint to determine priority based on context; or it could
   leave a sender will reuse the determination to same packet number across
   different packet number spaces.  ACK frames only acknowledge the application.

   A QUIC implementation SHOULD provide ways
   packet numbers that were transmitted by the sender in which an application can
   indicate the relative priority same packet
   number space of streams.  When deciding which
   streams to dedicate resources to, QUIC SHOULD use the information
   provided by packet that the application.  Failure to account for priority of
   streams can result in suboptimal performance.

   Stream priority is most relevant when deciding which stream data will
   be transmitted.  Often, there will ACK was received in.

   Version Negotiation and Retry packets cannot be limits acknowledged because
   they do not contain a packet number.  Rather than relying on what can be
   transmitted ACK
   frames, these packets are implicitly acknowledged by the next Initial
   packet sent by the client.

   An ACK frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Largest Acknowledged (i)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Delay (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ACK Block Count (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Blocks (*)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [ECN Section]                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 16: ACK Frame Format

   The fields in the ACK frame are as a result of connection flow control or follows:

   Largest Acknowledged:  A variable-length integer representing the current
   congestion controller state.

   Giving preference to
      largest packet number the transmission of its own management frames
   ensures that peer is acknowledging; this is usually
      the protocol functions efficiently.  That is,
   prioritizing frames other than STREAM frames ensures largest packet number that loss
   recovery, congestion control, and flow control operate effectively.

   CRYPTO frames SHOULD be prioritized over other streams the peer has received prior to
      generating the
   completion of ACK frame.  Unlike the cryptographic handshake.  This includes packet number in the
   retransmission of QUIC
      long or short header, the second flight of client handshake messages,
   that is, value in an ACK frame is not truncated.

   ACK Delay:  A variable-length integer including the TLS Finished and any client authentication messages.

   STREAM data time in frames determined to be lost SHOULD be retransmitted
   before sending new data, unless application priorities indicate
   otherwise.  Retransmitting lost stream data can fill
      microseconds that the largest acknowledged packet, as indicated in gaps, which
   allows
      the Largest Acknowledged field, was received by this peer to consume already received data and free up when
      this ACK was sent.  The value of the flow
   control window.

10.  Flow Control

   It ACK Delay field is necessary to limit scaled by
      multiplying the amount of data that a sender may have
   outstanding at any time, so as to prevent a fast sender from
   overwhelming a slow receiver, or to prevent a malicious sender from
   consuming significant resources at a receiver.  This section
   describes QUIC's flow-control mechanisms.

   QUIC employs a credit-based flow-control scheme similar encoded value by 2 to HTTP/2's
   flow control [HTTP2].  A receiver advertises the number power of octets it
   is prepared to receive on a given stream and for the entire
   connection.  This leads to two levels value of flow control in QUIC: (i)
   Connection flow control, which prevents senders from exceeding a
   receiver's buffer capacity for the connection, and (ii) Stream flow
   control, which prevents a single stream from consuming
      the entire
   receive buffer for a connection.

   A data receiver sends MAX_STREAM_DATA or MAX_DATA frames to "ack_delay_exponent" transport parameter set by the sender to advertise additional credit.  MAX_STREAM_DATA frames send
   the maximum absolute byte offset of a stream, while MAX_DATA sends
   the maximum of the sum of
      the absolute byte offsets of all streams.

   A receiver MAY advertise a larger offset at any point by sending
   MAX_DATA ACK frame.  The "ack_delay_exponent" defaults to 3, or MAX_STREAM_DATA frames.  A receiver cannot renege on an
   advertisement; that is, once a receiver advertises an offset,
   advertising
      multiplier of 8 (see Section 18.1).  Scaling in this fashion
      allows for a smaller offset has no effect.  A sender MUST therefore
   ignore any MAX_DATA or MAX_STREAM_DATA frames that do not increase
   flow control limits.

   A receiver MUST close the connection larger range of values with a FLOW_CONTROL_ERROR error
   (Section 11) if the peer violates shorter encoding at the advertised connection or stream
   data limits.
      cost of lower resolution.

   ACK Block Count:  A sender SHOULD send BLOCKED variable-length integer specifying the number of
      Additional ACK Block (and Gap) fields after the First ACK Block.

   ACK Blocks:  Contains one or STREAM_BLOCKED frames to indicate it
   has data to write but is blocked by flow control limits.  These
   frames are expected to be sent infrequently in common cases, but they
   are considered useful for debugging more blocks of packet numbers which have
      been successfully received, see Section 19.15.1.

19.15.1.  ACK Block Section

   The ACK Block Section consists of alternating Gap and monitoring purposes. ACK Block
   fields in descending packet number order.  A receiver advertises credit for a stream First Ack Block field is
   followed by sending a
   MAX_STREAM_DATA frame with variable number of alternating Gap and Additional ACK
   Blocks.  The number of Gap and Additional ACK Block fields is
   determined by the Stream ID set appropriately.  A
   receiver could ACK Block Count field.

   Gap and ACK Block fields use a relative integer encoding for
   efficiency.  Though each encoded value is positive, the current offset values are
   subtracted, so that each ACK Block describes progressively lower-
   numbered packets.  As long as contiguous ranges of data consumed to determine packets are small,
   the flow control offset to variable-length integer encoding ensures that each range can be advertised.  A receiver MAY send
   MAX_STREAM_DATA frames in multiple packets
   expressed in order to make sure that
   the sender receives an update before running out of flow control
   credit, even if one a small number of octets.

   The ACK frame uses the packets is lost.

   Connection flow control is a limit least significant bit(bit (that is, type 0x1b)
   to the total bytes indicate ECN feedback and report receipt of stream data
   sent packets with ECN
   codepoints of ECT(0), ECT(1), or CE in STREAM frames on all streams.  A receiver advertises credit
   for the packet's IP header.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First ACK Block (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 17: ACK Block Section

   Each ACK Block acknowledges a connection contiguous range of packets by sending a MAX_DATA frame.  A receiver maintains a
   cumulative sum
   indicating the number of bytes received on all contributing streams, which
   are used to check for flow control violations. acknowledged packets that precede the
   largest packet number in that block.  A receiver might use
   a sum value of bytes consumed on all contributing streams to determine zero indicates that
   only the
   maximum data limit to be advertised.

10.1.  Edge Cases and Other Considerations

   There are some edge cases which must be considered when dealing largest packet number is acknowledged.  Larger ACK Block
   values indicate a larger range, with
   stream and connection level flow control.  Given enough time, both
   endpoints must agree on flow control state.  If one end believes it
   can send more than corresponding lower values for
   the other end is willing to receive, smallest packet number in the
   connection will be torn down when too much data arrives.

   Conversely if range.  Thus, given a sender believes it largest
   packet number for the ACK, the smallest value is blocked, while endpoint B
   expects more data can be received, then determined by the connection can be in a
   deadlock, with
   formula:

      smallest = largest - ack_block

   The range of packets that are acknowledged by the ACK Block include
   the range from the smallest packet number to the sender waiting largest, inclusive.

   The largest value for a MAX_DATA or MAX_STREAM_DATA
   frame which will never come.

   On receipt of a RST_STREAM frame, an endpoint will tear down state the First ACK Block is determined by the
   Largest Acknowledged field; the largest for Additional ACK Blocks is
   determined by cumulatively subtracting the matching stream size of all preceding ACK
   Blocks and ignore further data arriving on Gaps.

   Each Gap indicates a range of packets that
   stream.  This could result are not being
   acknowledged.  The number of packets in the endpoints getting out of sync,
   since gap is one higher than
   the RST_STREAM frame may have arrived out encoded value of order and there
   may be further bytes in flight.  The data sender would have counted the data against its connection level flow control budget, but a
   receiver that has not received these bytes would not know to include
   them as well. Gap Field.

   The receiver must learn value of the Gap field establishes the largest packet number of bytes
   value for the ACK Block that were
   sent on follows the stream to make gap using the same adjustment in its connection flow
   controller.

   To avoid this de-synchronization, a RST_STREAM sender following
   formula:

     largest = previous_smallest - gap - 2

   If the calculated value for largest or smallest packet number for any
   ACK Block is negative, an endpoint MUST include generate a connection error
   of type FRAME_ENCODING_ERROR indicating an error in an ACK frame.

   The fields in the final byte offset sent on ACK Block Section are:

   First ACK Block:  A variable-length integer indicating the stream in number of
      contiguous packets preceding the RST_STREAM frame.  On
   receiving a RST_STREAM frame, a receiver definitively knows how many
   bytes were sent on Largest Acknowledged that stream before the RST_STREAM frame, and are
      being acknowledged.

   Gap (repeated):  A variable-length integer indicating the
   receiver MUST use number of
      contiguous unacknowledged packets preceding the final offset to account for all bytes sent on packet number one
      lower than the stream smallest in its connection level flow controller.

10.1.1.  Response to a RST_STREAM

   RST_STREAM terminates one direction the preceding ACK Block.

   Additional ACK Block (repeated):  A variable-length integer
      indicating the number of a stream abruptly.  Whether
   any action or response can or contiguous acknowledged packets preceding
      the largest packet number, as determined by the preceding Gap.

19.15.2.  ECN section

   The ECN section should only be taken on parsed when the data already
   received ACK frame type byte is an application-specific issue, but it will often be the
   case that upon receipt
   0x1b.  The ECN section consists of a RST_STREAM an endpoint will choose to
   stop sending data in its own direction.  If 3 ECN counters as shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ECT(0) Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ECT(1) Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ECN-CE Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   ECT(0) Count:  A variable-length integer representing the total
      number packets received with the ECT(0) codepoint.

   ECT(1) Count:  A variable-length integer representing the total
      number packets received with the ECT(1) codepoint.

   CE Count:  A variable-length integer representing the total number
      packets received with the sender of a
   RST_STREAM wishes CE codepoint.

19.16.  PATH_CHALLENGE Frame

   Endpoints can use PATH_CHALLENGE frames (type=0x0e) to check
   reachability to explicitly state that no future data will be
   processed, that endpoint MAY send a STOP_SENDING frame at the same
   time.

10.1.2.  Data Limit Increments

   This document leaves when peer and how many bytes to advertise in a
   MAX_DATA or MAX_STREAM_DATA to implementations, but offers a few
   considerations.  These frames contribute to for path validation during connection overhead.
   Therefore frequently sending
   migration.

   PATH_CHALLENGE frames with small changes is
   undesirable.  At the same time, infrequent updates require larger
   increments contain an 8-byte payload.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            Data (8)                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data:  This 8-byte field contains arbitrary data.

   A PATH_CHALLENGE frame containing 8 octets that are hard to limits if blocking guess is
   sufficient to be avoided.  Thus, larger
   updates require a receiver to commit to larger resource commitments.
   Thus there is a trade-off between resource commitment and overhead
   when determining how large a limit ensure that it is advertised.

   A receiver MAY use an autotuning mechanism easier to tune receive the frequency and
   amount that packet than it increases data limits based on a round-trip time
   estimate and the rate at which the receiving application consumes
   data, similar
   is to common TCP implementations.

10.2.  Stream Limit Increment

   As with flow control, guess the value correctly.

   The recipient of this document leaves when and how many streams
   to make available to frame MUST generate a peer via MAX_STREAM_ID PATH_RESPONSE frame
   (Section 19.17) containing the same Data.

19.17.  PATH_RESPONSE Frame

   The PATH_RESPONSE frame (type=0x0f) is sent in response to implementations, but
   offers a few considerations.  MAX_STREAM_ID frames constitute minimal
   overhead, while withholding MAX_STREAM_ID frames can prevent the peer
   from using the available parallelism.

   Implementations will likely want
   PATH_CHALLENGE frame.  Its format is identical to increase the maximum stream ID as
   peer-initiated streams close.  A receiver MAY also advance the
   maximum stream ID based on current activity, system conditions, and
   other environmental factors.

10.2.1.  Blocking on Flow Control PATH_CHALLENGE
   frame (Section 19.16).

   If the content of a sender PATH_RESPONSE frame does not receive match the content of
   a MAX_DATA or MAX_STREAM_DATA PATH_CHALLENGE frame when
   it has run out of flow control credit, previously sent by the sender will be blocked and
   SHOULD send a BLOCKED or STREAM_BLOCKED frame.  These frames are
   expected to be useful for debugging at endpoint, the receiver; they do not
   require any other action. endpoint
   MAY generate a connection error of type PROTOCOL_VIOLATION.

19.18.  NEW_TOKEN frame

   A receiver SHOULD NOT wait for server sends a BLOCKED
   or STREAM_BLOCKED NEW_TOKEN frame before sending MAX_DATA or MAX_STREAM_DATA,
   since doing so will mean that (type=0x19) to provide the client a sender is unable
   token to send for in the header of an
   entire round trip.

   For smooth operation Initial packet for a future
   connection.

   The NEW_TOKEN frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Token Length (i)  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Token (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the congestion controller, it is generally
   considered best to not let the sender go into quiescence if
   avoidable.  To avoid blocking a sender, and to reasonably account for NEW_TOKEN frame are as follows:

   Token Length:  A variable-length integer specifying the possibility length of loss, the
      token in bytes.

   Token:  An opaque blob that the client may use with a receiver should send future Initial
      packet.

19.19.  STREAM Frames

   STREAM frames implicitly create a MAX_DATA or
   MAX_STREAM_DATA stream and carry stream data.  The
   STREAM frame at least two round trips before it expects takes the
   sender form 0b00010XXX (or the set of values from
   0x10 to get blocked.

   A sender sends a single BLOCKED or STREAM_BLOCKED 0x17).  The value of the three low-order bits of the frame only once
   when it reaches a data limit.  A sender SHOULD NOT send multiple
   BLOCKED or STREAM_BLOCKED frames for
   type determine the same data limit, unless fields that are present in the frame.

   o  The OFF bit (0x04) in the
   original frame type is determined set to be lost.  Another BLOCKED or
   STREAM_BLOCKED frame can be sent after indicate that there
      is an Offset field present.  When set to 1, the data limit Offset field is increased.

10.3.  Stream Final
      present; when set to 0, the Offset

   The final offset field is absent and the count Stream
      Data starts at an offset of 0 (that is, the number of frame contains the
      first octets that are
   transmitted on a stream.  For of the stream, or the end of a stream that is reset, includes
      no data).

   o  The LEN bit (0x02) in the final
   offset frame type is set to indicate that there
      is carried explicitly in a RST_STREAM frame.  Otherwise, Length field present.  If this bit is set to 0, the
   final offset Length
      field is absent and the offset of Stream Data field extends to the end of
      the data carried in a STREAM
   frame marked with a FIN flag, or 0 in packet.  If this bit is set to 1, the case Length field is present.

   o  The FIN bit (0x01) of incoming
   unidirectional streams.

   An endpoint will know the frame type is set only on frames that
      contain the final offset for a stream when of the receive
   stream enters stream.  Setting this bit
      indicates that the "Size Known" or "Reset Recvd" state. frame marks the end of the stream.

   An endpoint MUST NOT send data on a stream at or beyond the final
   offset.

   Once that receives a final offset STREAM frame for a send-only stream MUST
   terminate the connection with error PROTOCOL_VIOLATION.

   A STREAM frame is known, it cannot change.  If a
   RST_STREAM or shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Stream ID (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [Offset (i)]                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [Length (i)]                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream Data (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 18: STREAM Frame Format

   The STREAM frame causes contains the final offset to change for a
   stream, an endpoint SHOULD respond with a FINAL_OFFSET_ERROR error following fields:

   Stream ID:  A variable-length integer indicating the stream ID of the
      stream (see Section 11). 2.1).

   Offset:  A receiver SHOULD treat receipt of data at or
   beyond variable-length integer specifying the final byte offset as a FINAL_OFFSET_ERROR error, even after a in the
      stream for the data in this STREAM frame.  This field is closed.  Generating these errors is not mandatory, but only
   because requiring that an endpoint generate these errors also means
   that present
      when the endpoint needs OFF bit is set to maintain 1.  When the Offset field is absent,
      the final offset state for closed
   streams, which could mean a significant state commitment.

10.4.  Flow Control for Cryptographic Handshake is 0.

   Length:  A variable-length integer specifying the length of the
      Stream Data sent field in CRYPTO frames this STREAM frame.  This field is not flow controlled in present
      when the same way as
   STREAM frames.  QUIC relies on LEN bit is set to 1.  When the cryptographic protocol
   implementation LEN bit is set to avoid excessive buffering of data, see [QUIC-TLS]. 0, the
      Stream Data field consumes all the remaining octets in the packet.

   Stream Data:  The implementation SHOULD provide an interface to QUIC to tell it
   about its buffering limits so that there is not excessive buffering
   at multiple layers.

11.  Error Handling

   An endpoint that detects an error SHOULD signal bytes from the existence of that
   error designated stream to its peer.  Both transport-level and application-level errors
   can affect an entire connection (see Section 11.1), while only
   application-level errors can be isolated to delivered.

   When a single stream (see
   Section 11.2).

   The most appropriate error code (Section 11.3) SHOULD be included Stream Data field has a length of 0, the offset in the STREAM
   frame that signals is the error.  Where this specification
   identifies error conditions, it also identifies offset of the error code that
   is used.

   A stateless reset (Section 6.13.4) is not suitable for any error next byte that
   can would be signaled with sent.

   The first byte in the stream has an offset of 0.  The largest offset
   delivered on a CONNECTION_CLOSE, APPLICATION_CLOSE, or
   RST_STREAM frame.  A stateless reset stream - the sum of the re-constructed offset and data
   length - MUST NOT be less than 2^62.

19.20.  CRYPTO Frame

   The CRYPTO frame (type=0x18) is used by an endpoint
   that has the state necessary to send a frame on the connection.

11.1.  Connection Errors

   Errors that result transmit cryptographic
   handshake messages.  It can be sent in all packet types.  The CRYPTO
   frame offers the connection being unusable, such as an
   obvious violation of cryptographic protocol semantics or corruption an in-order stream of state bytes.
   CRYPTO frames are functionally identical to STREAM frames, except
   that
   affects an entire connection, MUST be signaled using they do not bear a
   CONNECTION_CLOSE or APPLICATION_CLOSE frame (Section 7.4,
   Section 7.5).  An endpoint MAY close stream identifier; they are not flow
   controlled; and they do not carry markers for optional offset,
   optional length, and the connection in this manner
   even if end of the error only affects a single stream.

   Application protocols can signal application-specific protocol errors
   using

   A CRYPTO frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Offset (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Length (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Crypto Data (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 19: CRYPTO Frame Format

   The CRYPTO frame contains the following fields:

   Offset:  A variable-length integer specifying the APPLICATION_CLOSE frame.  Errors that are specific to byte offset in the
   transport, including all those described
      stream for the data in this document, are
   carried in a CONNECTION_CLOSE CRYPTO frame.  Other than

   Length:  A variable-length integer specifying the type length of error
   code they carry, these frames are identical in format and semantics.

   A CONNECTION_CLOSE or APPLICATION_CLOSE frame could be sent the
      Crypto Data field in this CRYPTO frame.

   Crypto Data:  The cryptographic message data.

   There is a
   packet separate flow of cryptographic handshake data in each
   encryption level, each of which starts at an offset of 0.  This
   implies that each encryption level is lost.  An endpoint SHOULD be prepared to retransmit a
   packet containing either frame type if it receives more packets on treated as a
   terminated connection.  Limiting the number separate CRYPTO
   stream of retransmissions and
   the time over data.

   Unlike STREAM frames, which this final packet is sent limits the effort
   expended on terminated connections.

   An endpoint that chooses not to retransmit packets containing
   CONNECTION_CLOSE or APPLICATION_CLOSE risks include a peer missing the first
   such packet.  The only mechanism available to an endpoint that
   continues Stream ID indicating to receive which
   stream the data belongs, the CRYPTO frame carries data for a terminated connection is to single
   stream per encryption level.  The stream does not have an explicit
   end, so CRYPTO frames do not have a FIN bit.

19.21.  Extension Frames

   QUIC frames do not use the
   stateless reset process (Section 6.13.4). a self-describing encoding.  An endpoint
   therefore needs to understand the syntax of all frames before it can
   successfully process a packet.  This allows for efficient encoding of
   frames, but it means that receives an invalid CONNECTION_CLOSE or
   APPLICATION_CLOSE endpoint cannot send a frame MUST NOT signal the existence of the error a type
   that is unknown to its peer.

11.2.  Stream Errors

   If an application-level error affects

   An extension to QUIC that wishes to use a single stream, but otherwise
   leaves the connection in new type of frame MUST
   first ensure that a recoverable state, peer is able to understand the frame.  An
   endpoint can send use a
   RST_STREAM transport parameter to signal its willingness to
   receive one or more extension frame (Section 7.3) types with an appropriate error code to
   terminate just the affected stream.

   Other than STOPPING (Section 9.3), RST_STREAM one transport
   parameter.

   Extension frames MUST be instigated by
   the application congestion controlled and MUST carry an application error code.  Resetting
   a stream without knowledge of the application protocol could cause
   the protocol to enter an unrecoverable state.  Application protocols
   might require certain streams ACK
   frame to be reliably delivered sent.  The exception is extension frames that replace or
   supplement the ACK frame.  Extension frames are not included in order flow
   control unless specified in the extension.

   An IANA registry is used to
   guarantee consistent state between endpoints.

11.3. manage the assignment of frame types, see
   Section 22.2.

20.  Transport Error Codes

   QUIC error codes are 16-bit unsigned integers.

   This section lists the defined QUIC transport error codes that may be
   used in a CONNECTION_CLOSE frame.  These errors apply to the entire
   connection.

   NO_ERROR (0x0):  An endpoint uses this with CONNECTION_CLOSE to
      signal that the connection is being closed abruptly in the absence
      of any error.

   INTERNAL_ERROR (0x1):  The endpoint encountered an internal error and
      cannot continue with the connection.

   SERVER_BUSY (0x2):  The server is currently busy and does not accept
      any new connections.

   FLOW_CONTROL_ERROR (0x3):  An endpoint received more data than it
      permitted in its advertised data limits (see Section 10). 4).

   STREAM_ID_ERROR (0x4):  An endpoint received a frame for a stream
      identifier that exceeded its advertised maximum stream ID.

   STREAM_STATE_ERROR (0x5):  An endpoint received a frame for a stream
      that was not in a state that permitted that frame (see Section 9.2). 3).

   FINAL_OFFSET_ERROR (0x6):  An endpoint received a STREAM frame
      containing data that exceeded the previously established final
      offset.  Or an endpoint received a RST_STREAM frame containing a
      final offset that was lower than the maximum offset of data that
      was already received.  Or an endpoint received a RST_STREAM frame
      containing a different final offset to the one already
      established.

   FRAME_ENCODING_ERROR (0x7):  An endpoint received a frame that was
      badly formatted.  For instance, a frame of an unknown type, or an
      ACK frame that has more acknowledgment ranges than the remainder
      of the packet could carry.

   TRANSPORT_PARAMETER_ERROR (0x8):  An endpoint received transport
      parameters that were badly formatted, included an invalid value,
      was absent even though it is mandatory, was present though it is
      forbidden, or is otherwise in error.

   VERSION_NEGOTIATION_ERROR (0x9):  An endpoint received transport
      parameters that contained version negotiation parameters that
      disagreed with the version negotiation that it performed.  This
      error code indicates a potential version downgrade attack.

   PROTOCOL_VIOLATION (0xA):  An endpoint detected an error with
      protocol compliance that was not covered by more specific error
      codes.

   INVALID_MIGRATION (0xC):  A peer has migrated to a different network
      when the endpoint had disabled migration.

   CRYPTO_ERROR (0x1XX):  The cryptographic handshake failed.  A range
      of 256 values is reserved for carrying error codes specific to the
      cryptographic handshake that is used.  Codes for errors occurring
      when TLS is used for the crypto handshake are described in
      Section 4.8 of [QUIC-TLS].

   See Section 13.3 22.3 for details of registering new error codes.

11.4.

20.1.  Application Protocol Error Codes

   Application protocol error codes are 16-bit unsigned integers, but
   the management of application error codes are left to application
   protocols.  Application protocol error codes are used for the
   RST_STREAM (Section 7.3) 19.2) and APPLICATION_CLOSE (Section 7.5) 19.4)
   frames.

   There is no restriction on the use of the 16-bit error code space for
   application protocols.  However, QUIC reserves the error code with a
   value of 0 to mean STOPPING.  The application error code of STOPPING
   (0) is used by the transport to cancel a stream in response to
   receipt of a STOP_SENDING frame.

12.

21.  Security Considerations

12.1.

21.1.  Handshake Denial of Service

   As an encrypted and authenticated transport QUIC provides a range of
   protections against denial of service.  Once the cryptographic
   handshake is complete, QUIC endpoints discard most packets that are
   not authenticated, greatly limiting the ability of an attacker to
   interfere with existing connections.

   Once a connection is established QUIC endpoints might accept some
   unauthenticated ICMP packets (see Section 8.4.1), 14.1.1), but the use of
   these packets is extremely limited.  The only other type of packet
   that an endpoint might accept is a stateless reset (Section 6.13.4) 10.4)
   which relies on the token being kept secret until it is used.

   During the creation of a connection, QUIC only provides protection
   against attack from off the network path.  All QUIC packets contain
   proof that the recipient saw a preceding packet from its peer.

   The first mechanism used is the source and destination connection
   IDs, which are required to match those set by a peer.  Except for an
   Initial and stateless reset packets, an endpoint only accepts packets
   that include a destination connection that matches a connection ID
   the endpoint previously chose.  This is the only protection offered
   for Version Negotiation packets.

   The destination connection ID in an Initial packet is selected by a
   client to be unpredictable, which serves an additional purpose.  The
   packets that carry the cryptographic handshake are protected with a
   key that is derived from this connection ID and salt specific to the
   QUIC version.  This allows endpoints to use the same process for
   authenticating packets that they receive as they use after the
   cryptographic handshake completes.  Packets that cannot be
   authenticated are discarded.  Protecting packets in this fashion
   provides a strong assurance that the sender of the packet saw the
   Initial packet and understood it.

   These protections are not intended to be effective against an
   attacker that is able to receive QUIC packets prior to the connection
   being established.  Such an attacker can potentially send packets
   that will be accepted by QUIC endpoints.  This version of QUIC
   attempts to detect this sort of attack, but it expects that endpoints
   will fail to establish a connection rather than recovering.  For the
   most part, the cryptographic handshake protocol [QUIC-TLS] is
   responsible for detecting tampering during the handshake, though
   additional validation is required for version negotiation (see
   Section 6.6.4). 7.3.3).

   Endpoints are permitted to use other methods to detect and attempt to
   recover from interference with the handshake.  Invalid packets may be
   identified and discarded using other methods, but no specific method
   is mandated in this document.

12.2.

21.2.  Spoofed ACK Attack

   An attacker might be able to receive an address validation token
   (Section 6.9) 8) from the server and then release the IP address it used
   to acquire that token.  The attacker may, in the future, spoof this
   same address (which now presumably addresses a different endpoint),
   and initiate a 0-RTT connection with a server on the victim's behalf.
   The attacker can then spoof ACK frames to the server which cause the
   server to send excessive amounts of data toward the new owner of the
   IP address.

   There are two possible mitigations to this attack.  The simplest one
   is that a server can unilaterally create a gap in packet-number
   space.  In the non-attack scenario, the client will send an ACK frame
   with the larger value for largest acknowledged.  In the attack
   scenario, the attacker could acknowledge a packet in the gap.  If the
   server sees an acknowledgment for a packet that was never sent, the
   connection can be aborted.

   The second mitigation is that the server can require that
   acknowledgments for sent packets match the encryption level of the
   sent packet.  This mitigation is useful if the connection has an
   ephemeral forward-secure key that is generated and used for every new
   connection.  If a packet sent is protected with a forward-secure key,
   then any acknowledgments that are received for them MUST also be
   forward-secure protected.  Since the attacker will not have the
   forward-secure key, the attacker will not be able to generate
   forward-secure protected packets with ACK frames.

12.3.

21.3.  Optimistic ACK Attack

   An endpoint that acknowledges packets it has not received might cause
   a congestion controller to permit sending at rates beyond what the
   network supports.  An endpoint MAY skip packet numbers when sending
   packets to detect this behavior.  An endpoint can then immediately
   close the connection with a connection error of type
   PROTOCOL_VIOLATION (see Section 6.13.3).

12.4. 10.3).

21.4.  Slowloris Attacks

   The attacks commonly known as Slowloris [SLOWLORIS] try to keep many
   connections to the target endpoint open and hold them open as long as
   possible.  These attacks can be executed against a QUIC endpoint by
   generating the minimum amount of activity necessary to avoid being
   closed for inactivity.  This might involve sending small amounts of
   data, gradually opening flow control windows in order to control the
   sender rate, or manufacturing ACK frames that simulate a high loss
   rate.

   QUIC deployments SHOULD provide mitigations for the Slowloris
   attacks, such as increasing the maximum number of clients the server
   will allow, limiting the number of connections a single IP address is
   allowed to make, imposing restrictions on the minimum transfer speed
   a connection is allowed to have, and restricting the length of time
   an endpoint is allowed to stay connected.

12.5.

21.5.  Stream Fragmentation and Reassembly Attacks

   An adversarial sender might intentionally send fragments of stream
   data in order to cause disproportionate receive buffer memory
   commitment and/or creation of a large and inefficient data structure.

   An adversarial receiver might intentionally not acknowledge packets
   containing stream data in order to force the sender to store the
   unacknowledged stream data for retransmission.

   The attack on receivers is mitigated if flow control windows
   correspond to available memory.  However, some receivers will over-
   commit memory and advertise flow control offsets in the aggregate
   that exceed actual available memory.  The over-commitment strategy
   can lead to better performance when endpoints are well behaved, but
   renders endpoints vulnerable to the stream fragmentation attack.

   QUIC deployments SHOULD provide mitigations against stream
   fragmentation attacks.  Mitigations could consist of avoiding over-
   committing memory, limiting the size of tracking data structures,
   delaying reassembly of STREAM frames, implementing heuristics based
   on the age and duration of reassembly holes, or some combination.

12.6.

21.6.  Stream Commitment Attack

   An adversarial endpoint can open lots of streams, exhausting state on
   an endpoint.  The adversarial endpoint could repeat the process on a
   large number of connections, in a manner similar to SYN flooding
   attacks in TCP.

   Normally, clients will open streams sequentially, as explained in
   Section 9.1. 2.1.  However, when several streams are initiated at short
   intervals, transmission error may cause STREAM DATA frames opening
   streams to be received out of sequence.  A receiver is obligated to
   open intervening streams if a higher-numbered stream ID is received.
   Thus, on a new connection, opening stream 2000001 opens 1 million
   streams, as required by the specification.

   The number of active streams is limited by the concurrent stream
   limit transport parameter, as explained in Section 9.4. 2.2.  If chosen
   judiciously, this limit mitigates the effect of the stream commitment
   attack.  However, setting the limit too low could affect performance
   when applications expect to open large number of streams.

12.7.

21.7.  Explicit Congestion Notification Attacks

   An on-path attacker could manipulate the value of ECN codepoints in
   the IP header to influence the sender's rate.  [RFC3168] discusses
   manipulations and their effects in more detail.

   An on-the-side attacker can duplicate and send packets with modified
   ECN codepoints to affect the sender's rate.  If duplicate packets are
   discarded by a receiver, an off-path attacker will need to race the
   duplicate packet against the original to be successful in this
   attack.  Therefore, QUIC receivers ignore ECN codepoints set in
   duplicate packets (see Section 6.8).

12.8. 13.3).

21.8.  Stateless Reset Oracle

   Stateless resets create a possible denial of service attack analogous
   to a TCP reset injection.  This attack is possible if an attacker is
   able to cause a stateless reset token to be generated for a
   connection with a selected connection ID.  An attacker that can cause
   this token to be generated can reset an active connection with the
   same connection ID.

   If a packet can be routed to different instances that share a static
   key, for example by changing an IP address or port, then an attacker
   can cause the server to send a stateless reset.  To defend against
   this style of denial service, endpoints that share a static key for
   stateless reset (see Section 6.13.4.2) 10.4.2) MUST be arranged so that packets
   with a given connection ID always arrive at an instance that has
   connection state, unless that connection is no longer active.

   In the case of a cluster that uses dynamic load balancing, it's
   possible that a change in load balancer configuration could happen
   while an active instance retains connection state; even if an
   instance retains connection state, the change in routing and
   resulting stateless reset will result in the connection being
   terminated.  If there is no chance in the packet being routed to the
   correct instance, it is better to send a stateless reset than wait
   for connections to time out.  However, this is acceptable only if the
   routing cannot be influenced by an attacker.

13.

22.  IANA Considerations

13.1.

22.1.  QUIC Transport Parameter Registry

   IANA [SHALL add/has added] a registry for "QUIC Transport Parameters"
   under a "QUIC Protocol" heading.

   The "QUIC Transport Parameters" registry governs a 16-bit space.
   This space is split into two spaces that are governed by different
   policies.  Values with the first byte in the range 0x00 to 0xfe (in
   hexadecimal) are assigned via the Specification Required policy
   [RFC8126].  Values with the first byte 0xff are reserved for Private
   Use [RFC8126].

   Registrations MUST include the following fields:

   Value:  The numeric value of the assignment (registrations will be
      between 0x0000 and 0xfeff).

   Parameter Name:  A short mnemonic for the parameter.

   Specification:  A reference to a publicly available specification for
      the value.

   The nominated expert(s) verify that a specification exists and is
   readily accessible.  Expert(s) are encouraged to be biased towards
   approving registrations unless they are abusive, frivolous, or
   actively harmful (not merely aesthetically displeasing, or
   architecturally dubious).

   The initial contents of this registry are shown in Table 7.

     +--------+-------------------------------------+---------------+
     | Value  | Parameter Name                      | Specification |
     +--------+-------------------------------------+---------------+
     | 0x0000 | initial_max_stream_data_bidi_local  | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0001 | initial_max_data                    | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0002 | initial_max_bidi_streams            | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0003 | idle_timeout                        | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0004 | preferred_address                   | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0005 | max_packet_size                     | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0006 | stateless_reset_token               | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0007 | ack_delay_exponent                  | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0008 | initial_max_uni_streams             | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x0009 | disable_migration                   | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x000a | initial_max_stream_data_bidi_remote | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x000b | initial_max_stream_data_uni         | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x000c | max_ack_delay                       | Section 6.6.1 18.1  |
     |        |                                     |               |
     | 0x000d | original_connection_id              | Section 6.6.1 18.1  |
     +--------+-------------------------------------+---------------+

            Table 7: Initial QUIC Transport Parameters Entries

13.2.

22.2.  QUIC Frame Type Registry

   IANA [SHALL add/has added] a registry for "QUIC Frame Types" under a
   "QUIC Protocol" heading.

   The "QUIC Frame Types" registry governs a 62-bit space.  This space
   is split into three spaces that are governed by different policies.
   Values between 0x00 and 0x3f (in hexadecimal) are assigned via the
   Standards Action or IESG Review policies [RFC8126].  Values from 0x40
   to 0x3fff operate on the Specification Required policy [RFC8126].
   All other values are assigned to Private Use [RFC8126].

   Registrations MUST include the following fields:

   Value:  The numeric value of the assignment (registrations will be
      between 0x00 and 0x3fff).  A range of values MAY be assigned.

   Frame Name:  A short mnemonic for the frame type.

   Specification:  A reference to a publicly available specification for
      the value.

   The nominated expert(s) verify that a specification exists and is
   readily accessible.  Specifications for new registrations need to
   describe the means by which an endpoint might determine that it can
   send the identified type of frame.  An accompanying transport
   parameter registration (see Section 13.1) 22.1) is expected for most
   registrations.  The specification needs to describe the format and
   assigned semantics of any fields in the frame.

   Expert(s) are encouraged to be biased towards approving registrations
   unless they are abusive, frivolous, or actively harmful (not merely
   aesthetically displeasing, or architecturally dubious).

   The initial contents of this registry are tabulated in Table 3.

13.3.

22.3.  QUIC Transport Error Codes Registry

   IANA [SHALL add/has added] a registry for "QUIC Transport Error
   Codes" under a "QUIC Protocol" heading.

   The "QUIC Transport Error Codes" registry governs a 16-bit space.
   This space is split into two spaces that are governed by different
   policies.  Values with the first byte in the range 0x00 to 0xfe (in
   hexadecimal) are assigned via the Specification Required policy
   [RFC8126].  Values with the first byte 0xff are reserved for Private
   Use [RFC8126].

   Registrations MUST include the following fields:

   Value:  The numeric value of the assignment (registrations will be
      between 0x0000 and 0xfeff).

   Code:  A short mnemonic for the parameter.

   Description:  A brief description of the error code semantics, which
      MAY be a summary if a specification reference is provided.

   Specification:  A reference to a publicly available specification for
      the value.

   The initial contents of this registry are shown in Table 8.  Values
   from 0xFF00 to 0xFFFF are reserved for Private Use [RFC8126].

   +------+---------------------------+----------------+---------------+
   | Valu | Error                     | Description    | Specification |
   | e    |                           |                |               |
   +------+---------------------------+----------------+---------------+
   | 0x0  | NO_ERROR                  | No error       | Section 11.3 20    |
   |      |                           |                |               |
   | 0x1  | INTERNAL_ERROR            | Implementation | Section 11.3 20    |
   |      |                           | error          |               |
   |      |                           |                |               |
   | 0x2  | SERVER_BUSY               | Server         | Section 11.3 20    |
   |      |                           | currently busy |               |
   |      |                           |                |               |
   | 0x3  | FLOW_CONTROL_ERROR        | Flow control   | Section 11.3 20    |
   |      |                           | error          |               |
   |      |                           |                |               |
   | 0x4  | STREAM_ID_ERROR           | Invalid stream | Section 11.3 20    |
   |      |                           | ID             |               |
   |      |                           |                |               |
   | 0x5  | STREAM_STATE_ERROR        | Frame received | Section 11.3 20    |
   |      |                           | in invalid     |               |
   |      |                           | stream state   |               |
   |      |                           |                |               |
   | 0x6  | FINAL_OFFSET_ERROR        | Change to      | Section 11.3 20    |
   |      |                           | final stream   |               |
   |      |                           | offset         |               |
   |      |                           |                |               |
   | 0x7  | FRAME_ENCODING_ERROR      | Frame encoding | Section 11.3 20    |
   |      |                           | error          |               |
   |      |                           |                |               |
   | 0x8  | TRANSPORT_PARAMETER_ERROR | Error in       | Section 11.3 20    |
   |      |                           | transport      |               |
   |      |                           | parameters     |               |
   |      |                           |                |               |
   | 0x9  | VERSION_NEGOTIATION_ERROR | Version        | Section 11.3 20    |
   |      |                           | negotiation    |               |
   |      |                           | failure        |               |
   |      |                           |                |               |
   | 0xA  | PROTOCOL_VIOLATION        | Generic        | Section 11.3 20    |
   |      |                           | protocol       |               |
   |      |                           | violation      |               |
   |      |                           |                |               |
   | 0xC  | INVALID_MIGRATION         | Violated       | Section 11.3 20    |
   |      |                           | disabled       |               |
   |      |                           | migration      |               |
   +------+---------------------------+----------------+---------------+

            Table 8: Initial QUIC Transport Error Codes Entries

14.

23.  References

14.1.

23.1.  Normative References

   [PLPMTUD]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [PMTUDv4]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [PMTUDv6]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [QUIC-RECOVERY]
              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", draft-ietf-quic-recovery-15 draft-ietf-quic-recovery-16 (work
              in progress), October 2018.

   [QUIC-TLS]
              Thomson, M., Ed. and S. Turner, Ed., "Using Transport
              Layer Security (TLS) to Secure QUIC", draft-ietf-quic-
              tls-15
              tls-16 (work in progress), October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC5119]  Edwards, T., "A Uniform Resource Name (URN) Namespace for
              the Society of Motion Picture and Television Engineers
              (SMPTE)", RFC 5119, DOI 10.17487/RFC5119, February 2008,
              <https://www.rfc-editor.org/info/rfc5119>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

14.2.

23.2.  Informative References

   [EARLY-DESIGN]
              Roskind, J., "QUIC: Multiplexed Transport Over UDP",
              December 2013, <https://goo.gl/dMVtFi>.

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [QUIC-INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              draft-ietf-quic-invariants-03 (work in progress), October
              2018.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,
              <https://www.rfc-editor.org/info/rfc2018>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2360]  Scott, G., "Guide for Internet Standards Writers", BCP 22,
              RFC 2360, DOI 10.17487/RFC2360, June 1998,
              <https://www.rfc-editor.org/info/rfc2360>.

   [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, DOI 10.17487/RFC2406, November
              1998, <https://www.rfc-editor.org/info/rfc2406>.

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <https://www.rfc-editor.org/info/rfc4787>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [SLOWLORIS]
              RSnake Hansen, R., "Welcome to Slowloris...", June 2009,
              <https://web.archive.org/web/20150315054838/
              http://ha.ckers.org/slowloris/>.

   [SST]      Ford, B., "Structured streams", ACM SIGCOMM Computer
              Communication Review Vol. 37, pp. 361,
              DOI 10.1145/1282427.1282421, October 2007.

Appendix A.  Sample Packet Number Decoding Algorithm

   The following pseudo-code shows how an implementation can decode
   packet numbers after packet number protection has been removed.

   DecodePacketNumber(largest_pn, truncated_pn, pn_nbits):
      expected_pn  = largest_pn + 1
      pn_win       = 1 << pn_nbits
      pn_hwin      = pn_win / 2
      pn_mask      = pn_win - 1
      // The incoming packet number should be greater than
      // expected_pn - pn_hwin and less than or equal to
      // expected_pn + pn_hwin
      //
      // This means we can't just strip the trailing bits from
      // expected_pn and add the truncated_pn because that might
      // yield a value outside the window.
      //
      // The following code calculates a candidate value and
      // makes sure it's within the packet number window.
      candidate_pn = (expected_pn & ~pn_mask) | truncated_pn
      if candidate_pn <= expected_pn - pn_hwin:
         return candidate_pn + pn_win
      // Note the extra check for underflow when candidate_pn
      // is near zero.
      if candidate_pn > expected_pn + pn_hwin and
         candidate_pn > pn_win:
         return candidate_pn - pn_win
      return candidate_pn

Appendix B.  Change Log

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

   Issue and pull request numbers are listed with a leading octothorp.

B.1.  Since draft-ietf-quic-transport-15

   Substantial editorial reorganization; no technical changes.

B.2.  Since draft-ietf-quic-transport-14

   o  Merge ACK and ACK_ECN (#1778, #1801)

   o  Explicitly communicate max_ack_delay (#981, #1781)

   o  Validate original connection ID after Retry packets (#1710, #1486,
      #1793)

   o  Idle timeout is optional and has no specified maximum (#1520,
      #1521) (#1765)
   o  Update connection ID handling; add RETIRE_CONNECTION_ID type
      (#1464, #1468, #1483, #1484, #1486, #1495, #1729, #1742, #1799,
      #1821)

   o  Include a Token in all Initial packets (#1649, #1794)

   o  Prevent handshake deadlock (#1764, #1824)

B.2.

B.3.  Since draft-ietf-quic-transport-13

   o  Streams open when higher-numbered streams of the same type open
      (#1342, #1549)

   o  Split initial stream flow control limit into 3 transport
      parameters (#1016, #1542)

   o  All flow control transport parameters are optional (#1610)

   o  Removed UNSOLICITED_PATH_RESPONSE error code (#1265, #1539)

   o  Permit stateless reset in response to any packet (#1348, #1553)

   o  Recommended defense against stateless reset spoofing (#1386,
      #1554)

   o  Prevent infinite stateless reset exchanges (#1443, #1627)

   o  Forbid processing of the same packet number twice (#1405, #1624)

   o  Added a packet number decoding example (#1493)

   o  More precisely define idle timeout (#1429, #1614, #1652)

   o  Corrected format of Retry packet and prevented looping (#1492,
      #1451, #1448, #1498)

   o  Permit 0-RTT after receiving Version Negotiation or Retry (#1507,
      #1514, #1621)

   o  Permit Retry in response to 0-RTT (#1547, #1552)

   o  Looser verification of ECN counters to account for ACK loss
      (#1555, #1481, #1565)

   o  Remove frame type field from APPLICATION_CLOSE (#1508, #1528)

B.3.

B.4.  Since draft-ietf-quic-transport-12

   o  Changes to integration of the TLS handshake (#829, #1018, #1094,
      #1165, #1190, #1233, #1242, #1252, #1450, #1458)

      *  The cryptographic handshake uses CRYPTO frames, not stream 0

      *  QUIC packet protection is used in place of TLS record
         protection

      *  Separate QUIC packet number spaces are used for the handshake

      *  Changed Retry to be independent of the cryptographic handshake

      *  Added NEW_TOKEN frame and Token fields to Initial packet

      *  Limit the use of HelloRetryRequest to address TLS needs (like
         key shares)

   o  Enable server to transition connections to a preferred address
      (#560, #1251, #1373)

   o  Added ECN feedback mechanisms and handling; new ACK_ECN frame
      (#804, #805, #1372)

   o  Changed rules and recommendations for use of new connection IDs
      (#1258, #1264, #1276, #1280, #1419, #1452, #1453, #1465)

   o  Added a transport parameter to disable intentional connection
      migration (#1271, #1447)

   o  Packets from different connection ID can't be coalesced (#1287,
      #1423)

   o  Fixed sampling method for packet number encryption; the length
      field in long headers includes the packet number field in addition
      to the packet payload (#1387, #1389)

   o  Stateless Reset is now symmetric and subject to size constraints
      (#466, #1346)

   o  Added frame type extension mechanism (#58, #1473)

B.4.

B.5.  Since draft-ietf-quic-transport-11

   o  Enable server to transition connections to a preferred address
      (#560, #1251)

   o  Packet numbers are encrypted (#1174, #1043, #1048, #1034, #850,
      #990, #734, #1317, #1267, #1079)

   o  Packet numbers use a variable-length encoding (#989, #1334)

   o  STREAM frames can now be empty (#1350)

B.5.

B.6.  Since draft-ietf-quic-transport-10

   o  Swap payload length and packed number fields in long header
      (#1294)

   o  Clarified that CONNECTION_CLOSE is allowed in Handshake packet
      (#1274)

   o  Spin bit reserved (#1283)

   o  Coalescing multiple QUIC packets in a UDP datagram (#1262, #1285)

   o  A more complete connection migration (#1249)

   o  Refine opportunistic ACK defense text (#305, #1030, #1185)

   o  A Stateless Reset Token isn't mandatory (#818, #1191)

   o  Removed implicit stream opening (#896, #1193)

   o  An empty STREAM frame can be used to open a stream without sending
      data (#901, #1194)

   o  Define stream counts in transport parameters rather than a maximum
      stream ID (#1023, #1065)

   o  STOP_SENDING is now prohibited before streams are used (#1050)

   o  Recommend including ACK in Retry packets and allow PADDING (#1067,
      #882)

   o  Endpoints now become closing after an idle timeout (#1178, #1179)

   o  Remove implication that Version Negotiation is sent when a packet
      of the wrong version is received (#1197)

B.6.

B.7.  Since draft-ietf-quic-transport-09

   o  Added PATH_CHALLENGE and PATH_RESPONSE frames to replace PING with
      Data and PONG frame.  Changed ACK frame type from 0x0e to 0x0d.
      (#1091, #725, #1086)

   o  A server can now only send 3 packets without validating the client
      address (#38, #1090)

   o  Delivery order of stream data is no longer strongly specified
      (#252, #1070)

   o  Rework of packet handling and version negotiation (#1038)

   o  Stream 0 is now exempt from flow control until the handshake
      completes (#1074, #725, #825, #1082)

   o  Improved retransmission rules for all frame types: information is
      retransmitted, not packets or frames (#463, #765, #1095, #1053)

   o  Added an error code for server busy signals (#1137)

   o  Endpoints now set the connection ID that their peer uses.
      Connection IDs are variable length.  Removed the
      omit_connection_id transport parameter and the corresponding short
      header flag. (#1089, #1052, #1146, #821, #745, #821, #1166, #1151)

B.7.

B.8.  Since draft-ietf-quic-transport-08

   o  Clarified requirements for BLOCKED usage (#65, #924)

   o  BLOCKED frame now includes reason for blocking (#452, #924, #927,
      #928)

   o  GAP limitation in ACK Frame (#613)

   o  Improved PMTUD description (#614, #1036)

   o  Clarified stream state machine (#634, #662, #743, #894)

   o  Reserved versions don't need to be generated deterministically
      (#831, #931)

   o  You don't always need the draining period (#871)

   o  Stateless reset clarified as version-specific (#930, #986)

   o  initial_max_stream_id_x transport parameters are optional (#970,
      #971)

   o  Ack Delay assumes a default value during the handshake (#1007,
      #1009)

   o  Removed transport parameters from NewSessionTicket (#1015)

B.8.

B.9.  Since draft-ietf-quic-transport-07

   o  The long header now has version before packet number (#926, #939)

   o  Rename and consolidate packet types (#846, #822, #847)

   o  Packet types are assigned new codepoints and the Connection ID
      Flag is inverted (#426, #956)

   o  Removed type for Version Negotiation and use Version 0 (#963,
      #968)

   o  Streams are split into unidirectional and bidirectional (#643,
      #656, #720, #872, #175, #885)

      *  Stream limits now have separate uni- and bi-directional
         transport parameters (#909, #958)

      *  Stream limit transport parameters are now optional and default
         to 0 (#970, #971)

   o  The stream state machine has been split into read and write (#634,
      #894)

   o  Employ variable-length integer encodings throughout (#595)

   o  Improvements to connection close

      *  Added distinct closing and draining states (#899, #871)

      *  Draining period can terminate early (#869, #870)

      *  Clarifications about stateless reset (#889, #890)

   o  Address validation for connection migration (#161, #732, #878)

   o  Clearly defined retransmission rules for BLOCKED (#452, #65, #924)

   o  negotiated_version is sent in server transport parameters (#710,
      #959)

   o  Increased the range over which packet numbers are randomized
      (#864, #850, #964)

B.9.

B.10.  Since draft-ietf-quic-transport-06

   o  Replaced FNV-1a with AES-GCM for all "Cleartext" packets (#554)

   o  Split error code space between application and transport (#485)

   o  Stateless reset token moved to end (#820)

   o  1-RTT-protected long header types removed (#848)

   o  No acknowledgments during draining period (#852)

   o  Remove "application close" as a separate close type (#854)

   o  Remove timestamps from the ACK frame (#841)

   o  Require transport parameters to only appear once (#792)

B.10.

B.11.  Since draft-ietf-quic-transport-05

   o  Stateless token is server-only (#726)

   o  Refactor section on connection termination (#733, #748, #328,
      #177)

   o  Limit size of Version Negotiation packet (#585)

   o  Clarify when and what to ack (#736)

   o  Renamed STREAM_ID_NEEDED to STREAM_ID_BLOCKED

   o  Clarify Keep-alive requirements (#729)

B.11.

B.12.  Since draft-ietf-quic-transport-04

   o  Introduce STOP_SENDING frame, RST_STREAM only resets in one
      direction (#165)

   o  Removed GOAWAY; application protocols are responsible for graceful
      shutdown (#696)

   o  Reduced the number of error codes (#96, #177, #184, #211)

   o  Version validation fields can't move or change (#121)

   o  Removed versions from the transport parameters in a
      NewSessionTicket message (#547)

   o  Clarify the meaning of "bytes in flight" (#550)

   o  Public reset is now stateless reset and not visible to the path
      (#215)

   o  Reordered bits and fields in STREAM frame (#620)

   o  Clarifications to the stream state machine (#572, #571)

   o  Increased the maximum length of the Largest Acknowledged field in
      ACK frames to 64 bits (#629)

   o  truncate_connection_id is renamed to omit_connection_id (#659)

   o  CONNECTION_CLOSE terminates the connection like TCP RST (#330,
      #328)

   o  Update labels used in HKDF-Expand-Label to match TLS 1.3 (#642)

B.12.

B.13.  Since draft-ietf-quic-transport-03

   o  Change STREAM and RST_STREAM layout

   o  Add MAX_STREAM_ID settings

B.13.

B.14.  Since draft-ietf-quic-transport-02

   o  The size of the initial packet payload has a fixed minimum (#267,
      #472)

   o  Define when Version Negotiation packets are ignored (#284, #294,
      #241, #143, #474)

   o  The 64-bit FNV-1a algorithm is used for integrity protection of
      unprotected packets (#167, #480, #481, #517)

   o  Rework initial packet types to change how the connection ID is
      chosen (#482, #442, #493)

   o  No timestamps are forbidden in unprotected packets (#542, #429)

   o  Cryptographic handshake is now on stream 0 (#456)

   o  Remove congestion control exemption for cryptographic handshake
      (#248, #476)

   o  Version 1 of QUIC uses TLS; a new version is needed to use a
      different handshake protocol (#516)

   o  STREAM frames have a reduced number of offset lengths (#543, #430)

   o  Split some frames into separate connection- and stream- level
      frames (#443)

      *  WINDOW_UPDATE split into MAX_DATA and MAX_STREAM_DATA (#450)

      *  BLOCKED split to match WINDOW_UPDATE split (#454)

      *  Define STREAM_ID_NEEDED frame (#455)

   o  A NEW_CONNECTION_ID frame supports connection migration without
      linkability (#232, #491, #496)

   o  Transport parameters for 0-RTT are retained from a previous
      connection (#405, #513, #512)

      *  A client in 0-RTT no longer required to reset excess streams
         (#425, #479)

   o  Expanded security considerations (#440, #444, #445, #448)

B.14.

B.15.  Since draft-ietf-quic-transport-01

   o  Defined short and long packet headers (#40, #148, #361)

   o  Defined a versioning scheme and stable fields (#51, #361)

   o  Define reserved version values for "greasing" negotiation (#112,
      #278)

   o  The initial packet number is randomized (#35, #283)

   o  Narrow the packet number encoding range requirement (#67, #286,
      #299, #323, #356)

   o  Defined client address validation (#52, #118, #120, #275)

   o  Define transport parameters as a TLS extension (#49, #122)

   o  SCUP and COPT parameters are no longer valid (#116, #117)

   o  Transport parameters for 0-RTT are either remembered from before,
      or assume default values (#126)

   o  The server chooses connection IDs in its final flight (#119, #349,
      #361)

   o  The server echoes the Connection ID and packet number fields when
      sending a Version Negotiation packet (#133, #295, #244)

   o  Defined a minimum packet size for the initial handshake packet
      from the client (#69, #136, #139, #164)

   o  Path MTU Discovery (#64, #106)

   o  The initial handshake packet from the client needs to fit in a
      single packet (#338)

   o  Forbid acknowledgment of packets containing only ACK and PADDING
      (#291)

   o  Require that frames are processed when packets are acknowledged
      (#381, #341)

   o  Removed the STOP_WAITING frame (#66)

   o  Don't require retransmission of old timestamps for lost ACK frames
      (#308)

   o  Clarified that frames are not retransmitted, but the information
      in them can be (#157, #298)

   o  Error handling definitions (#335)

   o  Split error codes into four sections (#74)

   o  Forbid the use of Public Reset where CONNECTION_CLOSE is possible
      (#289)

   o  Define packet protection rules (#336)

   o  Require that stream be entirely delivered or reset, including
      acknowledgment of all STREAM frames or the RST_STREAM, before it
      closes (#381)

   o  Remove stream reservation from state machine (#174, #280)

   o  Only stream 1 does not contribute to connection-level flow control
      (#204)

   o  Stream 1 counts towards the maximum concurrent stream limit (#201,
      #282)

   o  Remove connection-level flow control exclusion for some streams
      (except 1) (#246)

   o  RST_STREAM affects connection-level flow control (#162, #163)

   o  Flow control accounting uses the maximum data offset on each
      stream, rather than bytes received (#378)

   o  Moved length-determining fields to the start of STREAM and ACK
      (#168, #277)

   o  Added the ability to pad between frames (#158, #276)

   o  Remove error code and reason phrase from GOAWAY (#352, #355)

   o  GOAWAY includes a final stream number for both directions (#347)

   o  Error codes for RST_STREAM and CONNECTION_CLOSE are now at a
      consistent offset (#249)

   o  Defined priority as the responsibility of the application protocol
      (#104, #303)

B.15.

B.16.  Since draft-ietf-quic-transport-00

   o  Replaced DIVERSIFICATION_NONCE flag with KEY_PHASE flag

   o  Defined versioning

   o  Reworked description of packet and frame layout

   o  Error code space is divided into regions for each component

   o  Use big endian for all numeric values

B.16.

B.17.  Since draft-hamilton-quic-transport-protocol-01

   o  Adopted as base for draft-ietf-quic-tls

   o  Updated authors/editors list

   o  Added IANA Considerations section

   o  Moved Contributors and Acknowledgments to appendices

Acknowledgments

   Special thanks are due to the following for helping shape pre-IETF
   QUIC and its deployment: Chris Bentzel, Misha Efimov, Roberto Peon,
   Alistair Riddoch, Siddharth Vijayakrishnan, and Assar Westerlund.

   This document has benefited immensely from various private
   discussions and public ones on the quic@ietf.org and proto-
   quic@chromium.org mailing lists.  Our thanks to all.

Contributors

   The original authors of this specification were Ryan Hamilton, Jana
   Iyengar, Ian Swett, and Alyssa Wilk.

   The original design and rationale behind this protocol draw
   significantly from work by Jim Roskind [EARLY-DESIGN].  In
   alphabetical order, the contributors to the pre-IETF QUIC project at
   Google are: Britt Cyr, Jeremy Dorfman, Ryan Hamilton, Jana Iyengar,
   Fedor Kouranov, Charles Krasic, Jo Kulik, Adam Langley, Jim Roskind,
   Robbie Shade, Satyam Shekhar, Cherie Shi, Ian Swett, Raman Tenneti,
   Victor Vasiliev, Antonio Vicente, Patrik Westin, Alyssa Wilk, Dale
   Worley, Fan Yang, Dan Zhang, Daniel Ziegler.

Authors' Addresses

   Jana Iyengar (editor)
   Fastly

   Email: jri.ietf@gmail.com

   Martin Thomson (editor)
   Mozilla

   Email: martin.thomson@gmail.com