QUIC                                                     J. Iyengar, Ed.
Internet-Draft                                                    Fastly
Intended status: Standards Track                           I. Swett, Ed.
Expires: April 26, June 21, 2019                                            Google
                                                        October 23,
                                                       December 18, 2018

               QUIC Loss Detection and Congestion Control
                      draft-ietf-quic-recovery-16
                      draft-ietf-quic-recovery-17

Abstract

   This document describes loss detection and congestion control
   mechanisms for QUIC.

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 [1].

   Working Group information can be found at https://github.com/quicwg
   [2]; source code and issues list for this draft can be found at
   https://github.com/quicwg/base-drafts/labels/-recovery [3].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 26, June 21, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Design of the QUIC Transmission Machinery . . . . . . . . . .   4
     3.1.  Relevant Differences Between QUIC and TCP . . . . . . . .   5
       3.1.1.  Separate Packet Number Spaces . . . . . . . . . . . .   5
       3.1.2.  Monotonically Increasing Packet Numbers . . . . . . .   6
       3.1.3.  No Reneging . . . . . . . . . . . . . . . . . . . . .   6
       3.1.4.  More ACK Ranges . . . . . . . . . . . . . . . . . . .   6
       3.1.5.  Explicit Correction For Delayed ACKs  . . . . . . . .   6
   4.  Loss Detection  Generating Acknowledgements . . . . . . . . . . . . . . . . .   7
     4.1.  Crypto Handshake Data . . . . . . .   7
     4.1.  Computing the RTT estimate . . . . . . . . . . .   7
     4.2.  ACK Ranges  . . . .   7
     4.2.  Ack-based Detection . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Fast Retransmit
     4.3.  Receiver Tracking of ACK Frames . . . . . . . . . . . . .   8
   5.  Computing the RTT estimate  . . . . . . . .   7
       4.2.2.  Early Retransmit . . . . . . . . .   8
   6.  Loss Detection  . . . . . . . . . . . . . . . . . .   8
     4.3.  Timer-based Detection . . . . .   9
     6.1.  Acknowledgement-based Detection . . . . . . . . . . . . .   9
       4.3.1.  Crypto Retransmission Timeout
       6.1.1.  Packet Threshold  . . . . . . . . . . . . . . . . . .   9
       4.3.2.  Tail Loss Probe
       6.1.2.  Time Threshold  . . . . . . . . . . . . . . . . . . .  10
       4.3.3.  Retransmission
     6.2.  Timeout Loss Detection  . . . . . . . . . . . . . . .  11
     4.4.  Generating Acknowledgements . .  10
       6.2.1.  Crypto Retransmission Timeout . . . . . . . . . . . .  10
       6.2.2.  Probe Timeout .  12
       4.4.1.  Crypto Handshake Data . . . . . . . . . . . . . . . .  13
       4.4.2.  ACK Ranges . . .  12
     6.3.  Tracking Sent Packets . . . . . . . . . . . . . . . . . .  13
       4.4.3.  Receiver Tracking of ACK Frames
       6.3.1.  Sent Packet Fields  . . . . . . . . . . .  13
     4.5. . . . . . .  14
     6.4.  Pseudocode  . . . . . . . . . . . . . . . . . . . . . . .  14
       4.5.1.
       6.4.1.  Constants of interest . . . . . . . . . . . . . . . .  14
       4.5.2.
       6.4.2.  Variables of interest . . . . . . . . . . . . . . . .  14
       4.5.3.  15
       6.4.3.  Initialization  . . . . . . . . . . . . . . . . . . .  16
       4.5.4.
       6.4.4.  On Sending a Packet . . . . . . . . . . . . . . . . .  16
       4.5.5.
       6.4.5.  On Receiving an Acknowledgment  . . . . . . . . . . .  17
       4.5.6.  16
       6.4.6.  On Packet Acknowledgment  . . . . . . . . . . . . . .  19
       4.5.7.  18
       6.4.7.  Setting the Loss Detection Timer  . . . . . . . . . .  19
       4.5.8.  18
       6.4.8.  On Timeout  . . . . . . . . . . . . . . . . . . . . .  20
       4.5.9.  19
       6.4.9.  Detecting Lost Packets  . . . . . . . . . . . . . . .  21
     4.6.  20
     6.5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  22
   5.  21
   7.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  22
     5.1.
     7.1.  Explicit Congestion Notification  . . . . . . . . . . . .  23
     5.2.  22
     7.2.  Slow Start  . . . . . . . . . . . . . . . . . . . . . . .  23
     5.3.  22
     7.3.  Congestion Avoidance  . . . . . . . . . . . . . . . . . .  23
     5.4.  22
     7.4.  Recovery Period . . . . . . . . . . . . . . . . . . . . .  23
     5.5.  Tail Loss
     7.5.  Probe Timeout . . . . . . . . . . . . . . . . . . . . .  24
     5.6.  Retransmission Timeout .  23
     7.6.  Pacing  . . . . . . . . . . . . . . . . . . .  24
     5.7.  Pacing . . . . . .  23
     7.7.  Sending data after an idle period . . . . . . . . . . . .  24
     7.8.  Discarding Packet Number Space State  . . . . . . . . . .  24
     5.8.
     7.9.  Pseudocode  . . . . . . . . . . . . . . . . . . . . . . .  25
       5.8.1.  24
       7.9.1.  Constants of interest . . . . . . . . . . . . . . . .  25
       5.8.2.  24
       7.9.2.  Variables of interest . . . . . . . . . . . . . . . .  25
       5.8.3.
       7.9.3.  Initialization  . . . . . . . . . . . . . . . . . . .  26
       5.8.4.
       7.9.4.  On Packet Sent  . . . . . . . . . . . . . . . . . . .  26
       5.8.5.
       7.9.5.  On Packet Acknowledgement . . . . . . . . . . . . . .  26
       5.8.6.
       7.9.6.  On New Congestion Event . . . . . . . . . . . . . . .  27
       5.8.7.  26
       7.9.7.  Process ECN Information . . . . . . . . . . . . . . .  27
       5.8.8.
       7.9.8.  On Packets Lost . . . . . . . . . . . . . . . . . . .  27
       5.8.9.  On Retransmission Timeout Verified  . . . . . . . . .  28
   6.
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     6.1.  27
     8.1.  Congestion Signals  . . . . . . . . . . . . . . . . . . .  28
     6.2.
     8.2.  Traffic Analysis  . . . . . . . . . . . . . . . . . . . .  28
     6.3.
     8.3.  Misreporting ECN Markings . . . . . . . . . . . . . . . .  28
   7.
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   8.  28
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.1.  28
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     8.2.
     10.2.  Informative References . . . . . . . . . . . . . . . . .  29
     8.3.
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  30  31
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  31
     A.1.  Since draft-ietf-quic-recovery-14 draft-ietf-quic-recovery-16 . . . . . . . . . . . .  31
     A.2.  Since draft-ietf-quic-recovery-13 draft-ietf-quic-recovery-14 . . . . . . . . . . . .  31  32
     A.3.  Since draft-ietf-quic-recovery-12 draft-ietf-quic-recovery-13 . . . . . . . . . . . .  31  32
     A.4.  Since draft-ietf-quic-recovery-11 draft-ietf-quic-recovery-12 . . . . . . . . . . . .  31  32
     A.5.  Since draft-ietf-quic-recovery-10 draft-ietf-quic-recovery-11 . . . . . . . . . . . .  31  32
     A.6.  Since draft-ietf-quic-recovery-09 draft-ietf-quic-recovery-10 . . . . . . . . . . . .  32
     A.7.  Since draft-ietf-quic-recovery-08 draft-ietf-quic-recovery-09 . . . . . . . . . . . .  32  33
     A.8.  Since draft-ietf-quic-recovery-07 draft-ietf-quic-recovery-08 . . . . . . . . . . . .  32  33
     A.9.  Since draft-ietf-quic-recovery-06 draft-ietf-quic-recovery-07 . . . . . . . . . . . .  32  33
     A.10. Since draft-ietf-quic-recovery-05 draft-ietf-quic-recovery-06 . . . . . . . . . . . .  32  33
     A.11. Since draft-ietf-quic-recovery-04 draft-ietf-quic-recovery-05 . . . . . . . . . . . .  32  33
     A.12. Since draft-ietf-quic-recovery-03 draft-ietf-quic-recovery-04 . . . . . . . . . . . .  32  33
     A.13. Since draft-ietf-quic-recovery-02 draft-ietf-quic-recovery-03 . . . . . . . . . . . .  32  33
     A.14. Since draft-ietf-quic-recovery-01 draft-ietf-quic-recovery-02 . . . . . . . . . . . .  33
     A.15. Since draft-ietf-quic-recovery-00 draft-ietf-quic-recovery-01 . . . . . . . . . . . .  33  34
     A.16. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . .  34
     A.17. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . .  33  34
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  33  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33  34

1.  Introduction

   QUIC is a new multiplexed and secure transport atop UDP.  QUIC builds
   on decades of transport and security experience, and implements
   mechanisms that make it attractive as a modern general-purpose
   transport.  The QUIC protocol is described in [QUIC-TRANSPORT].

   QUIC implements the spirit of known TCP loss recovery mechanisms,
   described in RFCs, various Internet-drafts, and also those prevalent
   in the Linux TCP implementation.  This document describes QUIC
   congestion control and loss recovery, and where applicable,
   attributes the TCP equivalent in RFCs, Internet-drafts, academic
   papers, and/or TCP implementations.

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:

   ACK-only:  Any packet containing only an one or more ACK frame. frame(s).

   In-flight:  Packets are considered in-flight when they have been sent
      and neither acknowledged nor declared lost, and they are not ACK-
      only.

   Retransmittable

   Ack-eliciting Frames:  All frames besides ACK or PADDING are
      considered retransmittable.

   Retransmittable ack-eliciting.

   Ack-eliciting Packets:  Packets that contain retransmittable ack-eliciting frames
      elicit an ACK from the receiver within the maximum ack delay and
      are called retransmittable ack-eliciting packets.

   Crypto Packets:  Packets containing CRYPTO data sent in Initial or
      Handshake packets.

3.  Design of the QUIC Transmission Machinery

   All transmissions in QUIC are sent with a packet-level header, which
   indicates the encryption level and includes a packet sequence number
   (referred to below as a packet number).  The encryption level
   indicates the packet number space, as described in [QUIC-TRANSPORT].
   Packet numbers never repeat within a packet number space for the
   lifetime of a connection.  Packet numbers monotonically increase
   within a space, preventing ambiguity.

   This design obviates the need for disambiguating between
   transmissions and retransmissions and eliminates significant
   complexity from QUIC's interpretation of TCP loss detection
   mechanisms.

   QUIC packets can contain multiple frames of different types.  The
   recovery mechanisms ensure that data and frames that need reliable
   delivery are acknowledged or declared lost and sent in new packets as
   necessary.  The types of frames contained in a packet affect recovery
   and congestion control logic:

   o  All packets are acknowledged, though packets that contain only ACK
      and PADDING no ack-
      eliciting frames are not only acknowledged immediately. along with ack-eliciting
      packets.

   o  Long header packets that contain CRYPTO frames are critical to the
      performance of the QUIC handshake and use shorter timers for
      acknowledgement and retransmission.

   o  Packets that contain only ACK frames do not count toward
      congestion control limits and are not considered in-flight.  Note
      that this means PADDING frames cause packets to contribute toward
      bytes in flight without directly causing an acknowledgment to be
      sent.

3.1.  Relevant Differences Between QUIC and TCP

   Readers familiar with TCP's loss detection and congestion control
   will find algorithms here that parallel well-known TCP ones.
   Protocol differences between QUIC and TCP however contribute to
   algorithmic differences.  We briefly describe these protocol
   differences below.

3.1.1.  Separate Packet Number Spaces

   QUIC uses separate packet number spaces for each encryption level,
   except 0-RTT and all generations of 1-RTT keys use the same packet
   number space.  Separate packet number spaces ensures acknowledgement
   of packets sent with one level of encryption will not cause spurious
   retransmission of packets sent with a different encryption level.
   Congestion control and RTT measurement are unified across packet
   number spaces.

3.1.2.  Monotonically Increasing Packet Numbers

   TCP conflates transmission sequence number order at the sender with delivery sequence number order at
   the receiver, which results in retransmissions of the same data
   carrying the same sequence number, and consequently leads to problems caused by
   "retransmission ambiguity".  QUIC separates the two: QUIC uses a
   packet number for transmissions, to indicate transmission order, and any application
   data is sent in one or more streams, with delivery order determined
   by stream offsets encoded within STREAM frames.

   QUIC's packet number is strictly increasing, increasing within a packet number
   space, and directly encodes transmission order.  A higher QUIC packet
   number signifies that the packet was sent later, and a lower QUIC packet
   number signifies that the packet was sent earlier.  When a packet
   containing ack-eliciting frames is
   deemed detected lost, QUIC rebundles
   necessary frames in a new packet with a new packet number, removing
   ambiguity about which packet is acknowledged when an ACK is received.
   Consequently, more accurate RTT measurements can be made, spurious
   retransmissions are trivially detected, and mechanisms such as Fast
   Retransmit can be applied universally, based only on packet number.

   This design point significantly simplifies loss detection mechanisms
   for QUIC.  Most TCP mechanisms implicitly attempt to infer
   transmission ordering based on TCP sequence numbers - a non-trivial
   task, especially when TCP timestamps are not available.

3.1.3.  No Reneging

   QUIC ACKs contain information that is similar to TCP SACK, but QUIC
   does not allow any acked packet to be reneged, greatly simplifying
   implementations on both sides and reducing memory pressure on the
   sender.

3.1.4.  More ACK Ranges

   QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges.  In
   high loss environments, this speeds recovery, reduces spurious
   retransmits, and ensures forward progress without relying on
   timeouts.

3.1.5.  Explicit Correction For Delayed ACKs

   QUIC ACKs explicitly encode the delay incurred at the receiver
   between when a packet is received and when the corresponding ACK is
   sent.  This allows the receiver of the ACK to adjust for receiver
   delays, specifically the delayed ack timer, when estimating the path
   RTT.  This mechanism also allows a receiver to measure and report the
   delay from when a packet was received by the OS kernel, which is
   useful in receivers which may incur delays such as context-switch
   latency before a userspace QUIC receiver processes a received packet.

4.  Loss Detection  Generating Acknowledgements

   QUIC senders use both ack information and timeouts SHOULD delay sending acknowledgements in response to detect lost packets, and this section provides a description
   but MUST NOT excessively delay acknowledgements of these algorithms.
   Estimating ack-eliciting
   packets.  Specifically, implementations MUST attempt to enforce a
   maximum ack delay to avoid causing the network round-trip time (RTT) peer spurious timeouts.  The
   maximum ack delay is critical to these
   algorithms communicated in the "max_ack_delay" transport
   parameter and is described first.

4.1.  Computing the RTT estimate

   RTT default value is calculated when an ACK frame arrives by computing 25ms.

   An acknowledgement SHOULD be sent immediately upon receipt of a
   second packet but the
   difference between delay SHOULD NOT exceed the current time and maximum ack delay.
   QUIC recovery algorithms do not assume the time peer generates an
   acknowledgement immediately when receiving a second full-packet.

   Out-of-order packets SHOULD be acknowledged more quickly, in order to
   accelerate loss recovery.  The receiver SHOULD send an immediate ACK
   when it receives a new packet which is not one greater than the
   largest newly
   acked received packet was sent.  If no number.

   Similarly, packets are newly acknowledged, RTT
   cannot be calculated.  When RTT is calculated, marked with the ack delay field
   from ECN Congestion Experienced (CE)
   codepoint in the ACK frame IP header SHOULD be acknowledged immediately, to
   reduce the peer's response time to congestion events.

   As an optimization, a receiver MAY process multiple packets before
   sending any ACK frames in response.  In this case they can determine
   whether an immediate or delayed acknowledgement should be generated
   after processing incoming packets.

4.1.  Crypto Handshake Data

   In order to quickly complete the handshake and avoid spurious
   retransmissions due to crypto retransmission timeouts, crypto packets
   SHOULD use a very short ack delay, such as 1ms.  ACK frames MAY be
   sent immediately when the crypto stack indicates all data for that
   packet number space has been received.

4.2.  ACK Ranges

   When an ACK frame is sent, one or more ranges of acknowledged packets
   are included.  Including older packets reduces the chance of spurious
   retransmits caused by losing previously sent ACK frames, at the cost
   of larger ACK frames.

   ACK frames SHOULD always acknowledge the most recently received
   packets, and the more out-of-order the packets are, the more
   important it is to send an updated ACK frame quickly, to prevent the
   peer from declaring a packet as lost and spuriously retransmitting
   the frames it contains.

   Below is one recommended approach for determining what packets to
   include in an ACK frame.

4.3.  Receiver Tracking of ACK Frames

   When a packet containing an ACK frame is sent, the largest
   acknowledged in that frame may be saved.  When a packet containing an
   ACK frame is acknowledged, the receiver can stop acknowledging
   packets less than or equal to the largest acknowledged in the sent
   ACK frame.

   In cases without ACK frame loss, this algorithm allows for a minimum
   of 1 RTT of reordering.  In cases with ACK frame loss and reordering,
   this approach does not guarantee that every acknowledgement is seen
   by the sender before it is no longer included in the ACK frame.
   Packets could be received out of order and all subsequent ACK frames
   containing them could be lost.  In this case, the loss recovery
   algorithm may cause spurious retransmits, but the sender will
   continue making forward progress.

5.  Computing the RTT estimate

   RTT is calculated when an ACK frame arrives by computing the
   difference between the current time and the time the largest acked
   packet was sent.  An RTT sample MUST NOT be taken for a packet that
   is not newly acknowledged or not ack-eliciting.

   When RTT is calculated, the ack delay field from the ACK frame SHOULD
   be limited to the max_ack_delay specified by the peer.  Limiting
   ack_delay to max_ack_delay ensures a peer specifying an extremely
   small max_ack_delay doesn't cause more spurious timeouts than a peer
   that correctly specifies max_ack_delay.  It SHOULD be subtracted from
   the RTT as long as the result is larger than the Min RTT. min_rtt.  If the
   result is smaller than the min_rtt, the RTT should be used, but the
   ack delay field should be ignored.

   Like TCP, QUIC calculates both smoothed RTT and RTT variance similar
   to those specified in [RFC6298].

   Min RTT

   min_rtt is the minimum RTT measured over the connection, prior to
   adjusting by ack delay.  Ignoring ack delay for min RTT prevents
   intentional or unintentional underestimation of min RTT, which in
   turn prevents underestimating smoothed RTT.

4.2.  Ack-based

6.  Loss Detection

   QUIC senders use both ack information and timeouts to detect lost
   packets, and this section provides a description of these algorithms.
   Estimating the network round-trip time (RTT) is critical to these
   algorithms and is described first.

   If a packet is lost, the QUIC transport needs to recover from that
   loss, such as by retransmitting the data, sending an updated frame,
   or abandoning the frame.  For more information, see Section 13.2 of
   [QUIC-TRANSPORT].

6.1.  Acknowledgement-based Detection

   Ack-based

   Acknowledgement-based loss detection implements the spirit of TCP's
   Fast Retransmit [RFC5681], Early Retransmit [RFC5827], FACK, and FACK [FACK],
   SACK loss recovery [RFC6675]. [RFC6675], and RACK [RACK].  This section provides
   an overview of how these algorithms are implemented in QUIC.

4.2.1.  Fast Retransmit

   An unacknowledged

   A packet is marked as declared lost when if it meets all the following conditions:

   o  The packet is unacknowledged, in-flight, and was sent prior to an acknowledgment
      acknowledged packet.

   o  Either its packet number is
   received for a kPacketThreshold smaller than an
      acknowledged packet that (Section 6.1.1), or it was sent a threshold number of packets
   (kReorderingThreshold) and/or a threshold amount of time after the
   unacknowledged packet.  Receipt of long enough in
      the past (Section 6.1.2).

   The acknowledgement indicates that a later packet sent later was received, delivered,
   while the reordering threshold provides packet and time thresholds provide some tolerance for reordering of packets in the network.

   The RECOMMENDED initial value for kReorderingThreshold is 3, based on
   TCP loss recovery [RFC5681] [RFC6675].  Some networks may exhibit
   higher degrees of reordering, causing a sender to detect spurious
   losses.
   packet reordering.

   Spuriously declaring packets as lost leads to unnecessary
   retransmissions and may result in degraded performance due to the
   actions of the congestion controller upon detecting loss.
   Implementations that detect spurious retransmissions and increase the
   reordering threshold in packets or time MAY choose to start with
   smaller initial reordering thresholds to minimize recovery latency.

6.1.1.  Packet Threshold

   The RECOMMENDED initial value for the packet reordering threshold
   (kPacketThreshold) is 3, based on best practices for TCP loss
   detection [RFC5681] [RFC6675].

   Some networks may exhibit higher degrees of reordering, causing a
   sender to detect spurious losses.  Implementers MAY use algorithms
   developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's
   reordering resilience.

   QUIC implementations can use time-based loss detection to handle
   reordering based on time elapsed since the

6.1.2.  Time Threshold

   Once a later packet has been acknowledged, an endpoint SHOULD declare
   an earlier packet lost if it was sent.  This may
   be used either as a replacement for sent a packet reordering threshold or
   in addition to it.  The RECOMMENDED time threshold, expressed as a
   fraction amount of time in
   the round-trip past.  The time (kTimeReorderingFraction), threshold is 1/8.

4.2.2.  Early Retransmit

   Unacknowledged packets close to the tail may have fewer than
   kReorderingThreshold retransmittable computed as kTimeThreshold *
   max(SRTT, latest_RTT).  If packets sent after them.  Loss
   of such packets cannot be detected via Fast Retransmit.  To enable
   ack-based loss detection of such packets, receipt of an
   acknowledgment for prior to the last outstanding retransmittable largest
   acknowledged packet
   triggers the Early Retransmit process, as follows.

   If there are unacknowledged in-flight packets still pending, they
   should cannot yet be marked as lost.  To compensate for the reduced reordering
   resilience, the sender SHOULD set declared lost, then a timer for a small period of time.
   If the unacknowledged in-flight packets are not acknowledged during
   this time, then these packets MUST be marked as lost.

   An endpoint SHOULD
   be set for the timer such that remaining time.

   The RECOMMENDED time threshold (kTimeThreshold), expressed as a packet
   round-trip time multiplier, is marked as lost
   no earlier than 1.125 * max(SRTT, latest_RTT) since when it was sent. 9/8.

   Using max(SRTT, latest_RTT) protects from the two following cases:

   o  the latest RTT sample is lower than the SRTT, perhaps due to
      reordering where packet whose ack triggered the Early Retransit Retransmit
      process encountered a shorter path;

   o  the latest RTT sample is higher than the SRTT, perhaps due to a
      sustained increase in the actual RTT, but the smoothed SRTT has
      not yet caught up.

   The 1.125 multiplier increases reordering resilience.

   Implementers MAY experiment with using other multipliers, reordering thresholds,
   including absolute thresholds, bearing in mind that a lower
   multiplier reduces reordering resilience and increases spurious
   retransmissions, and a higher multiplier increases loss recovery
   delay.

   This mechanism is based on Early Retransmit for TCP [RFC5827].
   However, [RFC5827] does not include the timer described above.  Early
   Retransmit is prone to spurious retransmissions due to its reduced
   reordering resilence without the timer.  This observation led Linux
   TCP implementers to implement a timer for TCP as well, and this
   document incorporates this advancement.

4.3.  Timer-based spurious
   retransmissions, and a higher multiplier increases loss detection
   delay.

6.2.  Timeout Loss Detection

   Timer-based

   Timeout loss detection recovers from losses that cannot be handled by ack-based
   acknowledgement-based loss detection.  It uses a single timer which
   switches between a crypto retransmission timer, a Tail Loss Probe timer and Retransmission Timeout mechanisms.

4.3.1. a probe timer.

6.2.1.  Crypto Retransmission Timeout

   Data in CRYPTO frames is critical to QUIC transport and crypto
   negotiation, so a more aggressive timeout is used to retransmit it.

   The initial crypto retransmission timeout SHOULD be set to twice the
   initial RTT.

   At the beginning, there are no prior RTT samples within a connection.
   Resumed connections over the same network SHOULD use the previous
   connection's final smoothed RTT value as the resumed connection's
   initial RTT.  If no previous RTT is available, or if the network
   changes, the initial RTT SHOULD be set to 100ms.  When an
   acknowledgement is received, a new RTT is computed and the timer
   SHOULD be set for twice the newly computed smoothed RTT.

   When crypto packets are sent, the sender MUST set a timer for the
   crypto timeout period.  Upon timeout, the sender MUST retransmit all
   unacknowledged CRYPTO data if possible.

   Until the server has validated the client's address on the path, the
   number
   amount of bytes data it can send is limited, as specified in
   [QUIC-TRANSPORT].  If not all unacknowledged CRYPTO data can be sent,
   then all unacknowledged CRYPTO data sent in Initial packets should be
   retransmitted.  If no bytes data can be sent, then no alarm should be armed
   until bytes have data has been received from the client.

   Because the server could be blocked until more packets are received,
   the client MUST start the crypto retransmission timer even if there
   is no unacknowledged CRYPTO data.  If the timer expires and the
   client has no CRYPTO data to retransmit and does not have Handshake
   keys, it SHOULD send an Initial packet in a UDP datagram of at least
   1200 octets. bytes.  If the client has Handshake keys, it SHOULD send a
   Handshake packet.

   On each consecutive expiration of the crypto timer without receiving
   an acknowledgement for a new packet, the sender SHOULD double the
   crypto retransmission timeout and set a timer for this period.

   When crypto packets are outstanding, in flight, the TLP and RTO timers are probe timer (Section 6.2.2) is
   not active.

4.3.1.1.

6.2.1.1.  Retry and Version Negotiation

   A Retry or Version Negotiation packet causes a client to send another
   Initial packet, effectively restarting the connection process. process and
   resetting congestion control and loss recovery state, including
   resetting any pending timers.  Either packet indicates that the
   Initial was received but not processed.  Neither packet can be
   treated as an acknowledgment for the Initial, but they MAY be used to improve the RTT estimate.

4.3.2.  Tail Loss Probe

   The algorithm Initial.

6.2.1.2.  Discarding Initial State

   As described in this section is an adaptation of the Tail
   Loss Probe algorithm proposed for TCP [TLP].

   A packet sent at the tail is particularly vulnerable to slow loss
   detection, since acks of subsequent packets are needed to trigger
   ack-based detection.  To ameliorate this weakness of tail packets,
   the sender schedules a timer when the last retransmittable packet
   before quiescence is transmitted.  Upon timeout, a Tail Loss Probe
   (TLP) packet is sent to evoke an acknowledgement from the receiver.

   The timer duration, or Probe Timeout (PTO), is set based on the
   following conditions:

   o  PTO SHOULD be scheduled for max(1.5*SRTT+MaxAckDelay,
      kMinTLPTimeout)

   o  If RTO (Section 4.3.3) is earlier, schedule a TLP in its place.
      That is, PTO SHOULD be scheduled for min(RTO, PTO).

   QUIC includes MaxAckDelay in all probe timeouts, because it assumes
   the ack delay may come into play, regardless of the number of packets
   outstanding.  TCP's TLP assumes if at least 2 packets are
   outstanding, acks will not be delayed.

   A PTO value Section 17.5.1 of at least 1.5*SRTT ensures that the ACK is overdue.
   The 1.5 is based on [TLP], but implementations MAY experiment with
   other constants.

   To reduce latency, it is RECOMMENDED that the sender set [QUIC-TRANSPORT], endpoints stop
   sending and allow
   the TLP timer to fire twice before setting an RTO timer.  In other
   words, when the TLP timer expires receiving Initial packets once they start exchanging
   Handshake packets.  At this point, all loss recovery state for the first time, a TLP
   Initial packet number space is
   sent, and it is RECOMMENDED also discarded.  Packets that the TLP timer be scheduled are in
   flight for a
   second time.  When the TLP timer expires the second time, a second
   TLP packet is sent, and an RTO timer SHOULD be scheduled
   Section 4.3.3.

   A TLP packet SHOULD carry new data when possible.  If new data is
   unavailable number space are not declared as either
   acknowledged or lost.  After discarding state, new data cannot Initial packets
   will not be sent due to flow control, a TLP
   packet sent.

   The client MAY retransmit unacknowledged data to potentially reduce
   recovery time.  Since a TLP timer is used however compute an RTT estimate to send a probe into the
   network prior to establishing any packet loss, prior unacknowledged
   packets SHOULD NOT be marked server as lost the
   time period from when the first Initial was sent to when a TLP timer expires.

   A sender may not know that Retry or a
   Version Negotiation packet being sent is received.  The client MAY use this
   value to seed the RTT estimator for a tail packet.
   Consequently, a sender may have subsequent connection attempt
   to arm or adjust the TLP timer on
   every sent retransmittable packet.

4.3.3.  Retransmission server.

6.2.2.  Probe Timeout

   A Retransmission Probe Timeout (RTO) timer (PTO) triggers a probe packet when ack-eliciting data
   is the final backstop for loss
   detection.  The algorithm used in QUIC is based on the RTO algorithm
   for TCP [RFC5681] and flight but an acknowledgement is additionally resilient to spurious RTO
   events [RFC5682].

   When not received within the last TLP packet is sent,
   expected period of time.  A PTO enables a timer is set for the RTO period.
   When this timer expires, the sender sends two packets, connection to evoke
   acknowledgements recover from
   loss of tail packets or acks.  The PTO algorithm used in QUIC
   implements the receiver, and restarts the reliability functions of Tail Loss Probe [TLP] [RACK],
   RTO timer.

   Similar to [RFC5681] and F-RTO algorithms for TCP [RFC6298], [RFC5682], and the RTO period timeout
   computation is set based on the
   following conditions:

   o TCP's retransmission timeout period
   [RFC6298].

6.2.2.1.  Computing PTO

   When the final TLP an ack-eliciting packet is sent, transmitted, the RTO sender schedules a
   timer for the PTO period is set to
      max(SRTT as follows:

   PTO = max(smoothed_rtt + 4*RTTVAR 4*rttvar + MaxAckDelay, kMinRTOTimeout)

   o  When an RTO timer expires, the RTO max_ack_delay, kGranularity)

   kGranularity, smoothed_rtt, rttvar, and max_ack_delay are defined in
   Section 6.4.1 and Section 6.4.2.

   The PTO period is doubled.

   The sender typically has incurred a high latency penalty by the amount of time
   an RTO timer expires, and this penalty increases exponentially in
   subsequent consecutive RTO events.  Sending that a single packet on an RTO
   event therefore makes the connection very sensitive to single packet
   loss.  Sending two packets instead of one significantly increases
   resilience sender ought to packet drop in both directions, thus reducing the
   probability of consecutive RTO events.

   QUIC's RTO algorithm differs from TCP in that the firing of wait for
   an RTO
   timer is not considered a strong enough signal acknowledgement of packet loss, so
   does not result in an immediate change to congestion window or
   recovery state.  An RTO timer expires only when there's a prolonged sent packet.  This time period of network silence, which could be caused by a change in includes the
   underlying
   estimated network RTT.

   QUIC also diverges from TCP by including MaxAckDelay in roundtrip-time (smoothed_rtt), the RTO
   period.  Since QUIC corrects for this delay in its SRTT and RTTVAR
   computations, it is necessary to add this delay explicitly variance in the TLP
   estimate (4*rttvar), and RTO computation.

   When an acknowledgment is received for a packet sent on an RTO event,
   any unacknowledged packets with lower packet numbers than those
   acknowledged MUST be marked as lost.  If an acknowledgement max_ack_delay, to account for the maximum
   time by which a
   packet sent on receiver might delay sending an RTO is received acknowledgement.

   The PTO value MUST be set to at the same time packets sent prior least kGranularity, to avoid the first RTO are acknowledged,
   timer expiring immediately.

   When a PTO timer expires, the RTO PTO period MUST be set to twice its
   current value.  This exponential reduction in the sender's rate is considered spurious and
   standard
   important because the PTOs might be caused by loss detection rules apply.

   A packet sent when an RTO timer expires MAY carry new data if
   available of packets or unacknowledged data
   acknowledgements due to potentially reduce recovery time.
   Since this severe congestion.

   A sender computes its PTO timer every time an ack-eliciting packet is sent as a probe into the network prior
   sent.  A sender might choose to
   establishing any packet loss, prior unacknowledged optimize this by setting the timer
   fewer times if it knows that more ack-eliciting packets SHOULD NOT will be marked as lost.

   A packet sent on an RTO
   within a short period of time.

6.2.2.2.  Sending Probe Packets

   When a PTO timer MUST NOT be blocked by expires, the sender's
   congestion controller.  A sender MUST however count these bytes as
   additional bytes in flight, since this packet adds network load
   without establishing send one ack-eliciting
   packet loss.

4.4.  Generating Acknowledgements

   QUIC SHOULD delay sending acknowledgements in response as a probe.  A sender MAY send up to two ack-eliciting
   packets,
   but MUST NOT excessively delay acknowledgements of packets containing
   frames other than ACK.  Specifically, implementations MUST attempt to
   enforce a maximum ack delay to avoid causing the peer spurious
   timeouts.  The maximum ack delay is communicated in the
   "max_ack_delay" transport parameter and the default value is 25ms.

   An acknowledgement SHOULD be sent immediately upon receipt of an expensive consecutive PTO expiration due to a
   second
   single packet but the delay SHOULD NOT exceed the maximum ack delay.
   QUIC recovery algorithms do not assume the peer generates an
   acknowledgement immediately when receiving loss.

   Consecutive PTO periods increase exponentially, and as a second full-packet.

   Out-of-order result,
   connection recovery latency increases exponentially as packets SHOULD
   continue to be acknowledged more quickly, dropped in order the network.  Sending two packets on PTO
   expiration increases resilience to
   accelerate loss recovery.  The receiver SHOULD send an immediate ACK
   when it receives packet drops, thus reducing the
   probability of consecutive PTO events.

   Probe packets sent on a PTO MUST be ack-eliciting.  A probe packet
   SHOULD carry new data when possible.  A probe packet which MAY carry
   retransmitted unacknowledged data when new data is unavailable, when
   flow control does not one greater than the
   largest received packet number.

   Similarly, packets marked with the ECN Congestion Experienced (CE)
   codepoint in the IP header SHOULD permit new data to be acknowledged immediately, sent, or to
   opportunistically reduce the peer's response time to congestion events.

   As an optimization, a receiver loss recovery delay.  Implementations MAY process multiple packets before
   use alternate strategies for determining the content of probe
   packets, including sending any ACK frames new or retransmitted data based on the
   application's priorities.

6.2.2.3.  Loss Detection

   Delivery or loss of packets in response.  In this case they can determine
   whether flight is established when an immediate ACK
   frame is received that newly acknowledges one or delayed acknowledgement should be generated
   after processing incoming more packets.

4.4.1.  Crypto Handshake Data

   In order

   A PTO timer expiration event does not indicate packet loss and MUST
   NOT cause prior unacknowledged packets to quickly complete be marked as lost.  After a
   PTO timer has expired, an endpoint uses the handshake and avoid spurious
   retransmissions due following rules to crypto retransmission timeouts, crypto mark
   packets
   SHOULD use a very short ack delay, such as 1ms.  ACK frames MAY be
   sent immediately lost when the crypto stack indicates all data for an acknowledgement is received that
   encryption level has been received.

4.4.2.  ACK Ranges newly
   acknowledges packets.

   When an ACK frame acknowledgement is sent, one or more ranges of acknowledged packets
   are included.  Including older packets reduces the chance of spurious
   retransmits caused by losing previously sent ACK frames, at the cost
   of larger ACK frames.

   ACK frames SHOULD always acknowledge the most recently received that newly acknowledges packets,
   loss detection proceeds as dictated by packet and time threshold
   mechanisms, see Section 6.1.

6.3.  Tracking Sent Packets

   To correctly implement congestion control, a QUIC sender tracks every
   ack-eliciting packet until the more out-of-order the packets are, the more
   important it packet is acknowledged or lost.  It is
   expected that implementations will be able to send access this information
   by packet number and crypto context and store the per-packet fields
   (Section 6.3.1) for loss recovery and congestion control.

   After a packet is declared lost, it SHOULD be tracked for an updated ACK frame quickly, amount
   of time comparable to prevent the
   peer from declaring a maximum expected packet reordering, such as lost and spuriously retransmitting
   the frames it contains.

   Below is one recommended approach
   1 RTT.  This allows for determining what detection of spurious retransmissions.

   Sent packets to
   include in an are tracked for each packet number space, and ACK frame.

4.4.3.  Receiver Tracking
   processing only applies to a single space.

6.3.1.  Sent Packet Fields

   packet_number:  The packet number of ACK Frames

   When the sent packet.

   ack_eliciting:  A boolean that indicates whether a packet containing is ack-
      eliciting.  If true, it is expected that an acknowledgement will
      be received, though the peer could delay sending the ACK frame is sent,
      containing it by up to the largest
   acknowledged in MaxAckDelay.

   in_flight:  A boolean that frame may be saved.  When a indicates whether the packet containing an
   ACK frame is acknowledged, counts
      towards bytes in flight.

   is_crypto_packet:  A boolean that indicates whether the receiver can stop acknowledging
   packets less than or equal packet
      contains cryptographic handshake messages critical to the largest acknowledged in
      completion of the sent
   ACK frame. QUIC handshake.  In cases without ACK frame loss, this algorithm allows for a minimum
   of 1 RTT version of reordering.  In cases with ACK frame loss, QUIC, this approach
   does not guarantee that every acknowledgement is seen by the sender
   before it is no longer included in
      includes any packet with the ACK long header that includes a CRYPTO
      frame.  Packets could be
   received out

   sent_bytes:  The number of order and all subsequent ACK frames containing them
   could be lost.  In this case, bytes sent in the loss recovery algorithm may cause
   spurious retransmits, packet, not including
      UDP or IP overhead, but including QUIC framing overhead.

   time_sent:  The time the sender will continue making forward
   progress.

4.5. packet was sent.

6.4.  Pseudocode

4.5.1.

6.4.1.  Constants of interest

   Constants used in loss recovery are based on a combination of RFCs,
   papers, and common practice.  Some may need to be changed or
   negotiated in order to better suit a variety of environments.

   kMaxTLPs:  Maximum number of tail loss probes before an RTO expires.
      The RECOMMENDED value is 2.

   kReorderingThreshold:

   kPacketThreshold:  Maximum reordering in packet number space packets before FACK style packet
      threshold loss detection considers a packet lost.  The RECOMMENDED
      value is 3.

   kTimeReorderingFraction:

   kTimeThreshold:  Maximum reordering in time space before time based threshold
      loss detection considers a packet lost.  In fraction of
      an RTT.  The RECOMMENDED value is 1/8.

   kUsingTimeLossDetection:  Whether time based loss detection is in
      use.  If false, uses FACK style loss detection.  The RECOMMENDED
      value is false.

   kMinTLPTimeout:  Minimum time in the future a tail loss probe timer
      may be set for.  The RECOMMENDED value is 10ms.

   kMinRTOTimeout:  Minimum time in the future an RTO timer may be set
      for.  The RECOMMENDED value is 200ms.

   kDelayedAckTimeout:  The length of the peer's delayed ack timer. lost.  Specified as an RTT
      multiplier.  The RECOMMENDED value is 25ms. 9/8.

   kGranularity:  Timer granularity.  This is a system-dependent value.
      However, implementations SHOULD use a value no smaller than 1ms.

   kInitialRtt:  The RTT used before an RTT sample is taken.  The
      RECOMMENDED value is 100ms.

4.5.2.

6.4.2.  Variables of interest

   Variables required to implement the congestion control mechanisms are
   described in this section.

   loss_detection_timer:  Multi-modal timer used for loss detection.

   crypto_count:  The number of times all unacknowledged CRYPTO data has
      been retransmitted without receiving an ack.

   tlp_count:

   pto_count:  The number of times a tail loss probe has been sent
      without receiving an ack.

   rto_count:  The number of times an RTO PTO has been sent without receiving
      an ack.

   largest_sent_before_rto:  The last packet number sent prior to the
      first retransmission timeout.

   time_of_last_sent_retransmittable_packet:

   time_of_last_sent_ack_eliciting_packet:  The time the most recent
      retransmittable
      ack-eliciting packet was sent.

   time_of_last_sent_crypto_packet:  The time the most recent crypto
      packet was sent.

   largest_sent_packet:  The packet number of the most recently sent
      packet.

   largest_acked_packet:  The largest packet number acknowledged in an
      ACK frame. the
      packet number space so far.

   latest_rtt:  The most recent RTT measurement made when receiving an
      ack for a previously unacked packet.

   smoothed_rtt:  The smoothed RTT of the connection, computed as
      described in [RFC6298]

   rttvar:  The RTT variance, computed as described in [RFC6298]

   min_rtt:  The minimum RTT seen in the connection, ignoring ack delay.

   max_ack_delay:  The maximum amount of time by which the receiver
      intends to delay acknowledgments, in milliseconds.  The actual
      ack_delay in a received ACK frame may be larger due to late
      timers, reordering, or lost ACKs.

   reordering_threshold:  The largest packet number gap between the
      largest acknowledged retransmittable packet and an unacknowledged
      retransmittable packet before it is declared lost.

   time_reordering_fraction:  The reordering window as a fraction of
      max(smoothed_rtt, latest_rtt).

   loss_time:  The time at which the next packet will be considered lost
      based on early transmit or exceeding the reordering window in
      time.

   sent_packets:  An association of packet numbers to information about
      them, including a number field indicating the packet number, a
      time field indicating the time a packet was sent, a boolean
      indicating whether the packet is ack-only, a boolean indicating
      whether it counts towards bytes
      them.  Described in flight, and a bytes field
      indicating the packet's size.  sent_packets is ordered by packet
      number, and packets remain detail above in sent_packets until acknowledged or
      lost.  A sent_packets data structure is maintained per packet
      number space, and ACK processing only applies to a single space.

4.5.3. Section 6.3.

6.4.3.  Initialization

   At the beginning of the connection, initialize the loss detection
   variables as follows:

      loss_detection_timer.reset()
      crypto_count = 0
      tlp_count = 0
      rto_count
      pto_count = 0
      if (kUsingTimeLossDetection)
        reordering_threshold = infinite
        time_reordering_fraction = kTimeReorderingFraction
      else:
        reordering_threshold = kReorderingThreshold
        time_reordering_fraction = infinite
      loss_time = 0
      smoothed_rtt = 0
      rttvar = 0
      min_rtt = infinite
      largest_sent_before_rto = 0
      time_of_last_sent_retransmittable_packet = 0
      time_of_last_sent_crypto_packet = 0
      largest_sent_packet
      time_of_last_sent_ack_eliciting_packet = 0

4.5.4.  On Sending a Packet

   After any packet is sent, be it a new transmission or a rebundled
   transmission, the following OnPacketSent function is called.  The
   parameters to OnPacketSent are as follows:

   o  packet_number: The packet number of the sent packet.

   o  ack_only: A boolean that indicates whether a packet contains only
      ACK or PADDING frame(s).  If true, it is still expected an ack
      will be received for this packet, but it is not retransmittable.

   o  in_flight: A boolean that indicates whether the packet counts
      towards bytes in flight.

   o  is_crypto_packet: A boolean that indicates whether the
      time_of_last_sent_crypto_packet = 0
      largest_sent_packet = 0
      largest_acked_packet = 0

6.4.4.  On Sending a Packet

   After a packet
      contains cryptographic handshake messages critical to the
      completion of is sent, information about the QUIC handshake.  In this version of QUIC, this
      includes any packet with the long header that includes a CRYPTO
      frame.

   o  sent_bytes: is stored.  The number of bytes sent
   parameters to OnPacketSent are described in the packet, not including
      UDP or IP overhead, but including QUIC framing overhead. detail above in
   Section 6.3.1.

   Pseudocode for OnPacketSent follows:

    OnPacketSent(packet_number, ack_only, ack_eliciting, in_flight,
                 is_crypto_packet, sent_bytes):
      largest_sent_packet = packet_number
      sent_packets[packet_number].packet_number = packet_number
      sent_packets[packet_number].time
      sent_packets[packet_number].time_sent = now
      sent_packets[packet_number].ack_only
      sent_packets[packet_number].ack_eliciting = ack_only ack_eliciting
      sent_packets[packet_number].in_flight = in_flight
      if !ack_only: (ack_eliciting):
        if is_crypto_packet: (is_crypto_packet):
          time_of_last_sent_crypto_packet = now
        time_of_last_sent_retransmittable_packet
        time_of_last_sent_ack_eliciting_packet = now
        OnPacketSentCC(sent_bytes)
        sent_packets[packet_number].bytes
        sent_packets[packet_number].size = sent_bytes
        SetLossDetectionTimer()

4.5.5.

6.4.5.  On Receiving an Acknowledgment

   When an ACK frame is received, it may newly acknowledge any number of
   packets.

   Pseudocode for OnAckReceived and UpdateRtt follow:

     OnAckReceived(ack):
       largest_acked_packet = ack.largest_acked max(largest_acked_packet,
                                  ack.largest_acked)

       // If the largest acknowledged is newly acked, acked and
       // ack-eliciting, update the RTT.
       if (sent_packets[ack.largest_acked]): (sent_packets[ack.largest_acked] &&
           sent_packets[ack.largest_acked].ack_eliciting):
         latest_rtt =
           now - sent_packets[ack.largest_acked].time sent_packets[ack.largest_acked].time_sent
         UpdateRtt(latest_rtt, ack.ack_delay)

       // Process ECN information if present.
       if (ACK frame contains ECN information):
          ProcessECN(ack)

       // Find all newly acked packets in this ACK frame
       newly_acked_packets = DetermineNewlyAckedPackets(ack)
       if (newly_acked_packets.empty()):
         return

       for acked_packet in newly_acked_packets:
         OnPacketAcked(acked_packet.packet_number)

       if !newly_acked_packets.empty():
         // Find the smallest newly acknowledged packet
         smallest_newly_acked =
           FindSmallestNewlyAcked(newly_acked_packets)
         // If any packets sent prior to RTO were acked, then the
         // RTO was spurious. Otherwise, inform congestion control.
         if (rto_count > 0 &&
               smallest_newly_acked > largest_sent_before_rto):
           OnRetransmissionTimeoutVerified(smallest_newly_acked)

       crypto_count = 0
         tlp_count = 0
         rto_count
       pto_count = 0

       DetectLostPackets(ack.largest_acked_packet)

       DetectLostPackets()
       SetLossDetectionTimer()

       // Process ECN information if present.
       if (ACK frame contains ECN information):
          ProcessECN(ack)

     UpdateRtt(latest_rtt, ack_delay):
       // min_rtt ignores ack delay.
       min_rtt = min(min_rtt, latest_rtt)
       // Limit ack_delay by max_ack_delay
       ack_delay = min(ack_delay, max_ack_delay)
       // Adjust for ack delay if it's plausible.
       if (latest_rtt - min_rtt > ack_delay):
         latest_rtt -= ack_delay
       // Based on {{RFC6298}}.
       if (smoothed_rtt == 0):
         smoothed_rtt = latest_rtt
         rttvar = latest_rtt / 2
       else:
         rttvar_sample = abs(smoothed_rtt - latest_rtt)
         rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
         smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt

4.5.6.

6.4.6.  On Packet Acknowledgment

   When a packet is acked acknowledged for the first time, the following
   OnPacketAcked function is called.  Note that a single ACK frame may
   newly acknowledge several packets.  OnPacketAcked must be called once
   for each of these newly acked acknowledged packets.

   OnPacketAcked takes one parameter, acked_packet, which is the struct
   of the newly acked packet.

   If this is the first acknowledgement following RTO, check if the
   smallest newly acknowledged packet is one sent by the RTO, and if so,
   inform congestion control of a verified RTO, similar to F-RTO
   [RFC5682].
   detailed in Section 6.3.1.

   Pseudocode for OnPacketAcked follows:

      OnPacketAcked(acked_packet):
        if (!acked_packet.is_ack_only): (acked_packet.ack_eliciting):
          OnPacketAckedCC(acked_packet)
        sent_packets.remove(acked_packet.packet_number)

4.5.7.

6.4.7.  Setting the Loss Detection Timer

   QUIC loss detection uses a single timer for all timer-based timeout loss
   detection.  The duration of the timer is based on the timer's mode,
   which is set in the packet and timer events further below.  The
   function SetLossDetectionTimer defined below shows how the single
   timer is set.

   This algorithm may result in the timer being set in the past,
   particularly if timers wake up late.  Timers set in the past SHOULD
   fire immediately.

   Pseudocode for SetLossDetectionTimer follows:

    SetLossDetectionTimer():
       // Don't arm timer if there are no retransmittable ack-eliciting packets
       // in flight.
       if (bytes_in_flight == 0): (no ack-eliciting packets in flight):
         loss_detection_timer.cancel()
         return

       if (crypto packets are outstanding): in flight):
         // Crypto retransmission timer.
         if (smoothed_rtt == 0):
           timeout = 2 * kInitialRtt
         else:
           timeout = 2 * smoothed_rtt
         timeout = max(timeout, kMinTLPTimeout) kGranularity)
         timeout = timeout * (2 ^ crypto_count)
         loss_detection_timer.set(
           time_of_last_sent_crypto_packet + timeout)
         return
       if (loss_time != 0):
         // Early retransmit timer or time Time threshold loss detection.
         timeout = loss_time -
           time_of_last_sent_retransmittable_packet
       else:
         // RTO or TLP timer
         loss_detection_timer.set(loss_time)
         return

       // Calculate RTO PTO duration
       timeout =
         smoothed_rtt + 4 * rttvar + max_ack_delay
       timeout = max(timeout, kMinRTOTimeout) kGranularity)
       timeout = timeout * (2 ^ rto_count)
         if (tlp_count < kMaxTLPs):
           // Tail Loss Probe
           tlp_timeout = max(1.5 * smoothed_rtt
                              + max_ack_delay, kMinTLPTimeout)
           timeout = min(tlp_timeout, timeout) pto_count)

       loss_detection_timer.set(
         time_of_last_sent_retransmittable_packet
         time_of_last_sent_ack_eliciting_packet + timeout)

4.5.8.

6.4.8.  On Timeout

   When the loss detection timer expires, the timer's mode determines
   the action to be performed.

   Pseudocode for OnLossDetectionTimeout follows:

      OnLossDetectionTimeout():
        if (crypto packets are outstanding): in flight):
          // Crypto retransmission timeout.
          RetransmitUnackedCryptoData()
          crypto_count++
        else if (loss_time != 0):
          // Early retransmit or Time Loss Detection
          DetectLostPackets(largest_acked_packet)
        else if (tlp_count < kMaxTLPs):
          // Tail Loss Probe.
          SendOnePacket()
          tlp_count++ Time threshold loss Detection
          DetectLostPackets()
        else:
          // RTO.
          if (rto_count == 0)
            largest_sent_before_rto = largest_sent_packet PTO
          SendTwoPackets()
          rto_count++
          pto_count++

        SetLossDetectionTimer()

4.5.9.

6.4.9.  Detecting Lost Packets

   Packets in QUIC are only considered lost once a larger packet number
   in the same packet number space is acknowledged.

   DetectLostPackets is called every time an ack ACK is received and
   operates on the sent_packets for that packet number space.  If the
   loss detection timer expires and the loss_time is set, the previous
   largest acked acknowledged packet is supplied.

4.5.9.1.  Pseudocode

   DetectLostPackets takes one parameter, acked, which is the largest
   acked packet.

   Pseudocode for DetectLostPackets follows:

   DetectLostPackets(largest_acked):

   DetectLostPackets():
     loss_time = 0
     lost_packets = {}
     delay_until_lost = infinite
     if (kUsingTimeLossDetection):
       delay_until_lost
     loss_delay =
         (1 + time_reordering_fraction) kTimeThreshold * max(latest_rtt, smoothed_rtt)
     else if (largest_acked.packet_number == largest_sent_packet):

     // Early retransmit timer.
       delay_until_lost = 9/8 * max(latest_rtt, smoothed_rtt)
     foreach (unacked < largest_acked.packet_number):
       time_since_sent Packets sent before this time are deemed lost.
     lost_send_time = now() - unacked.time_sent
       delta loss_delay

     // Packets with packet numbers before this are deemed lost.
     lost_pn = largest_acked.packet_number largest_acked_packet - unacked.packet_number kPacketThreshold

     foreach unacked in sent_packets:
       if (time_since_sent (unacked.packet_number > delay_until_lost largest_acked_packet):
         continue

       // Mark packet as lost, or set time when it should be marked.
       if (unacked.time_sent <= lost_send_time ||
           delta > reordering_threshold):
           unacked.packet_number <= lost_pn):
         sent_packets.remove(unacked.packet_number)
         if (!unacked.is_ack_only): (unacked.in_flight):
           lost_packets.insert(unacked)
       else if (loss_time == 0 && delay_until_lost != infinite): 0):
         loss_time = now() unacked.time_sent + delay_until_lost - time_since_sent loss_delay
       else:
         loss_time = min(loss_time, unacked.time_sent + loss_delay)

     // Inform the congestion controller of lost packets and
     // lets let it decide whether to retransmit immediately.
     if (!lost_packets.empty()):
       OnPacketsLost(lost_packets)

4.6.

6.5.  Discussion

   The majority of constants were derived from best common practices
   among widely deployed TCP implementations on the internet.
   Exceptions follow.

   A shorter delayed ack time of 25ms was chosen because longer delayed
   acks can delay loss recovery and for the small number of connections
   where less than packet per 25ms is delivered, acking every packet is
   beneficial to congestion control and loss recovery.

   The default initial RTT of 100ms was chosen because it is slightly
   higher than both the median and mean min_rtt typically observed on
   the public internet.

5.

7.  Congestion Control

   QUIC's congestion control is based on TCP NewReno [RFC6582].  NewReno
   is a congestion window based congestion control.  QUIC specifies the
   congestion window in bytes rather than packets due to finer control
   and the ease of appropriate byte counting [RFC3465].

   QUIC hosts MUST NOT send packets if they would increase
   bytes_in_flight (defined in Section 5.8.2) 7.9.2) beyond the available
   congestion window, unless the packet is a probe packet sent after the
   TLP or RTO a
   PTO timer expires, as described in Section 4.3.2 and
   Section 4.3.3. 6.2.2.

   Implementations MAY use other congestion control algorithms, such as
   Cubic [RFC8312], and endpoints MAY use different algorithms from one
   another.  The signals QUIC provides for congestion control are
   generic and are designed to support different algorithms.

5.1.

7.1.  Explicit Congestion Notification

   If a path has been verified to support ECN, QUIC treats a Congestion
   Experienced codepoint in the IP header as a signal of congestion.
   This document specifies an endpoint's response when its peer receives
   packets with the Congestion Experienced codepoint.  As discussed in
   [RFC8311], endpoints are permitted to experiment with other response
   functions.

5.2.

7.2.  Slow Start

   QUIC begins every connection in slow start and exits slow start upon
   loss or upon increase in the ECN-CE counter.  QUIC re-enters slow
   start anytime the congestion window is less than ssthresh, which
   typically only occurs after an RTO. PTO.  While in slow start, QUIC
   increases the congestion window by the number of bytes acknowledged
   when each ack acknowledgment is processed.

5.3.

7.3.  Congestion Avoidance

   Slow start exits to congestion avoidance.  Congestion avoidance in
   NewReno uses an additive increase multiplicative decrease (AIMD)
   approach that increases the congestion window by one maximum packet
   size per congestion window acknowledged.  When a loss is detected,
   NewReno halves the congestion window and sets the slow start
   threshold to the new congestion window.

5.4.

7.4.  Recovery Period

   Recovery is a period of time beginning with detection of a lost
   packet or an increase in the ECN-CE counter.  Because QUIC
   retransmits stream data and control frames, does not
   retransmit packets, it defines the end of recovery as a packet sent
   after the start of recovery being acknowledged.  This is slightly
   different from TCP's definition of recovery, which ends when the lost
   packet that started recovery is acknowledged.

   The recovery period limits congestion window reduction to once per
   round trip.  During recovery, the congestion window remains unchanged
   irrespective of new losses or increases in the ECN-CE counter.

5.5.  Tail Loss

7.5.  Probe

   A TLP packet Timeout

   Probe packets MUST NOT be blocked by the sender's congestion controller.  The  A
   sender MUST however count these bytes packets as additional
   bytes-in-flight, being additionally in
   flight, since a TLP these packets adds network load without establishing
   packet loss.

   Acknowledgement or loss of tail loss probes are treated like any
   other packet.

5.6.  Retransmission Timeout

   When retransmissions are sent due to a retransmission timeout timer,
   no change is made  Note that sending probe packets might cause the
   sender's bytes in flight to exceed the congestion window until the next an
   acknowledgement arrives.  The retransmission timeout is considered
   spurious when this acknowledgement acknowledges packets sent prior to received that establishes loss or delivery of
   packets.

   If a threshold number of consecutive PTOs have occurred (pto_count is
   more than kPersistentCongestionThreshold, see Section 7.9.1), the first retransmission timeout.  The retransmission timeout
   network is considered valid when this acknowledgement acknowledges no packets
   sent prior to be experiencing persistent congestion, and
   the first retransmission timeout.  In this case, the sender's congestion window MUST be reduced to the minimum
   congestion window
   and slow start is re-entered.

5.7. window.

7.6.  Pacing

   This document does not specify a pacer, but it is RECOMMENDED that a
   sender pace sending of all in-flight packets based on input from the
   congestion controller.  For example, a pacer might distribute the
   congestion window over the SRTT when used with a window-based
   controller, and a pacer might use the rate estimate of a rate-based
   controller.

   An implementation should take care to architect its congestion
   controller to work well with a pacer.  For instance, a pacer might
   wrap the congestion controller and control the availability of the
   congestion window, or a pacer might pace out packets handed to it by
   the congestion controller.  Timely delivery of ACK frames is
   important for efficient loss recovery.  Packets containing only ACK
   frames should therefore not be paced, to avoid delaying their
   delivery to the peer.

   As an example of a well-known and publicly available implementation
   of a flow pacer, implementers are referred to the Fair Queue packet
   scheduler (fq qdisc) in Linux (3.11 onwards).

5.8.

7.7.  Sending data after an idle period

   A sender becomes idle if it ceases to send data and has no bytes in
   flight.  A sender's congestion window MUST not increase while it is
   idle.

   When sending data after becoming idle, a sender MUST reset its
   congestion window to the initial congestion window (see Section 4.1
   of [RFC5681]), unless it paces the sending of packets.  A sender MAY
   retain its congestion window if it paces the sending of any packets
   in excess of the initial congestion window.

   A sender MAY implement alternate mechanisms to update its congestion
   window after idle periods, such as those proposed for TCP in
   [RFC7661].

7.8.  Discarding Packet Number Space State

   When keys for an packet number space are discarded, any packets sent
   with those keys are removed from the count of bytes in flight.  No
   loss events will occur any in-flight packets from that space, as a
   result of discarding loss recovery state (see Section 6.2.1.2).  Note
   that it is expected that keys are discarded after those packets would
   be declared lost, but Initial secrets are destroyed earlier.

7.9.  Pseudocode

5.8.1.

7.9.1.  Constants of interest

   Constants used in congestion control are based on a combination of
   RFCs, papers, and common practice.  Some may need to be changed or
   negotiated in order to better suit a variety of environments.

   kMaxDatagramSize:  The sender's maximum payload size.  Does not
      include UDP or IP overhead.  The max packet size is used for
      calculating initial and minimum congestion windows.  The
      RECOMMENDED value is 1200 bytes.

   kInitialWindow:  Default limit on the initial amount of outstanding data in
      flight, in bytes.  Taken from [RFC6928].  The RECOMMENDED value is
      the minimum of 10 * kMaxDatagramSize and max(2* kMaxDatagramSize,
      14600)).

   kMinimumWindow:  Minimum congestion window in bytes.  The RECOMMENDED
      value is 2 * kMaxDatagramSize.

   kLossReductionFactor:  Reduction in congestion window when a new loss
      event is detected.  The RECOMMENDED value is 0.5.

5.8.2.

   kPersistentCongestionThreshold:  Number of consecutive PTOs after
      which network is considered to be experiencing persistent
      congestion.  The rationale for this threshold is to enable a
      sender to use initial PTOs for aggressive probing, similar to Tail
      Loss Probe (TLP) in TCP [TLP] [RACK].  Once the number of
      consecutive PTOs reaches this threshold - that is, persistent
      congestion is established - the sender responds by collapsing its
      congestion window to kMinimumWindow, similar to a Retransmission
      Timeout (RTO) in TCP [RFC5681].  The RECOMMENDED value for
      kPersistentCongestionThreshold is 2, which is equivalent to having
      two TLPs before an RTO in TCP.

7.9.2.  Variables of interest

   Variables required to implement the congestion control mechanisms are
   described in this section.

   ecn_ce_counter:  The highest value reported for the ECN-CE counter by
      the peer in an ACK frame.  This variable is used to detect
      increases in the reported ECN-CE counter.

   bytes_in_flight:  The sum of the size in bytes of all sent packets
      that contain at least one retransmittable ack-eliciting or PADDING frame, and have
      not been acked or declared lost.  The size does not include IP or
      UDP overhead, but does include the QUIC header and AEAD overhead.
      Packets only containing ACK frames do not count towards
      bytes_in_flight to ensure congestion control does not impede
      congestion feedback.

   congestion_window:  Maximum number of bytes-in-flight that may be
      sent.

   end_of_recovery:

   recovery_start_time:  The largest packet number sent time when QUIC first detects a
      loss. loss,
      causing it to enter recovery.  When a larger packet sent after this time
      is acknowledged, QUIC exits recovery.

   ssthresh:  Slow start threshold in bytes.  When the congestion window
      is below ssthresh, the mode is slow start and the window grows by
      the number of bytes acknowledged.

5.8.3.

7.9.3.  Initialization

   At the beginning of the connection, initialize the congestion control
   variables as follows:

      congestion_window = kInitialWindow
      bytes_in_flight = 0
      end_of_recovery
      recovery_start_time = 0
      ssthresh = infinite
      ecn_ce_counter = 0

5.8.4.

7.9.4.  On Packet Sent

   Whenever a packet is sent, and it contains non-ACK frames, the packet
   increases bytes_in_flight.

      OnPacketSentCC(bytes_sent):
        bytes_in_flight += bytes_sent

5.8.5.

7.9.5.  On Packet Acknowledgement

   Invoked from loss detection's OnPacketAcked and is supplied with the
   acked_packet from sent_packets.

      InRecovery(packet_number):

      InRecovery(sent_time):
        return packet_number sent_time <= end_of_recovery recovery_start_time

      OnPacketAckedCC(acked_packet):
        // Remove from bytes_in_flight.
        bytes_in_flight -= acked_packet.bytes acked_packet.size
        if (InRecovery(acked_packet.packet_number)): (InRecovery(acked_packet.time_sent)):
          // Do not increase congestion window in recovery period.
          return
        if (congestion_window < ssthresh):
          // Slow start.
          congestion_window += acked_packet.bytes acked_packet.size
        else:
          // Congestion avoidance.
          congestion_window += kMaxDatagramSize * acked_packet.bytes acked_packet.size
              / congestion_window

5.8.6.

7.9.6.  On New Congestion Event

   Invoked from ProcessECN and OnPacketsLost when a new congestion event
   is detected.  Starts  May start a new recovery period and reduces the
   congestion window.

      CongestionEvent(packet_number):

      CongestionEvent(sent_time):
        // Start a new congestion event if packet_number
        // the sent time is larger
        // than the end start time of the previous recovery epoch.
        if (!InRecovery(packet_number)):
          end_of_recovery (!InRecovery(sent_time)):
          recovery_start_time = largest_sent_packet Now()
          congestion_window *= kLossReductionFactor
          congestion_window = max(congestion_window, kMinimumWindow)
          ssthresh = congestion_window

5.8.7.
          // Collapse congestion window if persistent congestion
          if (pto_count > kPersistentCongestionThreshold):
            congestion_window = kMinimumWindow

7.9.7.  Process ECN Information

   Invoked when an ACK frame with an ECN section is received from the
   peer.

      ProcessECN(ack):
        // If the ECN-CE counter reported by the peer has increased,
        // this could be a new congestion event.
        if (ack.ce_counter > ecn_ce_counter):
          ecn_ce_counter = ack.ce_counter
          // Start a new congestion event if the last acknowledged
          // packet is past was sent after the end start of the previous
          // recovery epoch.
          CongestionEvent(ack.largest_acked_packet)

5.8.8.
          CongestionEvent(sent_packets[ack.largest_acked].time_sent)

7.9.8.  On Packets Lost

   Invoked by loss detection from DetectLostPackets when new packets are
   detected lost.

      OnPacketsLost(lost_packets):
        // Remove lost packets from bytes_in_flight.
        for (lost_packet : lost_packets):
          bytes_in_flight -= lost_packet.bytes lost_packet.size
        largest_lost_packet = lost_packets.last()

        // Start a new congestion epoch if the last lost packet
        // is past the end of the previous recovery epoch.
        CongestionEvent(largest_lost_packet.packet_number)

5.8.9.  On Retransmission Timeout Verified

   QUIC decreases the congestion window to the minimum value once the
   retransmission timeout has been verified and removes any packets sent
   before the newly acknowledged RTO packet.

      OnRetransmissionTimeoutVerified(packet_number)
        congestion_window = kMinimumWindow
        // Declare all packets prior to packet_number lost.
        for (sent_packet: sent_packets):
          if (sent_packet.packet_number < packet_number):
            bytes_in_flight -= sent_packet.bytes
            sent_packets.remove(sent_packet.packet_number)

6.
        CongestionEvent(largest_lost_packet.time_sent)

8.  Security Considerations

6.1.
8.1.  Congestion Signals

   Congestion control fundamentally involves the consumption of signals
   - both loss and ECN codepoints - from unauthenticated entities.  On-
   path attackers can spoof or alter these signals.  An attacker can
   cause endpoints to reduce their sending rate by dropping packets, or
   alter send rate by changing ECN codepoints.

6.2.

8.2.  Traffic Analysis

   Packets that carry only ACK frames can be heuristically identified by
   observing packet size.  Acknowledgement patterns may expose
   information about link characteristics or application behavior.
   Endpoints can use PADDING frames or bundle acknowledgments with other
   frames to reduce leaked information.

6.3.

8.3.  Misreporting ECN Markings

   A receiver can misreport ECN markings to alter the congestion
   response of a sender.  Suppressing reports of ECN-CE markings could
   cause a sender to increase their send rate.  This increase could
   result in congestion and loss.

   A sender MAY attempt to detect suppression of reports by marking
   occasional packets that they send with ECN-CE.  If a packet marked
   with ECN-CE is not reported as having been marked when the packet is
   acknowledged, the sender SHOULD then disable ECN for that path.

   Reporting additional ECN-CE markings will cause a sender to reduce
   their sending rate, which is similar in effect to advertising reduced
   connection flow control limits and so no advantage is gained by doing
   so.

   Endpoints choose the congestion controller that they use.  Though
   congestion controllers generally treat reports of ECN-CE markings as
   equivalent to loss [RFC8311], the exact response for each controller
   could be different.  Failure to correctly respond to information
   about ECN markings is therefore difficult to detect.

7.

9.  IANA Considerations

   This document has no IANA actions.  Yet.

8.

10.  References

8.1.
10.1.  Normative References

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport-16
              transport-17 (work in progress), October December 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>.

   [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>.

8.2.

10.2.  Informative References

   [FACK]     Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
              Refining TCP Congestion Control", ACM SIGCOMM , August
              1996.

   [RACK]     Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
              a time-based fast loss detection algorithm for TCP",
              draft-ietf-tcpm-rack-04 (work in progress), July 2018.

   [RFC3465]  Allman, M., "TCP Congestion Control with Appropriate Byte
              Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
              2003, <https://www.rfc-editor.org/info/rfc3465>.

   [RFC4653]  Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
              "Improving the Robustness of TCP to Non-Congestion
              Events", RFC 4653, DOI 10.17487/RFC4653, August 2006,
              <https://www.rfc-editor.org/info/rfc4653>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5682]  Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
              "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
              Spurious Retransmission Timeouts with TCP", RFC 5682,
              DOI 10.17487/RFC5682, September 2009,
              <https://www.rfc-editor.org/info/rfc5682>.

   [RFC5827]  Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
              P. Hurtig, "Early Retransmit for TCP and Stream Control
              Transmission Protocol (SCTP)", RFC 5827,
              DOI 10.17487/RFC5827, May 2010,
              <https://www.rfc-editor.org/info/rfc5827>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC6582]  Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
              NewReno Modification to TCP's Fast Recovery Algorithm",
              RFC 6582, DOI 10.17487/RFC6582, April 2012,
              <https://www.rfc-editor.org/info/rfc6582>.

   [RFC6675]  Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
              and Y. Nishida, "A Conservative Loss Recovery Algorithm
              Based on Selective Acknowledgment (SACK) for TCP",
              RFC 6675, DOI 10.17487/RFC6675, August 2012,
              <https://www.rfc-editor.org/info/rfc6675>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.

   [RFC7661]  Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
              TCP to Support Rate-Limited Traffic", RFC 7661,
              DOI 10.17487/RFC7661, October 2015,
              <https://www.rfc-editor.org/info/rfc7661>.

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,
              <https://www.rfc-editor.org/info/rfc8312>.

   [TLP]      Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
              "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
              Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
              in progress), February 2013.

8.3.

10.3.  URIs

   [1] https://mailarchive.ietf.org/arch/search/?email_list=quic

   [2] https://github.com/quicwg

   [3] https://github.com/quicwg/base-drafts/labels/-recovery

Appendix A.  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.

A.1.  Since draft-ietf-quic-recovery-16

   o  Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP
      and min crypto timeouts; eliminate timeout validation (#2114,
      #2166, #2168, #1017)

   o  Redefine how congestion avoidance in terms of when the period
      starts (#1928, #1930)

   o  Document what needs to be tracked for packets that are in flight
      (#765, #1724, #1939)

   o  Integrate both time and packet thresholds into loss detection
      (#1969, #1212, #934, #1974)

   o  Reduce congestion window after idle, unless pacing is used (#2007,
      #2023)

   o  Disable RTT calculation for packets that don't elicit
      acknowledgment (#2060, #2078)

   o  Limit ack_delay by max_ack_delay (#2060, #2099)

   o  Initial keys are discarded once Handshake are avaialble (#1951,
      #2045)

   o  Reorder ECN and loss detection in pseudocode (#2142)

   o  Only cancel loss detection timer if ack-eliciting packets are in
      flight (#2093, #2117)

A.2.  Since draft-ietf-quic-recovery-14

   o  Used max_ack_delay from transport params (#1796, #1782)

   o  Merge ACK and ACK_ECN (#1783)

A.2.

A.3.  Since draft-ietf-quic-recovery-13

   o  Corrected the lack of ssthresh reduction in CongestionEvent
      pseudocode (#1598)

   o  Considerations for ECN spoofing (#1426, #1626)

   o  Clarifications for PADDING and congestion control (#837, #838,
      #1517, #1531, #1540)

   o  Reduce early retransmission timer to RTT/8 (#945, #1581)

   o  Packets are declared lost after an RTO is verified (#935, #1582)

A.3.

A.4.  Since draft-ietf-quic-recovery-12

   o  Changes to manage separate packet number spaces and encryption
      levels (#1190, #1242, #1413, #1450)

   o  Added ECN feedback mechanisms and handling; new ACK_ECN frame
      (#804, #805, #1372)

A.4.

A.5.  Since draft-ietf-quic-recovery-11

   No significant changes.

A.5.

A.6.  Since draft-ietf-quic-recovery-10

   o  Improved text on ack generation (#1139, #1159)

   o  Make references to TCP recovery mechanisms informational (#1195)

   o  Define time_of_last_sent_handshake_packet (#1171)

   o  Added signal from TLS the data it includes needs to be sent in a
      Retry packet (#1061, #1199)

   o  Minimum RTT (min_rtt) is initialized with an infinite value
      (#1169)

A.6.

A.7.  Since draft-ietf-quic-recovery-09

   No significant changes.

A.7.

A.8.  Since draft-ietf-quic-recovery-08

   o  Clarified pacing and RTO (#967, #977)

A.8.

A.9.  Since draft-ietf-quic-recovery-07

   o  Include Ack Delay in RTO(and TLP) computations (#981)

   o  Ack Delay in SRTT computation (#961)

   o  Default RTT and Slow Start (#590)

   o  Many editorial fixes.

A.9.

A.10.  Since draft-ietf-quic-recovery-06

   No significant changes.

A.10.

A.11.  Since draft-ietf-quic-recovery-05

   o  Add more congestion control text (#776)

A.11.

A.12.  Since draft-ietf-quic-recovery-04

   No significant changes.

A.12.

A.13.  Since draft-ietf-quic-recovery-03

   No significant changes.

A.13.

A.14.  Since draft-ietf-quic-recovery-02

   o  Integrate F-RTO (#544, #409)

   o  Add congestion control (#545, #395)

   o  Require connection abort if a skipped packet was acknowledged
      (#415)

   o  Simplify RTO calculations (#142, #417)

A.14.

A.15.  Since draft-ietf-quic-recovery-01

   o  Overview added to loss detection

   o  Changes initial default RTT to 100ms

   o  Added time-based loss detection and fixes early retransmit

   o  Clarified loss recovery for handshake packets

   o  Fixed references and made TCP references informative

A.15.

A.16.  Since draft-ietf-quic-recovery-00

   o  Improved description of constants and ACK behavior

A.16.

A.17.  Since draft-iyengar-quic-loss-recovery-01

   o  Adopted as base for draft-ietf-quic-recovery

   o  Updated authors/editors list

   o  Added table of contents

Acknowledgments

Authors' Addresses

   Jana Iyengar (editor)
   Fastly

   Email: jri.ietf@gmail.com

   Ian Swett (editor)
   Google

   Email: ianswett@google.com