draft-ietf-quic-recovery-18.txt   draft-ietf-quic-recovery-19.txt 
QUIC J. Iyengar, Ed. QUIC J. Iyengar, Ed.
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track I. Swett, Ed. Intended status: Standards Track I. Swett, Ed.
Expires: July 27, 2019 Google Expires: September 12, 2019 Google
January 23, 2019 March 11, 2019
QUIC Loss Detection and Congestion Control QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-18 draft-ietf-quic-recovery-19
Abstract Abstract
This document describes loss detection and congestion control This document describes loss detection and congestion control
mechanisms for QUIC. mechanisms for QUIC.
Note to Readers Note to Readers
Discussion of this draft takes place on the QUIC working group Discussion of this draft takes place on the QUIC working group
mailing list (quic@ietf.org), which is archived at mailing list (quic@ietf.org), which is archived at
skipping to change at page 1, line 42 skipping to change at page 1, line 42
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 27, 2019. This Internet-Draft will expire on September 12, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Design of the QUIC Transmission Machinery . . . . . . . . . . 4 3. Design of the QUIC Transmission Machinery . . . . . . . . . . 5
3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5 3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5
3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 5 3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 6
3.1.2. Monotonically Increasing Packet Numbers . . . . . . . 6 3.1.2. Monotonically Increasing Packet Numbers . . . . . . . 6
3.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6 3.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6
3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6 3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6
3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 6 3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 7
4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7 4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7
4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 7 4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 8
4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8 4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8
5. Computing the RTT estimate . . . . . . . . . . . . . . . . . 8 5. Computing the RTT estimate . . . . . . . . . . . . . . . . . 8
6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9 6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 9 6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 9
6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10 6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10
6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10 6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10
6.2. Timeout Loss Detection . . . . . . . . . . . . . . . . . 10 6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 11
6.2.1. Crypto Retransmission Timeout . . . . . . . . . . . . 11 6.2.1. Retry and Version Negotiation . . . . . . . . . . . . 12
6.2.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . 12 6.2.2. Discarding Keys and Packet State . . . . . . . . . . 12
6.3. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 14 6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 12
6.3.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 14 6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 13
6.4. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 15 6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 13
6.4.1. Constants of interest . . . . . . . . . . . . . . . . 15 6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 14
6.4.2. Variables of interest . . . . . . . . . . . . . . . . 15 6.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 14
6.4.3. Initialization . . . . . . . . . . . . . . . . . . . 16 7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 15
6.4.4. On Sending a Packet . . . . . . . . . . . . . . . . . 17 7.1. Explicit Congestion Notification . . . . . . . . . . . . 15
6.4.5. On Receiving an Acknowledgment . . . . . . . . . . . 17 7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 15
6.4.6. On Packet Acknowledgment . . . . . . . . . . . . . . 19 7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 15
6.4.7. Setting the Loss Detection Timer . . . . . . . . . . 19 7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 16
6.4.8. On Timeout . . . . . . . . . . . . . . . . . . . . . 20 7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 16
6.4.9. Detecting Lost Packets . . . . . . . . . . . . . . . 21 7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 16
6.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 22 7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 16
7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 23 7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Explicit Congestion Notification . . . . . . . . . . . . 23 7.9. Sending data after an idle period . . . . . . . . . . . . 18
7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 23 7.10. Application Limited Sending . . . . . . . . . . . . . . . 18
7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 23 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 24 8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 18
7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 24 8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 19
7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 24 8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 19
7.7. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 24 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7.8. Sending data after an idle period . . . . . . . . . . . . 25 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.9. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 25 10.1. Normative References . . . . . . . . . . . . . . . . . . 19
7.9.1. Constants of interest . . . . . . . . . . . . . . . . 25 10.2. Informative References . . . . . . . . . . . . . . . . . 20
7.9.2. Variables of interest . . . . . . . . . . . . . . . . 26 10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.9.3. Initialization . . . . . . . . . . . . . . . . . . . 27 Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 22
7.9.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 27 A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 22
7.9.5. On Packet Acknowledgement . . . . . . . . . . . . . . 27 A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 22
7.9.6. On New Congestion Event . . . . . . . . . . . . . . . 27 A.2. Constants of interest . . . . . . . . . . . . . . . . . . 22
7.9.7. Process ECN Information . . . . . . . . . . . . . . . 28 A.3. Variables of interest . . . . . . . . . . . . . . . . . . 23
7.9.8. On Packets Lost . . . . . . . . . . . . . . . . . . . 28 A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 28 A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 24
8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 29 A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 25
8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 29 A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 27
8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 29 A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 27
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 29
10.1. Normative References . . . . . . . . . . . . . . . . . . 30 Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . 30 B.1. Constants of interest . . . . . . . . . . . . . . . . . . 30
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 32 B.2. Variables of interest . . . . . . . . . . . . . . . . . . 31
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 32 B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 32
A.1. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 32 B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 32
A.2. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 32 B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 32
A.3. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 33 B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 33
A.4. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 33 B.7. Process ECN Information . . . . . . . . . . . . . . . . . 33
A.5. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 34 B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 33
A.6. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 34 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 34
A.7. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 34 C.1. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 34
A.8. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 34 C.2. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 35
A.9. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 34 C.3. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 35
A.10. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 34 C.4. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 36
A.11. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 35 C.5. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 36
A.12. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 35 C.6. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 36
A.13. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 35 C.7. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 36
A.14. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 35 C.8. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 36
A.15. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 35 C.9. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 37
A.16. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 35 C.10. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 37
A.17. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 35 C.11. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 37
A.18. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 35 C.12. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 37
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36 C.13. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36 C.14. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 37
C.15. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 37
C.16. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 37
C.17. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 38
C.18. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 38
C.19. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 38
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction 1. Introduction
QUIC is a new multiplexed and secure transport atop UDP. QUIC builds QUIC is a new multiplexed and secure transport atop UDP. QUIC builds
on decades of transport and security experience, and implements on decades of transport and security experience, and implements
mechanisms that make it attractive as a modern general-purpose mechanisms that make it attractive as a modern general-purpose
transport. The QUIC protocol is described in [QUIC-TRANSPORT]. transport. The QUIC protocol is described in [QUIC-TRANSPORT].
QUIC implements the spirit of known TCP loss recovery mechanisms, QUIC implements the spirit of existing TCP loss recovery mechanisms,
described in RFCs, various Internet-drafts, and also those prevalent described in RFCs, various Internet-drafts, and also those prevalent
in the Linux TCP implementation. This document describes QUIC in the Linux TCP implementation. This document describes QUIC
congestion control and loss recovery, and where applicable, congestion control and loss recovery, and where applicable,
attributes the TCP equivalent in RFCs, Internet-drafts, academic attributes the TCP equivalent in RFCs, Internet-drafts, academic
papers, and/or TCP implementations. papers, and/or TCP implementations.
2. Conventions and Definitions 2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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Ack-eliciting Frames: All frames besides ACK or PADDING are Ack-eliciting Frames: All frames besides ACK or PADDING are
considered ack-eliciting. considered ack-eliciting.
Ack-eliciting Packets: Packets that contain ack-eliciting frames Ack-eliciting Packets: Packets that contain ack-eliciting frames
elicit an ACK from the receiver within the maximum ack delay and elicit an ACK from the receiver within the maximum ack delay and
are called ack-eliciting packets. are called ack-eliciting packets.
Crypto Packets: Packets containing CRYPTO data sent in Initial or Crypto Packets: Packets containing CRYPTO data sent in Initial or
Handshake packets. Handshake packets.
Out-of-order Packets: Packets that do not increase the largest
received packet number for its packet number space by exactly one.
Packets arrive out of order when earlier packets are lost or
delayed.
3. Design of the QUIC Transmission Machinery 3. Design of the QUIC Transmission Machinery
All transmissions in QUIC are sent with a packet-level header, which All transmissions in QUIC are sent with a packet-level header, which
indicates the encryption level and includes a packet sequence number indicates the encryption level and includes a packet sequence number
(referred to below as a packet number). The encryption level (referred to below as a packet number). The encryption level
indicates the packet number space, as described in [QUIC-TRANSPORT]. indicates the packet number space, as described in [QUIC-TRANSPORT].
Packet numbers never repeat within a packet number space for the Packet numbers never repeat within a packet number space for the
lifetime of a connection. Packet numbers monotonically increase lifetime of a connection. Packet numbers monotonically increase
within a space, preventing ambiguity. within a space, preventing ambiguity.
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o All packets are acknowledged, though packets that contain no ack- o All packets are acknowledged, though packets that contain no ack-
eliciting frames are only acknowledged along with ack-eliciting eliciting frames are only acknowledged along with ack-eliciting
packets. packets.
o Long header packets that contain CRYPTO frames are critical to the o Long header packets that contain CRYPTO frames are critical to the
performance of the QUIC handshake and use shorter timers for performance of the QUIC handshake and use shorter timers for
acknowledgement and retransmission. acknowledgement and retransmission.
o Packets that contain only ACK frames do not count toward o Packets that contain only ACK frames do not count toward
congestion control limits and are not considered in-flight. Note congestion control limits and are not considered in-flight.
that this means PADDING frames cause packets to contribute toward
bytes in flight without directly causing an acknowledgment to be o PADDING frames cause packets to contribute toward bytes in flight
sent. without directly causing an acknowledgment to be sent.
3.1. Relevant Differences Between QUIC and TCP 3.1. Relevant Differences Between QUIC and TCP
Readers familiar with TCP's loss detection and congestion control Readers familiar with TCP's loss detection and congestion control
will find algorithms here that parallel well-known TCP ones. will find algorithms here that parallel well-known TCP ones.
Protocol differences between QUIC and TCP however contribute to Protocol differences between QUIC and TCP however contribute to
algorithmic differences. We briefly describe these protocol algorithmic differences. We briefly describe these protocol
differences below. differences below.
3.1.1. Separate Packet Number Spaces 3.1.1. Separate Packet Number Spaces
QUIC uses separate packet number spaces for each encryption level, QUIC uses separate packet number spaces for each encryption level,
except 0-RTT and all generations of 1-RTT keys use the same packet except 0-RTT and all generations of 1-RTT keys use the same packet
number space. Separate packet number spaces ensures acknowledgement number space. Separate packet number spaces ensures acknowledgement
of packets sent with one level of encryption will not cause spurious of packets sent with one level of encryption will not cause spurious
retransmission of packets sent with a different encryption level. retransmission of packets sent with a different encryption level.
Congestion control and RTT measurement are unified across packet Congestion control and round-trip time (RTT) measurement are unified
number spaces. across packet number spaces.
3.1.2. Monotonically Increasing Packet Numbers 3.1.2. Monotonically Increasing Packet Numbers
TCP conflates transmission order at the sender with delivery order at TCP conflates transmission order at the sender with delivery order at
the receiver, which results in retransmissions of the same data the receiver, which results in retransmissions of the same data
carrying the same sequence number, and consequently leads to carrying the same sequence number, and consequently leads to
"retransmission ambiguity". QUIC separates the two: QUIC uses a "retransmission ambiguity". QUIC separates the two: QUIC uses a
packet number to indicate transmission order, and any application packet number to indicate transmission order, and any application
data is sent in one or more streams, with delivery order determined data is sent in one or more streams, with delivery order determined
by stream offsets encoded within STREAM frames. by stream offsets encoded within STREAM frames.
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4. Generating Acknowledgements 4. Generating Acknowledgements
QUIC SHOULD delay sending acknowledgements in response to packets, QUIC SHOULD delay sending acknowledgements in response to packets,
but MUST NOT excessively delay acknowledgements of ack-eliciting but MUST NOT excessively delay acknowledgements of ack-eliciting
packets. Specifically, implementations MUST attempt to enforce a packets. Specifically, implementations MUST attempt to enforce a
maximum ack delay to avoid causing the peer spurious timeouts. The maximum ack delay to avoid causing the peer spurious timeouts. The
maximum ack delay is communicated in the "max_ack_delay" transport maximum ack delay is communicated in the "max_ack_delay" transport
parameter and the default value is 25ms. parameter and the default value is 25ms.
An acknowledgement SHOULD be sent immediately upon receipt of a An acknowledgement SHOULD be sent immediately upon receipt of a
second packet but the delay SHOULD NOT exceed the maximum ack delay. second ack-eliciting packet. QUIC recovery algorithms do not assume
QUIC recovery algorithms do not assume the peer generates an the peer sends an ACK immediately when receiving a second ack-
acknowledgement immediately when receiving a second full-packet. eliciting packet.
Out-of-order packets SHOULD be acknowledged more quickly, in order to In order to accelerate loss recovery and reduce timeouts, the
accelerate loss recovery. The receiver SHOULD send an immediate ACK receiver SHOULD send an immediate ACK after it receives an out-of-
when it receives a new packet which is not one greater than the order packet. It could send immediate ACKs for in-order packets for
largest received packet number. a period of time that SHOULD NOT exceed 1/8 RTT unless more out-of-
order packets arrive. If every packet arrives out-of- order, then an
immediate ACK SHOULD be sent for every received packet.
Similarly, packets marked with the ECN Congestion Experienced (CE) Similarly, packets marked with the ECN Congestion Experienced (CE)
codepoint in the IP header SHOULD be acknowledged immediately, to codepoint in the IP header SHOULD be acknowledged immediately, to
reduce the peer's response time to congestion events. reduce the peer's response time to congestion events.
As an optimization, a receiver MAY process multiple packets before As an optimization, a receiver MAY process multiple packets before
sending any ACK frames in response. In this case they can determine sending any ACK frames in response. In this case the receiver can
whether an immediate or delayed acknowledgement should be generated determine whether an immediate or delayed acknowledgement should be
after processing incoming packets. generated after processing incoming packets.
4.1. Crypto Handshake Data 4.1. Crypto Handshake Data
In order to quickly complete the handshake and avoid spurious In order to quickly complete the handshake and avoid spurious
retransmissions due to crypto retransmission timeouts, crypto packets retransmissions due to crypto retransmission timeouts, crypto packets
SHOULD use a very short ack delay, such as 1ms. ACK frames MAY be SHOULD use a very short ack delay, such as the local timer
sent immediately when the crypto stack indicates all data for that granularity. ACK frames MAY be sent immediately when the crypto
packet number space has been received. stack indicates all data for that packet number space has been
received.
4.2. ACK Ranges 4.2. ACK Ranges
When an ACK frame is sent, one or more ranges of acknowledged packets When an ACK frame is sent, one or more ranges of acknowledged packets
are included. Including older packets reduces the chance of spurious are included. Including older packets reduces the chance of spurious
retransmits caused by losing previously sent ACK frames, at the cost retransmits caused by losing previously sent ACK frames, at the cost
of larger ACK frames. of larger ACK frames.
ACK frames SHOULD always acknowledge the most recently received ACK frames SHOULD always acknowledge the most recently received
packets, and the more out-of-order the packets are, the more packets, and the more out-of-order the packets are, the more
skipping to change at page 8, line 30 skipping to change at page 8, line 49
of 1 RTT of reordering. In cases with ACK frame loss and reordering, of 1 RTT of reordering. In cases with ACK frame loss and reordering,
this approach does not guarantee that every acknowledgement is seen this approach does not guarantee that every acknowledgement is seen
by the sender before it is no longer included in the ACK frame. 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 Packets could be received out of order and all subsequent ACK frames
containing them could be lost. In this case, the loss recovery containing them could be lost. In this case, the loss recovery
algorithm may cause spurious retransmits, but the sender will algorithm may cause spurious retransmits, but the sender will
continue making forward progress. continue making forward progress.
5. Computing the RTT estimate 5. Computing the RTT estimate
RTT is calculated when an ACK frame arrives by computing the Round-trip time (RTT) is calculated when an ACK frame arrives by
difference between the current time and the time the largest acked computing the difference between the current time and the time the
packet was sent. An RTT sample MUST NOT be taken for a packet that largest acked packet was sent. An RTT sample MUST NOT be taken for a
is not newly acknowledged or not ack-eliciting. packet that is not newly acknowledged or not ack-eliciting.
When RTT is calculated, the ack delay field from the ACK frame SHOULD 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 be limited to the max_ack_delay specified by the peer. Limiting
ack_delay to max_ack_delay ensures a peer specifying an extremely 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 small max_ack_delay doesn't cause more spurious timeouts than a peer
that correctly specifies max_ack_delay. It SHOULD be subtracted from that correctly specifies max_ack_delay. It SHOULD be subtracted from
the RTT as long as the result is larger than the min_rtt. If the the RTT as long as the result is larger than the min_rtt. If the
result is smaller than the min_rtt, the RTT should be used, but the result is smaller than the min_rtt, the RTT should be used, but the
ack delay field should be ignored. ack delay field should be ignored.
A sender calculates both smoothed RTT and RTT variance similar to A sender calculates both smoothed RTT (SRTT) and RTT variance
those specified in [RFC6298], see Section 6.4.5. (RTTVAR) similar to those specified in [RFC6298], see Appendix A.6.
A sender takes an RTT sample when an ACK frame is received that A sender takes an RTT sample when an ACK frame is received that
acknowledges a larger packet number than before (see Section 6.4.5). acknowledges a larger packet number than before (see Appendix A.6).
A sender will take multiple RTT samples per RTT when multiple such A sender will take multiple RTT samples per RTT when multiple such
ACK frames are received within an RTT. When multiple samples are ACK frames are received within an RTT. When multiple samples are
generated within an RTT, the smoothed RTT and RTT variance could generated within an RTT, the smoothed RTT and RTT variance could
retain inadequate history, as suggested in [RFC6298]. Changing these retain inadequate history, as suggested in [RFC6298]. Changing these
computations is currently an open research question. computations is currently an open research question.
min_rtt is the minimum RTT measured over the connection, prior to min_rtt is the minimum RTT measured over the connection, prior to
adjusting by ack delay. Ignoring ack delay for min RTT prevents adjusting by ack delay. Ignoring ack delay for min RTT prevents
intentional or unintentional underestimation of min RTT, which in intentional or unintentional underestimation of min RTT, which in
turn prevents underestimating smoothed RTT. turn prevents underestimating smoothed RTT.
6. Loss Detection 6. Loss Detection
QUIC senders use both ack information and timeouts to detect lost QUIC senders use both ack information and timeouts to detect lost
packets, and this section provides a description of these algorithms. 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 If a packet is lost, the QUIC transport needs to recover from that
loss, such as by retransmitting the data, sending an updated frame, loss, such as by retransmitting the data, sending an updated frame,
or abandoning the frame. For more information, see Section 13.2 of or abandoning the frame. For more information, see Section 13.2 of
[QUIC-TRANSPORT]. [QUIC-TRANSPORT].
6.1. Acknowledgement-based Detection 6.1. Acknowledgement-based Detection
Acknowledgement-based loss detection implements the spirit of TCP's Acknowledgement-based loss detection implements the spirit of TCP's
Fast Retransmit [RFC5681], Early Retransmit [RFC5827], FACK [FACK], Fast Retransmit [RFC5681], Early Retransmit [RFC5827], FACK [FACK],
skipping to change at page 10, line 31 skipping to change at page 10, line 46
max(SRTT, latest_RTT). If packets sent prior to the largest max(SRTT, latest_RTT). If packets sent prior to the largest
acknowledged packet cannot yet be declared lost, then a timer SHOULD acknowledged packet cannot yet be declared lost, then a timer SHOULD
be set for the remaining time. be set for the remaining time.
The RECOMMENDED time threshold (kTimeThreshold), expressed as a The RECOMMENDED time threshold (kTimeThreshold), expressed as a
round-trip time multiplier, is 9/8. round-trip time multiplier, is 9/8.
Using max(SRTT, latest_RTT) protects from the two following cases: Using max(SRTT, latest_RTT) protects from the two following cases:
o the latest RTT sample is lower than the SRTT, perhaps due to o the latest RTT sample is lower than the SRTT, perhaps due to
reordering where packet whose ack triggered the Early Retransmit reordering where the acknowledgement encountered a shorter path;
process encountered a shorter path;
o the latest RTT sample is higher than the SRTT, perhaps due to a 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 sustained increase in the actual RTT, but the smoothed SRTT has
not yet caught up. not yet caught up.
Implementations MAY experiment with absolute thresholds, thresholds Implementations MAY experiment with absolute thresholds, thresholds
from previous connections, adaptive thresholds, or including RTT from previous connections, adaptive thresholds, or including RTT
variance. Smaller thresholds reduce reordering resilience and variance. Smaller thresholds reduce reordering resilience and
increase spurious retransmissions, and larger thresholds increase increase spurious retransmissions, and larger thresholds increase
loss detection delay. loss detection delay.
6.2. Timeout Loss Detection 6.2. Crypto Retransmission Timeout
Timeout loss detection recovers from losses that cannot be handled by
acknowledgement-based loss detection. It uses a single timer which
switches between a crypto retransmission timer and a probe timer.
6.2.1. Crypto Retransmission Timeout
Data in CRYPTO frames is critical to QUIC transport and crypto Data in CRYPTO frames is critical to QUIC transport and crypto
negotiation, so a more aggressive timeout is used to retransmit it. negotiation, so a more aggressive timeout is used to retransmit it.
The initial crypto retransmission timeout SHOULD be set to twice the The initial crypto retransmission timeout SHOULD be set to twice the
initial RTT. initial RTT.
At the beginning, there are no prior RTT samples within a connection. At the beginning, there are no prior RTT samples within a connection.
Resumed connections over the same network SHOULD use the previous Resumed connections over the same network SHOULD use the previous
connection's final smoothed RTT value as the resumed connection's connection's final smoothed RTT value as the resumed connection's
initial RTT. If no previous RTT is available, or if the network initial RTT. If no previous RTT is available, or if the network
changes, the initial RTT SHOULD be set to 100ms. When an changes, the initial RTT SHOULD be set to 100ms. When an
acknowledgement is received, a new RTT is computed and the timer acknowledgement is received, a new RTT is computed and the timer
SHOULD be set for twice the newly computed smoothed RTT. SHOULD be set for twice the newly computed smoothed RTT.
When crypto packets are sent, the sender MUST set a timer for the When a crypto packet is sent, the sender MUST set a timer for the
crypto timeout period. Upon timeout, the sender MUST retransmit all crypto timeout period. This timer MUST be updated when a new crypto
packet is sent. Upon timeout, the sender MUST retransmit all
unacknowledged CRYPTO data if possible. unacknowledged CRYPTO data if possible.
Until the server has validated the client's address on the path, the Until the server has validated the client's address on the path, the
amount of data it can send is limited, as specified in amount of data it can send is limited, as specified in
[QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent, [QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent,
then all unacknowledged CRYPTO data sent in Initial packets should be then all unacknowledged CRYPTO data sent in Initial packets should be
retransmitted. If no data can be sent, then no alarm should be armed retransmitted. If no data can be sent, then no alarm should be armed
until data has been received from the client. until data has been received from the client.
Because the server could be blocked until more packets are received, Because the server could be blocked until more packets are received,
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is no unacknowledged CRYPTO data. If the timer expires and the is no unacknowledged CRYPTO data. If the timer expires and the
client has no CRYPTO data to retransmit and does not have Handshake 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 keys, it SHOULD send an Initial packet in a UDP datagram of at least
1200 bytes. If the client has Handshake keys, it SHOULD send a 1200 bytes. If the client has Handshake keys, it SHOULD send a
Handshake packet. Handshake packet.
On each consecutive expiration of the crypto timer without receiving On each consecutive expiration of the crypto timer without receiving
an acknowledgement for a new packet, the sender SHOULD double the an acknowledgement for a new packet, the sender SHOULD double the
crypto retransmission timeout and set a timer for this period. crypto retransmission timeout and set a timer for this period.
When crypto packets are in flight, the probe timer (Section 6.2.2) is When crypto packets are in flight, the probe timer (Section 6.3) is
not active. not active.
6.2.1.1. Retry and Version Negotiation 6.2.1. Retry and Version Negotiation
A Retry or Version Negotiation packet causes a client to send another A Retry or Version Negotiation packet causes a client to send another
Initial packet, effectively restarting the connection process and Initial packet, effectively restarting the connection process and
resetting congestion control and loss recovery state, including resetting congestion control and loss recovery state, including
resetting any pending timers. Either packet indicates that the resetting any pending timers. Either packet indicates that the
Initial was received but not processed. Neither packet can be Initial was received but not processed. Neither packet can be
treated as an acknowledgment for the Initial. treated as an acknowledgment for the Initial.
The client MAY however compute an RTT estimate to the server as the The client MAY however compute an RTT estimate to the server as the
time period from when the first Initial was sent to when a Retry or a time period from when the first Initial was sent to when a Retry or a
Version Negotiation packet is received. The client MAY use this Version Negotiation packet is received. The client MAY use this
value to seed the RTT estimator for a subsequent connection attempt value to seed the RTT estimator for a subsequent connection attempt
to the server. to the server.
6.2.1.2. Discarding Keys and Packet State 6.2.2. Discarding Keys and Packet State
When packet protection keys are discarded (see Section 4.9 of When packet protection keys are discarded (see Section 4.9 of
[QUIC-TLS]), all packets that were sent with those keys can no longer [QUIC-TLS]), all packets that were sent with those keys can no longer
be acknowledged because their acknowledgements cannot be processed be acknowledged because their acknowledgements cannot be processed
anymore. The sender considers them no longer in flight. That is, anymore. The sender MUST discard all recovery state associated with
the sender SHOULD discard all recovery state associated with those those packets and MUST remove them from the count of bytes in flight.
packets and MUST remove them from the count of bytes in flight.
Endpoints stop sending and receiving Initial packets once they start Endpoints stop sending and receiving Initial packets once they start
exchanging Handshake packets (see Section 17.2.2.1 of exchanging Handshake packets (see Section 17.2.2.1 of
[QUIC-TRANSPORT]). At this point, recovery state for all in-flight [QUIC-TRANSPORT]). At this point, recovery state for all in-flight
Initial packets is discarded. Initial packets is discarded.
When 0-RTT is rejected, recovery state for all in-flight 0-RTT When 0-RTT is rejected, recovery state for all in-flight 0-RTT
packets is discarded. packets is discarded.
If a server accepts 0-RTT, but does not buffer 0-RTT packets that If a server accepts 0-RTT, but does not buffer 0-RTT packets that
arrive before Initial packets, early 0-RTT packets will be declared arrive before Initial packets, early 0-RTT packets will be declared
lost, but that is expected to be infrequent. lost, but that is expected to be infrequent.
It is expected that keys are discarded after packets encrypted with It is expected that keys are discarded after packets encrypted with
them would be acknowledged or declared lost. Initial secrets however them would be acknowledged or declared lost. Initial secrets however
might be destroyed sooner, as soon as handshake keys are available might be destroyed sooner, as soon as handshake keys are available
(see Section 4.10 of [QUIC-TLS]). (see Section 4.10 of [QUIC-TLS]).
6.2.2. Probe Timeout 6.3. Probe Timeout
A Probe Timeout (PTO) triggers a probe packet when ack-eliciting data A Probe Timeout (PTO) triggers a probe packet when ack-eliciting data
is in flight but an acknowledgement is not received within the is in flight but an acknowledgement is not received within the
expected period of time. A PTO enables a connection to recover from expected period of time. A PTO enables a connection to recover from
loss of tail packets or acks. The PTO algorithm used in QUIC loss of tail packets or acks. The PTO algorithm used in QUIC
implements the reliability functions of Tail Loss Probe [TLP] [RACK], implements the reliability functions of Tail Loss Probe [TLP] [RACK],
RTO [RFC5681] and F-RTO algorithms for TCP [RFC5682], and the timeout RTO [RFC5681] and F-RTO algorithms for TCP [RFC5682], and the timeout
computation is based on TCP's retransmission timeout period computation is based on TCP's retransmission timeout period
[RFC6298]. [RFC6298].
6.2.2.1. Computing PTO 6.3.1. Computing PTO
When an ack-eliciting packet is transmitted, the sender schedules a When an ack-eliciting packet is transmitted, the sender schedules a
timer for the PTO period as follows: timer for the PTO period as follows:
PTO = max(smoothed_rtt + 4*rttvar + max_ack_delay, kGranularity) PTO = smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay
kGranularity, smoothed_rtt, rttvar, and max_ack_delay are defined in kGranularity, smoothed_rtt, rttvar, and max_ack_delay are defined in
Section 6.4.1 and Section 6.4.2. Appendix A.2 and Appendix A.3.
The PTO period is the amount of time that a sender ought to wait for The PTO period is the amount of time that a sender ought to wait for
an acknowledgement of a sent packet. This time period includes the an acknowledgement of a sent packet. This time period includes the
estimated network roundtrip-time (smoothed_rtt), the variance in the estimated network roundtrip-time (smoothed_rtt), the variance in the
estimate (4*rttvar), and max_ack_delay, to account for the maximum estimate (4*rttvar), and max_ack_delay, to account for the maximum
time by which a receiver might delay sending an acknowledgement. time by which a receiver might delay sending an acknowledgement.
The PTO value MUST be set to at least kGranularity, to avoid the The PTO value MUST be set to at least kGranularity, to avoid the
timer expiring immediately. timer expiring immediately.
When a PTO timer expires, the PTO period MUST be set to twice its When a PTO timer expires, the sender probes the network as described
current value. This exponential reduction in the sender's rate is in the next section. The PTO period MUST be set to twice its current
important because the PTOs might be caused by loss of packets or value. This exponential reduction in the sender's rate is important
because the PTOs might be caused by loss of packets or
acknowledgements due to severe congestion. acknowledgements due to severe congestion.
A sender computes its PTO timer every time an ack-eliciting packet is A sender computes its PTO timer every time an ack-eliciting packet is
sent. A sender might choose to optimize this by setting the timer sent. A sender might choose to optimize this by setting the timer
fewer times if it knows that more ack-eliciting packets will be sent fewer times if it knows that more ack-eliciting packets will be sent
within a short period of time. within a short period of time.
6.2.2.2. Sending Probe Packets 6.3.2. Sending Probe Packets
When a PTO timer expires, the sender MUST send one ack-eliciting When a PTO timer expires, the sender MUST send one ack-eliciting
packet as a probe. A sender MAY send up to two ack-eliciting packet as a probe. A sender MAY send up to two ack-eliciting
packets, to avoid an expensive consecutive PTO expiration due to a packets, to avoid an expensive consecutive PTO expiration due to a
single packet loss. single packet loss.
Consecutive PTO periods increase exponentially, and as a result, Consecutive PTO periods increase exponentially, and as a result,
connection recovery latency increases exponentially as packets connection recovery latency increases exponentially as packets
continue to be dropped in the network. Sending two packets on PTO continue to be dropped in the network. Sending two packets on PTO
expiration increases resilience to packet drops, thus reducing the expiration increases resilience to packet drops, thus reducing the
skipping to change at page 14, line 7 skipping to change at page 14, line 11
Probe packets sent on a PTO MUST be ack-eliciting. A probe packet Probe packets sent on a PTO MUST be ack-eliciting. A probe packet
SHOULD carry new data when possible. A probe packet MAY carry SHOULD carry new data when possible. A probe packet MAY carry
retransmitted unacknowledged data when new data is unavailable, when retransmitted unacknowledged data when new data is unavailable, when
flow control does not permit new data to be sent, or to flow control does not permit new data to be sent, or to
opportunistically reduce loss recovery delay. Implementations MAY opportunistically reduce loss recovery delay. Implementations MAY
use alternate strategies for determining the content of probe use alternate strategies for determining the content of probe
packets, including sending new or retransmitted data based on the packets, including sending new or retransmitted data based on the
application's priorities. application's priorities.
When the PTO timer expires multiple times and new data cannot be
sent, implementations must choose between sending the same payload
every time or sending different payloads. Sending the same payload
may be simpler and ensures the highest priority frames arrive first.
Sending different payloads each time reduces the chances of spurious
retransmission.
When a PTO timer expires, new or previously-sent data may not be When a PTO timer expires, new or previously-sent data may not be
available to send and packets may still be in flight. A sender can available to send and packets may still be in flight. A sender can
be blocked from sending new data in the future if packets are left in be blocked from sending new data in the future if packets are left in
flight. Under these conditions, a sender SHOULD mark any packets flight. Under these conditions, a sender SHOULD mark any packets
still in flight as lost. If a sender wishes to establish delivery of still in flight as lost. If a sender wishes to establish delivery of
packets still in flight, it MAY send an ack-eliciting packet and re- packets still in flight, it MAY send an ack-eliciting packet and re-
arm the PTO timer instead. arm the PTO timer instead.
6.2.2.3. Loss Detection 6.3.3. Loss Detection
Delivery or loss of packets in flight is established when an ACK Delivery or loss of packets in flight is established when an ACK
frame is received that newly acknowledges one or more packets. frame is received that newly acknowledges one or more packets.
A PTO timer expiration event does not indicate packet loss and MUST A PTO timer expiration event does not indicate packet loss and MUST
NOT cause prior unacknowledged packets to be marked as lost. After a NOT cause prior unacknowledged packets to be marked as lost. When an
PTO timer has expired, an endpoint uses the following rules to mark acknowledgement is received that newly acknowledges packets, loss
packets as lost when an acknowledgement is received that newly detection proceeds as dictated by packet and time threshold
acknowledges packets.
When an acknowledgement is received that newly acknowledges packets,
loss detection proceeds as dictated by packet and time threshold
mechanisms, see Section 6.1. mechanisms, see Section 6.1.
6.3. Tracking Sent Packets 6.4. 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.
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 Appendix B.2) beyond the available
congestion window, unless the packet is a probe packet sent after a
PTO timer expires, as described in Section 6.3.
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.
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.
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 PTO. While in slow start, QUIC
increases the congestion window by the number of bytes acknowledged
when each acknowledgment is processed.
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.
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 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.
7.5. Ignoring Loss of Undecryptable Packets
During the handshake, some packet protection keys might not be
available when a packet arrives. In particular, Handshake and 0-RTT
packets cannot be processed until the Initial packets arrive, and
1-RTT packets cannot be processed until the handshake completes.
Endpoints MAY ignore the loss of Handshake, 0-RTT, and 1-RTT packets
that might arrive before the peer has packet protection keys to
process those packets.
7.6. Probe Timeout
Probe packets MUST NOT be blocked by the congestion controller. A
sender MUST however count these packets as being additionally in
flight, since these packets add network load without establishing
packet loss. Note that sending probe packets might cause the
sender's bytes in flight to exceed the congestion window until an
acknowledgement is received that establishes loss or delivery of
packets.
7.7. Persistent Congestion
When an ACK frame is received that establishes loss of all in-flight
packets sent over a long enough period of time, the network is
considered to be experiencing persistent congestion. Commonly, this
can be established by consecutive PTOs, but since the PTO timer is
reset when a new ack-eliciting packet is sent, an explicit duration
must be used to account for those cases where PTOs do not occur or
are substantially delayed. This duration is the equivalent of
kPersistentCongestionThreshold consecutive PTOs, and is computed as
follows: ~~~ (smoothed_rtt + 4 * rttvar + max_ack_delay) * ((2 ^
kPersistentCongestionThreshold) - 1) ~~~
For example, assume:
smoothed_rtt = 1 rttvar = 0 max_ack_delay = 0
kPersistentCongestionThreshold = 2
If an eck-eliciting packet is sent at time = 0, the following
scenario would illustrate persistent congestion:
+-----+------------------------+
| t=0 | Send Pkt #1 (App Data) |
+-----+------------------------+
| t=1 | Send Pkt #2 (PTO 1) |
| | |
| t=3 | Send Pkt #3 (PTO 2) |
| | |
| t=7 | Send Pkt #4 (PTO 3) |
| | |
| t=8 | Recv ACK of Pkt #4 |
+-----+------------------------+
The first three packets are determined to be lost when the ACK of
packet 4 is received at t=8. The congestion period is calculated as
the time between the oldest and newest lost packets: (3 - 0) = 3.
The duration for persistent congestion is equal to: (1 * ((2 ^
kPersistentCongestionThreshold) - 1)) = 3. Because the threshold was
reached and because none of the packets between the oldest and the
newest packets are acknowledged, the network is considered to have
experienced persistent congestion.
When persistent congestion is established, the sender's congestion
window MUST be reduced to the minimum congestion window
(kMinimumWindow). This response of collapsing the congestion window
on persistent congestion is functionally similar to a sender's
response on a Retransmission Timeout (RTO) in TCP [RFC5681] after
Tail Loss Probes (TLP) [TLP].
7.8. 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).
7.9. 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].
7.10. Application Limited Sending
The congestion window should not be increased in slow start or
congestion avoidance when it is not fully utilized. The congestion
window could be under-utilized due to insufficient application data
or flow control credit.
A sender that paces packets (see Section 7.8) might delay sending
packets and not fully utilize the congestion window due to this
delay. A sender should not consider itself application limited if it
would have fully utilized the congestion window without pacing delay.
8. Security Considerations
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.
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.
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.
9. IANA Considerations
This document has no IANA actions. Yet.
10. References
10.1. Normative References
[QUIC-TLS]
Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", draft-ietf-quic-tls-19 (work in progress), March
2019.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport-19 (work in progress), March 2019.
[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>.
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.
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. Loss Recovery Pseudocode
We now describe an example implementation of the loss detection
mechanisms described in Section 6.
A.1. Tracking Sent Packets
To correctly implement congestion control, a QUIC sender tracks every To correctly implement congestion control, a QUIC sender tracks every
ack-eliciting packet until the packet is acknowledged or lost. It is ack-eliciting packet until the packet is acknowledged or lost. It is
expected that implementations will be able to access this information expected that implementations will be able to access this information
by packet number and crypto context and store the per-packet fields by packet number and crypto context and store the per-packet fields
(Section 6.3.1) for loss recovery and congestion control. (Appendix A.1.1) for loss recovery and congestion control.
After a packet is declared lost, it SHOULD be tracked for an amount After a packet is declared lost, it SHOULD be tracked for an amount
of time comparable to the maximum expected packet reordering, such as of time comparable to the maximum expected packet reordering, such as
1 RTT. This allows for detection of spurious retransmissions. 1 RTT. This allows for detection of spurious retransmissions.
Sent packets are tracked for each packet number space, and ACK Sent packets are tracked for each packet number space, and ACK
processing only applies to a single space. processing only applies to a single space.
6.3.1. Sent Packet Fields A.1.1. Sent Packet Fields
packet_number: The packet number of the sent packet. packet_number: The packet number of the sent packet.
ack_eliciting: A boolean that indicates whether a packet is ack- ack_eliciting: A boolean that indicates whether a packet is ack-
eliciting. If true, it is expected that an acknowledgement will eliciting. If true, it is expected that an acknowledgement will
be received, though the peer could delay sending the ACK frame be received, though the peer could delay sending the ACK frame
containing it by up to the MaxAckDelay. containing it by up to the MaxAckDelay.
in_flight: A boolean that indicates whether the packet counts in_flight: A boolean that indicates whether the packet counts
towards bytes in flight. towards bytes in flight.
skipping to change at page 15, line 21 skipping to change at page 22, line 48
contains cryptographic handshake messages critical to the contains cryptographic handshake messages critical to the
completion of the QUIC handshake. In this version of QUIC, this completion of the QUIC handshake. In this version of QUIC, this
includes any packet with the long header that includes a CRYPTO includes any packet with the long header that includes a CRYPTO
frame. frame.
sent_bytes: The number of bytes sent in the packet, not including sent_bytes: The number of bytes sent in the packet, not including
UDP or IP overhead, but including QUIC framing overhead. UDP or IP overhead, but including QUIC framing overhead.
time_sent: The time the packet was sent. time_sent: The time the packet was sent.
6.4. Pseudocode A.2. Constants of interest
6.4.1. Constants of interest
Constants used in loss recovery are based on a combination of RFCs, Constants used in loss recovery are based on a combination of RFCs,
papers, and common practice. Some may need to be changed or papers, and common practice. Some may need to be changed or
negotiated in order to better suit a variety of environments. negotiated in order to better suit a variety of environments.
kPacketThreshold: Maximum reordering in packets before packet kPacketThreshold: Maximum reordering in packets before packet
threshold loss detection considers a packet lost. The RECOMMENDED threshold loss detection considers a packet lost. The RECOMMENDED
value is 3. value is 3.
kTimeThreshold: Maximum reordering in time before time threshold kTimeThreshold: Maximum reordering in time before time threshold
loss detection considers a packet lost. Specified as an RTT loss detection considers a packet lost. Specified as an RTT
multiplier. The RECOMMENDED value is 9/8. multiplier. The RECOMMENDED value is 9/8.
kGranularity: Timer granularity. This is a system-dependent value. kGranularity: Timer granularity. This is a system-dependent value.
However, implementations SHOULD use a value no smaller than 1ms. However, implementations SHOULD use a value no smaller than 1ms.
kInitialRtt: The RTT used before an RTT sample is taken. The kInitialRtt: The RTT used before an RTT sample is taken. The
RECOMMENDED value is 100ms. RECOMMENDED value is 100ms.
6.4.2. Variables of interest kPacketNumberSpace: An enum to enumerate the three packet number
spaces. ~~~ enum kPacketNumberSpace { Initial, Handshake,
ApplicationData, } ~~~
A.3. Variables of interest
Variables required to implement the congestion control mechanisms are Variables required to implement the congestion control mechanisms are
described in this section. described in this section.
loss_detection_timer: Multi-modal timer used for loss detection. loss_detection_timer: Multi-modal timer used for loss detection.
crypto_count: The number of times all unacknowledged CRYPTO data has crypto_count: The number of times all unacknowledged CRYPTO data has
been retransmitted without receiving an ack. been retransmitted without receiving an ack.
pto_count: The number of times a PTO has been sent without receiving pto_count: The number of times a PTO has been sent without receiving
an ack. an ack.
time_of_last_sent_ack_eliciting_packet: The time the most recent time_of_last_sent_ack_eliciting_packet: The time the most recent
ack-eliciting packet was sent. ack-eliciting packet was sent.
time_of_last_sent_crypto_packet: The time the most recent crypto time_of_last_sent_crypto_packet: The time the most recent crypto
packet was sent. packet was sent.
largest_sent_packet: The packet number of the most recently sent largest_acked_packet[kPacketNumberSpace]: The largest packet number
packet. acknowledged in the packet number space so far.
largest_acked_packet: The largest packet number acknowledged in the
packet number space so far.
latest_rtt: The most recent RTT measurement made when receiving an latest_rtt: The most recent RTT measurement made when receiving an
ack for a previously unacked packet. ack for a previously unacked packet.
smoothed_rtt: The smoothed RTT of the connection, computed as smoothed_rtt: The smoothed RTT of the connection, computed as
described in [RFC6298] described in [RFC6298]
rttvar: The RTT variance, 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. min_rtt: The minimum RTT seen in the connection, ignoring ack delay.
max_ack_delay: The maximum amount of time by which the receiver max_ack_delay: The maximum amount of time by which the receiver
intends to delay acknowledgments, in milliseconds. The actual intends to delay acknowledgments, in milliseconds. The actual
ack_delay in a received ACK frame may be larger due to late ack_delay in a received ACK frame may be larger due to late
timers, reordering, or lost ACKs. timers, reordering, or lost ACKs.
loss_time: The time at which the next packet will be considered lost loss_time[kPacketNumberSpace]: The time at which the next packet in
based on early transmit or exceeding the reordering window in that packet number space will be considered lost based on
time. exceeding the reordering window in time.
sent_packets: An association of packet numbers to information about sent_packets[kPacketNumberSpace]: An association of packet numbers
them. Described in detail above in Section 6.3. in a packet number space to information about them. Described in
detail above in Appendix A.1.
6.4.3. Initialization A.4. Initialization
At the beginning of the connection, initialize the loss detection At the beginning of the connection, initialize the loss detection
variables as follows: variables as follows:
loss_detection_timer.reset() loss_detection_timer.reset()
crypto_count = 0 crypto_count = 0
pto_count = 0 pto_count = 0
loss_time = 0
smoothed_rtt = 0 smoothed_rtt = 0
rttvar = 0 rttvar = 0
min_rtt = infinite min_rtt = infinite
time_of_last_sent_ack_eliciting_packet = 0 time_of_last_sent_ack_eliciting_packet = 0
time_of_last_sent_crypto_packet = 0 time_of_last_sent_crypto_packet = 0
largest_sent_packet = 0 for pn_space in [ Initial, Handshake, ApplicatonData ]:
largest_acked_packet = 0 largest_acked_packet[pn_space] = 0
loss_time[pn_space] = 0
6.4.4. On Sending a Packet A.5. On Sending a Packet
After a packet is sent, information about the packet is stored. The After a packet is sent, information about the packet is stored. The
parameters to OnPacketSent are described in detail above in parameters to OnPacketSent are described in detail above in
Section 6.3.1. Appendix A.1.1.
Pseudocode for OnPacketSent follows: Pseudocode for OnPacketSent follows:
OnPacketSent(packet_number, ack_eliciting, in_flight, OnPacketSent(packet_number, pn_space, ack_eliciting,
is_crypto_packet, sent_bytes): in_flight, is_crypto_packet, sent_bytes):
largest_sent_packet = packet_number sent_packets[pn_space][packet_number].packet_number =
sent_packets[packet_number].packet_number = packet_number packet_number
sent_packets[packet_number].time_sent = now sent_packets[pn_space][packet_number].time_sent = now
sent_packets[packet_number].ack_eliciting = ack_eliciting sent_packets[pn_space][packet_number].ack_eliciting =
sent_packets[packet_number].in_flight = in_flight ack_eliciting
sent_packets[pn_space][packet_number].in_flight = in_flight
if (in_flight): if (in_flight):
if (is_crypto_packet): if (is_crypto_packet):
time_of_last_sent_crypto_packet = now time_of_last_sent_crypto_packet = now
if (ack_eliciting): if (ack_eliciting):
time_of_last_sent_ack_eliciting_packet = now time_of_last_sent_ack_eliciting_packet = now
OnPacketSentCC(sent_bytes) OnPacketSentCC(sent_bytes)
sent_packets[packet_number].size = sent_bytes sent_packets[pn_space][packet_number].size = sent_bytes
SetLossDetectionTimer() SetLossDetectionTimer()
6.4.5. On Receiving an Acknowledgment A.6. On Receiving an Acknowledgment
When an ACK frame is received, it may newly acknowledge any number of When an ACK frame is received, it may newly acknowledge any number of
packets. packets.
Pseudocode for OnAckReceived and UpdateRtt follow: Pseudocode for OnAckReceived and UpdateRtt follow:
OnAckReceived(ack): OnAckReceived(ack, pn_space):
largest_acked_packet = max(largest_acked_packet, largest_acked_packet[pn_space] =
ack.largest_acked) max(largest_acked_packet[pn_space], ack.largest_acked)
// If the largest acknowledged is newly acked and // If the largest acknowledged is newly acked and
// ack-eliciting, update the RTT. // ack-eliciting, update the RTT.
if (sent_packets[ack.largest_acked] && if (sent_packets[pn_space][ack.largest_acked] &&
sent_packets[ack.largest_acked].ack_eliciting): sent_packets[pn_space][ack.largest_acked].ack_eliciting):
latest_rtt = latest_rtt =
now - sent_packets[ack.largest_acked].time_sent now - sent_packets[pn_space][ack.largest_acked].time_sent
UpdateRtt(latest_rtt, ack.ack_delay) UpdateRtt(latest_rtt, ack.ack_delay)
// Process ECN information if present. // Process ECN information if present.
if (ACK frame contains ECN information): if (ACK frame contains ECN information):
ProcessECN(ack) ProcessECN(ack)
// Find all newly acked packets in this ACK frame // Find all newly acked packets in this ACK frame
newly_acked_packets = DetermineNewlyAckedPackets(ack) newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)
if (newly_acked_packets.empty()): if (newly_acked_packets.empty()):
return return
for acked_packet in newly_acked_packets: for acked_packet in newly_acked_packets:
OnPacketAcked(acked_packet.packet_number) OnPacketAcked(acked_packet.packet_number, pn_space)
DetectLostPackets() DetectLostPackets(pn_space)
crypto_count = 0 crypto_count = 0
pto_count = 0 pto_count = 0
SetLossDetectionTimer() SetLossDetectionTimer()
UpdateRtt(latest_rtt, ack_delay): UpdateRtt(latest_rtt, ack_delay):
// min_rtt ignores ack delay. // min_rtt ignores ack delay.
min_rtt = min(min_rtt, latest_rtt) min_rtt = min(min_rtt, latest_rtt)
// Limit ack_delay by max_ack_delay // Limit ack_delay by max_ack_delay
ack_delay = min(ack_delay, max_ack_delay) ack_delay = min(ack_delay, max_ack_delay)
// Adjust for ack delay if it's plausible. // Adjust for ack delay if it's plausible.
if (latest_rtt - min_rtt > ack_delay): if (latest_rtt - min_rtt > ack_delay):
latest_rtt -= ack_delay latest_rtt -= ack_delay
// Based on {{RFC6298}}. // Based on {{RFC6298}}.
if (smoothed_rtt == 0): if (smoothed_rtt == 0):
smoothed_rtt = latest_rtt smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2 rttvar = latest_rtt / 2
else: else:
rttvar_sample = abs(smoothed_rtt - latest_rtt) rttvar_sample = abs(smoothed_rtt - latest_rtt)
rttvar = 3/4 * rttvar + 1/4 * rttvar_sample rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt
6.4.6. On Packet Acknowledgment A.7. On Packet Acknowledgment
When a packet is acknowledged for the first time, the following When a packet is acknowledged for the first time, the following
OnPacketAcked function is called. Note that a single ACK frame may OnPacketAcked function is called. Note that a single ACK frame may
newly acknowledge several packets. OnPacketAcked must be called once newly acknowledge several packets. OnPacketAcked must be called once
for each of these newly acknowledged packets. for each of these newly acknowledged packets.
OnPacketAcked takes one parameter, acked_packet, which is the struct OnPacketAcked takes two parameters: acked_packet, which is the struct
detailed in Section 6.3.1. detailed in Appendix A.1.1, and the packet number space that this ACK
frame was sent for.
Pseudocode for OnPacketAcked follows: Pseudocode for OnPacketAcked follows:
OnPacketAcked(acked_packet): OnPacketAcked(acked_packet, pn_space):
if (acked_packet.ack_eliciting): if (acked_packet.ack_eliciting):
OnPacketAckedCC(acked_packet) OnPacketAckedCC(acked_packet)
sent_packets.remove(acked_packet.packet_number) sent_packets[pn_space].remove(acked_packet.packet_number)
6.4.7. Setting the Loss Detection Timer A.8. Setting the Loss Detection Timer
QUIC loss detection uses a single timer for all timeout loss QUIC loss detection uses a single timer for all timeout loss
detection. The duration of the timer is based on the timer's mode, 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 which is set in the packet and timer events further below. The
function SetLossDetectionTimer defined below shows how the single function SetLossDetectionTimer defined below shows how the single
timer is set. timer is set.
This algorithm may result in the timer being set in the past, This algorithm may result in the timer being set in the past,
particularly if timers wake up late. Timers set in the past SHOULD particularly if timers wake up late. Timers set in the past SHOULD
fire immediately. fire immediately.
Pseudocode for SetLossDetectionTimer follows: Pseudocode for SetLossDetectionTimer follows:
SetLossDetectionTimer(): // Returns the earliest loss_time and the packet number
// Don't arm timer if there are no ack-eliciting packets // space it's from. Returns 0 if all times are 0.
// in flight. GetEarliestLossTime():
if (no ack-eliciting packets in flight): time = loss_time[Initial]
loss_detection_timer.cancel() space = Initial
return for pn_space in [ Handshake, ApplicatonData ]:
if loss_time[pn_space] != 0 &&
(time == 0 || loss_time[pn_space] < time):
time = loss_time[pn_space];
space = pn_space
return time, space
if (crypto packets are in flight): SetLossDetectionTimer():
// Crypto retransmission timer. // Don't arm timer if there are no ack-eliciting packets
if (smoothed_rtt == 0): // in flight.
timeout = 2 * kInitialRtt if (no ack-eliciting packets in flight):
else: loss_detection_timer.cancel()
timeout = 2 * smoothed_rtt return
timeout = max(timeout, kGranularity)
timeout = timeout * (2 ^ crypto_count)
loss_detection_timer.set(
time_of_last_sent_crypto_packet + timeout)
return
if (loss_time != 0):
// Time threshold loss detection.
loss_detection_timer.set(loss_time)
return
// Calculate PTO duration loss_time, _ = GetEarliestLossTime()
timeout = if (loss_time != 0):
smoothed_rtt + 4 * rttvar + max_ack_delay // Time threshold loss detection.
loss_detection_timer.update(loss_time)
return
if (crypto packets are in flight):
// Crypto retransmission timer.
if (smoothed_rtt == 0):
timeout = 2 * kInitialRtt
else:
timeout = 2 * smoothed_rtt
timeout = max(timeout, kGranularity) timeout = max(timeout, kGranularity)
timeout = timeout * (2 ^ pto_count) timeout = timeout * (2 ^ crypto_count)
loss_detection_timer.update(
time_of_last_sent_crypto_packet + timeout)
return
loss_detection_timer.set( // Calculate PTO duration
time_of_last_sent_ack_eliciting_packet + timeout) timeout =
smoothed_rtt + max(4 * rttvar, kGranularity) + max_ack_delay
timeout = timeout * (2 ^ pto_count)
6.4.8. On Timeout loss_detection_timer.update(
time_of_last_sent_ack_eliciting_packet + timeout)
A.9. On Timeout
When the loss detection timer expires, the timer's mode determines When the loss detection timer expires, the timer's mode determines
the action to be performed. the action to be performed.
Pseudocode for OnLossDetectionTimeout follows: Pseudocode for OnLossDetectionTimeout follows:
OnLossDetectionTimeout(): OnLossDetectionTimeout():
if (crypto packets are in flight): loss_time, pn_space = GetEarliestLossTime()
// Crypto retransmission timeout. if (loss_time != 0):
RetransmitUnackedCryptoData() // Time threshold loss Detection
crypto_count++ DetectLostPackets(pn_space)
else if (loss_time != 0): // Retransmit crypto data if no packets were lost
// Time threshold loss Detection // and there are still crypto packets in flight.
DetectLostPackets() else if (crypto packets are in flight):
else: // Crypto retransmission timeout.
// PTO RetransmitUnackedCryptoData()
SendTwoPackets() crypto_count++
pto_count++ else:
// PTO
SendOneOrTwoPackets()
pto_count++
SetLossDetectionTimer() SetLossDetectionTimer()
6.4.9. Detecting Lost Packets A.10. Detecting Lost Packets
DetectLostPackets is called every time an ACK is received and DetectLostPackets is called every time an ACK is received and
operates on the sent_packets for that packet number space. If the operates on the sent_packets for that packet number space. If the
loss detection timer expires and the loss_time is set, the previous loss detection timer expires and the loss_time is set, the previous
largest acknowledged packet is supplied. largest acknowledged packet is supplied.
Pseudocode for DetectLostPackets follows: Pseudocode for DetectLostPackets follows:
DetectLostPackets(): DetectLostPackets(pn_space):
loss_time = 0 loss_time[pn_space] = 0
lost_packets = {} lost_packets = {}
loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt) loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)
// Packets sent before this time are deemed lost. // Packets sent before this time are deemed lost.
lost_send_time = now() - loss_delay lost_send_time = now() - loss_delay
// Packets with packet numbers before this are deemed lost. // Packets with packet numbers before this are deemed lost.
lost_pn = largest_acked_packet - kPacketThreshold lost_pn = largest_acked_packet[pn_space] - kPacketThreshold
foreach unacked in sent_packets: foreach unacked in sent_packets:
if (unacked.packet_number > largest_acked_packet): if (unacked.packet_number > largest_acked_packet[pn_space]):
continue continue
// Mark packet as lost, or set time when it should be marked. // Mark packet as lost, or set time when it should be marked.
if (unacked.time_sent <= lost_send_time || if (unacked.time_sent <= lost_send_time ||
unacked.packet_number <= lost_pn): unacked.packet_number <= lost_pn):
sent_packets.remove(unacked.packet_number) sent_packets.remove(unacked.packet_number)
if (unacked.in_flight): if (unacked.in_flight):
lost_packets.insert(unacked) lost_packets.insert(unacked)
else if (loss_time == 0):
loss_time = unacked.time_sent + loss_delay
else: else:
loss_time = min(loss_time, unacked.time_sent + loss_delay) if (loss_time[pn_space] == 0):
loss_time[pn_space] = unacked.time_sent + loss_delay
else:
loss_time[pn_space] = min(loss_time[pn_space],
unacked.time_sent + loss_delay)
// Inform the congestion controller of lost packets and // Inform the congestion controller of lost packets and
// let it decide whether to retransmit immediately. // let it decide whether to retransmit immediately.
if (!lost_packets.empty()): if (!lost_packets.empty()):
OnPacketsLost(lost_packets) OnPacketsLost(lost_packets)
6.5. Discussion Appendix B. Congestion Control Pseudocode
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.
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 7.9.2) beyond the available
congestion window, unless the packet is a probe packet sent after a
PTO timer expires, as described in Section 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.
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.
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 PTO. While in slow start, QUIC
increases the congestion window by the number of bytes acknowledged
when each acknowledgment is processed.
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.
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 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.
7.5. Ignoring Loss of Undecryptable Packets
During the handshake, some packet protection keys might not be
available when a packet arrives. In particular, Handshake and 0-RTT
packets cannot be processed until the Initial packets arrive, and
1-RTT packets cannot be processed until the handshake completes.
Endpoints MAY ignore the loss of Handshake, 0-RTT, and 1-RTT packets
that might arrive before the peer has packet protection keys to
process those packets.
7.6. Probe Timeout
Probe packets MUST NOT be blocked by the congestion controller. A
sender MUST however count these packets as being additionally in
flight, since these packets adds network load without establishing
packet loss. Note that sending probe packets might cause the
sender's bytes in flight to exceed the congestion window until an
acknowledgement is received that establishes loss or delivery of
packets.
When an ACK frame is received that establishes loss of all in-flight
packets sent prior to a threshold number of consecutive PTOs
(pto_count is more than kPersistentCongestionThreshold, see
Section 7.9.1), the network is considered to be experiencing
persistent congestion, and the sender's congestion window MUST be
reduced to the minimum congestion window (kMinimumWindow). This
response of collapsing the congestion window on persistent congestion
is functionally similar to a sender's response on a Retransmission
Timeout (RTO) in TCP [RFC5681].
7.7. 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).
7.8. 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].
7.9. Pseudocode We now describe an example implementation of the congestion
controller described in Section 7.
7.9.1. Constants of interest B.1. Constants of interest
Constants used in congestion control are based on a combination of Constants used in congestion control are based on a combination of
RFCs, papers, and common practice. Some may need to be changed or RFCs, papers, and common practice. Some may need to be changed or
negotiated in order to better suit a variety of environments. negotiated in order to better suit a variety of environments.
kMaxDatagramSize: The sender's maximum payload size. Does not kMaxDatagramSize: The sender's maximum payload size. Does not
include UDP or IP overhead. The max packet size is used for include UDP or IP overhead. The max packet size is used for
calculating initial and minimum congestion windows. The calculating initial and minimum congestion windows. The
RECOMMENDED value is 1200 bytes. RECOMMENDED value is 1200 bytes.
kInitialWindow: Default limit on the initial amount of data in kInitialWindow: Default limit on the initial amount of data in
flight, in bytes. Taken from [RFC6928]. The RECOMMENDED value is flight, in bytes. Taken from [RFC6928], but increased slightly to
the minimum of 10 * kMaxDatagramSize and max(2* kMaxDatagramSize, account for the smaller 8 byte overhead of UDP vs 20 bytes for
14600)). TCP. The RECOMMENDED value is the minimum of 10 *
kMaxDatagramSize and max(2* kMaxDatagramSize, 14720)).
kMinimumWindow: Minimum congestion window in bytes. The RECOMMENDED kMinimumWindow: Minimum congestion window in bytes. The RECOMMENDED
value is 2 * kMaxDatagramSize. value is 2 * kMaxDatagramSize.
kLossReductionFactor: Reduction in congestion window when a new loss kLossReductionFactor: Reduction in congestion window when a new loss
event is detected. The RECOMMENDED value is 0.5. event is detected. The RECOMMENDED value is 0.5.
kPersistentCongestionThreshold: Number of consecutive PTOs required kPersistentCongestionThreshold: Number of consecutive PTOs required
for persistent congestion to be established. The rationale for for persistent congestion to be established. The rationale for
this threshold is to enable a sender to use initial PTOs for this threshold is to enable a sender to use initial PTOs for
aggressive probing, as TCP does with Tail Loss Probe (TLP) [TLP] aggressive probing, as TCP does with Tail Loss Probe (TLP) [TLP]
[RACK], before establishing persistent congestion, as TCP does [RACK], before establishing persistent congestion, as TCP does
with a Retransmission Timeout (RTO) [RFC5681]. The RECOMMENDED with a Retransmission Timeout (RTO) [RFC5681]. The RECOMMENDED
value for kPersistentCongestionThreshold is 2, which is equivalent value for kPersistentCongestionThreshold is 2, which is equivalent
to having two TLPs before an RTO in TCP. to having two TLPs before an RTO in TCP.
7.9.2. Variables of interest B.2. Variables of interest
Variables required to implement the congestion control mechanisms are Variables required to implement the congestion control mechanisms are
described in this section. described in this section.
ecn_ce_counter: The highest value reported for the ECN-CE counter by 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 the peer in an ACK frame. This variable is used to detect
increases in the reported ECN-CE counter. increases in the reported ECN-CE counter.
bytes_in_flight: The sum of the size in bytes of all sent packets bytes_in_flight: The sum of the size in bytes of all sent packets
that contain at least one ack-eliciting or PADDING frame, and have that contain at least one ack-eliciting or PADDING frame, and have
skipping to change at page 27, line 5 skipping to change at page 32, line 5
sent. sent.
recovery_start_time: The time when QUIC first detects a loss, recovery_start_time: The time when QUIC first detects a loss,
causing it to enter recovery. When a packet sent after this time causing it to enter recovery. When a packet sent after this time
is acknowledged, QUIC exits recovery. is acknowledged, QUIC exits recovery.
ssthresh: Slow start threshold in bytes. When the congestion window ssthresh: Slow start threshold in bytes. When the congestion window
is below ssthresh, the mode is slow start and the window grows by is below ssthresh, the mode is slow start and the window grows by
the number of bytes acknowledged. the number of bytes acknowledged.
7.9.3. Initialization B.3. Initialization
At the beginning of the connection, initialize the congestion control At the beginning of the connection, initialize the congestion control
variables as follows: variables as follows:
congestion_window = kInitialWindow congestion_window = kInitialWindow
bytes_in_flight = 0 bytes_in_flight = 0
recovery_start_time = 0 recovery_start_time = 0
ssthresh = infinite ssthresh = infinite
ecn_ce_counter = 0 ecn_ce_counter = 0
7.9.4. On Packet Sent B.4. On Packet Sent
Whenever a packet is sent, and it contains non-ACK frames, the packet Whenever a packet is sent, and it contains non-ACK frames, the packet
increases bytes_in_flight. increases bytes_in_flight.
OnPacketSentCC(bytes_sent): OnPacketSentCC(bytes_sent):
bytes_in_flight += bytes_sent bytes_in_flight += bytes_sent
7.9.5. On Packet Acknowledgement B.5. On Packet Acknowledgement
Invoked from loss detection's OnPacketAcked and is supplied with the Invoked from loss detection's OnPacketAcked and is supplied with the
acked_packet from sent_packets. acked_packet from sent_packets.
InRecovery(sent_time): InRecovery(sent_time):
return sent_time <= recovery_start_time return sent_time <= recovery_start_time
OnPacketAckedCC(acked_packet): OnPacketAckedCC(acked_packet):
// Remove from bytes_in_flight. // Remove from bytes_in_flight.
bytes_in_flight -= acked_packet.size bytes_in_flight -= acked_packet.size
if (InRecovery(acked_packet.time_sent)): if (InRecovery(acked_packet.time_sent)):
// Do not increase congestion window in recovery period. // Do not increase congestion window in recovery period.
return return
if (IsAppLimited())
// Do not increase congestion_window if application
// limited.
return
if (congestion_window < ssthresh): if (congestion_window < ssthresh):
// Slow start. // Slow start.
congestion_window += acked_packet.size congestion_window += acked_packet.size
else: else:
// Congestion avoidance. // Congestion avoidance.
congestion_window += kMaxDatagramSize * acked_packet.size congestion_window += kMaxDatagramSize * acked_packet.size
/ congestion_window / congestion_window
7.9.6. On New Congestion Event B.6. On New Congestion Event
Invoked from ProcessECN and OnPacketsLost when a new congestion event Invoked from ProcessECN and OnPacketsLost when a new congestion event
is detected. May start a new recovery period and reduces the is detected. May start a new recovery period and reduces the
congestion window. congestion window.
CongestionEvent(sent_time): CongestionEvent(sent_time):
// Start a new congestion event if the sent time is larger // Start a new congestion event if the sent time is larger
// than the start time of the previous recovery epoch. // than the start time of the previous recovery epoch.
if (!InRecovery(sent_time)): if (!InRecovery(sent_time)):
recovery_start_time = Now() recovery_start_time = Now()
congestion_window *= kLossReductionFactor congestion_window *= kLossReductionFactor
congestion_window = max(congestion_window, kMinimumWindow) congestion_window = max(congestion_window, kMinimumWindow)
ssthresh = congestion_window ssthresh = congestion_window
// Collapse congestion window if persistent congestion
if (pto_count > kPersistentCongestionThreshold):
congestion_window = kMinimumWindow
7.9.7. Process ECN Information B.7. Process ECN Information
Invoked when an ACK frame with an ECN section is received from the Invoked when an ACK frame with an ECN section is received from the
peer. peer.
ProcessECN(ack): ProcessECN(ack):
// If the ECN-CE counter reported by the peer has increased, // If the ECN-CE counter reported by the peer has increased,
// this could be a new congestion event. // this could be a new congestion event.
if (ack.ce_counter > ecn_ce_counter): if (ack.ce_counter > ecn_ce_counter):
ecn_ce_counter = ack.ce_counter ecn_ce_counter = ack.ce_counter
// Start a new congestion event if the last acknowledged // Start a new congestion event if the last acknowledged
// packet was sent after the start of the previous // packet was sent after the start of the previous
// recovery epoch. // recovery epoch.
CongestionEvent(sent_packets[ack.largest_acked].time_sent) CongestionEvent(sent_packets[ack.largest_acked].time_sent)
7.9.8. On Packets Lost B.8. On Packets Lost
Invoked by loss detection from DetectLostPackets when new packets are Invoked by loss detection from DetectLostPackets when new packets are
detected lost. detected lost.
InPersistentCongestion(largest_lost_packet):
pto = smoothed_rtt + max(4 * rttvar, kGranularity) +
max_ack_delay
congestion_period =
pto * (2 ^ kPersistentCongestionThreshold - 1)
// Determine if all packets in the window before the
// newest lost packet, including the edges, are marked
// lost
return IsWindowLost(largest_lost_packet, congestion_period)
OnPacketsLost(lost_packets): OnPacketsLost(lost_packets):
// Remove lost packets from bytes_in_flight. // Remove lost packets from bytes_in_flight.
for (lost_packet : lost_packets): for (lost_packet : lost_packets):
bytes_in_flight -= lost_packet.size bytes_in_flight -= lost_packet.size
largest_lost_packet = lost_packets.last() largest_lost_packet = lost_packets.last()
// Start a new congestion epoch if the last lost packet // Start a new congestion epoch if the last lost packet
// is past the end of the previous recovery epoch. // is past the end of the previous recovery epoch.
CongestionEvent(largest_lost_packet.time_sent) CongestionEvent(largest_lost_packet.time_sent)
8. Security Considerations // Collapse congestion window if persistent congestion
8.1. Congestion Signals if (InPersistentCongestion(largest_lost_packet)):
congestion_window = kMinimumWindow
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.
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.
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.
9. IANA Considerations
This document has no IANA actions. Yet.
10. References
10.1. Normative References
[QUIC-TLS]
Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", draft-ietf-quic-tls-18 (work in progress), January
2019.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport-17 (work in progress), January 2019.
[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>.
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 Appendix C. Change Log
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 *RFC Editor's Note:* Please remove this section prior to
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", publication of a final version of this document.
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, Issue and pull request numbers are listed with a leading octothorp.
"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
in progress), February 2013.
10.3. URIs C.1. Since draft-ietf-quic-recovery-18
[1] https://mailarchive.ietf.org/arch/search/?email_list=quic o Change IW byte limit to 14720 from 14600 (#2494)
[2] https://github.com/quicwg o Update PTO calculation to match RFC6298 (#2480, #2489, #2490)
[3] https://github.com/quicwg/base-drafts/labels/-recovery o Improve loss detection's description of multiple packet number
spaces and pseudocode (#2485, #2451, #2417)
Appendix A. Change Log o Declare persistent congestion even if non-probe packets are sent
and don't make persistent congestion more aggressive than RTO
verified was (#2365, #2244)
*RFC Editor's Note:* Please remove this section prior to o Move pseudocode to the appendices (#2408)
publication of a final version of this document.
Issue and pull request numbers are listed with a leading octothorp. o What to send on multiple PTOs (#2380)
A.1. Since draft-ietf-quic-recovery-17 C.2. Since draft-ietf-quic-recovery-17
o After Probe Timeout discard in-flight packets or send another o After Probe Timeout discard in-flight packets or send another
(#2212, #1965) (#2212, #1965)
o Endpoints discard initial keys as soon as handshake keys are o Endpoints discard initial keys as soon as handshake keys are
available (#1951, #2045) available (#1951, #2045)
o 0-RTT state is discarded when 0-RTT is rejected (#2300) o 0-RTT state is discarded when 0-RTT is rejected (#2300)
o Loss detection timer is cancelled when ack-eliciting frames are in o Loss detection timer is cancelled when ack-eliciting frames are in
skipping to change at page 32, line 44 skipping to change at page 35, line 29
o After becoming idle, either pace packets or reset the congestion o After becoming idle, either pace packets or reset the congestion
controller (#2138, 2187) controller (#2138, 2187)
o Process ECN counts before marking packets lost (#2142) o Process ECN counts before marking packets lost (#2142)
o Mark packets lost before resetting crypto_count and pto_count o Mark packets lost before resetting crypto_count and pto_count
(#2208, #2209) (#2208, #2209)
o Congestion and loss recovery state are discarded when keys are o Congestion and loss recovery state are discarded when keys are
discarded (#2237) discarded (#2327)
A.2. Since draft-ietf-quic-recovery-16 C.3. Since draft-ietf-quic-recovery-16
o Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP o Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP
and min crypto timeouts; eliminate timeout validation (#2114, and min crypto timeouts; eliminate timeout validation (#2114,
#2166, #2168, #1017) #2166, #2168, #1017)
o Redefine how congestion avoidance in terms of when the period o Redefine how congestion avoidance in terms of when the period
starts (#1928, #1930) starts (#1928, #1930)
o Document what needs to be tracked for packets that are in flight o Document what needs to be tracked for packets that are in flight
(#765, #1724, #1939) (#765, #1724, #1939)
skipping to change at page 33, line 21 skipping to change at page 36, line 4
o Integrate both time and packet thresholds into loss detection o Integrate both time and packet thresholds into loss detection
(#1969, #1212, #934, #1974) (#1969, #1212, #934, #1974)
o Reduce congestion window after idle, unless pacing is used (#2007, o Reduce congestion window after idle, unless pacing is used (#2007,
#2023) #2023)
o Disable RTT calculation for packets that don't elicit o Disable RTT calculation for packets that don't elicit
acknowledgment (#2060, #2078) acknowledgment (#2060, #2078)
o Limit ack_delay by max_ack_delay (#2060, #2099) o Limit ack_delay by max_ack_delay (#2060, #2099)
o Initial keys are discarded once Handshake are avaialble (#1951, o Initial keys are discarded once Handshake are avaialble (#1951,
#2045) #2045)
o Reorder ECN and loss detection in pseudocode (#2142) o Reorder ECN and loss detection in pseudocode (#2142)
o Only cancel loss detection timer if ack-eliciting packets are in o Only cancel loss detection timer if ack-eliciting packets are in
flight (#2093, #2117) flight (#2093, #2117)
A.3. Since draft-ietf-quic-recovery-14 C.4. Since draft-ietf-quic-recovery-14
o Used max_ack_delay from transport params (#1796, #1782) o Used max_ack_delay from transport params (#1796, #1782)
o Merge ACK and ACK_ECN (#1783) o Merge ACK and ACK_ECN (#1783)
A.4. Since draft-ietf-quic-recovery-13 C.5. Since draft-ietf-quic-recovery-13
o Corrected the lack of ssthresh reduction in CongestionEvent o Corrected the lack of ssthresh reduction in CongestionEvent
pseudocode (#1598) pseudocode (#1598)
o Considerations for ECN spoofing (#1426, #1626) o Considerations for ECN spoofing (#1426, #1626)
o Clarifications for PADDING and congestion control (#837, #838, o Clarifications for PADDING and congestion control (#837, #838,
#1517, #1531, #1540) #1517, #1531, #1540)
o Reduce early retransmission timer to RTT/8 (#945, #1581) o Reduce early retransmission timer to RTT/8 (#945, #1581)
o Packets are declared lost after an RTO is verified (#935, #1582) o Packets are declared lost after an RTO is verified (#935, #1582)
A.5. Since draft-ietf-quic-recovery-12 C.6. Since draft-ietf-quic-recovery-12
o Changes to manage separate packet number spaces and encryption o Changes to manage separate packet number spaces and encryption
levels (#1190, #1242, #1413, #1450) levels (#1190, #1242, #1413, #1450)
o Added ECN feedback mechanisms and handling; new ACK_ECN frame o Added ECN feedback mechanisms and handling; new ACK_ECN frame
(#804, #805, #1372) (#804, #805, #1372)
A.6. Since draft-ietf-quic-recovery-11 C.7. Since draft-ietf-quic-recovery-11
No significant changes. No significant changes.
A.7. Since draft-ietf-quic-recovery-10 C.8. Since draft-ietf-quic-recovery-10
o Improved text on ack generation (#1139, #1159) o Improved text on ack generation (#1139, #1159)
o Make references to TCP recovery mechanisms informational (#1195) o Make references to TCP recovery mechanisms informational (#1195)
o Define time_of_last_sent_handshake_packet (#1171) o Define time_of_last_sent_handshake_packet (#1171)
o Added signal from TLS the data it includes needs to be sent in a o Added signal from TLS the data it includes needs to be sent in a
Retry packet (#1061, #1199) Retry packet (#1061, #1199)
o Minimum RTT (min_rtt) is initialized with an infinite value o Minimum RTT (min_rtt) is initialized with an infinite value
(#1169) (#1169)
A.8. Since draft-ietf-quic-recovery-09 C.9. Since draft-ietf-quic-recovery-09
No significant changes. No significant changes.
A.9. Since draft-ietf-quic-recovery-08 C.10. Since draft-ietf-quic-recovery-08
o Clarified pacing and RTO (#967, #977) o Clarified pacing and RTO (#967, #977)
A.10. Since draft-ietf-quic-recovery-07 C.11. Since draft-ietf-quic-recovery-07
o Include Ack Delay in RTO(and TLP) computations (#981) o Include Ack Delay in RTO(and TLP) computations (#981)
o Ack Delay in SRTT computation (#961) o Ack Delay in SRTT computation (#961)
o Default RTT and Slow Start (#590) o Default RTT and Slow Start (#590)
o Many editorial fixes. o Many editorial fixes.
A.11. Since draft-ietf-quic-recovery-06 C.12. Since draft-ietf-quic-recovery-06
No significant changes. No significant changes.
A.12. Since draft-ietf-quic-recovery-05 C.13. Since draft-ietf-quic-recovery-05
o Add more congestion control text (#776) o Add more congestion control text (#776)
A.13. Since draft-ietf-quic-recovery-04 C.14. Since draft-ietf-quic-recovery-04
No significant changes. No significant changes.
A.14. Since draft-ietf-quic-recovery-03 C.15. Since draft-ietf-quic-recovery-03
No significant changes. No significant changes.
A.15. Since draft-ietf-quic-recovery-02 C.16. Since draft-ietf-quic-recovery-02
o Integrate F-RTO (#544, #409) o Integrate F-RTO (#544, #409)
o Add congestion control (#545, #395) o Add congestion control (#545, #395)
o Require connection abort if a skipped packet was acknowledged o Require connection abort if a skipped packet was acknowledged
(#415) (#415)
o Simplify RTO calculations (#142, #417) o Simplify RTO calculations (#142, #417)
A.16. Since draft-ietf-quic-recovery-01 C.17. Since draft-ietf-quic-recovery-01
o Overview added to loss detection o Overview added to loss detection
o Changes initial default RTT to 100ms o Changes initial default RTT to 100ms
o Added time-based loss detection and fixes early retransmit o Added time-based loss detection and fixes early retransmit
o Clarified loss recovery for handshake packets o Clarified loss recovery for handshake packets
o Fixed references and made TCP references informative o Fixed references and made TCP references informative
A.17. Since draft-ietf-quic-recovery-00 C.18. Since draft-ietf-quic-recovery-00
o Improved description of constants and ACK behavior o Improved description of constants and ACK behavior
A.18. Since draft-iyengar-quic-loss-recovery-01 C.19. Since draft-iyengar-quic-loss-recovery-01
o Adopted as base for draft-ietf-quic-recovery o Adopted as base for draft-ietf-quic-recovery
o Updated authors/editors list o Updated authors/editors list
o Added table of contents o Added table of contents
Acknowledgments Acknowledgments
Authors' Addresses Authors' Addresses
Jana Iyengar (editor) Jana Iyengar (editor)
Fastly Fastly
Email: jri.ietf@gmail.com Email: jri.ietf@gmail.com
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