draft-ietf-quic-recovery-22.txt   draft-ietf-quic-recovery-23.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: January 10, 2020 Google Expires: March 15, 2020 Google
July 09, 2019 September 12, 2019
QUIC Loss Detection and Congestion Control QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-22 draft-ietf-quic-recovery-23
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 January 10, 2020. This Internet-Draft will expire on March 15, 2020.
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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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
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 . . . . . . . . . . 5 3. Design of the QUIC Transmission Machinery . . . . . . . . . . 5
3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 6 3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5
3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 6 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. Clearer Loss Epoch . . . . . . . . . . . . . . . . . 7 3.1.3. Clearer Loss Epoch . . . . . . . . . . . . . . . . . 6
3.1.4. No Reneging . . . . . . . . . . . . . . . . . . . . . 7 3.1.4. No Reneging . . . . . . . . . . . . . . . . . . . . . 7
3.1.5. More ACK Ranges . . . . . . . . . . . . . . . . . . . 7 3.1.5. More ACK Ranges . . . . . . . . . . . . . . . . . . . 7
3.1.6. Explicit Correction For Delayed Acknowledgements . . 7 3.1.6. Explicit Correction For Delayed Acknowledgements . . 7
4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7 4. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 7
4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 8 4.1. Generating RTT samples . . . . . . . . . . . . . . . . . 7
4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 8 4.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 8
4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8 4.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 8
4.4. Measuring and Reporting Host Delay . . . . . . . . . . . 9 5. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9
5. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 9 5.1. Acknowledgement-based Detection . . . . . . . . . . . . . 10
5.1. Generating RTT samples . . . . . . . . . . . . . . . . . 9 5.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10
5.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 10 5.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10
5.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 10 5.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 11
6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 11 5.2.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 11
6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 12 5.3. Handshakes and New Paths . . . . . . . . . . . . . . . . 12
6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 12 5.3.1. Sending Probe Packets . . . . . . . . . . . . . . . . 13
6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 12 5.3.2. Loss Detection . . . . . . . . . . . . . . . . . . . 14
6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 13 5.4. Retry and Version Negotiation . . . . . . . . . . . . . . 14
6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 14 5.5. Discarding Keys and Packet State . . . . . . . . . . . . 14
6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 15 5.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . 15
6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 15 6. Congestion Control . . . . . . . . . . . . . . . . . . . . . 15
6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 16 6.1. Explicit Congestion Notification . . . . . . . . . . . . 15
6.4. Retry and Version Negotiation . . . . . . . . . . . . . . 16 6.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 16
6.5. Discarding Keys and Packet State . . . . . . . . . . . . 17 6.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 16
6.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . 17 6.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 16
7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17 6.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 16
7.1. Explicit Congestion Notification . . . . . . . . . . . . 18 6.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 17
7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 18 6.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 17
7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 18 6.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 18 6.9. Under-utilizing the Congestion Window . . . . . . . . . . 18
7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 19
7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 19 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 19 7.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 19
7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 20 7.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 19
7.9. Under-utilizing the Congestion Window . . . . . . . . . . 21 7.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 21 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 21 9.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 22 9.2. Informative References . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . 22 A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . 23 A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 22
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24 A.2. Constants of interest . . . . . . . . . . . . . . . . . . 23
Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 24 A.3. Variables of interest . . . . . . . . . . . . . . . . . . 23
A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 25 A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 24
A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 25 A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 25
A.2. Constants of interest . . . . . . . . . . . . . . . . . . 25 A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 25
A.3. Variables of interest . . . . . . . . . . . . . . . . . . 26 A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 26
A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 27 A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 27
A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 27 A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 29
A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 28 A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 29
A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 29 Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 30
A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 30 B.1. Constants of interest . . . . . . . . . . . . . . . . . . 30
A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 32 B.2. Variables of interest . . . . . . . . . . . . . . . . . . 31
A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 32 B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 32
Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 33 B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 32
B.1. Constants of interest . . . . . . . . . . . . . . . . . . 33 B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 32
B.2. Variables of interest . . . . . . . . . . . . . . . . . . 34 B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 33
B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 35 B.7. Process ECN Information . . . . . . . . . . . . . . . . . 33
B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 35 B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 34
B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 35 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 34
B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 36 C.1. Since draft-ietf-quic-recovery-22 . . . . . . . . . . . . 34
B.7. Process ECN Information . . . . . . . . . . . . . . . . . 36 C.2. Since draft-ietf-quic-recovery-21 . . . . . . . . . . . . 34
B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 37 C.3. Since draft-ietf-quic-recovery-20 . . . . . . . . . . . . 35
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 37 C.4. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 35
C.1. Since draft-ietf-quic-recovery-21 . . . . . . . . . . . . 37 C.5. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 35
C.2. Since draft-ietf-quic-recovery-20 . . . . . . . . . . . . 37 C.6. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 36
C.3. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 38 C.7. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 36
C.4. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 38 C.8. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 37
C.5. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 38 C.9. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 37
C.6. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 39 C.10. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 37
C.7. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 40 C.11. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 37
C.8. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 40 C.12. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 37
C.9. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 40 C.13. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 38
C.10. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 40 C.14. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 38
C.11. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 40 C.15. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 38
C.12. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 41 C.16. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 38
C.13. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 41 C.17. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 38
C.14. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 41 C.18. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 38
C.15. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 41 C.19. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 38
C.16. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 41 C.20. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 38
C.17. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 41 C.21. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 39
C.18. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 41 C.22. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 39
C.19. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 41 C.23. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 39
C.20. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 42 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 39
C.21. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 42 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
C.22. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 42
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
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 existing 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
skipping to change at page 5, line 44 skipping to change at page 5, line 37
delivery are acknowledged or declared lost and sent in new packets as delivery are acknowledged or declared lost and sent in new packets as
necessary. The types of frames contained in a packet affect recovery necessary. The types of frames contained in a packet affect recovery
and congestion control logic: and congestion control logic:
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.
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. congestion control limits and are not considered in-flight.
o PADDING frames cause packets to contribute toward bytes in flight o PADDING frames cause packets to contribute toward bytes in flight
without directly causing an acknowledgment to be 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
skipping to change at page 7, line 34 skipping to change at page 7, line 24
QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In
high loss environments, this speeds recovery, reduces spurious high loss environments, this speeds recovery, reduces spurious
retransmits, and ensures forward progress without relying on retransmits, and ensures forward progress without relying on
timeouts. timeouts.
3.1.6. Explicit Correction For Delayed Acknowledgements 3.1.6. Explicit Correction For Delayed Acknowledgements
QUIC endpoints measure the delay incurred between when a packet is QUIC endpoints measure the delay incurred between when a packet is
received and when the corresponding acknowledgment is sent, allowing received and when the corresponding acknowledgment is sent, allowing
a peer to maintain a more accurate round-trip time estimate (see a peer to maintain a more accurate round-trip time estimate (see
Section 4.4). Section 13.2 of [QUIC-TRANSPORT]).
4. Generating Acknowledgements
An acknowledgement SHOULD be sent immediately upon receipt of a
second ack-eliciting packet. QUIC recovery algorithms do not assume
the peer sends an ACK immediately when receiving a second ack-
eliciting packet.
In order to accelerate loss recovery and reduce timeouts, the
receiver SHOULD send an immediate ACK after it receives an out-of-
order packet. It could send immediate ACKs for in-order packets for
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)
codepoint in the IP header SHOULD be acknowledged immediately, to
reduce the peer's response time to congestion events.
As an optimization, a receiver MAY process multiple packets before
sending any ACK frames in response. In this case the receiver can
determine whether an immediate or delayed acknowledgement should be
generated after processing incoming packets.
4.1. Crypto Handshake Data
In order to quickly complete the handshake and avoid spurious
retransmissions due to crypto retransmission timeouts, crypto packets
SHOULD use a very short ack delay, such as the local timer
granularity. ACK frames SHOULD be sent immediately when the crypto
stack indicates all data for that packet number space has been
received.
4.2. ACK Ranges
When an ACK frame is sent, one or more ranges of acknowledged packets
are included. Including older packets reduces the chance of spurious
retransmits caused by losing previously sent ACK frames, at the cost
of larger ACK frames.
ACK frames SHOULD always acknowledge the most recently received
packets, and the more out-of-order the packets are, the more
important it is to send an updated ACK frame quickly, to prevent the
peer from declaring a packet as lost and spuriously retransmitting
the frames it contains.
Below is one recommended approach for determining what packets to
include in an ACK frame.
4.3. Receiver Tracking of ACK Frames
When a packet containing an ACK frame is sent, the largest
acknowledged in that frame may be saved. When a packet containing an
ACK frame is acknowledged, the receiver can stop acknowledging
packets less than or equal to the largest acknowledged in the sent
ACK frame.
In cases without ACK frame loss, this algorithm allows for a minimum
of 1 RTT of reordering. In cases with ACK frame loss and reordering,
this approach does not guarantee that every acknowledgement is seen
by the sender before it is no longer included in the ACK frame.
Packets could be received out of order and all subsequent ACK frames
containing them could be lost. In this case, the loss recovery
algorithm may cause spurious retransmits, but the sender will
continue making forward progress.
4.4. Measuring and Reporting Host Delay
An endpoint measures the delays intentionally introduced between when
an ACK-eliciting packet is received and the corresponding
acknowledgment is sent. The endpoint encodes this delay for the
largest acknowledged packet in the Ack Delay field of an ACK frame
(see Section 19.3 of [QUIC-TRANSPORT]). This allows the receiver of
the ACK to adjust for any intentional delays, which is important for
delayed acknowledgements, when estimating the path RTT. A packet
might be held in the OS kernel or elsewhere on the host before being
processed. An endpoint SHOULD NOT include these unintentional delays
when populating the Ack Delay field in an ACK frame.
An endpoint MUST NOT excessively delay acknowledgements of ack-
eliciting packets. The maximum ack delay is communicated in the
max_ack_delay transport parameter; see Section 18.1 of
[QUIC-TRANSPORT]. max_ack_delay implies an explicit contract: an
endpoint promises to never delay acknowledgments of an ack-eliciting
packet by more than the indicated value. If it does, any excess
accrues to the RTT estimate and could result in spurious
retransmissions from the peer. For Initial and Handshake packets, a
max_ack_delay of 0 is used.
5. Estimating the Round-Trip Time 4. Estimating the Round-Trip Time
At a high level, an endpoint measures the time from when a packet was At a high level, an endpoint measures the time from when a packet was
sent to when it is acknowledged as a round-trip time (RTT) sample. sent to when it is acknowledged as a round-trip time (RTT) sample.
The endpoint uses RTT samples and peer-reported host delays The endpoint uses RTT samples and peer-reported host delays (see
(Section 4.4) to generate a statistical description of the Section 13.2 of [QUIC-TRANSPORT]) to generate a statistical
connection's RTT. An endpoint computes the following three values: description of the connection's RTT. An endpoint computes the
the minimum value observed over the lifetime of the connection following three values: the minimum value observed over the lifetime
(min_rtt), an exponentially-weighted moving average (smoothed_rtt), of the connection (min_rtt), an exponentially-weighted moving average
and the variance in the observed RTT samples (rttvar). (smoothed_rtt), and the variance in the observed RTT samples
(rttvar).
5.1. Generating RTT samples 4.1. Generating RTT samples
An endpoint generates an RTT sample on receiving an ACK frame that An endpoint generates an RTT sample on receiving an ACK frame that
meets the following two conditions: meets the following two conditions:
o the largest acknowledged packet number is newly acknowledged, and o the largest acknowledged packet number is newly acknowledged, and
o at least one of the newly acknowledged packets was ack-eliciting. o at least one of the newly acknowledged packets was ack-eliciting.
The RTT sample, latest_rtt, is generated as the time elapsed since The RTT sample, latest_rtt, is generated as the time elapsed since
the largest acknowledged packet was sent: the largest acknowledged packet was sent:
latest_rtt = ack_time - send_time_of_largest_acked latest_rtt = ack_time - send_time_of_largest_acked
An RTT sample is generated using only the largest acknowledged packet An RTT sample is generated using only the largest acknowledged packet
in the received ACK frame. This is because a peer reports host in the received ACK frame. This is because a peer reports host
delays for only the largest acknowledged packet in an ACK frame. delays for only the largest acknowledged packet in an ACK frame.
While the reported host delay is not used by the RTT sample While the reported host delay is not used by the RTT sample
measurement, it is used to adjust the RTT sample in subsequent measurement, it is used to adjust the RTT sample in subsequent
computations of smoothed_rtt and rttvar Section 5.3. computations of smoothed_rtt and rttvar Section 4.3.
To avoid generating multiple RTT samples using the same packet, an To avoid generating multiple RTT samples using the same packet, an
ACK frame SHOULD NOT be used to update RTT estimates if it does not ACK frame SHOULD NOT be used to update RTT estimates if it does not
newly acknowledge the largest acknowledged packet. newly acknowledge the largest acknowledged packet.
An RTT sample MUST NOT be generated on receiving an ACK frame that An RTT sample MUST NOT be generated on receiving an ACK frame that
does not newly acknowledge at least one ack-eliciting packet. A peer does not newly acknowledge at least one ack-eliciting packet. A peer
does not send an ACK frame on receiving only non-ack-eliciting does not send an ACK frame on receiving only non-ack-eliciting
packets, so an ACK frame that is subsequently sent can include an packets, so an ACK frame that is subsequently sent can include an
arbitrarily large Ack Delay field. Ignoring such ACK frames avoids arbitrarily large Ack Delay field. Ignoring such ACK frames avoids
complications in subsequent smoothed_rtt and rttvar computations. complications in subsequent smoothed_rtt and rttvar computations.
A sender might generate multiple RTT samples per RTT when multiple A sender might generate multiple RTT samples per RTT when multiple
ACK frames are received within an RTT. As suggested in [RFC6298], ACK frames are received within an RTT. As suggested in [RFC6298],
doing so might result in inadequate history in smoothed_rtt and doing so might result in inadequate history in smoothed_rtt and
rttvar. Ensuring that RTT estimates retain sufficient history is an rttvar. Ensuring that RTT estimates retain sufficient history is an
open research question. open research question.
5.2. Estimating min_rtt 4.2. Estimating min_rtt
min_rtt is the minimum RTT observed over the lifetime of the min_rtt is the minimum RTT observed over the lifetime of the
connection. min_rtt is set to the latest_rtt on the first sample in connection. min_rtt is set to the latest_rtt on the first sample in
a connection, and to the lesser of min_rtt and latest_rtt on a connection, and to the lesser of min_rtt and latest_rtt on
subsequent samples. subsequent samples.
An endpoint uses only locally observed times in computing the min_rtt An endpoint uses only locally observed times in computing the min_rtt
and does not adjust for host delays reported by the peer and does not adjust for host delays reported by the peer. Doing so
(Section 4.4). Doing so allows the endpoint to set a lower bound for allows the endpoint to set a lower bound for the smoothed_rtt based
the smoothed_rtt based entirely on what it observes (see entirely on what it observes (see Section 4.3), and limits potential
Section 5.3), and limits potential underestimation due to underestimation due to erroneously-reported delays by the peer.
erroneously-reported delays by the peer.
5.3. Estimating smoothed_rtt and rttvar 4.3. Estimating smoothed_rtt and rttvar
smoothed_rtt is an exponentially-weighted moving average of an smoothed_rtt is an exponentially-weighted moving average of an
endpoint's RTT samples, and rttvar is the endpoint's estimated endpoint's RTT samples, and rttvar is the endpoint's estimated
variance in the RTT samples. variance in the RTT samples.
The calculation of smoothed_rtt uses path latency after adjusting RTT The calculation of smoothed_rtt uses path latency after adjusting RTT
samples for host delays (Section 4.4). For packets sent in the samples for host delays. For packets sent in the ApplicationData
ApplicationData packet number space, a peer limits any delay in packet number space, a peer limits any delay in sending an
sending an acknowledgement for an ack-eliciting packet to no greater acknowledgement for an ack-eliciting packet to no greater than the
than the value it advertised in the max_ack_delay transport value it advertised in the max_ack_delay transport parameter.
parameter. Consequently, when a peer reports an Ack Delay that is
greater than its max_ack_delay, the delay is attributed to reasons Consequently, when a peer reports an Ack Delay that is greater than
out of the peer's control, such as scheduler latency at the peer or its max_ack_delay, the delay is attributed to reasons out of the
loss of previous ACK frames. Any delays beyond the peer's peer's control, such as scheduler latency at the peer or loss of
max_ack_delay are therefore considered effectively part of path delay previous ACK frames. Any delays beyond the peer's max_ack_delay are
and incorporated into the smoothed_rtt estimate. therefore considered effectively part of path delay and incorporated
into the smoothed_rtt estimate.
When adjusting an RTT sample using peer-reported acknowledgement When adjusting an RTT sample using peer-reported acknowledgement
delays, an endpoint: delays, an endpoint:
o MUST ignore the Ack Delay field of the ACK frame for packets sent o MUST ignore the Ack Delay field of the ACK frame for packets sent
in the Initial and Handshake packet number space. in the Initial and Handshake packet number space.
o MUST use the lesser of the value reported in Ack Delay field of o MUST use the lesser of the value reported in Ack Delay field of
the ACK frame and the peer's max_ack_delay transport parameter the ACK frame and the peer's max_ack_delay transport parameter.
(Section 4.4).
o MUST NOT apply the adjustment if the resulting RTT sample is o MUST NOT apply the adjustment if the resulting RTT sample is
smaller than the min_rtt. This limits the underestimation that a smaller than the min_rtt. This limits the underestimation that a
misreporting peer can cause to the smoothed_rtt. misreporting peer can cause to the smoothed_rtt.
On the first RTT sample in a connection, the smoothed_rtt is set to On the first RTT sample in a connection, the smoothed_rtt is set to
the latest_rtt. the latest_rtt.
smoothed_rtt and rttvar are computed as follows, similar to smoothed_rtt and rttvar are computed as follows, similar to
[RFC6298]. On the first RTT sample in a connection: [RFC6298]. On the first RTT sample in a connection:
skipping to change at page 11, line 45 skipping to change at page 9, line 44
On subsequent RTT samples, smoothed_rtt and rttvar evolve as follows: On subsequent RTT samples, smoothed_rtt and rttvar evolve as follows:
ack_delay = min(Ack Delay in ACK Frame, max_ack_delay) ack_delay = min(Ack Delay in ACK Frame, max_ack_delay)
adjusted_rtt = latest_rtt adjusted_rtt = latest_rtt
if (min_rtt + ack_delay < latest_rtt): if (min_rtt + ack_delay < latest_rtt):
adjusted_rtt = latest_rtt - ack_delay adjusted_rtt = latest_rtt - ack_delay
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
rttvar_sample = abs(smoothed_rtt - adjusted_rtt) rttvar_sample = abs(smoothed_rtt - adjusted_rtt)
rttvar = 3/4 * rttvar + 1/4 * rttvar_sample rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
6. Loss Detection 5. 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.
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 13.3 of
[QUIC-TRANSPORT]. [QUIC-TRANSPORT].
6.1. Acknowledgement-based Detection 5.1. 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],
SACK loss recovery [RFC6675], and RACK [RACK]. This section provides SACK loss recovery [RFC6675], and RACK [RACK]. This section provides
an overview of how these algorithms are implemented in QUIC. an overview of how these algorithms are implemented in QUIC.
A packet is declared lost if it meets all the following conditions: A packet is declared lost if it meets all the following conditions:
o The packet is unacknowledged, in-flight, and was sent prior to an o The packet is unacknowledged, in-flight, and was sent prior to an
acknowledged packet. acknowledged packet.
o Either its packet number is kPacketThreshold smaller than an o Either its packet number is kPacketThreshold smaller than an
acknowledged packet (Section 6.1.1), or it was sent long enough in acknowledged packet (Section 5.1.1), or it was sent long enough in
the past (Section 6.1.2). the past (Section 5.1.2).
The acknowledgement indicates that a packet sent later was delivered, The acknowledgement indicates that a packet sent later was delivered,
while the packet and time thresholds provide some tolerance for while the packet and time thresholds provide some tolerance for
packet reordering. packet reordering.
Spuriously declaring packets as lost leads to unnecessary Spuriously declaring packets as lost leads to unnecessary
retransmissions and may result in degraded performance due to the retransmissions and may result in degraded performance due to the
actions of the congestion controller upon detecting loss. actions of the congestion controller upon detecting loss.
Implementations that detect spurious retransmissions and increase the Implementations that detect spurious retransmissions and increase the
reordering threshold in packets or time MAY choose to start with reordering threshold in packets or time MAY choose to start with
smaller initial reordering thresholds to minimize recovery latency. smaller initial reordering thresholds to minimize recovery latency.
6.1.1. Packet Threshold 5.1.1. Packet Threshold
The RECOMMENDED initial value for the packet reordering threshold The RECOMMENDED initial value for the packet reordering threshold
(kPacketThreshold) is 3, based on best practices for TCP loss (kPacketThreshold) is 3, based on best practices for TCP loss
detection [RFC5681] [RFC6675]. detection [RFC5681] [RFC6675].
Some networks may exhibit higher degrees of reordering, causing a Some networks may exhibit higher degrees of reordering, causing a
sender to detect spurious losses. Implementers MAY use algorithms sender to detect spurious losses. Implementers MAY use algorithms
developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's
reordering resilience. reordering resilience.
6.1.2. Time Threshold 5.1.2. Time Threshold
Once a later packet packet within the same packet number space has Once a later packet packet within the same packet number space has
been acknowledged, an endpoint SHOULD declare an earlier packet lost been acknowledged, an endpoint SHOULD declare an earlier packet lost
if it was sent a threshold amount of time in the past. To avoid if it was sent a threshold amount of time in the past. To avoid
declaring packets as lost too early, this time threshold MUST be set declaring packets as lost too early, this time threshold MUST be set
to at least kGranularity. The time threshold is: to at least kGranularity. The time threshold is:
kTimeThreshold * max(SRTT, latest_RTT, kGranularity) kTimeThreshold * max(SRTT, latest_RTT, kGranularity)
If packets sent prior to the largest acknowledged packet cannot yet If packets sent prior to the largest acknowledged packet cannot yet
be declared lost, then a timer SHOULD be set for the remaining time. be declared lost, then a timer SHOULD be set for the remaining time.
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 the acknowledgement encountered a shorter path; reordering where the acknowledgement 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
skipping to change at page 13, line 28 skipping to change at page 11, line 25
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.
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. Crypto Retransmission Timeout 5.2. Probe Timeout
Data in CRYPTO frames is critical to QUIC transport and crypto
negotiation, so a more aggressive timeout is used to retransmit it.
The initial crypto retransmission timeout SHOULD be set to twice the
initial RTT.
At the beginning, there are no prior RTT samples within a connection.
Resumed connections over the same network SHOULD use the previous
connection's final smoothed RTT value as the resumed connection's
initial RTT. If no previous RTT is available, or if the network
changes, the initial RTT SHOULD be set to 500ms, resulting in a 1
second initial handshake timeout as recommended in [RFC6298].
A connection MAY use the delay between sending a PATH_CHALLENGE and
receiving a PATH_RESPONSE to seed initial_rtt for a new path, but the
delay SHOULD NOT be considered an RTT sample.
When a crypto packet is sent, the sender MUST set a timer for twice
the smoothed RTT. This timer MUST be updated when a new crypto
packet is sent and when an acknowledgement is received which computes
a new RTT sample. Upon timeout, the sender MUST retransmit all
unacknowledged CRYPTO data if possible. The sender MUST NOT declare
in-flight crypto packets as lost when the crypto timer expires.
On each consecutive expiration of the crypto timer without receiving
an acknowledgement for a new packet, the sender MUST double the
crypto retransmission timeout and set a timer for this period.
Until the server has validated the client's address on the path, the
amount of data it can send is limited, as specified in Section 8.1 of
[QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent,
then all unacknowledged CRYPTO data sent in Initial packets should be
retransmitted. If no data can be sent, then no alarm should be armed
until data has been received from the client.
Because the server could be blocked until more packets are received,
the client MUST ensure that the crypto retransmission timer is set if
there is unacknowledged crypto data or if the client does not yet
have 1-RTT keys. If the crypto retransmission timer expires before
the client has 1-RTT keys, it is possible that the client may not
have any crypto data to retransmit. However, the client MUST send a
new packet, containing only PADDING frames if necessary, to allow the
server to continue sending data. If Handshake keys are available to
the client, it MUST send a Handshake packet, and otherwise it MUST
send an Initial packet in a UDP datagram of at least 1200 bytes.
Because packets only containing PADDING do not elicit an
acknowledgement, they may never be acknowledged, but they are removed
from bytes in flight when the client gets Handshake keys and the
Initial keys are discarded.
The crypto retransmission timer is not set if the time threshold
Section 6.1.2 loss detection timer is set. The time threshold loss
detection timer is expected to both expire earlier than the crypto
retransmission timeout and be less likely to spuriously retransmit
data. The Initial and Handshake packet number spaces will typically
contain a small number of packets, so losses are less likely to be
detected using packet-threshold loss detection.
When the crypto retransmission timer is active, the probe timer
(Section 6.3) is not active.
6.3. Probe Timeout
A Probe Timeout (PTO) triggers a probe packet when ack-eliciting data A Probe Timeout (PTO) triggers sending one or two probe datagrams
is in flight but an acknowledgement is not received within the when ack-eliciting packets are not acknowledged within the expected
expected period of time. A PTO enables a connection to recover from period of time or the handshake has not been completed. A PTO
loss of tail packets or acks. The PTO algorithm used in QUIC enables a connection to recover from loss of tail packets or
implements the reliability functions of Tail Loss Probe [TLP] [RACK], acknowledgements. The PTO algorithm used in QUIC implements the
RTO [RFC5681] and F-RTO algorithms for TCP [RFC5682], and the timeout reliability functions of Tail Loss Probe [TLP] [RACK], RTO [RFC5681]
computation is based on TCP's retransmission timeout period and F-RTO algorithms for TCP [RFC5682], and the timeout computation
[RFC6298]. is based on TCP's retransmission timeout period [RFC6298].
6.3.1. Computing PTO 5.2.1. 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 = smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay 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
Appendix A.2 and Appendix A.3. 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 sender probes the network as described When a PTO timer expires, the PTO period MUST be set to twice its
in the next section. The PTO period MUST be set to twice its current current value. This exponential reduction in the sender's rate is
value. This exponential reduction in the sender's rate is important important because the PTOs might be caused by loss of packets or
because the PTOs might be caused by loss of packets or acknowledgements due to severe congestion. The life of a connection
acknowledgements due to severe congestion. that is experiencing consecutive PTOs is limited by the endpoint's
idle timeout.
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.
6.3.2. Sending Probe Packets The probe timer is not set if the time threshold Section 5.1.2 loss
detection timer is set. The time threshold loss detection timer is
expected to both expire earlier than the PTO and be less likely to
spuriously retransmit data.
5.3. Handshakes and New Paths
The initial probe timeout for a new connection or new path SHOULD be
set to twice the initial RTT. Resumed connections over the same
network SHOULD use the previous connection's final smoothed RTT value
as the resumed connection's initial RTT. If no previous RTT is
available, the initial RTT SHOULD be set to 500ms, resulting in a 1
second initial timeout as recommended in [RFC6298].
A connection MAY use the delay between sending a PATH_CHALLENGE and
receiving a PATH_RESPONSE to seed initial_rtt for a new path, but the
delay SHOULD NOT be considered an RTT sample.
Until the server has validated the client's address on the path, the
amount of data it can send is limited, as specified in Section 8.1 of
[QUIC-TRANSPORT]. Data at Initial encryption MUST be retransmitted
before Handshake data and data at Handshake encryption MUST be
retransmitted before any ApplicationData data. If no data can be
sent, then the PTO alarm MUST NOT be armed until data has been
received from the client.
Since the server could be blocked until more packets are received
from the client, it is the client's responsibility to send packets to
unblock the server until it is certain that the server has finished
its address validation (see Section 8 of [QUIC-TRANSPORT]). That is,
the client MUST set the probe timer if the client has not received an
acknowledgement for one of its Handshake or 1-RTT packets.
Prior to handshake completion, when few to none RTT samples have been
generated, it is possible that the probe timer expiration is due to
an incorrect RTT estimate at the client. To allow the client to
improve its RTT estimate, the new packet that it sends MUST be ack-
eliciting. If Handshake keys are available to the client, it MUST
send a Handshake packet, and otherwise it MUST send an Initial packet
in a UDP datagram of at least 1200 bytes.
Initial packets and Handshake packets may never be acknowledged, but
they are removed from bytes in flight when the Initial and Handshake
keys are discarded.
5.3.1. Sending Probe Packets
When a PTO timer expires, a sender MUST send at least one ack- When a PTO timer expires, a sender MUST send at least one ack-
eliciting packet as a probe, unless there is no data available to eliciting packet as a probe, unless there is no data available to
send. An endpoint MAY send up to two ack-eliciting packets, to avoid send. An endpoint MAY send up to two full-sized datagrams containing
an expensive consecutive PTO expiration due to a single packet loss. ack-eliciting packets, to avoid an expensive consecutive PTO
expiration due to a single lost datagram.
It is possible that the sender has no new or previously-sent data to It is possible that the sender has no new or previously-sent data to
send. As an example, consider the following sequence of events: new send. As an example, consider the following sequence of events: new
application data is sent in a STREAM frame, deemed lost, then application data is sent in a STREAM frame, deemed lost, then
retransmitted in a new packet, and then the original transmission is retransmitted in a new packet, and then the original transmission is
acknowledged. In the absence of any new application data, a PTO acknowledged. In the absence of any new application data, a PTO
timer expiration now would find the sender with no new or previously- timer expiration now would find the sender with no new or previously-
sent data to send. sent data to send.
When there is no data to send, the sender SHOULD send a PING or other When there is no data to send, the sender SHOULD send a PING or other
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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 When the PTO timer expires multiple times and new data cannot be
sent, implementations must choose between sending the same payload sent, implementations must choose between sending the same payload
every time or sending different payloads. 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. may be simpler and ensures the highest priority frames arrive first.
Sending different payloads each time reduces the chances of spurious Sending different payloads each time reduces the chances of spurious
retransmission. retransmission.
6.3.3. Loss Detection 5.3.2. 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. When an NOT cause prior unacknowledged packets to be marked as lost. When an
acknowledgement is received that newly acknowledges packets, loss acknowledgement is received that newly acknowledges packets, loss
detection proceeds as dictated by packet and time threshold detection proceeds as dictated by packet and time threshold
mechanisms; see Section 6.1. mechanisms; see Section 5.1.
6.4. Retry and Version Negotiation 5.4. 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.
6.5. Discarding Keys and Packet State 5.5. 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 MUST discard all recovery state associated with anymore. The sender MUST discard all recovery state associated with
those packets and MUST remove them from the count of bytes in flight. those 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.
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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 4.9.1 of [QUIC-TLS]). (see Section 4.9.1 of [QUIC-TLS]).
6.6. Discussion 5.6. Discussion
The majority of constants were derived from best common practices The majority of constants were derived from best common practices
among widely deployed TCP implementations on the internet. among widely deployed TCP implementations on the internet.
Exceptions follow. Exceptions follow.
A shorter delayed ack time of 25ms was chosen because longer delayed A shorter delayed ack time of 25ms was chosen because longer delayed
acks can delay loss recovery and for the small number of connections acks can delay loss recovery and for the small number of connections
where less than packet per 25ms is delivered, acking every packet is where less than packet per 25ms is delivered, acking every packet is
beneficial to congestion control and loss recovery. beneficial to congestion control and loss recovery.
7. Congestion Control 6. Congestion Control
QUIC's congestion control is based on TCP NewReno [RFC6582]. NewReno QUIC's congestion control is based on TCP NewReno [RFC6582]. NewReno
is a congestion window based congestion control. QUIC specifies the is a congestion window based congestion control. QUIC specifies the
congestion window in bytes rather than packets due to finer control congestion window in bytes rather than packets due to finer control
and the ease of appropriate byte counting [RFC3465]. and the ease of appropriate byte counting [RFC3465].
QUIC hosts MUST NOT send packets if they would increase QUIC hosts MUST NOT send packets if they would increase
bytes_in_flight (defined in Appendix B.2) beyond the available bytes_in_flight (defined in Appendix B.2) beyond the available
congestion window, unless the packet is a probe packet sent after a congestion window, unless the packet is a probe packet sent after a
PTO timer expires, as described in Section 6.3. PTO timer expires, as described in Section 5.2.
Implementations MAY use other congestion control algorithms, such as Implementations MAY use other congestion control algorithms, such as
Cubic [RFC8312], and endpoints MAY use different algorithms from one Cubic [RFC8312], and endpoints MAY use different algorithms from one
another. The signals QUIC provides for congestion control are another. The signals QUIC provides for congestion control are
generic and are designed to support different algorithms. generic and are designed to support different algorithms.
7.1. Explicit Congestion Notification 6.1. Explicit Congestion Notification
If a path has been verified to support ECN, QUIC treats a Congestion If a path has been verified to support ECN, QUIC treats a Congestion
Experienced codepoint in the IP header as a signal of congestion. Experienced codepoint in the IP header as a signal of congestion.
This document specifies an endpoint's response when its peer receives This document specifies an endpoint's response when its peer receives
packets with the Congestion Experienced codepoint. As discussed in packets with the Congestion Experienced codepoint. As discussed in
[RFC8311], endpoints are permitted to experiment with other response [RFC8311], endpoints are permitted to experiment with other response
functions. functions.
7.2. Slow Start 6.2. Slow Start
QUIC begins every connection in slow start and exits slow start upon 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 loss or upon increase in the ECN-CE counter. QUIC re-enters slow
start anytime the congestion window is less than ssthresh, which only start anytime the congestion window is less than ssthresh, which only
occurs after persistent congestion is declared. While in slow start, occurs after persistent congestion is declared. While in slow start,
QUIC increases the congestion window by the number of bytes QUIC increases the congestion window by the number of bytes
acknowledged when each acknowledgment is processed. acknowledged when each acknowledgment is processed.
7.3. Congestion Avoidance 6.3. Congestion Avoidance
Slow start exits to congestion avoidance. Congestion avoidance in Slow start exits to congestion avoidance. Congestion avoidance in
NewReno uses an additive increase multiplicative decrease (AIMD) NewReno uses an additive increase multiplicative decrease (AIMD)
approach that increases the congestion window by one maximum packet approach that increases the congestion window by one maximum packet
size per congestion window acknowledged. When a loss is detected, size per congestion window acknowledged. When a loss is detected,
NewReno halves the congestion window and sets the slow start NewReno halves the congestion window and sets the slow start
threshold to the new congestion window. threshold to the new congestion window.
7.4. Recovery Period 6.4. Recovery Period
Recovery is a period of time beginning with detection of a lost 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 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 retransmit packets, it defines the end of recovery as a packet sent
after the start of recovery being acknowledged. This is slightly after the start of recovery being acknowledged. This is slightly
different from TCP's definition of recovery, which ends when the lost different from TCP's definition of recovery, which ends when the lost
packet that started recovery is acknowledged. packet that started recovery is acknowledged.
The recovery period limits congestion window reduction to once per The recovery period limits congestion window reduction to once per
round trip. During recovery, the congestion window remains unchanged round trip. During recovery, the congestion window remains unchanged
irrespective of new losses or increases in the ECN-CE counter. irrespective of new losses or increases in the ECN-CE counter.
7.5. Ignoring Loss of Undecryptable Packets 6.5. Ignoring Loss of Undecryptable Packets
During the handshake, some packet protection keys might not be During the handshake, some packet protection keys might not be
available when a packet arrives. In particular, Handshake and 0-RTT available when a packet arrives. In particular, Handshake and 0-RTT
packets cannot be processed until the Initial packets arrive, and packets cannot be processed until the Initial packets arrive, and
1-RTT packets cannot be processed until the handshake completes. 1-RTT packets cannot be processed until the handshake completes.
Endpoints MAY ignore the loss of Handshake, 0-RTT, and 1-RTT packets Endpoints MAY ignore the loss of Handshake, 0-RTT, and 1-RTT packets
that might arrive before the peer has packet protection keys to that might arrive before the peer has packet protection keys to
process those packets. process those packets.
7.6. Probe Timeout 6.6. Probe Timeout
Probe packets MUST NOT be blocked by the congestion controller. A Probe packets MUST NOT be blocked by the congestion controller. A
sender MUST however count these packets as being additionally in sender MUST however count these packets as being additionally in
flight, since these packets add network load without establishing flight, since these packets add network load without establishing
packet loss. Note that sending probe packets might cause the packet loss. Note that sending probe packets might cause the
sender's bytes in flight to exceed the congestion window until an sender's bytes in flight to exceed the congestion window until an
acknowledgement is received that establishes loss or delivery of acknowledgement is received that establishes loss or delivery of
packets. packets.
7.7. Persistent Congestion 6.7. Persistent Congestion
When an ACK frame is received that establishes loss of all in-flight 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 packets sent over a long enough period of time, the network is
considered to be experiencing persistent congestion. Commonly, this considered to be experiencing persistent congestion. Commonly, this
can be established by consecutive PTOs, but since the PTO timer is can be established by consecutive PTOs, but since the PTO timer is
reset when a new ack-eliciting packet is sent, an explicit duration 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 must be used to account for those cases where PTOs do not occur or
are substantially delayed. This duration is computed as follows: are substantially delayed. This duration is computed as follows:
(smoothed_rtt + 4 * rttvar + max_ack_delay) * (smoothed_rtt + 4 * rttvar + max_ack_delay) *
skipping to change at page 20, line 33 skipping to change at page 18, line 15
newest packets are acknowledged, the network is considered to have newest packets are acknowledged, the network is considered to have
experienced persistent congestion. experienced persistent congestion.
When persistent congestion is established, the sender's congestion When persistent congestion is established, the sender's congestion
window MUST be reduced to the minimum congestion window window MUST be reduced to the minimum congestion window
(kMinimumWindow). This response of collapsing the congestion window (kMinimumWindow). This response of collapsing the congestion window
on persistent congestion is functionally similar to a sender's on persistent congestion is functionally similar to a sender's
response on a Retransmission Timeout (RTO) in TCP [RFC5681] after response on a Retransmission Timeout (RTO) in TCP [RFC5681] after
Tail Loss Probes (TLP) [TLP]. Tail Loss Probes (TLP) [TLP].
7.8. Pacing 6.8. Pacing
This document does not specify a pacer, but it is RECOMMENDED that a 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 sender pace sending of all in-flight packets based on input from the
congestion controller. For example, a pacer might distribute the congestion controller. For example, a pacer might distribute the
congestion window over the SRTT when used with a window-based 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, and a pacer might use the rate estimate of a rate-based
controller. controller.
An implementation should take care to architect its congestion An implementation should take care to architect its congestion
controller to work well with a pacer. For instance, a pacer might controller to work well with a pacer. For instance, a pacer might
skipping to change at page 21, line 9 skipping to change at page 18, line 37
congestion window, or a pacer might pace out packets handed to it by congestion window, or a pacer might pace out packets handed to it by
the congestion controller. Timely delivery of ACK frames is the congestion controller. Timely delivery of ACK frames is
important for efficient loss recovery. Packets containing only ACK important for efficient loss recovery. Packets containing only ACK
frames should therefore not be paced, to avoid delaying their frames should therefore not be paced, to avoid delaying their
delivery to the peer. delivery to the peer.
As an example of a well-known and publicly available implementation As an example of a well-known and publicly available implementation
of a flow pacer, implementers are referred to the Fair Queue packet of a flow pacer, implementers are referred to the Fair Queue packet
scheduler (fq qdisc) in Linux (3.11 onwards). scheduler (fq qdisc) in Linux (3.11 onwards).
7.9. Under-utilizing the Congestion Window 6.9. Under-utilizing the Congestion Window
A congestion window that is under-utilized SHOULD NOT be increased in A congestion window that is under-utilized SHOULD NOT be increased in
either slow start or congestion avoidance. This can happen due to either slow start or congestion avoidance. This can happen due to
insufficient application data or flow control credit. insufficient application data or flow control credit.
A sender MAY use the pipeACK method described in section 4.3 of A sender MAY use the pipeACK method described in section 4.3 of
[RFC7661] to determine if the congestion window is sufficiently [RFC7661] to determine if the congestion window is sufficiently
utilized. utilized.
A sender that paces packets (see Section 7.8) might delay sending A sender that paces packets (see Section 6.8) might delay sending
packets and not fully utilize the congestion window due to this packets and not fully utilize the congestion window due to this
delay. A sender should not consider itself application limited if it delay. A sender should not consider itself application limited if it
would have fully utilized the congestion window without pacing delay. would have fully utilized the congestion window without pacing delay.
Bursting more than an intial window's worth of data into the network Bursting more than an initial window's worth of data into the network
might cause short-term congestion and losses. Implemementations might cause short-term congestion and losses. Implemementations
SHOULD either use pacing or reduce their congestion window to limit SHOULD either use pacing or reduce their congestion window to limit
such bursts. such bursts.
A sender MAY implement alternate mechanisms to update its congestion A sender MAY implement alternate mechanisms to update its congestion
window after periods of under-utilization, such as those proposed for window after periods of under-utilization, such as those proposed for
TCP in [RFC7661]. TCP in [RFC7661].
8. Security Considerations 7. Security Considerations
8.1. Congestion Signals 7.1. Congestion Signals
Congestion control fundamentally involves the consumption of signals Congestion control fundamentally involves the consumption of signals
- both loss and ECN codepoints - from unauthenticated entities. On- - both loss and ECN codepoints - from unauthenticated entities. On-
path attackers can spoof or alter these signals. An attacker can path attackers can spoof or alter these signals. An attacker can
cause endpoints to reduce their sending rate by dropping packets, or cause endpoints to reduce their sending rate by dropping packets, or
alter send rate by changing ECN codepoints. alter send rate by changing ECN codepoints.
8.2. Traffic Analysis 7.2. Traffic Analysis
Packets that carry only ACK frames can be heuristically identified by Packets that carry only ACK frames can be heuristically identified by
observing packet size. Acknowledgement patterns may expose observing packet size. Acknowledgement patterns may expose
information about link characteristics or application behavior. information about link characteristics or application behavior.
Endpoints can use PADDING frames or bundle acknowledgments with other Endpoints can use PADDING frames or bundle acknowledgments with other
frames to reduce leaked information. frames to reduce leaked information.
8.3. Misreporting ECN Markings 7.3. Misreporting ECN Markings
A receiver can misreport ECN markings to alter the congestion A receiver can misreport ECN markings to alter the congestion
response of a sender. Suppressing reports of ECN-CE markings could response of a sender. Suppressing reports of ECN-CE markings could
cause a sender to increase their send rate. This increase could cause a sender to increase their send rate. This increase could
result in congestion and loss. result in congestion and loss.
A sender MAY attempt to detect suppression of reports by marking A sender MAY attempt to detect suppression of reports by marking
occasional packets that they send with ECN-CE. If a packet marked 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 with ECN-CE is not reported as having been marked when the packet is
acknowledged, the sender SHOULD then disable ECN for that path. acknowledged, the sender SHOULD then disable ECN for that path.
skipping to change at page 22, line 28 skipping to change at page 20, line 7
their sending rate, which is similar in effect to advertising reduced their sending rate, which is similar in effect to advertising reduced
connection flow control limits and so no advantage is gained by doing connection flow control limits and so no advantage is gained by doing
so. so.
Endpoints choose the congestion controller that they use. Though Endpoints choose the congestion controller that they use. Though
congestion controllers generally treat reports of ECN-CE markings as congestion controllers generally treat reports of ECN-CE markings as
equivalent to loss [RFC8311], the exact response for each controller equivalent to loss [RFC8311], the exact response for each controller
could be different. Failure to correctly respond to information could be different. Failure to correctly respond to information
about ECN markings is therefore difficult to detect. about ECN markings is therefore difficult to detect.
9. IANA Considerations 8. IANA Considerations
This document has no IANA actions. Yet. This document has no IANA actions. Yet.
10. References 9. References
10.1. Normative References 9.1. Normative References
[QUIC-TLS] [QUIC-TLS]
Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", draft-ietf-quic-tls-22 (work in progress), July QUIC", draft-ietf-quic-tls-23 (work in progress),
2019. September 2019.
[QUIC-TRANSPORT] [QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic- Multiplexed and Secure Transport", draft-ietf-quic-
transport-22 (work in progress), July 2019. transport-23 (work in progress), September 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion [RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311, Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018, DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>. <https://www.rfc-editor.org/info/rfc8311>.
10.2. Informative References 9.2. Informative References
[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement: [FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
Refining TCP Congestion Control", ACM SIGCOMM , August Refining TCP Congestion Control", ACM SIGCOMM , August
1996. 1996.
[RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK: [RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
a time-based fast loss detection algorithm for TCP", a time-based fast loss detection algorithm for TCP",
draft-ietf-tcpm-rack-05 (work in progress), April 2019. draft-ietf-tcpm-rack-05 (work in progress), April 2019.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte [RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
skipping to change at page 24, line 36 skipping to change at page 22, line 15
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and [RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018, RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>. <https://www.rfc-editor.org/info/rfc8312>.
[TLP] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis, [TLP] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
in progress), February 2013. in progress), February 2013.
10.3. URIs 9.3. URIs
[1] https://mailarchive.ietf.org/arch/search/?email_list=quic [1] https://mailarchive.ietf.org/arch/search/?email_list=quic
[2] https://github.com/quicwg [2] https://github.com/quicwg
[3] https://github.com/quicwg/base-drafts/labels/-recovery [3] https://github.com/quicwg/base-drafts/labels/-recovery
Appendix A. Loss Recovery Pseudocode Appendix A. Loss Recovery Pseudocode
We now describe an example implementation of the loss detection We now describe an example implementation of the loss detection
mechanisms described in Section 6. mechanisms described in Section 5.
A.1. Tracking Sent Packets 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
(Appendix A.1.1) for loss recovery and congestion control. (Appendix A.1.1) for loss recovery and congestion control.
After a packet is declared lost, the endpoint can track it for an After a packet is declared lost, the endpoint can track it for an
skipping to change at page 25, line 33 skipping to change at page 23, line 8
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.
is_crypto_packet: A boolean that indicates whether the packet
contains cryptographic handshake messages critical to the
completion of the QUIC handshake. In this version of QUIC, this
includes any packet with the long header that includes a CRYPTO
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.
A.2. Constants of interest A.2. 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.
skipping to change at page 26, line 29 skipping to change at page 23, line 47
Initial, Initial,
Handshake, Handshake,
ApplicationData, ApplicationData,
} }
A.3. Variables of interest 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.
crypto_count: The number of times all unacknowledged CRYPTO data has
been retransmitted without receiving an ack.
pto_count: The number of times a PTO has been sent without receiving
an ack.
time_of_last_sent_ack_eliciting_packet: The time the most recent
ack-eliciting packet was sent.
time_of_last_sent_crypto_packet: The time the most recent crypto
packet was sent.
largest_acked_packet[kPacketNumberSpace]: 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 for packets in the intends to delay acknowledgments for packets in the
ApplicationData packet number space. The actual ack_delay in a ApplicationData packet number space. The actual ack_delay in a
received ACK frame may be larger due to late timers, reordering, received ACK frame may be larger due to late timers, reordering,
or lost ACKs. or lost ACKs.
loss_detection_timer: Multi-modal timer used for loss detection.
pto_count: The number of times a PTO has been sent without receiving
an ack.
time_of_last_sent_ack_eliciting_packet: The time the most recent
ack-eliciting packet was sent.
largest_acked_packet[kPacketNumberSpace]: The largest packet number
acknowledged in the packet number space so far.
loss_time[kPacketNumberSpace]: The time at which the next packet in loss_time[kPacketNumberSpace]: The time at which the next packet in
that packet number space will be considered lost based on that packet number space will be considered lost based on
exceeding the reordering window in time. exceeding the reordering window in time.
sent_packets[kPacketNumberSpace]: An association of packet numbers sent_packets[kPacketNumberSpace]: An association of packet numbers
in a packet number space to information about them. Described in in a packet number space to information about them. Described in
detail above in Appendix A.1. detail above in Appendix A.1.
A.4. 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
pto_count = 0 pto_count = 0
latest_rtt = 0 latest_rtt = 0
smoothed_rtt = 0 smoothed_rtt = 0
rttvar = 0 rttvar = 0
min_rtt = 0 min_rtt = 0
max_ack_delay = 0 max_ack_delay = 0
time_of_last_sent_ack_eliciting_packet = 0 time_of_last_sent_ack_eliciting_packet = 0
time_of_last_sent_crypto_packet = 0
for pn_space in [ Initial, Handshake, ApplicationData ]: for pn_space in [ Initial, Handshake, ApplicationData ]:
largest_acked_packet[pn_space] = infinite largest_acked_packet[pn_space] = infinite
loss_time[pn_space] = 0 loss_time[pn_space] = 0
A.5. 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
Appendix A.1.1. Appendix A.1.1.
Pseudocode for OnPacketSent follows: Pseudocode for OnPacketSent follows:
OnPacketSent(packet_number, pn_space, ack_eliciting, OnPacketSent(packet_number, pn_space, ack_eliciting,
in_flight, is_crypto_packet, sent_bytes): in_flight, sent_bytes):
sent_packets[pn_space][packet_number].packet_number = sent_packets[pn_space][packet_number].packet_number =
packet_number packet_number
sent_packets[pn_space][packet_number].time_sent = now sent_packets[pn_space][packet_number].time_sent = now
sent_packets[pn_space][packet_number].ack_eliciting = sent_packets[pn_space][packet_number].ack_eliciting =
ack_eliciting ack_eliciting
sent_packets[pn_space][packet_number].in_flight = in_flight sent_packets[pn_space][packet_number].in_flight = in_flight
if (in_flight): if (in_flight):
if (is_crypto_packet):
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[pn_space][packet_number].size = sent_bytes sent_packets[pn_space][packet_number].size = sent_bytes
SetLossDetectionTimer() SetLossDetectionTimer()
A.6. 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.
skipping to change at page 28, line 43 skipping to change at page 25, line 49
largest_acked_packet[pn_space] = largest_acked_packet[pn_space] =
max(largest_acked_packet[pn_space], ack.largest_acked) max(largest_acked_packet[pn_space], ack.largest_acked)
// Nothing to do if there are no newly acked packets. // Nothing to do if there are no newly acked packets.
newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space) newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)
if (newly_acked_packets.empty()): if (newly_acked_packets.empty()):
return return
// If the largest acknowledged is newly acked and // If the largest acknowledged is newly acked and
// at least one ack-eliciting was newly acked, update the RTT. // at least one ack-eliciting was newly acked, update the RTT.
if (sent_packets[pn_space][ack.largest_acked] && if (sent_packets[pn_space].contains(ack.largest_acked) &&
IncludesAckEliciting(newly_acked_packets)) IncludesAckEliciting(newly_acked_packets)):
latest_rtt = latest_rtt =
now - sent_packets[pn_space][ack.largest_acked].time_sent now - sent_packets[pn_space][ack.largest_acked].time_sent
ack_delay = 0 ack_delay = 0
if pn_space == ApplicationData: if (pn_space == ApplicationData):
ack_delay = ack.ack_delay ack_delay = ack.ack_delay
UpdateRtt(ack_delay) UpdateRtt(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, pn_space)
for acked_packet in newly_acked_packets: for acked_packet in newly_acked_packets:
OnPacketAcked(acked_packet.packet_number, pn_space) OnPacketAcked(acked_packet.packet_number, pn_space)
DetectLostPackets(pn_space) DetectLostPackets(pn_space)
crypto_count = 0
pto_count = 0 pto_count = 0
SetLossDetectionTimer() SetLossDetectionTimer()
UpdateRtt(ack_delay): UpdateRtt(ack_delay):
// First RTT sample. // First RTT sample.
if (smoothed_rtt == 0): if (smoothed_rtt == 0):
min_rtt = latest_rtt min_rtt = latest_rtt
smoothed_rtt = latest_rtt smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2 rttvar = latest_rtt / 2
skipping to change at page 31, line 11 skipping to change at page 28, line 11
fire immediately. fire immediately.
Pseudocode for SetLossDetectionTimer follows: Pseudocode for SetLossDetectionTimer follows:
// Returns the earliest loss_time and the packet number // Returns the earliest loss_time and the packet number
// space it's from. Returns 0 if all times are 0. // space it's from. Returns 0 if all times are 0.
GetEarliestLossTime(): GetEarliestLossTime():
time = loss_time[Initial] time = loss_time[Initial]
space = Initial space = Initial
for pn_space in [ Handshake, ApplicationData ]: for pn_space in [ Handshake, ApplicationData ]:
if loss_time[pn_space] != 0 && if (loss_time[pn_space] != 0 &&
(time == 0 || loss_time[pn_space] < time): (time == 0 || loss_time[pn_space] < time)):
time = loss_time[pn_space]; time = loss_time[pn_space];
space = pn_space space = pn_space
return time, space return time, space
PeerNotAwaitingAddressValidation():
# Assume clients validate the server's address implicitly.
if (endpoint is server):
return true
# Servers complete address validation when a
# protected packet is received.
return has received Handshake ACK ||
has received 1-RTT ACK
SetLossDetectionTimer(): SetLossDetectionTimer():
loss_time, _ = GetEarliestLossTime() loss_time, _ = GetEarliestLossTime()
if (loss_time != 0): if (loss_time != 0):
// Time threshold loss detection. // Time threshold loss detection.
loss_detection_timer.update(loss_time) loss_detection_timer.update(loss_time)
return return
if (has unacknowledged crypto data if (no ack-eliciting packets in flight &&
|| endpoint is client without 1-RTT keys): PeerNotAwaitingAddressValidation()):
// Crypto retransmission timer.
if (smoothed_rtt == 0):
timeout = 2 * kInitialRtt
else:
timeout = 2 * smoothed_rtt
timeout = max(timeout, kGranularity)
timeout = timeout * (2 ^ crypto_count)
loss_detection_timer.update(
time_of_last_sent_crypto_packet + timeout)
return
// Don't arm timer if there are no ack-eliciting packets
// in flight.
if (no ack-eliciting packets in flight):
loss_detection_timer.cancel() loss_detection_timer.cancel()
return return
// Calculate PTO duration // Use a default timeout if there are no RTT measurements
timeout = if (smoothed_rtt == 0):
smoothed_rtt + max(4 * rttvar, kGranularity) + max_ack_delay timeout = 2 * kInitialRtt
else:
// Calculate PTO duration
timeout = smoothed_rtt + max(4 * rttvar, kGranularity) +
max_ack_delay
timeout = timeout * (2 ^ pto_count) timeout = timeout * (2 ^ pto_count)
loss_detection_timer.update( loss_detection_timer.update(
time_of_last_sent_ack_eliciting_packet + timeout) time_of_last_sent_ack_eliciting_packet + timeout)
A.9. On 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():
loss_time, pn_space = GetEarliestLossTime() loss_time, pn_space = GetEarliestLossTime()
if (loss_time != 0): if (loss_time != 0):
// Time threshold loss Detection // Time threshold loss Detection
DetectLostPackets(pn_space) DetectLostPackets(pn_space)
// Retransmit crypto data if no packets were lost SetLossDetectionTimer()
// and there is crypto data to retransmit. return
else if (has unacknowledged crypto data):
// Crypto retransmission timeout. if (endpoint is client without 1-RTT keys):
RetransmitUnackedCryptoData()
crypto_count++
else if (endpoint is client without 1-RTT keys):
// Client sends an anti-deadlock packet: Initial is padded // Client sends an anti-deadlock packet: Initial is padded
// to earn more anti-amplification credit, // to earn more anti-amplification credit,
// a Handshake packet proves address ownership. // a Handshake packet proves address ownership.
if (has Handshake keys): if (has Handshake keys):
SendOneHandshakePacket() SendOneAckElicitingHandshakePacket()
else: else:
SendOnePaddedInitialPacket() SendOneAckElicitingPaddedInitialPacket()
crypto_count++
else: else:
// PTO. Send new data if available, else retransmit old data. // PTO. Send new data if available, else retransmit old data.
// If neither is available, send a single PING frame. // If neither is available, send a single PING frame.
SendOneOrTwoPackets() SendOneOrTwoAckElicitingPackets()
pto_count++
pto_count++
SetLossDetectionTimer() SetLossDetectionTimer()
A.10. 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. operates on the sent_packets for that packet number space.
Pseudocode for DetectLostPackets follows: Pseudocode for DetectLostPackets follows:
DetectLostPackets(pn_space): DetectLostPackets(pn_space):
skipping to change at page 33, line 43 skipping to change at page 30, line 43
unacked.time_sent + loss_delay) 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)
Appendix B. Congestion Control Pseudocode Appendix B. Congestion Control Pseudocode
We now describe an example implementation of the congestion We now describe an example implementation of the congestion
controller described in Section 7. controller described in Section 6.
B.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
skipping to change at page 34, line 34 skipping to change at page 31, line 34
TCP does with a Retransmission Timeout (RTO) [RFC5681]. The TCP does with a Retransmission Timeout (RTO) [RFC5681]. The
RECOMMENDED value for kPersistentCongestionThreshold is 3, which RECOMMENDED value for kPersistentCongestionThreshold is 3, which
is approximately equivalent to having two TLPs before an RTO in is approximately equivalent to having two TLPs before an RTO in
TCP. TCP.
B.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_counters[kPacketNumberSpace]: The highest value reported for
the peer in an ACK frame. This variable is used to detect the ECN-CE counter in the packet number space by the peer in an
increases in the reported ECN-CE counter. ACK frame. This value is used to detect increases in the reported
ECN-CE counter.
bytes_in_flight: The sum of the size in bytes of all sent packets 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
not been acked or declared lost. The size does not include IP or not been acked or declared lost. The size does not include IP or
UDP overhead, but does include the QUIC header and AEAD overhead. UDP overhead, but does include the QUIC header and AEAD overhead.
Packets only containing ACK frames do not count towards Packets only containing ACK frames do not count towards
bytes_in_flight to ensure congestion control does not impede bytes_in_flight to ensure congestion control does not impede
congestion feedback. congestion feedback.
congestion_window: Maximum number of bytes-in-flight that may be congestion_window: Maximum number of bytes-in-flight that may be
skipping to change at page 35, line 20 skipping to change at page 32, line 20
B.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
congestion_recovery_start_time = 0 congestion_recovery_start_time = 0
ssthresh = infinite ssthresh = infinite
ecn_ce_counter = 0 for pn_space in [ Initial, Handshake, ApplicationData ]:
ecn_ce_counters[pn_space] = 0
B.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
B.5. On Packet Acknowledgement B.5. On Packet Acknowledgement
skipping to change at page 36, line 14 skipping to change at page 33, line 14
InCongestionRecovery(sent_time): InCongestionRecovery(sent_time):
return sent_time <= congestion_recovery_start_time return sent_time <= congestion_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 (InCongestionRecovery(acked_packet.time_sent)): if (InCongestionRecovery(acked_packet.time_sent)):
// Do not increase congestion window in recovery period. // Do not increase congestion window in recovery period.
return return
if (IsAppLimited()) if (IsAppLimited()):
// Do not increase congestion_window if application // Do not increase congestion_window if application
// limited. // limited.
return 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
skipping to change at page 36, line 46 skipping to change at page 33, line 46
congestion_recovery_start_time = Now() congestion_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
B.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, pn_space):
// 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_counters[pn_space]):
ecn_ce_counter = ack.ce_counter ecn_ce_counters[pn_space] = ack.ce_counter
CongestionEvent(sent_packets[ack.largest_acked].time_sent) CongestionEvent(sent_packets[ack.largest_acked].time_sent)
B.8. On Packets Lost B.8. On Packets Lost
Invoked from DetectLostPackets when packets are deemed lost. Invoked from DetectLostPackets when packets are deemed lost.
InPersistentCongestion(largest_lost_packet): InPersistentCongestion(largest_lost_packet):
pto = smoothed_rtt + max(4 * rttvar, kGranularity) + pto = smoothed_rtt + max(4 * rttvar, kGranularity) +
max_ack_delay max_ack_delay
congestion_period = pto * kPersistentCongestionThreshold congestion_period = pto * kPersistentCongestionThreshold
skipping to change at page 37, line 37 skipping to change at page 34, line 37
if (InPersistentCongestion(largest_lost_packet)): if (InPersistentCongestion(largest_lost_packet)):
congestion_window = kMinimumWindow congestion_window = kMinimumWindow
Appendix C. Change Log Appendix C. Change Log
*RFC Editor's Note:* Please remove this section prior to *RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document. publication of a final version of this document.
Issue and pull request numbers are listed with a leading octothorp. Issue and pull request numbers are listed with a leading octothorp.
C.1. Since draft-ietf-quic-recovery-21 C.1. Since draft-ietf-quic-recovery-22
o PTO should always send an ack-eliciting packet (#2895)
o Unify the Handshake Timer with the PTO timer (#2648, #2658, #2886)
o Move ACK generation text to transport draft (#1860, #2916)
C.2. Since draft-ietf-quic-recovery-21
o No changes o No changes
C.2. Since draft-ietf-quic-recovery-20 C.3. Since draft-ietf-quic-recovery-20
o Path validation can be used as initial RTT value (#2644, #2687) o Path validation can be used as initial RTT value (#2644, #2687)
o max_ack_delay transport parameter defaults to 0 (#2638, #2646) o max_ack_delay transport parameter defaults to 0 (#2638, #2646)
o Ack Delay only measures intentional delays induced by the o Ack Delay only measures intentional delays induced by the
implementation (#2596, #2786) implementation (#2596, #2786)
C.3. Since draft-ietf-quic-recovery-19 C.4. Since draft-ietf-quic-recovery-19
o Change kPersistentThreshold from an exponent to a multiplier o Change kPersistentThreshold from an exponent to a multiplier
(#2557) (#2557)
o Send a PING if the PTO timer fires and there's nothing to send o Send a PING if the PTO timer fires and there's nothing to send
(#2624) (#2624)
o Set loss delay to at least kGranularity (#2617) o Set loss delay to at least kGranularity (#2617)
o Merge application limited and sending after idle sections. Always o Merge application limited and sending after idle sections. Always
skipping to change at page 38, line 30 skipping to change at page 35, line 39
packet is ack-eliciting but the largest_acked is not (#2592) packet is ack-eliciting but the largest_acked is not (#2592)
o Don't arm the handshake timer if there is no handshake data o Don't arm the handshake timer if there is no handshake data
(#2590) (#2590)
o Clarify that the time threshold loss alarm takes precedence over o Clarify that the time threshold loss alarm takes precedence over
the crypto handshake timer (#2590, #2620) the crypto handshake timer (#2590, #2620)
o Change initial RTT to 500ms to align with RFC6298 (#2184) o Change initial RTT to 500ms to align with RFC6298 (#2184)
C.4. Since draft-ietf-quic-recovery-18 C.5. Since draft-ietf-quic-recovery-18
o Change IW byte limit to 14720 from 14600 (#2494) o Change IW byte limit to 14720 from 14600 (#2494)
o Update PTO calculation to match RFC6298 (#2480, #2489, #2490) o Update PTO calculation to match RFC6298 (#2480, #2489, #2490)
o Improve loss detection's description of multiple packet number o Improve loss detection's description of multiple packet number
spaces and pseudocode (#2485, #2451, #2417) spaces and pseudocode (#2485, #2451, #2417)
o Declare persistent congestion even if non-probe packets are sent o Declare persistent congestion even if non-probe packets are sent
and don't make persistent congestion more aggressive than RTO and don't make persistent congestion more aggressive than RTO
skipping to change at page 38, line 44 skipping to change at page 36, line 4
o Update PTO calculation to match RFC6298 (#2480, #2489, #2490) o Update PTO calculation to match RFC6298 (#2480, #2489, #2490)
o Improve loss detection's description of multiple packet number o Improve loss detection's description of multiple packet number
spaces and pseudocode (#2485, #2451, #2417) spaces and pseudocode (#2485, #2451, #2417)
o Declare persistent congestion even if non-probe packets are sent o Declare persistent congestion even if non-probe packets are sent
and don't make persistent congestion more aggressive than RTO and don't make persistent congestion more aggressive than RTO
verified was (#2365, #2244) verified was (#2365, #2244)
o Move pseudocode to the appendices (#2408) o Move pseudocode to the appendices (#2408)
o What to send on multiple PTOs (#2380) o What to send on multiple PTOs (#2380)
C.5. Since draft-ietf-quic-recovery-17 C.6. 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 39, line 26 skipping to change at page 36, line 32
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 (#2327) discarded (#2327)
C.6. Since draft-ietf-quic-recovery-16 C.7. 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 40, line 4 skipping to change at page 37, line 11
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)
C.7. Since draft-ietf-quic-recovery-14 C.8. 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)
C.8. Since draft-ietf-quic-recovery-13 C.9. 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)
C.9. Since draft-ietf-quic-recovery-12 C.10. 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)
C.10. Since draft-ietf-quic-recovery-11 C.11. Since draft-ietf-quic-recovery-11
No significant changes. No significant changes.
C.11. Since draft-ietf-quic-recovery-10 C.12. 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)
C.12. Since draft-ietf-quic-recovery-09 C.13. Since draft-ietf-quic-recovery-09
No significant changes. No significant changes.
C.13. Since draft-ietf-quic-recovery-08 C.14. Since draft-ietf-quic-recovery-08
o Clarified pacing and RTO (#967, #977) o Clarified pacing and RTO (#967, #977)
C.14. Since draft-ietf-quic-recovery-07 C.15. 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.
C.15. Since draft-ietf-quic-recovery-06 C.16. Since draft-ietf-quic-recovery-06
No significant changes. No significant changes.
C.16. Since draft-ietf-quic-recovery-05 C.17. Since draft-ietf-quic-recovery-05
o Add more congestion control text (#776) o Add more congestion control text (#776)
C.17. Since draft-ietf-quic-recovery-04 C.18. Since draft-ietf-quic-recovery-04
No significant changes. No significant changes.
C.18. Since draft-ietf-quic-recovery-03 C.19. Since draft-ietf-quic-recovery-03
No significant changes. No significant changes.
C.19. Since draft-ietf-quic-recovery-02 C.20. 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)
C.20. Since draft-ietf-quic-recovery-01 C.21. 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
C.21. Since draft-ietf-quic-recovery-00 C.22. Since draft-ietf-quic-recovery-00
o Improved description of constants and ACK behavior o Improved description of constants and ACK behavior
C.22. Since draft-iyengar-quic-loss-recovery-01 C.23. 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
 End of changes. 115 change blocks. 
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