draft-ietf-quic-recovery-19.txt   draft-ietf-quic-recovery-latest.txt 
QUIC Working Group J. Iyengar, Ed. QUIC Working Group J. Iyengar, Ed.
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track I. Swett, Ed. Intended status: Standards Track I. Swett, Ed.
Expires: September 12, 2019 Google Expires: October 20, 2019 Google
March 11, 2019 April 18, 2019
QUIC Loss Detection and Congestion Control QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-19 draft-ietf-quic-recovery-latest
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 September 12, 2019. This Internet-Draft will expire on October 20, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Design of the QUIC Transmission Machinery . . . . . . . . . . 5 3. Design of the QUIC Transmission Machinery . . . . . . . . . . 5
3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5 3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5
3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 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. No Reneging . . . . . . . . . . . . . . . . . . . . . 6 3.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6
3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6 3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 7
3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 7 3.1.5. Explicit Correction For Delayed Acknowledgements . . 7
4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7 4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7
4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 8 4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 7
4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 8 4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8 4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8
5. Computing the RTT estimate . . . . . . . . . . . . . . . . . 8 4.4. Measuring and Reporting Host Delay . . . . . . . . . . . 8
6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9 5. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 9
6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 9 5.1. Generating RTT samples . . . . . . . . . . . . . . . . . 9
6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10 5.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 10
6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10 5.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 10
6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 11 6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 11
6.2.1. Retry and Version Negotiation . . . . . . . . . . . . 12 6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 11
6.2.2. Discarding Keys and Packet State . . . . . . . . . . 12 6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 12
6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 12 6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 12
6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 13 6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 13
6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 13 6.2.1. Retry and Version Negotiation . . . . . . . . . . . . 14
6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 14 6.2.2. Discarding Keys and Packet State . . . . . . . . . . 14
6.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 14 6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 15
7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 15 6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 15
7.1. Explicit Congestion Notification . . . . . . . . . . . . 15 6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 16
7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 15 6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 16
7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 15 6.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 17
7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 16 7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17
7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 16 7.1. Explicit Congestion Notification . . . . . . . . . . . . 17
7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 16 7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 17
7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 16 7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 18
7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 18
7.9. Sending data after an idle period . . . . . . . . . . . . 18 7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 18
7.10. Application Limited Sending . . . . . . . . . . . . . . . 18 7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 18
8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 18 7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 19 7.9. Under-utilizing the Congestion Window . . . . . . . . . . 20
8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 19 8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 19 8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 20 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 22 10.1. Normative References . . . . . . . . . . . . . . . . . . 21
A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 22 10.2. Informative References . . . . . . . . . . . . . . . . . 22
A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 22 10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 23
A.2. Constants of interest . . . . . . . . . . . . . . . . . . 22 Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 24
A.3. Variables of interest . . . . . . . . . . . . . . . . . . 23 A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 24
A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 24 A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 24
A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 24 A.2. Constants of interest . . . . . . . . . . . . . . . . . . 25
A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 25 A.3. Variables of interest . . . . . . . . . . . . . . . . . . 25
A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 27 A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 26
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 . . . . . . . . . . . . . 27
A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 29 A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 28
Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 30 A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 29
B.1. Constants of interest . . . . . . . . . . . . . . . . . . 30 A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 31
B.2. Variables of interest . . . . . . . . . . . . . . . . . . 31 A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 31
B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 32 Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 32
B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 32 B.1. Constants of interest . . . . . . . . . . . . . . . . . . 32
B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 32 B.2. Variables of interest . . . . . . . . . . . . . . . . . . 33
B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 33 B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 34
B.7. Process ECN Information . . . . . . . . . . . . . . . . . 33 B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 34
B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 33 B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 34
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 34 B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 35
C.1. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 34 B.7. Process ECN Information . . . . . . . . . . . . . . . . . 35
C.2. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 35 B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 36
C.3. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 35 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 36
C.4. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 36 C.1. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 36
C.5. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 36 C.2. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 36
C.6. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 36 C.3. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 37
C.7. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 36 C.4. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 37
C.8. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 36 C.5. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 38
C.9. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 37 C.6. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 38
C.10. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 37 C.7. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 38
C.11. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 37 C.8. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 38
C.12. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 37 C.9. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 39
C.13. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 37 C.10. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 39
C.14. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 37 C.11. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 39
C.15. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 37 C.12. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 39
C.16. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 37 C.13. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 39
C.17. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 38 C.14. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 39
C.18. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 38 C.15. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 39
C.19. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 38 C.16. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 40
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 38 C.17. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38 C.18. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 40
C.19. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 40
C.20. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 40
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
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
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implementations on both sides and reducing memory pressure on the implementations on both sides and reducing memory pressure on the
sender. sender.
3.1.4. More ACK Ranges 3.1.4. More ACK Ranges
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.
3.1.5. Explicit Correction For Delayed ACKs 3.1.5. Explicit Correction For Delayed Acknowledgements
QUIC ACKs explicitly encode the delay incurred at the receiver QUIC endpoints measure the delay incurred between when a packet is
between when a packet is received and when the corresponding ACK is received and when the corresponding acknowledgment is sent, allowing
sent. This allows the receiver of the ACK to adjust for receiver a peer to maintain a more accurate round-trip time estimate (see
delays, specifically the delayed ack timer, when estimating the path Section 4.4).
RTT. This mechanism also allows a receiver to measure and report the
delay from when a packet was received by the OS kernel, which is
useful in receivers which may incur delays such as context-switch
latency before a userspace QUIC receiver processes a received packet.
4. Generating Acknowledgements 4. Generating Acknowledgements
QUIC SHOULD delay sending acknowledgements in response to packets,
but MUST NOT excessively delay acknowledgements of ack-eliciting
packets. Specifically, implementations MUST attempt to enforce a
maximum ack delay to avoid causing the peer spurious timeouts. The
maximum ack delay is communicated in the "max_ack_delay" transport
parameter and the default value is 25ms.
An acknowledgement SHOULD be sent immediately upon receipt of a An acknowledgement SHOULD be sent immediately upon receipt of a
second ack-eliciting packet. QUIC recovery algorithms do not assume second ack-eliciting packet. QUIC recovery algorithms do not assume
the peer sends an ACK immediately when receiving a second ack- the peer sends an ACK immediately when receiving a second ack-
eliciting packet. eliciting packet.
In order to accelerate loss recovery and reduce timeouts, the In order to accelerate loss recovery and reduce timeouts, the
receiver SHOULD send an immediate ACK after it receives an out-of- receiver SHOULD send an immediate ACK after it receives an out-of-
order packet. It could send immediate ACKs for in-order packets for 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- 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 order packets arrive. If every packet arrives out-of- order, then an
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As an optimization, a receiver MAY process multiple packets before As an optimization, a receiver MAY process multiple packets before
sending any ACK frames in response. In this case the receiver can sending any ACK frames in response. In this case the receiver can
determine whether an immediate or delayed acknowledgement should be determine whether an immediate or delayed acknowledgement should be
generated after processing incoming packets. generated after processing incoming packets.
4.1. Crypto Handshake Data 4.1. Crypto Handshake Data
In order to quickly complete the handshake and avoid spurious In order to quickly complete the handshake and avoid spurious
retransmissions due to crypto retransmission timeouts, crypto packets retransmissions due to crypto retransmission timeouts, crypto packets
SHOULD use a very short ack delay, such as the local timer SHOULD use a very short ack delay, such as the local timer
granularity. ACK frames MAY be sent immediately when the crypto granularity. ACK frames SHOULD be sent immediately when the crypto
stack indicates all data for that packet number space has been stack indicates all data for that packet number space has been
received. received.
4.2. ACK Ranges 4.2. ACK Ranges
When an ACK frame is sent, one or more ranges of acknowledged packets When an ACK frame is sent, one or more ranges of acknowledged packets
are included. Including older packets reduces the chance of spurious are included. Including older packets reduces the chance of spurious
retransmits caused by losing previously sent ACK frames, at the cost retransmits caused by losing previously sent ACK frames, at the cost
of larger ACK frames. of larger ACK frames.
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In cases without ACK frame loss, this algorithm allows for a minimum 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, of 1 RTT of reordering. In cases with ACK frame loss and reordering,
this approach does not guarantee that every acknowledgement is seen this approach does not guarantee that every acknowledgement is seen
by the sender before it is no longer included in the ACK frame. by the sender before it is no longer included in the ACK frame.
Packets could be received out of order and all subsequent ACK frames Packets could be received out of order and all subsequent ACK frames
containing them could be lost. In this case, the loss recovery containing them could be lost. In this case, the loss recovery
algorithm may cause spurious retransmits, but the sender will algorithm may cause spurious retransmits, but the sender will
continue making forward progress. continue making forward progress.
5. Computing the RTT estimate 4.4. Measuring and Reporting Host Delay
Round-trip time (RTT) is calculated when an ACK frame arrives by An endpoint measures the delay incurred between when a packet is
computing the difference between the current time and the time the received and when the corresponding acknowledgment is sent. The
largest acked packet was sent. An RTT sample MUST NOT be taken for a endpoint encodes this host delay for the largest acknowledged packet
packet that is not newly acknowledged or not ack-eliciting. 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 host delays, which is important for delayed acknowledgements,
when estimating the path RTT. In certain deployments, a packet might
be held in the OS kernel or elsewhere on the host before being
processed by the QUIC stack. Where possible, an endpoint MAY include
these delays when populating the Ack Delay field in an ACK frame.
When RTT is calculated, the ack delay field from the ACK frame SHOULD An endpoint MUST NOT excessively delay acknowledgements of ack-
be limited to the max_ack_delay specified by the peer. Limiting eliciting packets. The maximum ack delay is communicated in the
ack_delay to max_ack_delay ensures a peer specifying an extremely max_ack_delay transport parameter, see Section 18.1 of
small max_ack_delay doesn't cause more spurious timeouts than a peer [QUIC-TRANSPORT]. max_ack_delay implies an explicit contract: an
that correctly specifies max_ack_delay. It SHOULD be subtracted from endpoint promises to never delay acknowledgments of an ack-eliciting
the RTT as long as the result is larger than the min_rtt. If the packet by more than the indicated value. If it does, any excess
result is smaller than the min_rtt, the RTT should be used, but the accrues to the RTT estimate and could result in spurious
ack delay field should be ignored. retransmissions from the peer.
A sender calculates both smoothed RTT (SRTT) and RTT variance 5. Estimating the Round-Trip Time
(RTTVAR) similar to those specified in [RFC6298], see Appendix A.6.
A sender takes an RTT sample when an ACK frame is received that At a high level, an endpoint measures the time from when a packet was
acknowledges a larger packet number than before (see Appendix A.6). sent to when it is acknowledged as a round-trip time (RTT) sample.
A sender will take multiple RTT samples per RTT when multiple such The endpoint uses RTT samples and peer-reported host delays
ACK frames are received within an RTT. When multiple samples are (Section 4.4) to generate a statistical description of the
generated within an RTT, the smoothed RTT and RTT variance could connection's RTT. An endpoint computes the following three values:
retain inadequate history, as suggested in [RFC6298]. Changing these the minimum value observed over the lifetime of the connection
computations is currently an open research question. (min_rtt), an exponentially-weighted moving average (smoothed_rtt),
and the variance in the observed RTT samples (rttvar).
min_rtt is the minimum RTT measured over the connection, prior to 5.1. Generating RTT samples
adjusting by ack delay. Ignoring ack delay for min RTT prevents
intentional or unintentional underestimation of min RTT, which in An endpoint generates an RTT sample on receiving an ACK frame that
turn prevents underestimating smoothed RTT. meets the following two conditions:
o the largest acknowledged packet number is newly acknowledged, and
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 largest acknowledged packet was sent:
latest_rtt = ack_time - send_time_of_largest_acked
An RTT sample is generated using only the largest acknowledged packet
in the received ACK frame. This is because a peer reports host
delays for only the largest acknowledged packet in an ACK frame.
While the reported host delay is not used by the RTT sample
measurement, it is used to adjust the RTT sample in subsequent
computations of smoothed_rtt and rttvar Section 5.3.
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
newly acknowledge the largest acknowledged packet.
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 send an ACK frame on receiving only non-ack-eliciting
packets, so an ACK frame that is subsequently sent can include an
arbitrarily large Ack Delay field. Ignoring such ACK frames avoids
complications in subsequent smoothed_rtt and rttvar computations.
A sender might generate multiple RTT samples per RTT when multiple
ACK frames are received within an RTT. As suggested in [RFC6298],
doing so might result in inadequate history in smoothed_rtt and
rttvar. Ensuring that RTT estimates retain sufficient history is an
open research question.
5.2. Estimating min_rtt
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 a
connection, and to the lesser of min_rtt and latest_rtt on subsequent
samples.
An endpoint uses only locally observed times in computing the min_rtt
and does not adjust for host delays reported by the peer
(Section 4.4). Doing so allows the endpoint to set a lower bound for
the smoothed_rtt based entirely on what it observes (see
Section 5.3), and limits potential underestimation due to
erroneously-reported delays by the peer.
5.3. Estimating smoothed_rtt and rttvar
smoothed_rtt is an exponentially-weighted moving average of an
endpoint's RTT samples, and rttvar is the endpoint's estimated
variance in the RTT samples.
smoothed_rtt uses path latency after adjusting RTT samples for peer-
reported host delays (Section 4.4). A peer limits any delay in
sending an acknowledgement for an ack-eliciting packet to no greater
than the advertised max_ack_delay transport parameter. Consequently,
when a peer reports an Ack Delay that is greater than its
max_ack_delay, the delay is attributed to reasons out of the peer's
control, such as scheduler latency at the peer or loss of previous
ACK frames. Any delays beyond the peer's max_ack_delay are therefore
considered effectively part of path delay and incorporated into the
smoothed_rtt estimate.
When adjusting an RTT sample using peer-reported acknowledgement
delays, an endpoint:
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
(Section 4.4).
o MUST NOT apply the adjustment if the resulting RTT sample is
smaller than the min_rtt. This limits the underestimation that a
misreporting peer can cause to the smoothed_rtt.
On the first RTT sample in a connection, the smoothed_rtt is set to
the latest_rtt.
smoothed_rtt and rttvar are computed as follows, similar to
[RFC6298]. On the first RTT sample in a connection:
smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2
On subsequent RTT samples, smoothed_rtt and rttvar evolve as follows:
ack_delay = min(Ack Delay in ACK Frame, max_ack_delay)
adjusted_rtt = latest_rtt
if (min_rtt + ack_delay < latest_rtt):
adjusted_rtt = latest_rtt - ack_delay
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
rttvar_sample = abs(smoothed_rtt - adjusted_rtt)
rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
6. Loss Detection 6. Loss Detection
QUIC senders use both ack information and timeouts to detect lost QUIC senders use both ack information and timeouts to detect lost
packets, and this section provides a description of these algorithms. packets, and this section provides a description of these algorithms.
If a packet is lost, the QUIC transport needs to recover from that If a packet is lost, the QUIC transport needs to recover from that
loss, such as by retransmitting the data, sending an updated frame, loss, such as by retransmitting the data, sending an updated frame,
or abandoning the frame. For more information, see Section 13.2 of or abandoning the frame. For more information, see Section 13.2 of
[QUIC-TRANSPORT]. [QUIC-TRANSPORT].
skipping to change at page 11, line 5 skipping to change at page 12, line 48
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
not yet caught up. not yet caught up.
An endpoint might consistently record RTT samples as 0 in extremely
low latency networks, leading to a smoothed_rtt of 0. Consequently,
the endpoint could declare all earlier packets as lost immediately
upon receiving an acknowledgement for a later packet. That is, the
endpoint would not provide any reordering tolerance. To avoid
declaring packets as lost too early, the time threshold MUST be set
to at least kGranularity (defined in Appendix A.2).
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 6.2. Crypto Retransmission Timeout
Data in CRYPTO frames is critical to QUIC transport and crypto Data in CRYPTO frames is critical to QUIC transport and crypto
negotiation, so a more aggressive timeout is used to retransmit it. negotiation, so a more aggressive timeout is used to retransmit it.
The initial crypto retransmission timeout SHOULD be set to twice the The initial crypto retransmission timeout SHOULD be set to twice the
initial RTT. initial RTT.
At the beginning, there are no prior RTT samples within a connection. At the beginning, there are no prior RTT samples within a connection.
Resumed connections over the same network SHOULD use the previous Resumed connections over the same network SHOULD use the previous
connection's final smoothed RTT value as the resumed connection's connection's final smoothed RTT value as the resumed connection's
initial RTT. If no previous RTT is available, or if the network initial RTT. If no previous RTT is available, or if the network
changes, the initial RTT SHOULD be set to 100ms. When an changes, the initial RTT SHOULD be set to 500ms, resulting in a 1
acknowledgement is received, a new RTT is computed and the timer second initial handshake timeout as recommended in [RFC6298].
SHOULD be set for twice the newly computed smoothed RTT.
When a crypto packet is sent, the sender MUST set a timer for the When a crypto packet is sent, the sender MUST set a timer for twice
crypto timeout period. This timer MUST be updated when a new crypto the smoothed RTT. This timer MUST be updated when a new crypto
packet is sent. Upon timeout, the sender MUST retransmit all packet is sent and when an acknowledgement is received which computes
unacknowledged CRYPTO data if possible. 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 Until the server has validated the client's address on the path, the
amount of data it can send is limited, as specified in amount of data it can send is limited, as specified in Section 8.1 of
[QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent, [QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent,
then all unacknowledged CRYPTO data sent in Initial packets should be then all unacknowledged CRYPTO data sent in Initial packets should be
retransmitted. If no data can be sent, then no alarm should be armed retransmitted. If no data can be sent, then no alarm should be armed
until data has been received from the client. until data has been received from the client.
Because the server could be blocked until more packets are received, Because the server could be blocked until more packets are received,
the client MUST start the crypto retransmission timer even if there the client MUST ensure that the crypto retransmission timer is set if
is no unacknowledged CRYPTO data. If the timer expires and the there is unacknowledged crypto data or if the client does not yet
client has no CRYPTO data to retransmit and does not have Handshake have 1-RTT keys. If the crypto retransmission timer expires before
keys, it SHOULD send an Initial packet in a UDP datagram of at least the client has 1-RTT keys, it is possible that the client may not
1200 bytes. If the client has Handshake keys, it SHOULD send a have any crypto data to retransmit. However, the client MUST send a
Handshake packet. new packet, containing only PING or PADDDING 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.
On each consecutive expiration of the crypto timer without receiving The crypto retransmission timer is not set if the time threshold
an acknowledgement for a new packet, the sender SHOULD double the Section 6.1.2 loss detection timer is set. The time threshold loss
crypto retransmission timeout and set a timer for this period. 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 crypto packets are in flight, the probe timer (Section 6.3) is When the crypto retransmission timer is active, the probe timer
not active. (Section 6.3) is not active.
6.2.1. Retry and Version Negotiation 6.2.1. Retry and Version Negotiation
A Retry or Version Negotiation packet causes a client to send another A Retry or Version Negotiation packet causes a client to send another
Initial packet, effectively restarting the connection process and Initial packet, effectively restarting the connection process and
resetting congestion control and loss recovery state, including resetting congestion control and loss recovery state, including
resetting any pending timers. Either packet indicates that the resetting any pending timers. Either packet indicates that the
Initial was received but not processed. Neither packet can be Initial was received but not processed. Neither packet can be
treated as an acknowledgment for the Initial. treated as an acknowledgment for the Initial.
skipping to change at page 13, line 40 skipping to change at page 16, line 10
acknowledgements due to severe congestion. acknowledgements due to severe congestion.
A sender computes its PTO timer every time an ack-eliciting packet is A sender computes its PTO timer every time an ack-eliciting packet is
sent. A sender might choose to optimize this by setting the timer sent. A sender might choose to optimize this by setting the timer
fewer times if it knows that more ack-eliciting packets will be sent fewer times if it knows that more ack-eliciting packets will be sent
within a short period of time. within a short period of time.
6.3.2. Sending Probe Packets 6.3.2. Sending Probe Packets
When a PTO timer expires, the sender MUST send one ack-eliciting When a PTO timer expires, the sender MUST send one ack-eliciting
packet as a probe. A sender MAY send up to two ack-eliciting packet as a probe, unless there is nothing to send. A sender MAY
packets, to avoid an expensive consecutive PTO expiration due to a send up to two ack-eliciting packets, to avoid an expensive
single packet loss. consecutive PTO expiration due to a single packet loss.
Consecutive PTO periods increase exponentially, and as a result, Consecutive PTO periods increase exponentially, and as a result,
connection recovery latency increases exponentially as packets connection recovery latency increases exponentially as packets
continue to be dropped in the network. Sending two packets on PTO continue to be dropped in the network. Sending two packets on PTO
expiration increases resilience to packet drops, thus reducing the expiration increases resilience to packet drops, thus reducing the
probability of consecutive PTO events. probability of consecutive PTO events.
Probe packets sent on a PTO MUST be ack-eliciting. A probe packet Probe packets sent on a PTO MUST be ack-eliciting. A probe packet
SHOULD carry new data when possible. A probe packet MAY carry SHOULD carry new data when possible. A probe packet MAY carry
retransmitted unacknowledged data when new data is unavailable, when retransmitted unacknowledged data when new data is unavailable, when
skipping to change at page 14, line 48 skipping to change at page 17, line 18
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.
The default initial RTT of 100ms was chosen because it is slightly
higher than both the median and mean min_rtt typically observed on
the public internet.
7. Congestion Control 7. 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
skipping to change at page 16, line 46 skipping to change at page 19, line 6
packets. packets.
7.7. Persistent Congestion 7.7. 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 the equivalent of are substantially delayed. This duration is computed as follows:
kPersistentCongestionThreshold consecutive PTOs, and is computed as
follows: ~~~ (smoothed_rtt + 4 * rttvar + max_ack_delay) * ((2 ^ (smoothed_rtt + 4 * rttvar + max_ack_delay) *
kPersistentCongestionThreshold) - 1) ~~~ kPersistentCongestionThreshold
For example, assume: For example, assume:
smoothed_rtt = 1 rttvar = 0 max_ack_delay = 0 smoothed_rtt = 1 rttvar = 0 max_ack_delay = 0
kPersistentCongestionThreshold = 2 kPersistentCongestionThreshold = 3
If an eck-eliciting packet is sent at time = 0, the following If an eck-eliciting packet is sent at time = 0, the following
scenario would illustrate persistent congestion: scenario would illustrate persistent congestion:
+-----+------------------------+ +-----+------------------------+
| t=0 | Send Pkt #1 (App Data) | | t=0 | Send Pkt #1 (App Data) |
+-----+------------------------+ +-----+------------------------+
| t=1 | Send Pkt #2 (PTO 1) | | t=1 | Send Pkt #2 (PTO 1) |
| | | | | |
| t=3 | Send Pkt #3 (PTO 2) | | t=3 | Send Pkt #3 (PTO 2) |
| | | | | |
| t=7 | Send Pkt #4 (PTO 3) | | t=7 | Send Pkt #4 (PTO 3) |
| | | | | |
| t=8 | Recv ACK of Pkt #4 | | t=8 | Recv ACK of Pkt #4 |
+-----+------------------------+ +-----+------------------------+
The first three packets are determined to be lost when the ACK of The first three packets are determined to be lost when the ACK of
packet 4 is received at t=8. The congestion period is calculated as packet 4 is received at t=8. The congestion period is calculated as
the time between the oldest and newest lost packets: (3 - 0) = 3. the time between the oldest and newest lost packets: (3 - 0) = 3.
The duration for persistent congestion is equal to: (1 * ((2 ^ The duration for persistent congestion is equal to: (1 *
kPersistentCongestionThreshold) - 1)) = 3. Because the threshold was kPersistentCongestionThreshold) = 3. Because the threshold was
reached and because none of the packets between the oldest and the reached and because none of the packets between the oldest and the
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].
skipping to change at page 18, line 12 skipping to change at page 20, line 21
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. Sending data after an idle period 7.9. Under-utilizing the Congestion Window
A sender becomes idle if it ceases to send data and has no bytes in
flight. A sender's congestion window MUST NOT increase while it is
idle.
When sending data after becoming idle, a sender MUST reset its
congestion window to the initial congestion window (see Section 4.1
of [RFC5681]), unless it paces the sending of packets. A sender MAY
retain its congestion window if it paces the sending of any packets
in excess of the initial congestion window.
A sender MAY implement alternate mechanisms to update its congestion
window after idle periods, such as those proposed for TCP in
[RFC7661].
7.10. Application Limited Sending A congestion window that is under-utilized SHOULD NOT be increased in
either slow start or congestion avoidance. This can happen due to
insufficient application data or flow control credit.
The congestion window should not be increased in slow start or A sender MAY use the pipeACK method described in section 4.3 of
congestion avoidance when it is not fully utilized. The congestion [RFC7661] to determine if the congestion window is sufficiently
window could be under-utilized due to insufficient application data utilized.
or flow control credit.
A sender that paces packets (see Section 7.8) might delay sending A sender that paces packets (see Section 7.8) 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
might cause short-term congestion and losses. Implemementations
SHOULD either use pacing or reduce their congestion window to limit
such bursts.
A sender MAY implement alternate mechanisms to update its congestion
window after periods of under-utilization, such as those proposed for
TCP in [RFC7661].
8. Security Considerations 8. Security Considerations
8.1. Congestion Signals 8.1. 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.
skipping to change at page 19, line 46 skipping to change at page 21, line 48
9. IANA Considerations 9. IANA Considerations
This document has no IANA actions. Yet. This document has no IANA actions. Yet.
10. References 10. References
10.1. Normative References 10.1. 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-19 (work in progress). QUIC", draft-ietf-quic-tls-latest (work in progress).
[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-19 (work in progress). transport-latest (work in progress).
[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>.
skipping to change at page 23, line 17 skipping to change at page 25, line 23
value is 3. value is 3.
kTimeThreshold: Maximum reordering in time before time threshold kTimeThreshold: Maximum reordering in time before time threshold
loss detection considers a packet lost. Specified as an RTT loss detection considers a packet lost. Specified as an RTT
multiplier. The RECOMMENDED value is 9/8. multiplier. The RECOMMENDED value is 9/8.
kGranularity: Timer granularity. This is a system-dependent value. kGranularity: Timer granularity. This is a system-dependent value.
However, implementations SHOULD use a value no smaller than 1ms. However, implementations SHOULD use a value no smaller than 1ms.
kInitialRtt: The RTT used before an RTT sample is taken. The kInitialRtt: The RTT used before an RTT sample is taken. The
RECOMMENDED value is 100ms. RECOMMENDED value is 500ms.
kPacketNumberSpace: An enum to enumerate the three packet number kPacketNumberSpace: An enum to enumerate the three packet number
spaces. ~~~ enum kPacketNumberSpace { Initial, Handshake, spaces.
ApplicationData, } ~~~
enum kPacketNumberSpace {
Initial,
Handshake,
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. loss_detection_timer: Multi-modal timer used for loss detection.
crypto_count: The number of times all unacknowledged CRYPTO data has crypto_count: The number of times all unacknowledged CRYPTO data has
been retransmitted without receiving an ack. been retransmitted without receiving an ack.
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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 crypto_count = 0
pto_count = 0 pto_count = 0
latest_rtt = 0
smoothed_rtt = 0 smoothed_rtt = 0
rttvar = 0 rttvar = 0
min_rtt = infinite min_rtt = 0
time_of_last_sent_ack_eliciting_packet = 0 time_of_last_sent_ack_eliciting_packet = 0
time_of_last_sent_crypto_packet = 0 time_of_last_sent_crypto_packet = 0
for pn_space in [ Initial, Handshake, ApplicatonData ]: for pn_space in [ Initial, Handshake, ApplicationData ]:
largest_acked_packet[pn_space] = 0 largest_acked_packet[pn_space] = 0
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:
skipping to change at page 26, line 9 skipping to change at page 27, line 41
When an ACK frame is received, it may newly acknowledge any number of When an ACK frame is received, it may newly acknowledge any number of
packets. packets.
Pseudocode for OnAckReceived and UpdateRtt follow: Pseudocode for OnAckReceived and UpdateRtt follow:
OnAckReceived(ack, pn_space): OnAckReceived(ack, pn_space):
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.
newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)
if (newly_acked_packets.empty()):
return
// If the largest acknowledged is newly acked and // If the largest acknowledged is newly acked and
// ack-eliciting, 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][ack.largest_acked] &&
sent_packets[pn_space][ack.largest_acked].ack_eliciting): 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
UpdateRtt(latest_rtt, ack.ack_delay) UpdateRtt(ack.ack_delay)
// Process ECN information if present. // Process ECN information if present.
if (ACK frame contains ECN information): if (ACK frame contains ECN information):
ProcessECN(ack) ProcessECN(ack)
// Find all newly acked packets in this ACK frame
newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)
if (newly_acked_packets.empty()):
return
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 crypto_count = 0
pto_count = 0 pto_count = 0
SetLossDetectionTimer() SetLossDetectionTimer()
UpdateRtt(latest_rtt, ack_delay): UpdateRtt(ack_delay):
// First RTT sample.
if (smoothed_rtt == 0):
min_rtt = latest_rtt
smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2
return
// min_rtt ignores ack delay. // min_rtt ignores ack delay.
min_rtt = min(min_rtt, latest_rtt) min_rtt = min(min_rtt, latest_rtt)
// Limit ack_delay by max_ack_delay // Limit ack_delay by max_ack_delay
ack_delay = min(ack_delay, max_ack_delay) ack_delay = min(ack_delay, max_ack_delay)
// Adjust for ack delay if it's plausible. // Adjust for ack delay if plausible.
if (latest_rtt - min_rtt > ack_delay): adjusted_rtt = latest_rtt
latest_rtt -= ack_delay if (latest_rtt > min_rtt + ack_delay):
// Based on {{RFC6298}}. adjusted_rtt = latest_rtt - ack_delay
if (smoothed_rtt == 0):
smoothed_rtt = latest_rtt rttvar = 3/4 * rttvar + 1/4 * abs(smoothed_rtt - adjusted_rtt)
rttvar = latest_rtt / 2 smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
else:
rttvar_sample = abs(smoothed_rtt - latest_rtt)
rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt
A.7. On Packet Acknowledgment A.7. On Packet Acknowledgment
When a packet is acknowledged for the first time, the following When a packet is acknowledged for the first time, the following
OnPacketAcked function is called. Note that a single ACK frame may OnPacketAcked function is called. Note that a single ACK frame may
newly acknowledge several packets. OnPacketAcked must be called once newly acknowledge several packets. OnPacketAcked must be called once
for each of these newly acknowledged packets. for each of these newly acknowledged packets.
OnPacketAcked takes two parameters: acked_packet, which is the struct OnPacketAcked takes two parameters: acked_packet, which is the struct
detailed in Appendix A.1.1, and the packet number space that this ACK detailed in Appendix A.1.1, and the packet number space that this ACK
frame was sent for. frame was sent for.
Pseudocode for OnPacketAcked follows: Pseudocode for OnPacketAcked follows:
OnPacketAcked(acked_packet, pn_space): OnPacketAcked(acked_packet, pn_space):
if (acked_packet.ack_eliciting): if (acked_packet.in_flight):
OnPacketAckedCC(acked_packet) OnPacketAckedCC(acked_packet)
sent_packets[pn_space].remove(acked_packet.packet_number) sent_packets[pn_space].remove(acked_packet.packet_number)
A.8. Setting the Loss Detection Timer A.8. Setting the Loss Detection Timer
QUIC loss detection uses a single timer for all timeout loss QUIC loss detection uses a single timer for all timeout loss
detection. The duration of the timer is based on the timer's mode, detection. The duration of the timer is based on the timer's mode,
which is set in the packet and timer events further below. The which is set in the packet and timer events further below. The
function SetLossDetectionTimer defined below shows how the single function SetLossDetectionTimer defined below shows how the single
timer is set. timer is set.
skipping to change at page 28, line 10 skipping to change at page 30, line 10
particularly if timers wake up late. Timers set in the past SHOULD particularly if timers wake up late. Timers set in the past SHOULD
fire immediately. fire immediately.
Pseudocode for SetLossDetectionTimer follows: Pseudocode for SetLossDetectionTimer follows:
// 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, ApplicatonData ]: 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
SetLossDetectionTimer(): SetLossDetectionTimer():
// Don't arm timer if there are no ack-eliciting packets
// in flight.
if (no ack-eliciting packets in flight):
loss_detection_timer.cancel()
return
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 (crypto packets are in flight): if (has unacknowledged crypto data
|| endpoint is client without 1-RTT keys):
// Crypto retransmission timer. // Crypto retransmission timer.
if (smoothed_rtt == 0): if (smoothed_rtt == 0):
timeout = 2 * kInitialRtt timeout = 2 * kInitialRtt
else: else:
timeout = 2 * smoothed_rtt timeout = 2 * smoothed_rtt
timeout = max(timeout, kGranularity) timeout = max(timeout, kGranularity)
timeout = timeout * (2 ^ crypto_count) timeout = timeout * (2 ^ crypto_count)
loss_detection_timer.update( loss_detection_timer.update(
time_of_last_sent_crypto_packet + timeout) time_of_last_sent_crypto_packet + timeout)
return 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()
return
// Calculate PTO duration // Calculate PTO duration
timeout = timeout =
smoothed_rtt + max(4 * rttvar, kGranularity) + max_ack_delay 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
skipping to change at page 29, line 18 skipping to change at page 31, line 18
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 // Retransmit crypto data if no packets were lost
// and there are still crypto packets in flight. // and there is crypto data to retransmit.
else if (crypto packets are in flight): else if (has unacknowledged crypto data):
// Crypto retransmission timeout. // Crypto retransmission timeout.
RetransmitUnackedCryptoData() RetransmitUnackedCryptoData()
crypto_count++ crypto_count++
else if (endpoint is client without 1-RTT keys):
// Client sends an anti-deadlock packet: Initial is padded
// to earn more anti-amplification credit,
// a Handshake packet proves address ownership.
if (has Handshake keys):
SendOneHandshakePacket()
else:
SendOnePaddedInitialPacket()
crypto_count++
else: else:
// PTO // PTO
SendOneOrTwoPackets() SendOneOrTwoPackets()
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
skipping to change at page 30, line 10 skipping to change at page 32, line 10
loss detection timer expires and the loss_time is set, the previous loss detection timer expires and the loss_time is set, the previous
largest acknowledged packet is supplied. largest acknowledged packet is supplied.
Pseudocode for DetectLostPackets follows: Pseudocode for DetectLostPackets follows:
DetectLostPackets(pn_space): DetectLostPackets(pn_space):
loss_time[pn_space] = 0 loss_time[pn_space] = 0
lost_packets = {} lost_packets = {}
loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt) loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)
// Minimum time of kGranularity before packets are deemed lost.
loss_delay = max(loss_delay, kGranularity)
// Packets sent before this time are deemed lost. // Packets sent before this time are deemed lost.
lost_send_time = now() - loss_delay lost_send_time = now() - loss_delay
// Packets with packet numbers before this are deemed lost. // Packets with packet numbers before this are deemed lost.
lost_pn = largest_acked_packet[pn_space] - kPacketThreshold lost_pn = largest_acked_packet[pn_space] - kPacketThreshold
foreach unacked in sent_packets: foreach unacked in sent_packets[pn_space]:
if (unacked.packet_number > largest_acked_packet[pn_space]): if (unacked.packet_number > largest_acked_packet[pn_space]):
continue continue
// Mark packet as lost, or set time when it should be marked. // Mark packet as lost, or set time when it should be marked.
if (unacked.time_sent <= lost_send_time || if (unacked.time_sent <= lost_send_time ||
unacked.packet_number <= lost_pn): unacked.packet_number <= lost_pn):
sent_packets.remove(unacked.packet_number) sent_packets[pn_space].remove(unacked.packet_number)
if (unacked.in_flight): if (unacked.in_flight):
lost_packets.insert(unacked) lost_packets.insert(unacked)
else: else:
if (loss_time[pn_space] == 0): if (loss_time[pn_space] == 0):
loss_time[pn_space] = unacked.time_sent + loss_delay loss_time[pn_space] = unacked.time_sent + loss_delay
else: else:
loss_time[pn_space] = min(loss_time[pn_space], loss_time[pn_space] = min(loss_time[pn_space],
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
skipping to change at page 31, line 17 skipping to change at page 33, line 22
account for the smaller 8 byte overhead of UDP vs 20 bytes for account for the smaller 8 byte overhead of UDP vs 20 bytes for
TCP. The RECOMMENDED value is the minimum of 10 * TCP. The RECOMMENDED value is the minimum of 10 *
kMaxDatagramSize and max(2* kMaxDatagramSize, 14720)). kMaxDatagramSize and max(2* kMaxDatagramSize, 14720)).
kMinimumWindow: Minimum congestion window in bytes. The RECOMMENDED kMinimumWindow: Minimum congestion window in bytes. The RECOMMENDED
value is 2 * kMaxDatagramSize. value is 2 * kMaxDatagramSize.
kLossReductionFactor: Reduction in congestion window when a new loss kLossReductionFactor: Reduction in congestion window when a new loss
event is detected. The RECOMMENDED value is 0.5. event is detected. The RECOMMENDED value is 0.5.
kPersistentCongestionThreshold: Number of consecutive PTOs required kPersistentCongestionThreshold: Period of time for persistent
for persistent congestion to be established. The rationale for congestion to be established, specified as a PTO multiplier. The
this threshold is to enable a sender to use initial PTOs for rationale for this threshold is to enable a sender to use initial
aggressive probing, as TCP does with Tail Loss Probe (TLP) [TLP] PTOs for aggressive probing, as TCP does with Tail Loss Probe
[RACK], before establishing persistent congestion, as TCP does (TLP) [TLP] [RACK], before establishing persistent congestion, as
with a Retransmission Timeout (RTO) [RFC5681]. The RECOMMENDED TCP does with a Retransmission Timeout (RTO) [RFC5681]. The
value for kPersistentCongestionThreshold is 2, which is equivalent RECOMMENDED value for kPersistentCongestionThreshold is 3, which
to having two TLPs before an RTO in TCP. is approximately equivalent to having two TLPs before an RTO in
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_counter: The highest value reported for the ECN-CE counter by
the peer in an ACK frame. This variable is used to detect the peer in an ACK frame. This variable is used to detect
increases in the reported ECN-CE counter. increases in the reported ECN-CE counter.
skipping to change at page 31, line 46 skipping to change at page 34, line 5
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
sent. sent.
recovery_start_time: The time when QUIC first detects a loss, congestion_recovery_start_time: The time when QUIC first detects
causing it to enter recovery. When a packet sent after this time congestion due to loss or ECN, causing it to enter congestion
is acknowledged, QUIC exits recovery. recovery. When a packet sent after this time is acknowledged,
QUIC exits congestion recovery.
ssthresh: Slow start threshold in bytes. When the congestion window ssthresh: Slow start threshold in bytes. When the congestion window
is below ssthresh, the mode is slow start and the window grows by is below ssthresh, the mode is slow start and the window grows by
the number of bytes acknowledged. the number of bytes acknowledged.
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
recovery_start_time = 0 congestion_recovery_start_time = 0
ssthresh = infinite ssthresh = infinite
ecn_ce_counter = 0 ecn_ce_counter = 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
Invoked from loss detection's OnPacketAcked and is supplied with the Invoked from loss detection's OnPacketAcked and is supplied with the
acked_packet from sent_packets. acked_packet from sent_packets.
InRecovery(sent_time): InCongestionRecovery(sent_time):
return sent_time <= 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 (InRecovery(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:
skipping to change at page 33, line 12 skipping to change at page 35, line 33
congestion_window += kMaxDatagramSize * acked_packet.size congestion_window += kMaxDatagramSize * acked_packet.size
/ congestion_window / congestion_window
B.6. On New Congestion Event B.6. On New Congestion Event
Invoked from ProcessECN and OnPacketsLost when a new congestion event Invoked from ProcessECN and OnPacketsLost when a new congestion event
is detected. May start a new recovery period and reduces the is detected. May start a new recovery period and reduces the
congestion window. congestion window.
CongestionEvent(sent_time): CongestionEvent(sent_time):
// Start a new congestion event if the sent time is larger // Start a new congestion event if packet was sent after the
// than the start time of the previous recovery epoch. // start of the previous congestion recovery period.
if (!InRecovery(sent_time)): if (!InCongestionRecovery(sent_time)):
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):
// If the ECN-CE counter reported by the peer has increased, // If the ECN-CE counter reported by the peer has increased,
// this could be a new congestion event. // this could be a new congestion event.
if (ack.ce_counter > ecn_ce_counter): if (ack.ce_counter > ecn_ce_counter):
ecn_ce_counter = ack.ce_counter ecn_ce_counter = ack.ce_counter
// Start a new congestion event if the last acknowledged
// packet was sent after the start of the previous
// recovery epoch.
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 by loss detection from DetectLostPackets when new packets are Invoked by loss detection from DetectLostPackets when new packets are
detected lost. detected lost.
InPersistentCongestion(largest_lost_packet): 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 = congestion_period = pto * kPersistentCongestionThreshold
pto * (2 ^ kPersistentCongestionThreshold - 1)
// Determine if all packets in the window before the // Determine if all packets in the window before the
// newest lost packet, including the edges, are marked // newest lost packet, including the edges, are marked
// lost // lost
return IsWindowLost(largest_lost_packet, congestion_period) return IsWindowLost(largest_lost_packet, congestion_period)
OnPacketsLost(lost_packets): OnPacketsLost(lost_packets):
// Remove lost packets from bytes_in_flight. // Remove lost packets from bytes_in_flight.
for (lost_packet : lost_packets): for (lost_packet : lost_packets):
bytes_in_flight -= lost_packet.size bytes_in_flight -= lost_packet.size
largest_lost_packet = lost_packets.last() largest_lost_packet = lost_packets.last()
// Start a new congestion epoch if the last lost packet
// is past the end of the previous recovery epoch.
CongestionEvent(largest_lost_packet.time_sent) CongestionEvent(largest_lost_packet.time_sent)
// Collapse congestion window if persistent congestion // Collapse congestion window if persistent congestion
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-18 C.1. Since draft-ietf-quic-recovery-19
o Change initial RTT to 500ms to align with RFC6298 (#2184)
C.2. 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
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.2. Since draft-ietf-quic-recovery-17 C.3. 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 35, line 31 skipping to change at page 37, line 35
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.3. Since draft-ietf-quic-recovery-16 C.4. 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 36, line 4 skipping to change at page 38, line 9
o Integrate both time and packet thresholds into loss detection o Integrate both time and packet thresholds into loss detection
(#1969, #1212, #934, #1974) (#1969, #1212, #934, #1974)
o Reduce congestion window after idle, unless pacing is used (#2007, o Reduce congestion window after idle, unless pacing is used (#2007,
#2023) #2023)
o Disable RTT calculation for packets that don't elicit o Disable RTT calculation for packets that don't elicit
acknowledgment (#2060, #2078) acknowledgment (#2060, #2078)
o Limit ack_delay by max_ack_delay (#2060, #2099) o Limit ack_delay by max_ack_delay (#2060, #2099)
o Initial keys are discarded once Handshake are avaialble (#1951, o Initial keys are discarded once Handshake are avaialble (#1951,
#2045) #2045)
o Reorder ECN and loss detection in pseudocode (#2142) o Reorder ECN and loss detection in pseudocode (#2142)
o Only cancel loss detection timer if ack-eliciting packets are in o Only cancel loss detection timer if ack-eliciting packets are in
flight (#2093, #2117) flight (#2093, #2117)
C.4. Since draft-ietf-quic-recovery-14 C.5. 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.5. Since draft-ietf-quic-recovery-13 C.6. 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.6. Since draft-ietf-quic-recovery-12 C.7. 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.7. Since draft-ietf-quic-recovery-11 C.8. Since draft-ietf-quic-recovery-11
No significant changes. No significant changes.
C.8. Since draft-ietf-quic-recovery-10 C.9. 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.9. Since draft-ietf-quic-recovery-09 C.10. Since draft-ietf-quic-recovery-09
No significant changes. No significant changes.
C.10. Since draft-ietf-quic-recovery-08 C.11. Since draft-ietf-quic-recovery-08
o Clarified pacing and RTO (#967, #977) o Clarified pacing and RTO (#967, #977)
C.11. Since draft-ietf-quic-recovery-07 C.12. 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.12. Since draft-ietf-quic-recovery-06 C.13. Since draft-ietf-quic-recovery-06
No significant changes. No significant changes.
C.13. Since draft-ietf-quic-recovery-05 C.14. Since draft-ietf-quic-recovery-05
o Add more congestion control text (#776) o Add more congestion control text (#776)
C.14. Since draft-ietf-quic-recovery-04 C.15. Since draft-ietf-quic-recovery-04
No significant changes. No significant changes.
C.15. Since draft-ietf-quic-recovery-03 C.16. Since draft-ietf-quic-recovery-03
No significant changes. No significant changes.
C.16. Since draft-ietf-quic-recovery-02 C.17. 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.17. Since draft-ietf-quic-recovery-01 C.18. 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.18. Since draft-ietf-quic-recovery-00 C.19. Since draft-ietf-quic-recovery-00
o Improved description of constants and ACK behavior o Improved description of constants and ACK behavior
C.19. Since draft-iyengar-quic-loss-recovery-01 C.20. 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. 86 change blocks. 
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