draft-ietf-quic-recovery-22.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: January 10, 2020 Google Expires: January 20, 2020 Google
July 9, 2019 July 19, 2019
QUIC Loss Detection and Congestion Control QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-22 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 January 10, 2020. This Internet-Draft will expire on January 20, 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.
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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4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8 4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8
4.4. Measuring and Reporting Host Delay . . . . . . . . . . . 9 4.4. Measuring and Reporting Host Delay . . . . . . . . . . . 9
5. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 9 5. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 9
5.1. Generating RTT samples . . . . . . . . . . . . . . . . . 9 5.1. Generating RTT samples . . . . . . . . . . . . . . . . . 9
5.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 10 5.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 10
5.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 10 5.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 10
6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 11 6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 12 6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 12
6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 12 6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 12
6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 12 6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 12
6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 13 6.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 13
6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 14 6.2.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 13
6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 15 6.3. Handshakes and New Paths . . . . . . . . . . . . . . . . 14
6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 15 6.3.1. Sending Probe Packets . . . . . . . . . . . . . . . . 15
6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 16 6.3.2. Loss Detection . . . . . . . . . . . . . . . . . . . 16
6.4. Retry and Version Negotiation . . . . . . . . . . . . . . 16 6.4. Retry and Version Negotiation . . . . . . . . . . . . . . 16
6.5. Discarding Keys and Packet State . . . . . . . . . . . . 17 6.5. Discarding Keys and Packet State . . . . . . . . . . . . 16
6.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . 17 6.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . 17
7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17 7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17
7.1. Explicit Congestion Notification . . . . . . . . . . . . 18 7.1. Explicit Congestion Notification . . . . . . . . . . . . 17
7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 18 7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 18
7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 18 7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 18
7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 18 7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 18
7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 19 7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 18
7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 19 7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 18
7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 19 7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 19
7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 20 7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.9. Under-utilizing the Congestion Window . . . . . . . . . . 21 7.9. Under-utilizing the Congestion Window . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21 8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 21 8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 21
8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 21 8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 21
8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 22 8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . 22 10.1. Normative References . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . 23 10.2. Informative References . . . . . . . . . . . . . . . . . 22
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24 10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 24 Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 24
A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 25 A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 24
A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 25 A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 24
A.2. Constants of interest . . . . . . . . . . . . . . . . . . 25 A.2. Constants of interest . . . . . . . . . . . . . . . . . . 25
A.3. Variables of interest . . . . . . . . . . . . . . . . . . 26 A.3. Variables of interest . . . . . . . . . . . . . . . . . . 25
A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 27 A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 26
A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 27 A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 27
A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 28 A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 27
A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 29 A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 28
A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 30 A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 29
A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 32 A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 30
A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 32 A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 31
Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 33 Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 32
B.1. Constants of interest . . . . . . . . . . . . . . . . . . 33 B.1. Constants of interest . . . . . . . . . . . . . . . . . . 32
B.2. Variables of interest . . . . . . . . . . . . . . . . . . 34 B.2. Variables of interest . . . . . . . . . . . . . . . . . . 33
B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 35 B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 34
B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 35 B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 34
B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 35 B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 34
B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 36 B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 35
B.7. Process ECN Information . . . . . . . . . . . . . . . . . 36 B.7. Process ECN Information . . . . . . . . . . . . . . . . . 35
B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 37 B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 36
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 37 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 36
C.1. Since draft-ietf-quic-recovery-21 . . . . . . . . . . . . 37 C.1. Since draft-ietf-quic-recovery-21 . . . . . . . . . . . . 36
C.2. Since draft-ietf-quic-recovery-20 . . . . . . . . . . . . 37 C.2. Since draft-ietf-quic-recovery-20 . . . . . . . . . . . . 36
C.3. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 38 C.3. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 37
C.4. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 38 C.4. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 37
C.5. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 38 C.5. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 37
C.6. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 39 C.6. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 38
C.7. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 40 C.7. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 39
C.8. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 40 C.8. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 39
C.9. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 40 C.9. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 39
C.10. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 40 C.10. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 39
C.11. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 40 C.11. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 39
C.12. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 41 C.12. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 40
C.13. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 41 C.13. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 40
C.14. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 41 C.14. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 40
C.15. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 41 C.15. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 40
C.16. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 41 C.16. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 40
C.17. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 41 C.17. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 40
C.18. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 41 C.18. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 40
C.19. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 41 C.19. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 40
C.20. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 42 C.20. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 41
C.21. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 42 C.21. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 41
C.22. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 42 C.22. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 41
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
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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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
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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 6.2. 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 packets when
is in flight but an acknowledgement is not received within the ack-eliciting packets are not acknowledged within the expected period
expected period of time. A PTO enables a connection to recover from of time or the handshake has not been completed. A PTO enables a
loss of tail packets or acks. The PTO algorithm used in QUIC connection to recover from loss of tail packets or acks. The PTO
implements the reliability functions of Tail Loss Probe [TLP] [RACK], algorithm used in QUIC implements the reliability functions of Tail
RTO [RFC5681] and F-RTO algorithms for TCP [RFC5682], and the timeout Loss Probe [TLP] [RACK], RTO [RFC5681] and F-RTO algorithms for TCP
computation is based on TCP's retransmission timeout period [RFC5682], and the timeout computation is based on TCP's
[RFC6298]. retransmission timeout period [RFC6298].
6.3.1. Computing PTO 6.2.1. 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 6.1.2 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.
6.3. 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.
Because the server could be blocked until more packets are received,
the client MUST ensure that the retransmission timer is set if the
client does not yet have 1-RTT keys. If the probe 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 Initial packets containing only 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.
6.3.1. 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 ack-eliciting packets, to avoid
an expensive consecutive PTO expiration due to a single packet loss. an expensive consecutive PTO expiration due to a single packet loss.
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
skipping to change at page 16, line 33 skipping to change at page 16, line 14
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 6.3.2. 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 6.1.
skipping to change at page 18, line 8 skipping to change at page 17, line 38
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
PTO timer expires, as described in Section 6.3. PTO timer expires, as described in Section 6.2.
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 7.1. 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.
skipping to change at page 22, line 38 skipping to change at page 22, line 15
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-22 (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-22 (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 25, line 33 skipping to change at page 25, 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 31 skipping to change at page 25, line 49
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. 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 pto_count: The number of times a PTO has been sent without receiving
an ack. an ack.
time_of_last_sent_ack_eliciting_packet: The time the most recent time_of_last_sent_ack_eliciting_packet: The time the most recent
ack-eliciting packet was sent. ack-eliciting packet was sent.
time_of_last_sent_crypto_packet: The time the most recent crypto
packet was sent.
largest_acked_packet[kPacketNumberSpace]: The largest packet number largest_acked_packet[kPacketNumberSpace]: The largest packet number
acknowledged in the packet number space so far. 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]
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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 47 skipping to change at page 28, line 4
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][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)
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
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space = pn_space space = pn_space
return time, space return time, space
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
|| endpoint is client without 1-RTT keys):
// 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 // Don't arm timer if there are no ack-eliciting packets
// in flight. // in flight and the handshake is complete.
if (no ack-eliciting packets in flight): if (endpoint is client with 1-RTT keys &&
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() SendOneHandshakePacket()
else: else:
SendOnePaddedInitialPacket() SendOnePaddedInitialPacket()
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() 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
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):
 End of changes. 40 change blocks. 
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