draft-ietf-quic-recovery-16.txt   draft-ietf-quic-recovery-17.txt 
QUIC J. Iyengar, Ed. QUIC J. Iyengar, Ed.
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track I. Swett, Ed. Intended status: Standards Track I. Swett, Ed.
Expires: April 26, 2019 Google Expires: June 21, 2019 Google
October 23, 2018 December 18, 2018
QUIC Loss Detection and Congestion Control QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-16 draft-ietf-quic-recovery-17
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 April 26, 2019. This Internet-Draft will expire on June 21, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 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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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Design of the QUIC Transmission Machinery . . . . . . . . . . 4 3. Design of the QUIC Transmission Machinery . . . . . . . . . . 4
3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5 3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5
3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 5 3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 5
3.1.2. Monotonically Increasing Packet Numbers . . . . . . . 6 3.1.2. Monotonically Increasing Packet Numbers . . . . . . . 6
3.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6 3.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6
3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6 3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6
3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 6 3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 6
4. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 7 4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7
4.1. Computing the RTT estimate . . . . . . . . . . . . . . . 7 4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 7
4.2. Ack-based Detection . . . . . . . . . . . . . . . . . . . 7 4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Fast Retransmit . . . . . . . . . . . . . . . . . . . 7 4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8
4.2.2. Early Retransmit . . . . . . . . . . . . . . . . . . 8 5. Computing the RTT estimate . . . . . . . . . . . . . . . . . 8
4.3. Timer-based Detection . . . . . . . . . . . . . . . . . . 9 6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9
4.3.1. Crypto Retransmission Timeout . . . . . . . . . . . . 9 6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 9
4.3.2. Tail Loss Probe . . . . . . . . . . . . . . . . . . . 10 6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 9
4.3.3. Retransmission Timeout . . . . . . . . . . . . . . . 11 6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10
4.4. Generating Acknowledgements . . . . . . . . . . . . . . . 12 6.2. Timeout Loss Detection . . . . . . . . . . . . . . . . . 10
4.4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . 13 6.2.1. Crypto Retransmission Timeout . . . . . . . . . . . . 10
4.4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . 13 6.2.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . 12
4.4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . 13 6.3. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 13
4.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 14 6.3.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 14
4.5.1. Constants of interest . . . . . . . . . . . . . . . . 14 6.4. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.2. Variables of interest . . . . . . . . . . . . . . . . 14 6.4.1. Constants of interest . . . . . . . . . . . . . . . . 14
4.5.3. Initialization . . . . . . . . . . . . . . . . . . . 16 6.4.2. Variables of interest . . . . . . . . . . . . . . . . 15
4.5.4. On Sending a Packet . . . . . . . . . . . . . . . . . 16 6.4.3. Initialization . . . . . . . . . . . . . . . . . . . 16
4.5.5. On Receiving an Acknowledgment . . . . . . . . . . . 17 6.4.4. On Sending a Packet . . . . . . . . . . . . . . . . . 16
4.5.6. On Packet Acknowledgment . . . . . . . . . . . . . . 19 6.4.5. On Receiving an Acknowledgment . . . . . . . . . . . 16
4.5.7. Setting the Loss Detection Timer . . . . . . . . . . 19 6.4.6. On Packet Acknowledgment . . . . . . . . . . . . . . 18
4.5.8. On Timeout . . . . . . . . . . . . . . . . . . . . . 20 6.4.7. Setting the Loss Detection Timer . . . . . . . . . . 18
4.5.9. Detecting Lost Packets . . . . . . . . . . . . . . . 21 6.4.8. On Timeout . . . . . . . . . . . . . . . . . . . . . 19
4.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . 22 6.4.9. Detecting Lost Packets . . . . . . . . . . . . . . . 20
5. Congestion Control . . . . . . . . . . . . . . . . . . . . . 22 6.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Explicit Congestion Notification . . . . . . . . . . . . 23 7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 22
5.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 23 7.1. Explicit Congestion Notification . . . . . . . . . . . . 22
5.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 23 7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 22
5.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 23 7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 22
5.5. Tail Loss Probe . . . . . . . . . . . . . . . . . . . . . 24 7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 23
5.6. Retransmission Timeout . . . . . . . . . . . . . . . . . 24 7.5. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 23
5.7. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 24 7.6. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.8. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 25 7.7. Sending data after an idle period . . . . . . . . . . . . 24
5.8.1. Constants of interest . . . . . . . . . . . . . . . . 25 7.8. Discarding Packet Number Space State . . . . . . . . . . 24
5.8.2. Variables of interest . . . . . . . . . . . . . . . . 25 7.9. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 24
5.8.3. Initialization . . . . . . . . . . . . . . . . . . . 26 7.9.1. Constants of interest . . . . . . . . . . . . . . . . 24
5.8.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 26 7.9.2. Variables of interest . . . . . . . . . . . . . . . . 25
5.8.5. On Packet Acknowledgement . . . . . . . . . . . . . . 26 7.9.3. Initialization . . . . . . . . . . . . . . . . . . . 26
5.8.6. On New Congestion Event . . . . . . . . . . . . . . . 27 7.9.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 26
5.8.7. Process ECN Information . . . . . . . . . . . . . . . 27 7.9.5. On Packet Acknowledgement . . . . . . . . . . . . . . 26
5.8.8. On Packets Lost . . . . . . . . . . . . . . . . . . . 27 7.9.6. On New Congestion Event . . . . . . . . . . . . . . . 26
5.8.9. On Retransmission Timeout Verified . . . . . . . . . 28 7.9.7. Process ECN Information . . . . . . . . . . . . . . . 27
6. Security Considerations . . . . . . . . . . . . . . . . . . . 28 7.9.8. On Packets Lost . . . . . . . . . . . . . . . . . . . 27
6.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 28 8. Security Considerations . . . . . . . . . . . . . . . . . . . 27
6.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 28 8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 28
6.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 28 8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 28
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 28
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
8.1. Normative References . . . . . . . . . . . . . . . . . . 29 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.2. Informative References . . . . . . . . . . . . . . . . . 29 10.1. Normative References . . . . . . . . . . . . . . . . . . 29
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 30 10.2. Informative References . . . . . . . . . . . . . . . . . 29
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 31 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 31
A.1. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 31 A.1. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 31
A.2. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 31 A.2. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 32
A.3. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 31 A.3. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 32
A.4. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 31 A.4. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 32
A.5. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 31 A.5. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 32
A.6. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 32 A.6. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 32
A.7. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 32 A.7. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 33
A.8. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 32 A.8. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 33
A.9. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 32 A.9. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 33
A.10. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 32 A.10. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 33
A.11. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 32 A.11. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 33
A.12. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 32 A.12. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 33
A.13. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 32 A.13. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 33
A.14. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 33 A.14. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 33
A.15. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 33 A.15. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 34
A.16. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 33 A.16. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 34
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 33 A.17. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction 1. Introduction
QUIC is a new multiplexed and secure transport atop UDP. QUIC builds QUIC is a new multiplexed and secure transport atop UDP. QUIC builds
on decades of transport and security experience, and implements on decades of transport and security experience, and implements
mechanisms that make it attractive as a modern general-purpose mechanisms that make it attractive as a modern general-purpose
transport. The QUIC protocol is described in [QUIC-TRANSPORT]. transport. The QUIC protocol is described in [QUIC-TRANSPORT].
QUIC implements the spirit of known TCP loss recovery mechanisms, QUIC implements the spirit of known 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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2. Conventions and Definitions 2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Definitions of terms that are used in this document: Definitions of terms that are used in this document:
ACK-only: Any packet containing only an ACK frame. ACK-only: Any packet containing only one or more ACK frame(s).
In-flight: Packets are considered in-flight when they have been sent In-flight: Packets are considered in-flight when they have been sent
and neither acknowledged nor declared lost, and they are not ACK- and neither acknowledged nor declared lost, and they are not ACK-
only. only.
Retransmittable Frames: All frames besides ACK or PADDING are Ack-eliciting Frames: All frames besides ACK or PADDING are
considered retransmittable. considered ack-eliciting.
Retransmittable Packets: Packets that contain retransmittable frames Ack-eliciting Packets: Packets that contain ack-eliciting frames
elicit an ACK from the receiver and are called retransmittable elicit an ACK from the receiver within the maximum ack delay and
packets. are called ack-eliciting packets.
Crypto Packets: Packets containing CRYPTO data sent in Initial or Crypto Packets: Packets containing CRYPTO data sent in Initial or
Handshake packets. Handshake packets.
3. Design of the QUIC Transmission Machinery 3. Design of the QUIC Transmission Machinery
All transmissions in QUIC are sent with a packet-level header, which All transmissions in QUIC are sent with a packet-level header, which
indicates the encryption level and includes a packet sequence number indicates the encryption level and includes a packet sequence number
(referred to below as a packet number). The encryption level (referred to below as a packet number). The encryption level
indicates the packet number space, as described in [QUIC-TRANSPORT]. indicates the packet number space, as described in [QUIC-TRANSPORT].
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transmissions and retransmissions and eliminates significant transmissions and retransmissions and eliminates significant
complexity from QUIC's interpretation of TCP loss detection complexity from QUIC's interpretation of TCP loss detection
mechanisms. mechanisms.
QUIC packets can contain multiple frames of different types. The QUIC packets can contain multiple frames of different types. The
recovery mechanisms ensure that data and frames that need reliable recovery mechanisms ensure that data and frames that need reliable
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 only ACK o All packets are acknowledged, though packets that contain no ack-
and PADDING frames are not acknowledged immediately. eliciting frames are only acknowledged along with ack-eliciting
packets.
o Long header packets that contain CRYPTO frames are critical to the o Long header packets that contain CRYPTO frames are critical to the
performance of the QUIC handshake and use shorter timers for performance of the QUIC handshake and use shorter timers for
acknowledgement and retransmission. acknowledgement and retransmission.
o Packets that contain only ACK frames do not count toward o Packets that contain only ACK frames do not count toward
congestion control limits and are not considered in-flight. Note congestion control limits and are not considered in-flight. Note
that this means PADDING frames cause packets to contribute toward that this means PADDING frames cause packets to contribute toward
bytes in flight without directly causing an acknowledgment to be bytes in flight without directly causing an acknowledgment to be
sent. sent.
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QUIC uses separate packet number spaces for each encryption level, QUIC uses separate packet number spaces for each encryption level,
except 0-RTT and all generations of 1-RTT keys use the same packet except 0-RTT and all generations of 1-RTT keys use the same packet
number space. Separate packet number spaces ensures acknowledgement number space. Separate packet number spaces ensures acknowledgement
of packets sent with one level of encryption will not cause spurious of packets sent with one level of encryption will not cause spurious
retransmission of packets sent with a different encryption level. retransmission of packets sent with a different encryption level.
Congestion control and RTT measurement are unified across packet Congestion control and RTT measurement are unified across packet
number spaces. number spaces.
3.1.2. Monotonically Increasing Packet Numbers 3.1.2. Monotonically Increasing Packet Numbers
TCP conflates transmission sequence number at the sender with TCP conflates transmission order at the sender with delivery order at
delivery sequence number at the receiver, which results in the receiver, which results in retransmissions of the same data
retransmissions of the same data carrying the same sequence number, carrying the same sequence number, and consequently leads to
and consequently to problems caused by "retransmission ambiguity". "retransmission ambiguity". QUIC separates the two: QUIC uses a
QUIC separates the two: QUIC uses a packet number for transmissions, packet number to indicate transmission order, and any application
and any application data is sent in one or more streams, with data is sent in one or more streams, with delivery order determined
delivery order determined by stream offsets encoded within STREAM by stream offsets encoded within STREAM frames.
frames.
QUIC's packet number is strictly increasing, and directly encodes QUIC's packet number is strictly increasing within a packet number
transmission order. A higher QUIC packet number signifies that the space, and directly encodes transmission order. A higher packet
packet was sent later, and a lower QUIC packet number signifies that number signifies that the packet was sent later, and a lower packet
the packet was sent earlier. When a packet containing frames is number signifies that the packet was sent earlier. When a packet
deemed lost, QUIC rebundles necessary frames in a new packet with a containing ack-eliciting frames is detected lost, QUIC rebundles
new packet number, removing ambiguity about which packet is necessary frames in a new packet with a new packet number, removing
acknowledged when an ACK is received. Consequently, more accurate ambiguity about which packet is acknowledged when an ACK is received.
RTT measurements can be made, spurious retransmissions are trivially Consequently, more accurate RTT measurements can be made, spurious
detected, and mechanisms such as Fast Retransmit can be applied retransmissions are trivially detected, and mechanisms such as Fast
universally, based only on packet number. Retransmit can be applied universally, based only on packet number.
This design point significantly simplifies loss detection mechanisms This design point significantly simplifies loss detection mechanisms
for QUIC. Most TCP mechanisms implicitly attempt to infer for QUIC. Most TCP mechanisms implicitly attempt to infer
transmission ordering based on TCP sequence numbers - a non-trivial transmission ordering based on TCP sequence numbers - a non-trivial
task, especially when TCP timestamps are not available. task, especially when TCP timestamps are not available.
3.1.3. No Reneging 3.1.3. No Reneging
QUIC ACKs contain information that is similar to TCP SACK, but QUIC QUIC ACKs contain information that is similar to TCP SACK, but QUIC
does not allow any acked packet to be reneged, greatly simplifying does not allow any acked packet to be reneged, greatly simplifying
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QUIC ACKs explicitly encode the delay incurred at the receiver QUIC ACKs explicitly encode the delay incurred at the receiver
between when a packet is received and when the corresponding ACK is between when a packet is received and when the corresponding ACK is
sent. This allows the receiver of the ACK to adjust for receiver sent. This allows the receiver of the ACK to adjust for receiver
delays, specifically the delayed ack timer, when estimating the path delays, specifically the delayed ack timer, when estimating the path
RTT. This mechanism also allows a receiver to measure and report the 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 delay from when a packet was received by the OS kernel, which is
useful in receivers which may incur delays such as context-switch useful in receivers which may incur delays such as context-switch
latency before a userspace QUIC receiver processes a received packet. latency before a userspace QUIC receiver processes a received packet.
4. Loss Detection 4. Generating Acknowledgements
QUIC senders use both ack information and timeouts to detect lost QUIC SHOULD delay sending acknowledgements in response to packets,
packets, and this section provides a description of these algorithms. but MUST NOT excessively delay acknowledgements of ack-eliciting
Estimating the network round-trip time (RTT) is critical to these packets. Specifically, implementations MUST attempt to enforce a
algorithms and is described first. 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.
4.1. Computing the RTT estimate An acknowledgement SHOULD be sent immediately upon receipt of a
second packet but the delay SHOULD NOT exceed the maximum ack delay.
QUIC recovery algorithms do not assume the peer generates an
acknowledgement immediately when receiving a second full-packet.
Out-of-order packets SHOULD be acknowledged more quickly, in order to
accelerate loss recovery. The receiver SHOULD send an immediate ACK
when it receives a new packet which is not one greater than the
largest received packet number.
Similarly, packets marked with the ECN Congestion Experienced (CE)
codepoint in the IP header SHOULD be acknowledged immediately, to
reduce the peer's response time to congestion events.
As an optimization, a receiver MAY process multiple packets before
sending any ACK frames in response. In this case they can determine
whether an immediate or delayed acknowledgement should be generated
after processing incoming packets.
4.1. Crypto Handshake Data
In order to quickly complete the handshake and avoid spurious
retransmissions due to crypto retransmission timeouts, crypto packets
SHOULD use a very short ack delay, such as 1ms. ACK frames MAY be
sent immediately when the crypto stack indicates all data for that
packet number space has been received.
4.2. ACK Ranges
When an ACK frame is sent, one or more ranges of acknowledged packets
are included. Including older packets reduces the chance of spurious
retransmits caused by losing previously sent ACK frames, at the cost
of larger ACK frames.
ACK frames SHOULD always acknowledge the most recently received
packets, and the more out-of-order the packets are, the more
important it is to send an updated ACK frame quickly, to prevent the
peer from declaring a packet as lost and spuriously retransmitting
the frames it contains.
Below is one recommended approach for determining what packets to
include in an ACK frame.
4.3. Receiver Tracking of ACK Frames
When a packet containing an ACK frame is sent, the largest
acknowledged in that frame may be saved. When a packet containing an
ACK frame is acknowledged, the receiver can stop acknowledging
packets less than or equal to the largest acknowledged in the sent
ACK frame.
In cases without ACK frame loss, this algorithm allows for a minimum
of 1 RTT of reordering. In cases with ACK frame loss and reordering,
this approach does not guarantee that every acknowledgement is seen
by the sender before it is no longer included in the ACK frame.
Packets could be received out of order and all subsequent ACK frames
containing them could be lost. In this case, the loss recovery
algorithm may cause spurious retransmits, but the sender will
continue making forward progress.
5. Computing the RTT estimate
RTT is calculated when an ACK frame arrives by computing the RTT is calculated when an ACK frame arrives by computing the
difference between the current time and the time the largest newly difference between the current time and the time the largest acked
acked packet was sent. If no packets are newly acknowledged, RTT packet was sent. An RTT sample MUST NOT be taken for a packet that
cannot be calculated. When RTT is calculated, the ack delay field is not newly acknowledged or not ack-eliciting.
from the ACK frame SHOULD be subtracted from the RTT as long as the
result is larger than the Min RTT. If the result is smaller than the When RTT is calculated, the ack delay field from the ACK frame SHOULD
min_rtt, the RTT should be used, but the ack delay field should be be limited to the max_ack_delay specified by the peer. Limiting
ignored. ack_delay to max_ack_delay ensures a peer specifying an extremely
small max_ack_delay doesn't cause more spurious timeouts than a peer
that correctly specifies max_ack_delay. It SHOULD be subtracted from
the RTT as long as the result is larger than the min_rtt. If the
result is smaller than the min_rtt, the RTT should be used, but the
ack delay field should be ignored.
Like TCP, QUIC calculates both smoothed RTT and RTT variance similar Like TCP, QUIC calculates both smoothed RTT and RTT variance similar
to those specified in [RFC6298]. to those specified in [RFC6298].
Min RTT is the minimum RTT measured over the connection, prior to min_rtt is the minimum RTT measured over the connection, prior to
adjusting by ack delay. Ignoring ack delay for min RTT prevents adjusting by ack delay. Ignoring ack delay for min RTT prevents
intentional or unintentional underestimation of min RTT, which in intentional or unintentional underestimation of min RTT, which in
turn prevents underestimating smoothed RTT. turn prevents underestimating smoothed RTT.
4.2. Ack-based Detection 6. Loss Detection
Ack-based loss detection implements the spirit of TCP's Fast QUIC senders use both ack information and timeouts to detect lost
Retransmit [RFC5681], Early Retransmit [RFC5827], FACK, and SACK loss packets, and this section provides a description of these algorithms.
recovery [RFC6675]. This section provides an overview of how these Estimating the network round-trip time (RTT) is critical to these
algorithms are implemented in QUIC. algorithms and is described first.
4.2.1. Fast Retransmit If a packet is lost, the QUIC transport needs to recover from that
loss, such as by retransmitting the data, sending an updated frame,
or abandoning the frame. For more information, see Section 13.2 of
[QUIC-TRANSPORT].
An unacknowledged packet is marked as lost when an acknowledgment is 6.1. Acknowledgement-based Detection
received for a packet that was sent a threshold number of packets
(kReorderingThreshold) and/or a threshold amount of time after the
unacknowledged packet. Receipt of the acknowledgement indicates that
a later packet was received, while the reordering threshold provides
some tolerance for reordering of packets in the network.
The RECOMMENDED initial value for kReorderingThreshold is 3, based on Acknowledgement-based loss detection implements the spirit of TCP's
TCP loss recovery [RFC5681] [RFC6675]. Some networks may exhibit Fast Retransmit [RFC5681], Early Retransmit [RFC5827], FACK [FACK],
higher degrees of reordering, causing a sender to detect spurious SACK loss recovery [RFC6675], and RACK [RACK]. This section provides
losses. Spuriously declaring packets lost leads to unnecessary an overview of how these algorithms are implemented in QUIC.
A packet is declared lost if it meets all the following conditions:
o The packet is unacknowledged, in-flight, and was sent prior to an
acknowledged packet.
o Either its packet number is kPacketThreshold smaller than an
acknowledged packet (Section 6.1.1), or it was sent long enough in
the past (Section 6.1.2).
The acknowledgement indicates that a packet sent later was delivered,
while the packet and time thresholds provide some tolerance for
packet reordering.
Spuriously declaring packets as lost leads to unnecessary
retransmissions and may result in degraded performance due to the retransmissions and may result in degraded performance due to the
actions of the congestion controller upon detecting loss. actions of the congestion controller upon detecting loss.
Implementers MAY use algorithms developed for TCP, such as TCP-NCR Implementations that detect spurious retransmissions and increase the
[RFC4653], to improve QUIC's reordering resilience. reordering threshold in packets or time MAY choose to start with
smaller initial reordering thresholds to minimize recovery latency.
QUIC implementations can use time-based loss detection to handle 6.1.1. Packet Threshold
reordering based on time elapsed since the packet was sent. This may
be used either as a replacement for a packet reordering threshold or
in addition to it. The RECOMMENDED time threshold, expressed as a
fraction of the round-trip time (kTimeReorderingFraction), is 1/8.
4.2.2. Early Retransmit The RECOMMENDED initial value for the packet reordering threshold
(kPacketThreshold) is 3, based on best practices for TCP loss
detection [RFC5681] [RFC6675].
Unacknowledged packets close to the tail may have fewer than Some networks may exhibit higher degrees of reordering, causing a
kReorderingThreshold retransmittable packets sent after them. Loss sender to detect spurious losses. Implementers MAY use algorithms
of such packets cannot be detected via Fast Retransmit. To enable developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's
ack-based loss detection of such packets, receipt of an reordering resilience.
acknowledgment for the last outstanding retransmittable packet
triggers the Early Retransmit process, as follows.
If there are unacknowledged in-flight packets still pending, they 6.1.2. Time Threshold
should be marked as lost. To compensate for the reduced reordering
resilience, the sender SHOULD set a timer for a small period of time.
If the unacknowledged in-flight packets are not acknowledged during
this time, then these packets MUST be marked as lost.
An endpoint SHOULD set the timer such that a packet is marked as lost Once a later packet has been acknowledged, an endpoint SHOULD declare
no earlier than 1.125 * max(SRTT, latest_RTT) since when it was sent. an earlier packet lost if it was sent a threshold amount of time in
the past. The time threshold is computed as kTimeThreshold *
max(SRTT, latest_RTT). If packets sent prior to the largest
acknowledged packet cannot yet be declared lost, then a timer SHOULD
be set for the remaining time.
The RECOMMENDED time threshold (kTimeThreshold), expressed as a
round-trip time multiplier, is 9/8.
Using max(SRTT, latest_RTT) protects from the two following cases: Using max(SRTT, latest_RTT) protects from the two following cases:
o the latest RTT sample is lower than the SRTT, perhaps due to o the latest RTT sample is lower than the SRTT, perhaps due to
reordering where packet whose ack triggered the Early Retransit reordering where packet whose ack triggered the Early Retransmit
process encountered a shorter path; process encountered a shorter path;
o the latest RTT sample is higher than the SRTT, perhaps due to a o the latest RTT sample is higher than the SRTT, perhaps due to a
sustained increase in the actual RTT, but the smoothed SRTT has sustained increase in the actual RTT, but the smoothed SRTT has
not yet caught up. not yet caught up.
The 1.125 multiplier increases reordering resilience. Implementers Implementers MAY experiment with using other reordering thresholds,
MAY experiment with using other multipliers, bearing in mind that a including absolute thresholds, bearing in mind that a lower
lower multiplier reduces reordering resilience and increases spurious multiplier reduces reordering resilience and increases spurious
retransmissions, and a higher multiplier increases loss recovery retransmissions, and a higher multiplier increases loss detection
delay. delay.
This mechanism is based on Early Retransmit for TCP [RFC5827]. 6.2. Timeout Loss Detection
However, [RFC5827] does not include the timer described above. Early
Retransmit is prone to spurious retransmissions due to its reduced
reordering resilence without the timer. This observation led Linux
TCP implementers to implement a timer for TCP as well, and this
document incorporates this advancement.
4.3. Timer-based Detection
Timer-based loss detection recovers from losses that cannot be Timeout loss detection recovers from losses that cannot be handled by
handled by ack-based loss detection. It uses a single timer which acknowledgement-based loss detection. It uses a single timer which
switches between a crypto retransmission timer, a Tail Loss Probe switches between a crypto retransmission timer and a probe timer.
timer and Retransmission Timeout mechanisms.
4.3.1. Crypto Retransmission Timeout 6.2.1. Crypto Retransmission Timeout
Data in CRYPTO frames is critical to QUIC transport and crypto Data in CRYPTO frames is critical to QUIC transport and crypto
negotiation, so a more aggressive timeout is used to retransmit it. negotiation, so a more aggressive timeout is used to retransmit it.
The initial crypto retransmission timeout SHOULD be set to twice the The initial crypto retransmission timeout SHOULD be set to twice the
initial RTT. initial RTT.
At the beginning, there are no prior RTT samples within a connection. At the beginning, there are no prior RTT samples within a connection.
Resumed connections over the same network SHOULD use the previous Resumed connections over the same network SHOULD use the previous
connection's final smoothed RTT value as the resumed connection's connection's final smoothed RTT value as the resumed connection's
initial RTT. If no previous RTT is available, or if the network initial RTT. If no previous RTT is available, or if the network
changes, the initial RTT SHOULD be set to 100ms. When an changes, the initial RTT SHOULD be set to 100ms. When an
acknowledgement is received, a new RTT is computed and the timer acknowledgement is received, a new RTT is computed and the timer
SHOULD be set for twice the newly computed smoothed RTT. SHOULD be set for twice the newly computed smoothed RTT.
When crypto packets are sent, the sender MUST set a timer for the When crypto packets are sent, the sender MUST set a timer for the
crypto timeout period. Upon timeout, the sender MUST retransmit all crypto timeout period. Upon timeout, the sender MUST retransmit all
unacknowledged CRYPTO data if possible. unacknowledged CRYPTO data if possible.
Until the server has validated the client's address on the path, the Until the server has validated the client's address on the path, the
number of bytes it can send is limited, as specified in amount of data it can send is limited, as specified in
[QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent, [QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent,
then all unacknowledged CRYPTO data sent in Initial packets should be then all unacknowledged CRYPTO data sent in Initial packets should be
retransmitted. If no bytes can be sent, then no alarm should be retransmitted. If no data can be sent, then no alarm should be armed
armed until bytes have 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 start the crypto retransmission timer even if there
is no unacknowledged CRYPTO data. If the timer expires and the is no unacknowledged CRYPTO data. If the timer expires and the
client has no CRYPTO data to retransmit and does not have Handshake client has no CRYPTO data to retransmit and does not have Handshake
keys, it SHOULD send an Initial packet in a UDP datagram of at least keys, it SHOULD send an Initial packet in a UDP datagram of at least
1200 octets. If the client has Handshake keys, it SHOULD send a 1200 bytes. If the client has Handshake keys, it SHOULD send a
Handshake packet. Handshake packet.
On each consecutive expiration of the crypto timer without receiving On each consecutive expiration of the crypto timer without receiving
an acknowledgement for a new packet, the sender SHOULD double the an acknowledgement for a new packet, the sender SHOULD double the
crypto retransmission timeout and set a timer for this period. crypto retransmission timeout and set a timer for this period.
When crypto packets are outstanding, the TLP and RTO timers are not When crypto packets are in flight, the probe timer (Section 6.2.2) is
active. not active.
4.3.1.1. Retry and Version Negotiation 6.2.1.1. 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. Initial packet, effectively restarting the connection process and
resetting congestion control and loss recovery state, including
Either packet indicates that the Initial was received but not resetting any pending timers. Either packet indicates that the
processed. Neither packet can be treated as an acknowledgment for Initial was received but not processed. Neither packet can be
the Initial, but they MAY be used to improve the RTT estimate. treated as an acknowledgment for the Initial.
4.3.2. Tail Loss Probe
The algorithm described in this section is an adaptation of the Tail
Loss Probe algorithm proposed for TCP [TLP].
A packet sent at the tail is particularly vulnerable to slow loss
detection, since acks of subsequent packets are needed to trigger
ack-based detection. To ameliorate this weakness of tail packets,
the sender schedules a timer when the last retransmittable packet
before quiescence is transmitted. Upon timeout, a Tail Loss Probe
(TLP) packet is sent to evoke an acknowledgement from the receiver.
The timer duration, or Probe Timeout (PTO), is set based on the
following conditions:
o PTO SHOULD be scheduled for max(1.5*SRTT+MaxAckDelay,
kMinTLPTimeout)
o If RTO (Section 4.3.3) is earlier, schedule a TLP in its place.
That is, PTO SHOULD be scheduled for min(RTO, PTO).
QUIC includes MaxAckDelay in all probe timeouts, because it assumes 6.2.1.2. Discarding Initial State
the ack delay may come into play, regardless of the number of packets
outstanding. TCP's TLP assumes if at least 2 packets are
outstanding, acks will not be delayed.
A PTO value of at least 1.5*SRTT ensures that the ACK is overdue. As described in Section 17.5.1 of [QUIC-TRANSPORT], endpoints stop
The 1.5 is based on [TLP], but implementations MAY experiment with sending and receiving Initial packets once they start exchanging
other constants. Handshake packets. At this point, all loss recovery state for the
Initial packet number space is also discarded. Packets that are in
flight for the packet number space are not declared as either
acknowledged or lost. After discarding state, new Initial packets
will not be sent.
To reduce latency, it is RECOMMENDED that the sender set and allow The client MAY however compute an RTT estimate to the server as the
the TLP timer to fire twice before setting an RTO timer. In other time period from when the first Initial was sent to when a Retry or a
words, when the TLP timer expires the first time, a TLP packet is Version Negotiation packet is received. The client MAY use this
sent, and it is RECOMMENDED that the TLP timer be scheduled for a value to seed the RTT estimator for a subsequent connection attempt
second time. When the TLP timer expires the second time, a second to the server.
TLP packet is sent, and an RTO timer SHOULD be scheduled
Section 4.3.3.
A TLP packet SHOULD carry new data when possible. If new data is 6.2.2. Probe Timeout
unavailable or new data cannot be sent due to flow control, a TLP
packet MAY retransmit unacknowledged data to potentially reduce
recovery time. Since a TLP timer is used to send a probe into the
network prior to establishing any packet loss, prior unacknowledged
packets SHOULD NOT be marked as lost when a TLP timer expires.
A sender may not know that a packet being sent is a tail packet. A Probe Timeout (PTO) triggers a probe packet when ack-eliciting data
Consequently, a sender may have to arm or adjust the TLP timer on is in flight but an acknowledgement is not received within the
every sent retransmittable packet. expected period of time. A PTO enables a connection to recover from
loss of tail packets or acks. The PTO algorithm used in QUIC
implements the reliability functions of Tail Loss Probe [TLP] [RACK],
RTO [RFC5681] and F-RTO algorithms for TCP [RFC5682], and the timeout
computation is based on TCP's retransmission timeout period
[RFC6298].
4.3.3. Retransmission Timeout 6.2.2.1. Computing PTO
A Retransmission Timeout (RTO) timer is the final backstop for loss When an ack-eliciting packet is transmitted, the sender schedules a
detection. The algorithm used in QUIC is based on the RTO algorithm timer for the PTO period as follows:
for TCP [RFC5681] and is additionally resilient to spurious RTO
events [RFC5682].
When the last TLP packet is sent, a timer is set for the RTO period. PTO = max(smoothed_rtt + 4*rttvar + max_ack_delay, kGranularity)
When this timer expires, the sender sends two packets, to evoke
acknowledgements from the receiver, and restarts the RTO timer.
Similar to TCP [RFC6298], the RTO period is set based on the kGranularity, smoothed_rtt, rttvar, and max_ack_delay are defined in
following conditions: Section 6.4.1 and Section 6.4.2.
o When the final TLP packet is sent, the RTO period is set to The PTO period is the amount of time that a sender ought to wait for
max(SRTT + 4*RTTVAR + MaxAckDelay, kMinRTOTimeout) an acknowledgement of a sent packet. This time period includes the
estimated network roundtrip-time (smoothed_rtt), the variance in the
estimate (4*rttvar), and max_ack_delay, to account for the maximum
time by which a receiver might delay sending an acknowledgement.
o When an RTO timer expires, the RTO period is doubled. The PTO value MUST be set to at least kGranularity, to avoid the
timer expiring immediately.
The sender typically has incurred a high latency penalty by the time When a PTO timer expires, the PTO period MUST be set to twice its
an RTO timer expires, and this penalty increases exponentially in current value. This exponential reduction in the sender's rate is
subsequent consecutive RTO events. Sending a single packet on an RTO important because the PTOs might be caused by loss of packets or
event therefore makes the connection very sensitive to single packet acknowledgements due to severe congestion.
loss. Sending two packets instead of one significantly increases
resilience to packet drop in both directions, thus reducing the
probability of consecutive RTO events.
QUIC's RTO algorithm differs from TCP in that the firing of an RTO A sender computes its PTO timer every time an ack-eliciting packet is
timer is not considered a strong enough signal of packet loss, so sent. A sender might choose to optimize this by setting the timer
does not result in an immediate change to congestion window or fewer times if it knows that more ack-eliciting packets will be sent
recovery state. An RTO timer expires only when there's a prolonged within a short period of time.
period of network silence, which could be caused by a change in the
underlying network RTT.
QUIC also diverges from TCP by including MaxAckDelay in the RTO 6.2.2.2. Sending Probe Packets
period. Since QUIC corrects for this delay in its SRTT and RTTVAR
computations, it is necessary to add this delay explicitly in the TLP
and RTO computation.
When an acknowledgment is received for a packet sent on an RTO event, When a PTO timer expires, the sender MUST send one ack-eliciting
any unacknowledged packets with lower packet numbers than those packet as a probe. A sender MAY send up to two ack-eliciting
acknowledged MUST be marked as lost. If an acknowledgement for a packets, to avoid an expensive consecutive PTO expiration due to a
packet sent on an RTO is received at the same time packets sent prior single packet loss.
to the first RTO are acknowledged, the RTO is considered spurious and
standard loss detection rules apply.
A packet sent when an RTO timer expires MAY carry new data if Consecutive PTO periods increase exponentially, and as a result,
available or unacknowledged data to potentially reduce recovery time. connection recovery latency increases exponentially as packets
Since this packet is sent as a probe into the network prior to continue to be dropped in the network. Sending two packets on PTO
establishing any packet loss, prior unacknowledged packets SHOULD NOT expiration increases resilience to packet drops, thus reducing the
be marked as lost. probability of consecutive PTO events.
A packet sent on an RTO timer MUST NOT be blocked by the sender's Probe packets sent on a PTO MUST be ack-eliciting. A probe packet
congestion controller. A sender MUST however count these bytes as SHOULD carry new data when possible. A probe packet MAY carry
additional bytes in flight, since this packet adds network load retransmitted unacknowledged data when new data is unavailable, when
without establishing packet loss. flow control does not permit new data to be sent, or to
opportunistically reduce loss recovery delay. Implementations MAY
use alternate strategies for determining the content of probe
packets, including sending new or retransmitted data based on the
application's priorities.
4.4. Generating Acknowledgements 6.2.2.3. Loss Detection
QUIC SHOULD delay sending acknowledgements in response to packets, Delivery or loss of packets in flight is established when an ACK
but MUST NOT excessively delay acknowledgements of packets containing frame is received that newly acknowledges one or more packets.
frames other than ACK. Specifically, implementations MUST attempt to
enforce a maximum ack delay to avoid causing the peer spurious
timeouts. The maximum ack delay is communicated in the
"max_ack_delay" transport parameter and the default value is 25ms.
An acknowledgement SHOULD be sent immediately upon receipt of a A PTO timer expiration event does not indicate packet loss and MUST
second packet but the delay SHOULD NOT exceed the maximum ack delay. NOT cause prior unacknowledged packets to be marked as lost. After a
QUIC recovery algorithms do not assume the peer generates an PTO timer has expired, an endpoint uses the following rules to mark
acknowledgement immediately when receiving a second full-packet. packets as lost when an acknowledgement is received that newly
acknowledges packets.
Out-of-order packets SHOULD be acknowledged more quickly, in order to When an acknowledgement is received that newly acknowledges packets,
accelerate loss recovery. The receiver SHOULD send an immediate ACK loss detection proceeds as dictated by packet and time threshold
when it receives a new packet which is not one greater than the mechanisms, see Section 6.1.
largest received packet number.
Similarly, packets marked with the ECN Congestion Experienced (CE) 6.3. Tracking Sent Packets
codepoint in the IP header SHOULD be acknowledged immediately, to
reduce the peer's response time to congestion events.
As an optimization, a receiver MAY process multiple packets before To correctly implement congestion control, a QUIC sender tracks every
sending any ACK frames in response. In this case they can determine ack-eliciting packet until the packet is acknowledged or lost. It is
whether an immediate or delayed acknowledgement should be generated expected that implementations will be able to access this information
after processing incoming packets. by packet number and crypto context and store the per-packet fields
(Section 6.3.1) for loss recovery and congestion control.
4.4.1. Crypto Handshake Data After a packet is declared lost, it SHOULD be tracked for an amount
of time comparable to the maximum expected packet reordering, such as
1 RTT. This allows for detection of spurious retransmissions.
In order to quickly complete the handshake and avoid spurious Sent packets are tracked for each packet number space, and ACK
retransmissions due to crypto retransmission timeouts, crypto packets processing only applies to a single space.
SHOULD use a very short ack delay, such as 1ms. ACK frames MAY be
sent immediately when the crypto stack indicates all data for that
encryption level has been received.
4.4.2. ACK Ranges 6.3.1. Sent Packet Fields
When an ACK frame is sent, one or more ranges of acknowledged packets packet_number: The packet number of the sent packet.
are included. Including older packets reduces the chance of spurious
retransmits caused by losing previously sent ACK frames, at the cost
of larger ACK frames.
ACK frames SHOULD always acknowledge the most recently received ack_eliciting: A boolean that indicates whether a packet is ack-
packets, and the more out-of-order the packets are, the more eliciting. If true, it is expected that an acknowledgement will
important it is to send an updated ACK frame quickly, to prevent the be received, though the peer could delay sending the ACK frame
peer from declaring a packet as lost and spuriously retransmitting containing it by up to the MaxAckDelay.
the frames it contains.
Below is one recommended approach for determining what packets to in_flight: A boolean that indicates whether the packet counts
include in an ACK frame. towards bytes in flight.
4.4.3. Receiver Tracking of ACK Frames 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.
When a packet containing an ACK frame is sent, the largest sent_bytes: The number of bytes sent in the packet, not including
acknowledged in that frame may be saved. When a packet containing an UDP or IP overhead, but including QUIC framing overhead.
ACK frame is acknowledged, the receiver can stop acknowledging
packets less than or equal to the largest acknowledged in the sent
ACK frame.
In cases without ACK frame loss, this algorithm allows for a minimum time_sent: The time the packet was sent.
of 1 RTT of reordering. In cases with ACK frame loss, this approach
does not guarantee that every acknowledgement is seen by the sender
before it is no longer included in the ACK frame. Packets could be
received out of order and all subsequent ACK frames containing them
could be lost. In this case, the loss recovery algorithm may cause
spurious retransmits, but the sender will continue making forward
progress.
4.5. Pseudocode 6.4. Pseudocode
4.5.1. Constants of interest 6.4.1. Constants of interest
Constants used in loss recovery are based on a combination of RFCs, Constants used in loss recovery are based on a combination of RFCs,
papers, and common practice. Some may need to be changed or papers, and common practice. Some may need to be changed or
negotiated in order to better suit a variety of environments. negotiated in order to better suit a variety of environments.
kMaxTLPs: Maximum number of tail loss probes before an RTO expires. kPacketThreshold: Maximum reordering in packets before packet
The RECOMMENDED value is 2. threshold loss detection considers a packet lost. The RECOMMENDED
value is 3.
kReorderingThreshold: Maximum reordering in packet number space
before FACK style loss detection considers a packet lost. The
RECOMMENDED value is 3.
kTimeReorderingFraction: Maximum reordering in time space before
time based loss detection considers a packet lost. In fraction of
an RTT. The RECOMMENDED value is 1/8.
kUsingTimeLossDetection: Whether time based loss detection is in
use. If false, uses FACK style loss detection. The RECOMMENDED
value is false.
kMinTLPTimeout: Minimum time in the future a tail loss probe timer
may be set for. The RECOMMENDED value is 10ms.
kMinRTOTimeout: Minimum time in the future an RTO timer may be set kTimeThreshold: Maximum reordering in time before time threshold
for. The RECOMMENDED value is 200ms. loss detection considers a packet lost. Specified as an RTT
multiplier. The RECOMMENDED value is 9/8.
kDelayedAckTimeout: The length of the peer's delayed ack timer. The kGranularity: Timer granularity. This is a system-dependent value.
RECOMMENDED value is 25ms. However, implementations SHOULD use a value no smaller than 1ms.
kInitialRtt: The RTT used before an RTT sample is taken. The kInitialRtt: The RTT used before an RTT sample is taken. The
RECOMMENDED value is 100ms. RECOMMENDED value is 100ms.
4.5.2. Variables of interest 6.4.2. 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.
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.
tlp_count: The number of times a tail loss probe has been sent pto_count: The number of times a PTO has been sent without receiving
without receiving an ack. an ack.
rto_count: The number of times an RTO has been sent without
receiving an ack.
largest_sent_before_rto: The last packet number sent prior to the
first retransmission timeout.
time_of_last_sent_retransmittable_packet: The time the most recent time_of_last_sent_ack_eliciting_packet: The time the most recent
retransmittable packet was sent. ack-eliciting packet was sent.
time_of_last_sent_crypto_packet: The time the most recent crypto time_of_last_sent_crypto_packet: The time the most recent crypto
packet was sent. packet was sent.
largest_sent_packet: The packet number of the most recently sent largest_sent_packet: The packet number of the most recently sent
packet. packet.
largest_acked_packet: The largest packet number acknowledged in an largest_acked_packet: The largest packet number acknowledged in the
ACK frame. packet number space so far.
latest_rtt: The most recent RTT measurement made when receiving an latest_rtt: The most recent RTT measurement made when receiving an
ack for a previously unacked packet. ack for a previously unacked packet.
smoothed_rtt: The smoothed RTT of the connection, computed as smoothed_rtt: The smoothed RTT of the connection, computed as
described in [RFC6298] described in [RFC6298]
rttvar: The RTT variance, computed as described in [RFC6298] rttvar: The RTT variance, computed as described in [RFC6298]
min_rtt: The minimum RTT seen in the connection, ignoring ack delay. min_rtt: The minimum RTT seen in the connection, ignoring ack delay.
max_ack_delay: The maximum amount of time by which the receiver max_ack_delay: The maximum amount of time by which the receiver
intends to delay acknowledgments, in milliseconds. The actual intends to delay acknowledgments, in milliseconds. The actual
ack_delay in a received ACK frame may be larger due to late ack_delay in a received ACK frame may be larger due to late
timers, reordering, or lost ACKs. timers, reordering, or lost ACKs.
reordering_threshold: The largest packet number gap between the
largest acknowledged retransmittable packet and an unacknowledged
retransmittable packet before it is declared lost.
time_reordering_fraction: The reordering window as a fraction of
max(smoothed_rtt, latest_rtt).
loss_time: The time at which the next packet will be considered lost loss_time: The time at which the next packet will be considered lost
based on early transmit or exceeding the reordering window in based on early transmit or exceeding the reordering window in
time. time.
sent_packets: An association of packet numbers to information about sent_packets: An association of packet numbers to information about
them, including a number field indicating the packet number, a them. Described in detail above in Section 6.3.
time field indicating the time a packet was sent, a boolean
indicating whether the packet is ack-only, a boolean indicating
whether it counts towards bytes in flight, and a bytes field
indicating the packet's size. sent_packets is ordered by packet
number, and packets remain in sent_packets until acknowledged or
lost. A sent_packets data structure is maintained per packet
number space, and ACK processing only applies to a single space.
4.5.3. Initialization 6.4.3. 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
tlp_count = 0 pto_count = 0
rto_count = 0
if (kUsingTimeLossDetection)
reordering_threshold = infinite
time_reordering_fraction = kTimeReorderingFraction
else:
reordering_threshold = kReorderingThreshold
time_reordering_fraction = infinite
loss_time = 0 loss_time = 0
smoothed_rtt = 0 smoothed_rtt = 0
rttvar = 0 rttvar = 0
min_rtt = infinite min_rtt = infinite
largest_sent_before_rto = 0 time_of_last_sent_ack_eliciting_packet = 0
time_of_last_sent_retransmittable_packet = 0
time_of_last_sent_crypto_packet = 0 time_of_last_sent_crypto_packet = 0
largest_sent_packet = 0 largest_sent_packet = 0
largest_acked_packet = 0
4.5.4. On Sending a Packet 6.4.4. On Sending a Packet
After any packet is sent, be it a new transmission or a rebundled
transmission, the following OnPacketSent function is called. The
parameters to OnPacketSent are as follows:
o packet_number: The packet number of the sent packet.
o ack_only: A boolean that indicates whether a packet contains only
ACK or PADDING frame(s). If true, it is still expected an ack
will be received for this packet, but it is not retransmittable.
o in_flight: A boolean that indicates whether the packet counts
towards bytes in flight.
o is_crypto_packet: A boolean that indicates whether the 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.
o sent_bytes: The number of bytes sent in the packet, not including After a packet is sent, information about the packet is stored. The
UDP or IP overhead, but including QUIC framing overhead. parameters to OnPacketSent are described in detail above in
Section 6.3.1.
Pseudocode for OnPacketSent follows: Pseudocode for OnPacketSent follows:
OnPacketSent(packet_number, ack_only, in_flight, OnPacketSent(packet_number, ack_eliciting, in_flight,
is_crypto_packet, sent_bytes): is_crypto_packet, sent_bytes):
largest_sent_packet = packet_number largest_sent_packet = packet_number
sent_packets[packet_number].packet_number = packet_number sent_packets[packet_number].packet_number = packet_number
sent_packets[packet_number].time = now sent_packets[packet_number].time_sent = now
sent_packets[packet_number].ack_only = ack_only sent_packets[packet_number].ack_eliciting = ack_eliciting
sent_packets[packet_number].in_flight = in_flight sent_packets[packet_number].in_flight = in_flight
if !ack_only: if (ack_eliciting):
if is_crypto_packet: if (is_crypto_packet):
time_of_last_sent_crypto_packet = now time_of_last_sent_crypto_packet = now
time_of_last_sent_retransmittable_packet = now time_of_last_sent_ack_eliciting_packet = now
OnPacketSentCC(sent_bytes) OnPacketSentCC(sent_bytes)
sent_packets[packet_number].bytes = sent_bytes sent_packets[packet_number].size = sent_bytes
SetLossDetectionTimer() SetLossDetectionTimer()
4.5.5. On Receiving an Acknowledgment 6.4.5. On Receiving an Acknowledgment
When an ACK frame is received, it may newly acknowledge any number of When an ACK frame is received, it may newly acknowledge any number of
packets. packets.
Pseudocode for OnAckReceived and UpdateRtt follow: Pseudocode for OnAckReceived and UpdateRtt follow:
OnAckReceived(ack): OnAckReceived(ack):
largest_acked_packet = ack.largest_acked largest_acked_packet = max(largest_acked_packet,
// If the largest acknowledged is newly acked, ack.largest_acked)
// update the RTT.
if (sent_packets[ack.largest_acked]): // If the largest acknowledged is newly acked and
latest_rtt = now - sent_packets[ack.largest_acked].time // ack-eliciting, update the RTT.
if (sent_packets[ack.largest_acked] &&
sent_packets[ack.largest_acked].ack_eliciting):
latest_rtt =
now - sent_packets[ack.largest_acked].time_sent
UpdateRtt(latest_rtt, ack.ack_delay) UpdateRtt(latest_rtt, ack.ack_delay)
// Process ECN information if present.
if (ACK frame contains ECN information):
ProcessECN(ack)
// Find all newly acked packets in this ACK frame // Find all newly acked packets in this ACK frame
newly_acked_packets = DetermineNewlyAckedPackets(ack) newly_acked_packets = DetermineNewlyAckedPackets(ack)
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) OnPacketAcked(acked_packet.packet_number)
if !newly_acked_packets.empty(): crypto_count = 0
// Find the smallest newly acknowledged packet pto_count = 0
smallest_newly_acked =
FindSmallestNewlyAcked(newly_acked_packets)
// If any packets sent prior to RTO were acked, then the
// RTO was spurious. Otherwise, inform congestion control.
if (rto_count > 0 &&
smallest_newly_acked > largest_sent_before_rto):
OnRetransmissionTimeoutVerified(smallest_newly_acked)
crypto_count = 0
tlp_count = 0
rto_count = 0
DetectLostPackets(ack.largest_acked_packet) DetectLostPackets()
SetLossDetectionTimer() SetLossDetectionTimer()
// Process ECN information if present.
if (ACK frame contains ECN information):
ProcessECN(ack)
UpdateRtt(latest_rtt, ack_delay): UpdateRtt(latest_rtt, ack_delay):
// min_rtt ignores ack delay. // min_rtt ignores ack delay.
min_rtt = min(min_rtt, latest_rtt) min_rtt = min(min_rtt, latest_rtt)
// Limit ack_delay by max_ack_delay
ack_delay = min(ack_delay, max_ack_delay)
// Adjust for ack delay if it's plausible. // Adjust for ack delay if it's plausible.
if (latest_rtt - min_rtt > ack_delay): if (latest_rtt - min_rtt > ack_delay):
latest_rtt -= ack_delay latest_rtt -= ack_delay
// Based on {{RFC6298}}. // Based on {{RFC6298}}.
if (smoothed_rtt == 0): if (smoothed_rtt == 0):
smoothed_rtt = latest_rtt smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2 rttvar = latest_rtt / 2
else: else:
rttvar_sample = abs(smoothed_rtt - latest_rtt) rttvar_sample = abs(smoothed_rtt - latest_rtt)
rttvar = 3/4 * rttvar + 1/4 * rttvar_sample rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt
4.5.6. On Packet Acknowledgment 6.4.6. On Packet Acknowledgment
When a packet is acked 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 acked packets. for each of these newly acknowledged packets.
OnPacketAcked takes one parameter, acked_packet, which is the struct OnPacketAcked takes one parameter, acked_packet, which is the struct
of the newly acked packet. detailed in Section 6.3.1.
If this is the first acknowledgement following RTO, check if the
smallest newly acknowledged packet is one sent by the RTO, and if so,
inform congestion control of a verified RTO, similar to F-RTO
[RFC5682].
Pseudocode for OnPacketAcked follows: Pseudocode for OnPacketAcked follows:
OnPacketAcked(acked_packet): OnPacketAcked(acked_packet):
if (!acked_packet.is_ack_only): if (acked_packet.ack_eliciting):
OnPacketAckedCC(acked_packet) OnPacketAckedCC(acked_packet)
sent_packets.remove(acked_packet.packet_number) sent_packets.remove(acked_packet.packet_number)
4.5.7. Setting the Loss Detection Timer 6.4.7. Setting the Loss Detection Timer
QUIC loss detection uses a single timer for all timer-based loss QUIC loss detection uses a single timer for all timeout loss
detection. The duration of the timer is based on the timer's mode, detection. The duration of the timer is based on the timer's mode,
which is set in the packet and timer events further below. The which is set in the packet and timer events further below. The
function SetLossDetectionTimer defined below shows how the single function SetLossDetectionTimer defined below shows how the single
timer is set. timer is set.
This algorithm may result in the timer being set in the past,
particularly if timers wake up late. Timers set in the past SHOULD
fire immediately.
Pseudocode for SetLossDetectionTimer follows: Pseudocode for SetLossDetectionTimer follows:
SetLossDetectionTimer(): SetLossDetectionTimer():
// Don't arm timer if there are no retransmittable packets // Don't arm timer if there are no ack-eliciting packets
// in flight. // in flight.
if (bytes_in_flight == 0): if (no ack-eliciting packets in flight):
loss_detection_timer.cancel() loss_detection_timer.cancel()
return return
if (crypto packets are outstanding): if (crypto packets are in flight):
// 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, kMinTLPTimeout) timeout = max(timeout, kGranularity)
timeout = timeout * (2 ^ crypto_count) timeout = timeout * (2 ^ crypto_count)
loss_detection_timer.set( loss_detection_timer.set(
time_of_last_sent_crypto_packet + timeout) time_of_last_sent_crypto_packet + timeout)
return return
if (loss_time != 0): if (loss_time != 0):
// Early retransmit timer or time loss detection. // Time threshold loss detection.
timeout = loss_time - loss_detection_timer.set(loss_time)
time_of_last_sent_retransmittable_packet return
else:
// RTO or TLP timer // Calculate PTO duration
// Calculate RTO duration timeout =
timeout = smoothed_rtt + 4 * rttvar + max_ack_delay
smoothed_rtt + 4 * rttvar + max_ack_delay timeout = max(timeout, kGranularity)
timeout = max(timeout, kMinRTOTimeout) timeout = timeout * (2 ^ pto_count)
timeout = timeout * (2 ^ rto_count)
if (tlp_count < kMaxTLPs):
// Tail Loss Probe
tlp_timeout = max(1.5 * smoothed_rtt
+ max_ack_delay, kMinTLPTimeout)
timeout = min(tlp_timeout, timeout)
loss_detection_timer.set( loss_detection_timer.set(
time_of_last_sent_retransmittable_packet + timeout) time_of_last_sent_ack_eliciting_packet + timeout)
4.5.8. On Timeout 6.4.8. On Timeout
When the loss detection timer expires, the timer's mode determines When the loss detection timer expires, the timer's mode determines
the action to be performed. the action to be performed.
Pseudocode for OnLossDetectionTimeout follows: Pseudocode for OnLossDetectionTimeout follows:
OnLossDetectionTimeout(): OnLossDetectionTimeout():
if (crypto packets are outstanding): if (crypto packets are in flight):
// Crypto retransmission timeout. // Crypto retransmission timeout.
RetransmitUnackedCryptoData() RetransmitUnackedCryptoData()
crypto_count++ crypto_count++
else if (loss_time != 0): else if (loss_time != 0):
// Early retransmit or Time Loss Detection // Time threshold loss Detection
DetectLostPackets(largest_acked_packet) DetectLostPackets()
else if (tlp_count < kMaxTLPs):
// Tail Loss Probe.
SendOnePacket()
tlp_count++
else: else:
// RTO. // PTO
if (rto_count == 0)
largest_sent_before_rto = largest_sent_packet
SendTwoPackets() SendTwoPackets()
rto_count++ pto_count++
SetLossDetectionTimer() SetLossDetectionTimer()
4.5.9. Detecting Lost Packets 6.4.9. Detecting Lost Packets
Packets in QUIC are only considered lost once a larger packet number
in the same packet number space is acknowledged. DetectLostPackets
is called every time an ack is received and operates on the
sent_packets for that packet number space. If the loss detection
timer expires and the loss_time is set, the previous largest acked
packet is supplied.
4.5.9.1. Pseudocode
DetectLostPackets takes one parameter, acked, which is the largest DetectLostPackets is called every time an ACK is received and
acked packet. operates on the sent_packets for that packet number space. If the
loss detection timer expires and the loss_time is set, the previous
largest acknowledged packet is supplied.
Pseudocode for DetectLostPackets follows: Pseudocode for DetectLostPackets follows:
DetectLostPackets(largest_acked): DetectLostPackets():
loss_time = 0 loss_time = 0
lost_packets = {} lost_packets = {}
delay_until_lost = infinite loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)
if (kUsingTimeLossDetection):
delay_until_lost = // Packets sent before this time are deemed lost.
(1 + time_reordering_fraction) * lost_send_time = now() - loss_delay
max(latest_rtt, smoothed_rtt)
else if (largest_acked.packet_number == largest_sent_packet): // Packets with packet numbers before this are deemed lost.
// Early retransmit timer. lost_pn = largest_acked_packet - kPacketThreshold
delay_until_lost = 9/8 * max(latest_rtt, smoothed_rtt)
foreach (unacked < largest_acked.packet_number): foreach unacked in sent_packets:
time_since_sent = now() - unacked.time_sent if (unacked.packet_number > largest_acked_packet):
delta = largest_acked.packet_number - unacked.packet_number continue
if (time_since_sent > delay_until_lost ||
delta > reordering_threshold): // Mark packet as lost, or set time when it should be marked.
if (unacked.time_sent <= lost_send_time ||
unacked.packet_number <= lost_pn):
sent_packets.remove(unacked.packet_number) sent_packets.remove(unacked.packet_number)
if (!unacked.is_ack_only): if (unacked.in_flight):
lost_packets.insert(unacked) lost_packets.insert(unacked)
else if (loss_time == 0 && delay_until_lost != infinite): else if (loss_time == 0):
loss_time = now() + delay_until_lost - time_since_sent loss_time = unacked.time_sent + loss_delay
else:
loss_time = min(loss_time, unacked.time_sent + loss_delay)
// Inform the congestion controller of lost packets and // Inform the congestion controller of lost packets and
// lets it decide whether to retransmit immediately. // let it decide whether to retransmit immediately.
if (!lost_packets.empty()): if (!lost_packets.empty()):
OnPacketsLost(lost_packets) OnPacketsLost(lost_packets)
4.6. Discussion 6.5. Discussion
The majority of constants were derived from best common practices The majority of constants were derived from best common practices
among widely deployed TCP implementations on the internet. among widely deployed TCP implementations on the internet.
Exceptions follow. Exceptions follow.
A shorter delayed ack time of 25ms was chosen because longer delayed A shorter delayed ack time of 25ms was chosen because longer delayed
acks can delay loss recovery and for the small number of connections acks can delay loss recovery and for the small number of connections
where less than packet per 25ms is delivered, acking every packet is where less than packet per 25ms is delivered, acking every packet is
beneficial to congestion control and loss recovery. beneficial to congestion control and loss recovery.
The default initial RTT of 100ms was chosen because it is slightly The default initial RTT of 100ms was chosen because it is slightly
higher than both the median and mean min_rtt typically observed on higher than both the median and mean min_rtt typically observed on
the public internet. the public internet.
5. 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 Section 5.8.2) beyond the available bytes_in_flight (defined in Section 7.9.2) beyond the available
congestion window, unless the packet is a probe packet sent after the congestion window, unless the packet is a probe packet sent after a
TLP or RTO timer expires, as described in Section 4.3.2 and PTO timer expires, as described in Section 6.2.2.
Section 4.3.3.
Implementations MAY use other congestion control algorithms, and Implementations MAY use other congestion control algorithms, such as
endpoints MAY use different algorithms from one another. The signals Cubic [RFC8312], and endpoints MAY use different algorithms from one
QUIC provides for congestion control are generic and are designed to another. The signals QUIC provides for congestion control are
support different algorithms. generic and are designed to support different algorithms.
5.1. 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.
This document specifies an endpoint's response when its peer receives This document specifies an endpoint's response when its peer receives
packets with the Congestion Experienced codepoint. As discussed in packets with the Congestion Experienced codepoint. As discussed in
[RFC8311], endpoints are permitted to experiment with other response [RFC8311], endpoints are permitted to experiment with other response
functions. functions.
5.2. Slow Start 7.2. Slow Start
QUIC begins every connection in slow start and exits slow start upon QUIC begins every connection in slow start and exits slow start upon
loss or upon increase in the ECN-CE counter. QUIC re-enters slow loss or upon increase in the ECN-CE counter. QUIC re-enters slow
start anytime the congestion window is less than ssthresh, which start anytime the congestion window is less than ssthresh, which
typically only occurs after an RTO. While in slow start, QUIC typically only occurs after an PTO. While in slow start, QUIC
increases the congestion window by the number of bytes acknowledged increases the congestion window by the number of bytes acknowledged
when each ack is processed. when each acknowledgment is processed.
5.3. Congestion Avoidance 7.3. Congestion Avoidance
Slow start exits to congestion avoidance. Congestion avoidance in Slow start exits to congestion avoidance. Congestion avoidance in
NewReno uses an additive increase multiplicative decrease (AIMD) NewReno uses an additive increase multiplicative decrease (AIMD)
approach that increases the congestion window by one maximum packet approach that increases the congestion window by one maximum packet
size per congestion window acknowledged. When a loss is detected, size per congestion window acknowledged. When a loss is detected,
NewReno halves the congestion window and sets the slow start NewReno halves the congestion window and sets the slow start
threshold to the new congestion window. threshold to the new congestion window.
5.4. Recovery Period 7.4. Recovery Period
Recovery is a period of time beginning with detection of a lost Recovery is a period of time beginning with detection of a lost
packet or an increase in the ECN-CE counter. Because QUIC packet or an increase in the ECN-CE counter. Because QUIC does not
retransmits stream data and control frames, not packets, it defines retransmit packets, it defines the end of recovery as a packet sent
the end of recovery as a packet sent after the start of recovery after the start of recovery being acknowledged. This is slightly
being acknowledged. This is slightly different from TCP's definition different from TCP's definition of recovery, which ends when the lost
of recovery, which ends when the lost packet that started recovery is packet that started recovery is acknowledged.
acknowledged.
The recovery period limits congestion window reduction to once per The recovery period limits congestion window reduction to once per
round trip. During recovery, the congestion window remains unchanged round trip. During recovery, the congestion window remains unchanged
irrespective of new losses or increases in the ECN-CE counter. irrespective of new losses or increases in the ECN-CE counter.
5.5. Tail Loss Probe 7.5. Probe Timeout
A TLP packet MUST NOT be blocked by the sender's congestion
controller. The sender MUST however count these bytes as additional
bytes-in-flight, since a TLP adds network load without establishing
packet loss.
Acknowledgement or loss of tail loss probes are treated like any
other packet.
5.6. Retransmission Timeout Probe packets MUST NOT be blocked by the congestion controller. A
sender MUST however count these packets as being additionally in
flight, since these packets adds network load without establishing
packet loss. Note that sending probe packets might cause the
sender's bytes in flight to exceed the congestion window until an
acknowledgement is received that establishes loss or delivery of
packets.
When retransmissions are sent due to a retransmission timeout timer, If a threshold number of consecutive PTOs have occurred (pto_count is
no change is made to the congestion window until the next more than kPersistentCongestionThreshold, see Section 7.9.1), the
acknowledgement arrives. The retransmission timeout is considered network is considered to be experiencing persistent congestion, and
spurious when this acknowledgement acknowledges packets sent prior to the sender's congestion window MUST be reduced to the minimum
the first retransmission timeout. The retransmission timeout is congestion window.
considered valid when this acknowledgement acknowledges no packets
sent prior to the first retransmission timeout. In this case, the
congestion window MUST be reduced to the minimum congestion window
and slow start is re-entered.
5.7. Pacing 7.6. Pacing
This document does not specify a pacer, but it is RECOMMENDED that a This document does not specify a pacer, but it is RECOMMENDED that a
sender pace sending of all in-flight packets based on input from the sender pace sending of all in-flight packets based on input from the
congestion controller. For example, a pacer might distribute the congestion controller. For example, a pacer might distribute the
congestion window over the SRTT when used with a window-based congestion window over the SRTT when used with a window-based
controller, and a pacer might use the rate estimate of a rate-based controller, and a pacer might use the rate estimate of a rate-based
controller. controller.
An implementation should take care to architect its congestion An implementation should take care to architect its congestion
controller to work well with a pacer. For instance, a pacer might controller to work well with a pacer. For instance, a pacer might
skipping to change at page 25, line 5 skipping to change at page 24, line 9
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).
5.8. Pseudocode 7.7. Sending data after an idle period
5.8.1. Constants of interest A sender becomes idle if it ceases to send data and has no bytes in
flight. A sender's congestion window MUST not increase while it is
idle.
When sending data after becoming idle, a sender MUST reset its
congestion window to the initial congestion window (see Section 4.1
of [RFC5681]), unless it paces the sending of packets. A sender MAY
retain its congestion window if it paces the sending of any packets
in excess of the initial congestion window.
A sender MAY implement alternate mechanisms to update its congestion
window after idle periods, such as those proposed for TCP in
[RFC7661].
7.8. Discarding Packet Number Space State
When keys for an packet number space are discarded, any packets sent
with those keys are removed from the count of bytes in flight. No
loss events will occur any in-flight packets from that space, as a
result of discarding loss recovery state (see Section 6.2.1.2). Note
that it is expected that keys are discarded after those packets would
be declared lost, but Initial secrets are destroyed earlier.
7.9. Pseudocode
7.9.1. Constants of interest
Constants used in congestion control are based on a combination of Constants used in congestion control are based on a combination of
RFCs, papers, and common practice. Some may need to be changed or RFCs, papers, and common practice. Some may need to be changed or
negotiated in order to better suit a variety of environments. negotiated in order to better suit a variety of environments.
kMaxDatagramSize: The sender's maximum payload size. Does not kMaxDatagramSize: The sender's maximum payload size. Does not
include UDP or IP overhead. The max packet size is used for include UDP or IP overhead. The max packet size is used for
calculating initial and minimum congestion windows. The calculating initial and minimum congestion windows. The
RECOMMENDED value is 1200 bytes. RECOMMENDED value is 1200 bytes.
kInitialWindow: Default limit on the initial amount of outstanding kInitialWindow: Default limit on the initial amount of data in
data in bytes. Taken from [RFC6928]. The RECOMMENDED value is flight, in bytes. Taken from [RFC6928]. The RECOMMENDED value is
the minimum of 10 * kMaxDatagramSize and max(2* kMaxDatagramSize, the minimum of 10 * kMaxDatagramSize and max(2* kMaxDatagramSize,
14600)). 14600)).
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.
5.8.2. Variables of interest kPersistentCongestionThreshold: Number of consecutive PTOs after
which network is considered to be experiencing persistent
congestion. The rationale for this threshold is to enable a
sender to use initial PTOs for aggressive probing, similar to Tail
Loss Probe (TLP) in TCP [TLP] [RACK]. Once the number of
consecutive PTOs reaches this threshold - that is, persistent
congestion is established - the sender responds by collapsing its
congestion window to kMinimumWindow, similar to a Retransmission
Timeout (RTO) in TCP [RFC5681]. The RECOMMENDED value for
kPersistentCongestionThreshold is 2, which is equivalent to having
two TLPs before an RTO in TCP.
7.9.2. Variables of interest
Variables required to implement the congestion control mechanisms are Variables required to implement the congestion control mechanisms are
described in this section. described in this section.
ecn_ce_counter: The highest value reported for the ECN-CE counter by ecn_ce_counter: The highest value reported for the ECN-CE counter by
the peer in an ACK frame. This variable is used to detect the peer in an ACK frame. This variable is used to detect
increases in the reported ECN-CE counter. increases in the reported ECN-CE counter.
bytes_in_flight: The sum of the size in bytes of all sent packets bytes_in_flight: The sum of the size in bytes of all sent packets
that contain at least one retransmittable or PADDING frame, and that contain at least one ack-eliciting or PADDING frame, and have
have not been acked or declared lost. The size does not include not been acked or declared lost. The size does not include IP or
IP or UDP overhead, but does include the QUIC header and AEAD UDP overhead, but does include the QUIC header and AEAD overhead.
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.
end_of_recovery: The largest packet number sent when QUIC detects a recovery_start_time: The time when QUIC first detects a loss,
loss. When a larger packet is acknowledged, QUIC exits recovery. causing it to enter recovery. When a packet sent after this time
is acknowledged, QUIC exits recovery.
ssthresh: Slow start threshold in bytes. When the congestion window ssthresh: Slow start threshold in bytes. When the congestion window
is below ssthresh, the mode is slow start and the window grows by is below ssthresh, the mode is slow start and the window grows by
the number of bytes acknowledged. the number of bytes acknowledged.
5.8.3. Initialization 7.9.3. 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
end_of_recovery = 0 recovery_start_time = 0
ssthresh = infinite ssthresh = infinite
ecn_ce_counter = 0 ecn_ce_counter = 0
5.8.4. On Packet Sent 7.9.4. 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
5.8.5. On Packet Acknowledgement 7.9.5. On Packet Acknowledgement
Invoked from loss detection's OnPacketAcked and is supplied with Invoked from loss detection's OnPacketAcked and is supplied with the
acked_packet from sent_packets. acked_packet from sent_packets.
InRecovery(packet_number): InRecovery(sent_time):
return packet_number <= end_of_recovery return sent_time <= recovery_start_time
OnPacketAckedCC(acked_packet): OnPacketAckedCC(acked_packet):
// Remove from bytes_in_flight. // Remove from bytes_in_flight.
bytes_in_flight -= acked_packet.bytes bytes_in_flight -= acked_packet.size
if (InRecovery(acked_packet.packet_number)): if (InRecovery(acked_packet.time_sent)):
// Do not increase congestion window in recovery period. // Do not increase congestion window in recovery period.
return return
if (congestion_window < ssthresh): if (congestion_window < ssthresh):
// Slow start. // Slow start.
congestion_window += acked_packet.bytes congestion_window += acked_packet.size
else: else:
// Congestion avoidance. // Congestion avoidance.
congestion_window += kMaxDatagramSize * acked_packet.bytes congestion_window += kMaxDatagramSize * acked_packet.size
/ congestion_window / congestion_window
5.8.6. On New Congestion Event 7.9.6. 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. Starts a new recovery period and reduces the congestion is detected. May start a new recovery period and reduces the
window. congestion window.
CongestionEvent(packet_number): CongestionEvent(sent_time):
// Start a new congestion event if packet_number // Start a new congestion event if the sent time is larger
// is larger than the end of the previous recovery epoch. // than the start time of the previous recovery epoch.
if (!InRecovery(packet_number)): if (!InRecovery(sent_time)):
end_of_recovery = largest_sent_packet recovery_start_time = Now()
congestion_window *= kLossReductionFactor congestion_window *= kLossReductionFactor
congestion_window = max(congestion_window, kMinimumWindow) congestion_window = max(congestion_window, kMinimumWindow)
ssthresh = congestion_window ssthresh = congestion_window
// Collapse congestion window if persistent congestion
if (pto_count > kPersistentCongestionThreshold):
congestion_window = kMinimumWindow
5.8.7. Process ECN Information 7.9.7. Process ECN Information
Invoked when an ACK frame with an ECN section is received from the Invoked when an ACK frame with an ECN section is received from the
peer. peer.
ProcessECN(ack): ProcessECN(ack):
// If the ECN-CE counter reported by the peer has increased, // If the ECN-CE counter reported by the peer has increased,
// this could be a new congestion event. // this could be a new congestion event.
if (ack.ce_counter > ecn_ce_counter): if (ack.ce_counter > ecn_ce_counter):
ecn_ce_counter = ack.ce_counter ecn_ce_counter = ack.ce_counter
// Start a new congestion event if the last acknowledged // Start a new congestion event if the last acknowledged
// packet is past the end of the previous recovery epoch. // packet was sent after the start of the previous
CongestionEvent(ack.largest_acked_packet) // recovery epoch.
CongestionEvent(sent_packets[ack.largest_acked].time_sent)
5.8.8. On Packets Lost 7.9.8. 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.
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.bytes bytes_in_flight -= lost_packet.size
largest_lost_packet = lost_packets.last() largest_lost_packet = lost_packets.last()
// Start a new congestion epoch if the last lost packet // Start a new congestion epoch if the last lost packet
// is past the end of the previous recovery epoch. // is past the end of the previous recovery epoch.
CongestionEvent(largest_lost_packet.packet_number) CongestionEvent(largest_lost_packet.time_sent)
5.8.9. On Retransmission Timeout Verified
QUIC decreases the congestion window to the minimum value once the
retransmission timeout has been verified and removes any packets sent
before the newly acknowledged RTO packet.
OnRetransmissionTimeoutVerified(packet_number)
congestion_window = kMinimumWindow
// Declare all packets prior to packet_number lost.
for (sent_packet: sent_packets):
if (sent_packet.packet_number < packet_number):
bytes_in_flight -= sent_packet.bytes
sent_packets.remove(sent_packet.packet_number)
6. Security Considerations
6.1. Congestion Signals 8. Security Considerations
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.
6.2. Traffic Analysis 8.2. Traffic Analysis
Packets that carry only ACK frames can be heuristically identified by Packets that carry only ACK frames can be heuristically identified by
observing packet size. Acknowledgement patterns may expose observing packet size. Acknowledgement patterns may expose
information about link characteristics or application behavior. information about link characteristics or application behavior.
Endpoints can use PADDING frames or bundle acknowledgments with other Endpoints can use PADDING frames or bundle acknowledgments with other
frames to reduce leaked information. frames to reduce leaked information.
6.3. Misreporting ECN Markings 8.3. Misreporting ECN Markings
A receiver can misreport ECN markings to alter the congestion A receiver can misreport ECN markings to alter the congestion
response of a sender. Suppressing reports of ECN-CE markings could response of a sender. Suppressing reports of ECN-CE markings could
cause a sender to increase their send rate. This increase could cause a sender to increase their send rate. This increase could
result in congestion and loss. result in congestion and loss.
A sender MAY attempt to detect suppression of reports by marking A sender MAY attempt to detect suppression of reports by marking
occasional packets that they send with ECN-CE. If a packet marked occasional packets that they send with ECN-CE. If a packet marked
with ECN-CE is not reported as having been marked when the packet is with ECN-CE is not reported as having been marked when the packet is
acknowledged, the sender SHOULD then disable ECN for that path. acknowledged, the sender SHOULD then disable ECN for that path.
skipping to change at page 29, line 11 skipping to change at page 28, line 43
their sending rate, which is similar in effect to advertising reduced their sending rate, which is similar in effect to advertising reduced
connection flow control limits and so no advantage is gained by doing connection flow control limits and so no advantage is gained by doing
so. so.
Endpoints choose the congestion controller that they use. Though Endpoints choose the congestion controller that they use. Though
congestion controllers generally treat reports of ECN-CE markings as congestion controllers generally treat reports of ECN-CE markings as
equivalent to loss [RFC8311], the exact response for each controller equivalent to loss [RFC8311], the exact response for each controller
could be different. Failure to correctly respond to information could be different. Failure to correctly respond to information
about ECN markings is therefore difficult to detect. about ECN markings is therefore difficult to detect.
7. IANA Considerations 9. IANA Considerations
This document has no IANA actions. Yet. This document has no IANA actions. Yet.
8. References 10. References
10.1. Normative References
8.1. Normative References
[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-16 (work in progress), October 2018. transport-17 (work in progress), December 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion [RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311, Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018, DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>. <https://www.rfc-editor.org/info/rfc8311>.
8.2. Informative References 10.2. Informative References
[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
Refining TCP Congestion Control", ACM SIGCOMM , August
1996.
[RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
a time-based fast loss detection algorithm for TCP",
draft-ietf-tcpm-rack-04 (work in progress), July 2018.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte [RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>. 2003, <https://www.rfc-editor.org/info/rfc3465>.
[RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton, [RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
"Improving the Robustness of TCP to Non-Congestion "Improving the Robustness of TCP to Non-Congestion
Events", RFC 4653, DOI 10.17487/RFC4653, August 2006, Events", RFC 4653, DOI 10.17487/RFC4653, August 2006,
<https://www.rfc-editor.org/info/rfc4653>. <https://www.rfc-editor.org/info/rfc4653>.
skipping to change at page 30, line 38 skipping to change at page 30, line 38
and Y. Nishida, "A Conservative Loss Recovery Algorithm and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP", Based on Selective Acknowledgment (SACK) for TCP",
RFC 6675, DOI 10.17487/RFC6675, August 2012, RFC 6675, DOI 10.17487/RFC6675, August 2012,
<https://www.rfc-editor.org/info/rfc6675>. <https://www.rfc-editor.org/info/rfc6675>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis, [RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928, "Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013, DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>. <https://www.rfc-editor.org/info/rfc6928>.
[RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
TCP to Support Rate-Limited Traffic", RFC 7661,
DOI 10.17487/RFC7661, October 2015,
<https://www.rfc-editor.org/info/rfc7661>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>.
[TLP] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis, [TLP] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
in progress), February 2013. in progress), February 2013.
8.3. URIs 10.3. URIs
[1] https://mailarchive.ietf.org/arch/search/?email_list=quic [1] https://mailarchive.ietf.org/arch/search/?email_list=quic
[2] https://github.com/quicwg [2] https://github.com/quicwg
[3] https://github.com/quicwg/base-drafts/labels/-recovery [3] https://github.com/quicwg/base-drafts/labels/-recovery
Appendix A. Change Log Appendix A. 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.
A.1. Since draft-ietf-quic-recovery-14 Issue and pull request numbers are listed with a leading octothorp.
A.1. Since draft-ietf-quic-recovery-16
o Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP
and min crypto timeouts; eliminate timeout validation (#2114,
#2166, #2168, #1017)
o Redefine how congestion avoidance in terms of when the period
starts (#1928, #1930)
o Document what needs to be tracked for packets that are in flight
(#765, #1724, #1939)
o Integrate both time and packet thresholds into loss detection
(#1969, #1212, #934, #1974)
o Reduce congestion window after idle, unless pacing is used (#2007,
#2023)
o Disable RTT calculation for packets that don't elicit
acknowledgment (#2060, #2078)
o Limit ack_delay by max_ack_delay (#2060, #2099)
o Initial keys are discarded once Handshake are avaialble (#1951,
#2045)
o Reorder ECN and loss detection in pseudocode (#2142)
o Only cancel loss detection timer if ack-eliciting packets are in
flight (#2093, #2117)
A.2. Since draft-ietf-quic-recovery-14
o Used max_ack_delay from transport params (#1796, #1782) o Used max_ack_delay from transport params (#1796, #1782)
o Merge ACK and ACK_ECN (#1783) o Merge ACK and ACK_ECN (#1783)
A.2. Since draft-ietf-quic-recovery-13 A.3. Since draft-ietf-quic-recovery-13
o Corrected the lack of ssthresh reduction in CongestionEvent o Corrected the lack of ssthresh reduction in CongestionEvent
pseudocode (#1598) pseudocode (#1598)
o Considerations for ECN spoofing (#1426, #1626) o Considerations for ECN spoofing (#1426, #1626)
o Clarifications for PADDING and congestion control (#837, #838, o Clarifications for PADDING and congestion control (#837, #838,
#1517, #1531, #1540) #1517, #1531, #1540)
o Reduce early retransmission timer to RTT/8 (#945, #1581) o Reduce early retransmission timer to RTT/8 (#945, #1581)
o Packets are declared lost after an RTO is verified (#935, #1582) o Packets are declared lost after an RTO is verified (#935, #1582)
A.3. Since draft-ietf-quic-recovery-12 A.4. Since draft-ietf-quic-recovery-12
o Changes to manage separate packet number spaces and encryption o Changes to manage separate packet number spaces and encryption
levels (#1190, #1242, #1413, #1450) levels (#1190, #1242, #1413, #1450)
o Added ECN feedback mechanisms and handling; new ACK_ECN frame o Added ECN feedback mechanisms and handling; new ACK_ECN frame
(#804, #805, #1372) (#804, #805, #1372)
A.4. Since draft-ietf-quic-recovery-11 A.5. Since draft-ietf-quic-recovery-11
No significant changes. No significant changes.
A.5. Since draft-ietf-quic-recovery-10 A.6. Since draft-ietf-quic-recovery-10
o Improved text on ack generation (#1139, #1159) o Improved text on ack generation (#1139, #1159)
o Make references to TCP recovery mechanisms informational (#1195) o Make references to TCP recovery mechanisms informational (#1195)
o Define time_of_last_sent_handshake_packet (#1171) o Define time_of_last_sent_handshake_packet (#1171)
o Added signal from TLS the data it includes needs to be sent in a o Added signal from TLS the data it includes needs to be sent in a
Retry packet (#1061, #1199) Retry packet (#1061, #1199)
o Minimum RTT (min_rtt) is initialized with an infinite value o Minimum RTT (min_rtt) is initialized with an infinite value
(#1169) (#1169)
A.6. Since draft-ietf-quic-recovery-09 A.7. Since draft-ietf-quic-recovery-09
No significant changes. No significant changes.
A.7. Since draft-ietf-quic-recovery-08 A.8. Since draft-ietf-quic-recovery-08
o Clarified pacing and RTO (#967, #977) o Clarified pacing and RTO (#967, #977)
A.8. Since draft-ietf-quic-recovery-07 A.9. Since draft-ietf-quic-recovery-07
o Include Ack Delay in RTO(and TLP) computations (#981) o Include Ack Delay in RTO(and TLP) computations (#981)
o Ack Delay in SRTT computation (#961) o Ack Delay in SRTT computation (#961)
o Default RTT and Slow Start (#590) o Default RTT and Slow Start (#590)
o Many editorial fixes. o Many editorial fixes.
A.9. Since draft-ietf-quic-recovery-06 A.10. Since draft-ietf-quic-recovery-06
No significant changes. No significant changes.
A.10. Since draft-ietf-quic-recovery-05 A.11. Since draft-ietf-quic-recovery-05
o Add more congestion control text (#776) o Add more congestion control text (#776)
A.11. Since draft-ietf-quic-recovery-04 A.12. Since draft-ietf-quic-recovery-04
No significant changes. No significant changes.
A.12. Since draft-ietf-quic-recovery-03 A.13. Since draft-ietf-quic-recovery-03
No significant changes. No significant changes.
A.13. Since draft-ietf-quic-recovery-02 A.14. Since draft-ietf-quic-recovery-02
o Integrate F-RTO (#544, #409) o Integrate F-RTO (#544, #409)
o Add congestion control (#545, #395) o Add congestion control (#545, #395)
o Require connection abort if a skipped packet was acknowledged o Require connection abort if a skipped packet was acknowledged
(#415) (#415)
o Simplify RTO calculations (#142, #417) o Simplify RTO calculations (#142, #417)
A.14. Since draft-ietf-quic-recovery-01 A.15. Since draft-ietf-quic-recovery-01
o Overview added to loss detection o Overview added to loss detection
o Changes initial default RTT to 100ms o Changes initial default RTT to 100ms
o Added time-based loss detection and fixes early retransmit o Added time-based loss detection and fixes early retransmit
o Clarified loss recovery for handshake packets o Clarified loss recovery for handshake packets
o Fixed references and made TCP references informative o Fixed references and made TCP references informative
A.15. Since draft-ietf-quic-recovery-00 A.16. Since draft-ietf-quic-recovery-00
o Improved description of constants and ACK behavior o Improved description of constants and ACK behavior
A.16. Since draft-iyengar-quic-loss-recovery-01 A.17. 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. 190 change blocks. 
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