draft-ietf-quic-recovery-13.txt   draft-ietf-quic-recovery-14.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: December 30, 2018 Google Expires: February 16, 2019 Google
June 28, 2018 August 15, 2018
QUIC Loss Detection and Congestion Control QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-13 draft-ietf-quic-recovery-14
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
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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 December 30, 2018. This Internet-Draft will expire on February 16, 2019.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
2. Design of the QUIC Transmission Machinery . . . . . . . . . . 4 3. Design of the QUIC Transmission Machinery . . . . . . . . . . 4
2.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5 3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5
2.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 5 3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 5
2.1.2. Monotonically Increasing Packet Numbers . . . . . . . 5 3.1.2. Monotonically Increasing Packet Numbers . . . . . . . 6
2.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6 3.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6
2.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6 3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6
2.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 6 3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 6
3. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 6 4. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Computing the RTT estimate . . . . . . . . . . . . . . . 6 4.1. Computing the RTT estimate . . . . . . . . . . . . . . . 7
3.2. Ack-based Detection . . . . . . . . . . . . . . . . . . . 7 4.1.1. Maximum Ack Delay . . . . . . . . . . . . . . . . . . 7
3.2.1. Fast Retransmit . . . . . . . . . . . . . . . . . . . 7 4.2. Ack-based Detection . . . . . . . . . . . . . . . . . . . 7
3.2.2. Early Retransmit . . . . . . . . . . . . . . . . . . 7 4.2.1. Fast Retransmit . . . . . . . . . . . . . . . . . . . 8
3.3. Timer-based Detection . . . . . . . . . . . . . . . . . . 8 4.2.2. Early Retransmit . . . . . . . . . . . . . . . . . . 8
3.3.1. Crypto Handshake Timeout . . . . . . . . . . . . . . 8 4.3. Timer-based Detection . . . . . . . . . . . . . . . . . . 9
3.3.2. Tail Loss Probe . . . . . . . . . . . . . . . . . . . 9 4.3.1. Crypto Handshake Timeout . . . . . . . . . . . . . . 9
3.3.3. Retransmission Timeout . . . . . . . . . . . . . . . 10 4.3.2. Tail Loss Probe . . . . . . . . . . . . . . . . . . . 10
3.4. Generating Acknowledgements . . . . . . . . . . . . . . . 12 4.3.3. Retransmission Timeout . . . . . . . . . . . . . . . 11
3.4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . 12 4.4. Generating Acknowledgements . . . . . . . . . . . . . . . 12
3.4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . 12 4.4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . 13
3.4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . 13 4.4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . 13
3.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 13 4.4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . 13
3.5.1. Constants of interest . . . . . . . . . . . . . . . . 13 4.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 14
3.5.2. Variables of interest . . . . . . . . . . . . . . . . 14 4.5.1. Constants of interest . . . . . . . . . . . . . . . . 14
3.5.3. Initialization . . . . . . . . . . . . . . . . . . . 15 4.5.2. Variables of interest . . . . . . . . . . . . . . . . 14
3.5.4. On Sending a Packet . . . . . . . . . . . . . . . . . 16 4.5.3. Initialization . . . . . . . . . . . . . . . . . . . 16
3.5.5. On Receiving an Acknowledgment . . . . . . . . . . . 17 4.5.4. On Sending a Packet . . . . . . . . . . . . . . . . . 16
3.5.6. On Packet Acknowledgment . . . . . . . . . . . . . . 18 4.5.5. On Receiving an Acknowledgment . . . . . . . . . . . 17
3.5.7. Setting the Loss Detection Alarm . . . . . . . . . . 19 4.5.6. On Packet Acknowledgment . . . . . . . . . . . . . . 18
3.5.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . 21 4.5.7. Setting the Loss Detection Timer . . . . . . . . . . 19
3.5.9. Detecting Lost Packets . . . . . . . . . . . . . . . 22 4.5.8. On Timeout . . . . . . . . . . . . . . . . . . . . . 21
3.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . 23 4.5.9. Detecting Lost Packets . . . . . . . . . . . . . . . 22
4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 23 4.6. Discussion . . . . . . . . . . . . . . . . . . . . . . . 23
4.1. Explicit Congestion Notification . . . . . . . . . . . . 24 5. Congestion Control . . . . . . . . . . . . . . . . . . . . . 23
4.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 24 5.1. Explicit Congestion Notification . . . . . . . . . . . . 24
4.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 24 5.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 24
4.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 24 5.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 24
4.5. Tail Loss Probe . . . . . . . . . . . . . . . . . . . . . 25 5.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 24
4.6. Retransmission Timeout . . . . . . . . . . . . . . . . . 25 5.5. Tail Loss Probe . . . . . . . . . . . . . . . . . . . . . 25
4.7. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.6. Retransmission Timeout . . . . . . . . . . . . . . . . . 25
4.8. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 26 5.7. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.8.1. Constants of interest . . . . . . . . . . . . . . . . 26 5.8. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 26
4.8.2. Variables of interest . . . . . . . . . . . . . . . . 26 5.8.1. Constants of interest . . . . . . . . . . . . . . . . 26
4.8.3. Initialization . . . . . . . . . . . . . . . . . . . 27 5.8.2. Variables of interest . . . . . . . . . . . . . . . . 26
4.8.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 27 5.8.3. Initialization . . . . . . . . . . . . . . . . . . . 27
4.8.5. On Packet Acknowledgement . . . . . . . . . . . . . . 27 5.8.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 27
4.8.6. On New Congestion Event . . . . . . . . . . . . . . . 27 5.8.5. On Packet Acknowledgement . . . . . . . . . . . . . . 27
4.8.7. Process ECN Information . . . . . . . . . . . . . . . 28 5.8.6. On New Congestion Event . . . . . . . . . . . . . . . 28
4.8.8. On Packets Lost . . . . . . . . . . . . . . . . . . . 28 5.8.7. Process ECN Information . . . . . . . . . . . . . . . 28
4.8.9. On Retransmission Timeout Verified . . . . . . . . . 28 5.8.8. On Packets Lost . . . . . . . . . . . . . . . . . . . 28
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 5.8.9. On Retransmission Timeout Verified . . . . . . . . . 28
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
6.1. Normative References . . . . . . . . . . . . . . . . . . 29 6.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 29
6.2. Informative References . . . . . . . . . . . . . . . . . 29 6.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 29
6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 29
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 30 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
A.1. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 30 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
A.2. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 31 8.1. Normative References . . . . . . . . . . . . . . . . . . 30
A.3. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 31 8.2. Informative References . . . . . . . . . . . . . . . . . 30
A.4. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 31 8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 31
A.5. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 31 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 31
A.6. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 31 A.1. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 32
A.7. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 31 A.2. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 32
A.8. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 31 A.3. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 32
A.9. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 32 A.4. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 32
A.10. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 32 A.5. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 32
A.11. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 32 A.6. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 33
A.12. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 32 A.7. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 33
A.13. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 32 A.8. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 33
A.14. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 32 A.9. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 33
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 32 A.10. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 A.11. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 33
A.12. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 33
A.13. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 33
A.14. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 34
A.15. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 34
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
in the Linux TCP implementation. This document describes QUIC in the Linux TCP implementation. This document describes QUIC
congestion control and loss recovery, and where applicable, congestion control and loss recovery, and where applicable,
attributes the TCP equivalent in RFCs, Internet-drafts, academic attributes the TCP equivalent in RFCs, Internet-drafts, academic
papers, and/or TCP implementations. papers, and/or TCP implementations.
1.1. Notational Conventions 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.
2. Design of the QUIC Transmission Machinery Definitions of terms that are used in this document:
ACK frames: ACK frames refer to both ACK and ACK_ECN frames in this
document.
ACK-only: Any packet containing only an ACK or ACK_ECN frame.
In-flight: Packets are considered in-flight when they have been sent
and neither acknowledged nor declared lost, and they are not ACK-
only.
Retransmittable Frames: All frames besides ACK, ACK_ECN, or PADDING
are considered retransmittable.
Retransmittable Packets: Packets that contain retransmittable frames
elicit an ACK from the receiver and are called retransmittable
packets.
3. Design of the QUIC Transmission Machinery
All transmissions in QUIC are sent with a packet-level header, which All transmissions in QUIC are sent with a packet-level header, which
indicates the encryption level and includes a packet sequence number indicates the encryption level and includes a packet sequence number
(referred to below as a packet number). The encryption level (referred to below as a packet number). The encryption level
indicates the packet number space, as described in [QUIC-TRANSPORT]. indicates the packet number space, as described in [QUIC-TRANSPORT].
Packet numbers never repeat within a packet number space for the Packet numbers never repeat within a packet number space for the
lifetime of a connection. Packet numbers monotonically increase lifetime of a connection. Packet numbers monotonically increase
within a space, preventing ambiguity. within a space, preventing ambiguity.
This design obviates the need for disambiguating between This design obviates the need for disambiguating between
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.
Every packet may contain several frames. We outline the frames that QUIC packets can contain multiple frames of different types. The
are important to the loss detection and congestion control machinery recovery mechanisms ensure that data and frames that need reliable
below. delivery are acknowledged or declared lost and sent in new packets as
necessary. The types of frames contained in a packet affect recovery
o Retransmittable frames are those that count towards bytes in and congestion control logic:
flight and need acknowledgement. The most common are STREAM
frames, which typically contain application data.
o Retransmittable packets are those that contain at least one o All packets are acknowledged, though packets that contain only
retransmittable frame. ACK, ACK_ECN, and PADDING frames are not acknowledged immediately.
o Cryptographic handshake data is sent in CRYPTO frames, and uses o Long header packets that contain CRYPTO frames are critical to the
the reliability machinery of QUIC underneath. performance of the QUIC handshake and use shorter timers for
acknowledgement and retransmission.
o ACK and ACK_ECN frames contain acknowledgment information. o Packets that contain only ACK and ACK_ECN frames do not count
ACK_ECN frames additionally contain information about ECN toward congestion control limits and are not considered in-flight.
codepoints seen by the peer. (The rest of this document uses ACK Note that this means PADDING frames cause packets to contribute
frames to refer to both ACK and ACK_ECN frames.) toward bytes in flight without directly causing an acknowledgment
to be sent.
2.1. Relevant Differences Between QUIC and TCP 3.1. Relevant Differences Between QUIC and TCP
Readers familiar with TCP's loss detection and congestion control Readers familiar with TCP's loss detection and congestion control
will find algorithms here that parallel well-known TCP ones. will find algorithms here that parallel well-known TCP ones.
Protocol differences between QUIC and TCP however contribute to Protocol differences between QUIC and TCP however contribute to
algorithmic differences. We briefly describe these protocol algorithmic differences. We briefly describe these protocol
differences below. differences below.
2.1.1. Separate Packet Number Spaces 3.1.1. Separate Packet Number Spaces
QUIC uses separate packet number spaces for each encryption level, QUIC uses separate packet number spaces for each encryption level,
except 0-RTT and all generations of 1-RTT keys use the same packet except 0-RTT and all generations of 1-RTT keys use the same packet
number space. Separate packet number spaces ensures acknowledgement number space. Separate packet number spaces ensures acknowledgement
of packets sent with one level of encryption will not cause spurious of packets sent with one level of encryption will not cause spurious
retransmission of packets sent with a different encryption level. retransmission of packets sent with a different encryption level.
Congestion control and RTT measurement are unified across packet Congestion control and RTT measurement are unified across packet
number spaces. number spaces.
2.1.2. Monotonically Increasing Packet Numbers 3.1.2. Monotonically Increasing Packet Numbers
TCP conflates transmission sequence number at the sender with TCP conflates transmission sequence number at the sender with
delivery sequence number at the receiver, which results in delivery sequence number at the receiver, which results in
retransmissions of the same data carrying the same sequence number, retransmissions of the same data carrying the same sequence number,
and consequently to problems caused by "retransmission ambiguity". and consequently to problems caused by "retransmission ambiguity".
QUIC separates the two: QUIC uses a packet number for transmissions, QUIC separates the two: QUIC uses a packet number for transmissions,
and any data that is to be delivered to the receiving application(s) and any application data is sent in one or more streams, with
is sent in one or more streams, with delivery order determined by delivery order determined by stream offsets encoded within STREAM
stream offsets encoded within STREAM frames. frames.
QUIC's packet number is strictly increasing, and directly encodes QUIC's packet number is strictly increasing, and directly encodes
transmission order. A higher QUIC packet number signifies that the transmission order. A higher QUIC packet number signifies that the
packet was sent later, and a lower QUIC packet number signifies that packet was sent later, and a lower QUIC packet number signifies that
the packet was sent earlier. When a packet containing frames is the packet was sent earlier. When a packet containing frames is
deemed lost, QUIC rebundles necessary frames in a new packet with a deemed lost, QUIC rebundles necessary frames in a new packet with a
new packet number, removing ambiguity about which packet is new packet number, removing ambiguity about which packet is
acknowledged when an ACK is received. Consequently, more accurate acknowledged when an ACK is received. Consequently, more accurate
RTT measurements can be made, spurious retransmissions are trivially RTT measurements can be made, spurious retransmissions are trivially
detected, and mechanisms such as Fast Retransmit can be applied detected, and mechanisms such as Fast Retransmit can be applied
universally, based only on packet number. 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.
2.1.3. 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
implementations on both sides and reducing memory pressure on the implementations on both sides and reducing memory pressure on the
sender. sender.
2.1.4. More ACK Ranges 3.1.4. More ACK Ranges
QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In
high loss environments, this speeds recovery, reduces spurious high loss environments, this speeds recovery, reduces spurious
retransmits, and ensures forward progress without relying on retransmits, and ensures forward progress without relying on
timeouts. timeouts.
2.1.5. Explicit Correction For Delayed ACKs 3.1.5. Explicit Correction For Delayed ACKs
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.
3. Loss Detection 4. Loss Detection
QUIC senders use both ack information and timeouts to detect lost QUIC senders use both ack information and timeouts to detect lost
packets, and this section provides a description of these algorithms. packets, and this section provides a description of these algorithms.
Estimating the network round-trip time (RTT) is critical to these Estimating the network round-trip time (RTT) is critical to these
algorithms and is described first. algorithms and is described first.
3.1. Computing the RTT estimate 4.1. 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 newly
acked packet was sent. If no packets are newly acknowledged, RTT acked packet was sent. If no packets are newly acknowledged, RTT
cannot be calculated. When RTT is calculated, the ack delay field cannot be calculated. When RTT is calculated, the ack delay field
from the ACK frame SHOULD be subtracted from the RTT as long as the 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 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 min_rtt, the RTT should be used, but the ack delay field should be
ignored. 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.
3.2. Ack-based Detection 4.1.1. Maximum Ack Delay
QUIC is able to explicitly model delay at the receiver via the ack
delay field in the ACK frame. Therefore, QUIC diverges from TCP by
calculating a MaxAckDelay dynamically, instead of assuming a constant
delayed ack timeout for all connections.
MaxAckDelay is the maximum ack delay supplied in an all incoming ACK
frames. MaxAckDelay excludes ack delays that aren't included in an
RTT sample because they're too large or the largest acknowledged has
already been acknowledged. MaxAckDelay also excludes ack delays
where the largest acknowledged references an ACK-only packet.
4.2. Ack-based Detection
Ack-based loss detection implements the spirit of TCP's Fast Ack-based loss detection implements the spirit of TCP's Fast
Retransmit [RFC5681], Early Retransmit [RFC5827], FACK, and SACK loss Retransmit [RFC5681], Early Retransmit [RFC5827], FACK, and SACK loss
recovery [RFC6675]. This section provides an overview of how these recovery [RFC6675]. This section provides an overview of how these
algorithms are implemented in QUIC. algorithms are implemented in QUIC.
3.2.1. Fast Retransmit 4.2.1. Fast Retransmit
An unacknowledged packet is marked as lost when an acknowledgment is An unacknowledged packet is marked as lost when an acknowledgment is
received for a packet that was sent a threshold number of packets received for a packet that was sent a threshold number of packets
(kReorderingThreshold) after the unacknowledged packet. Receipt of (kReorderingThreshold) and/or a threshold amount of time after the
the ack indicates that a later packet was received, while unacknowledged packet. Receipt of the acknowledgement indicates that
kReorderingThreshold provides some tolerance for reordering of a later packet was received, while the reordering threshold provides
packets in the network. some tolerance for reordering of packets in the network.
The RECOMMENDED initial value for kReorderingThreshold is 3. The RECOMMENDED initial value for kReorderingThreshold is 3, based on
TCP loss recovery [RFC5681] [RFC6675]. Some networks may exhibit
higher degrees of reordering, causing a sender to detect spurious
losses. Spuriously declaring packets lost leads to unnecessary
retransmissions and may result in degraded performance due to the
actions of the congestion controller upon detecting loss.
Implementers MAY use algorithms developed for TCP, such as TCP-NCR
[RFC4653], to improve QUIC's reordering resilience.
We derive this recommendation from TCP loss recovery [RFC5681] QUIC implementations can use time-based loss detection to handle
[RFC6675]. It is possible for networks to exhibit higher degrees of reordering based on time elapsed since the packet was sent. This may
reordering, causing a sender to detect spurious losses. Detecting be used either as a replacement for a packet reordering threshold or
spurious losses leads to unnecessary retransmissions and may result in addition to it. The RECOMMENDED time threshold, expressed as a
in degraded performance due to the actions of the congestion fraction of the round-trip time (kTimeReorderingFraction), is 1/8.
controller upon detecting loss. Implementers MAY use algorithms
developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's
reordering resilience, though care should be taken to map TCP
specifics to QUIC correctly. Similarly, using time-based loss
detection to deal with reordering, such as in PR-TCP, should be more
readily usable in QUIC. Making QUIC deal with such networks is
important open research, and implementers are encouraged to explore
this space.
3.2.2. Early Retransmit 4.2.2. Early Retransmit
Unacknowledged packets close to the tail may have fewer than Unacknowledged packets close to the tail may have fewer than
kReorderingThreshold retransmittable packets sent after them. Loss kReorderingThreshold retransmittable packets sent after them. Loss
of such packets cannot be detected via Fast Retransmit. To enable of such packets cannot be detected via Fast Retransmit. To enable
ack-based loss detection of such packets, receipt of an ack-based loss detection of such packets, receipt of an
acknowledgment for the last outstanding retransmittable packet acknowledgment for the last outstanding retransmittable packet
triggers the Early Retransmit process, as follows. triggers the Early Retransmit process, as follows.
If there are unacknowledged retransmittable packets still pending, If there are unacknowledged in-flight packets still pending, they
they should be marked as lost. To compensate for the reduced should be marked as lost. To compensate for the reduced reordering
reordering resilience, the sender SHOULD set an alarm for a small resilience, the sender SHOULD set a timer for a small period of time.
period of time. If the unacknowledged retransmittable packets are If the unacknowledged in-flight packets are not acknowledged during
not acknowledged during this time, then these packets MUST be marked this time, then these packets MUST be marked as lost.
as lost.
An endpoint SHOULD set the alarm such that a packet is marked as lost An endpoint SHOULD set the timer such that a packet is marked as lost
no earlier than 1.25 * max(SRTT, latest_RTT) since when it was sent. no earlier than 1.125 * max(SRTT, latest_RTT) since when it was sent.
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 Retransit
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 1.25 multiplier increases reordering resilience. Implementers The 1.125 multiplier increases reordering resilience. Implementers
MAY experiment with using other multipliers, bearing in mind that a MAY experiment with using other multipliers, bearing in mind that a
lower multiplier reduces reordering resilience and increases spurious lower multiplier reduces reordering resilience and increases spurious
retransmissions, and a higher multipler increases loss recovery retransmissions, and a higher multipler increases loss recovery
delay. delay.
This mechanism is based on Early Retransmit for TCP [RFC5827]. This mechanism is based on Early Retransmit for TCP [RFC5827].
However, [RFC5827] does not include the alarm described above. Early However, [RFC5827] does not include the timer described above. Early
Retransmit is prone to spurious retransmissions due to its reduced Retransmit is prone to spurious retransmissions due to its reduced
reordering resilence without the alarm. This observation led Linux reordering resilence without the timer. This observation led Linux
TCP implementers to implement an alarm for TCP as well, and this TCP implementers to implement a timer for TCP as well, and this
document incorporates this advancement. document incorporates this advancement.
3.3. Timer-based Detection 4.3. Timer-based Detection
Timer-based loss detection implements a handshake retransmission Timer-based loss detection recovers from losses that cannot be
timer that is optimized for QUIC as well as the spirit of TCP's Tail handled by ack-based loss detection. It uses a single timer which
Loss Probe and Retransmission Timeout mechanisms. switches between a handshake retransmission timer, a Tail Loss Probe
timer and Retransmission Timeout mechanisms.
3.3.1. Crypto Handshake Timeout 4.3.1. Crypto Handshake 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.
Below, the term "handshake packet" is used to refer to packets Below, the term "handshake packet" is used to refer to packets
containing CRYPTO frames, not packets with the specific long header containing CRYPTO frames, not packets with the specific long header
packet type Handshake. packet type Handshake.
The initial handshake timeout SHOULD be set to twice the initial RTT. The initial handshake timeout SHOULD be set to twice the 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. initial RTT.
If no previous RTT is available, or if the network changes, the If no previous RTT is available, or if the network changes, the
initial RTT SHOULD be set to 100ms. initial RTT SHOULD be set to 100ms.
When CRYPTO frames are sent, the sender SHOULD set an alarm for the When CRYPTO frames are sent, the sender SHOULD set a timer for the
handshake timeout period. When the alarm fires, the sender MUST handshake timeout period. Upon timeout, the sender MUST retransmit
retransmit all unacknowledged CRYPTO data by calling all unacknowledged CRYPTO data by calling
RetransmitAllUnackedHandshakeData(). On each consecutive firing of RetransmitAllUnackedHandshakeData(). On each consecutive expiration
the handshake alarm without receiving an acknowledgement for a new of the handshake timer without receiving an acknowledgement for a new
packet, the sender SHOULD double the handshake timeout and set an packet, the sender SHOULD double the handshake timeout and set a
alarm for this period. timer for this period.
When CRYPTO frames are outstanding, the TLP and RTO timers are not When CRYPTO frames are outstanding, the TLP and RTO timers are not
active unless the CRYPTO frames were sent at 1RTT encryption. active unless the CRYPTO frames were sent at 1RTT encryption.
When an acknowledgement is received for a handshake packet, the new When an acknowledgement is received for a handshake packet, the new
RTT is computed and the alarm SHOULD be set for twice the newly RTT is computed and the timer SHOULD be set for twice the newly
computed smoothed RTT. computed smoothed RTT.
3.3.1.1. Retry 4.3.1.1. Retry and Version Negotiation
A Retry packet causes the content of the client's Initial packet to A Retry or Version Negotiation packet causes a client to send another
be immediately retransmitted along with the token present in the Initial packet, effectively restarting the connection process.
Retry.
The Retry indicates that the Initial was received but not processed. Either packet indicates that the Initial was received but not
It MUST NOT be treated as an acknowledgment for the Initial, but it processed. Neither packet can be treated as an acknowledgment for
MAY be used for an RTT measurement. the Initial, but they MAY be used to improve the RTT estimate.
3.3.2. Tail Loss Probe 4.3.2. Tail Loss Probe
The algorithm described in this section is an adaptation of the Tail The algorithm described in this section is an adaptation of the Tail
Loss Probe algorithm proposed for TCP [TLP]. Loss Probe algorithm proposed for TCP [TLP].
A packet sent at the tail is particularly vulnerable to slow loss A packet sent at the tail is particularly vulnerable to slow loss
detection, since acks of subsequent packets are needed to trigger detection, since acks of subsequent packets are needed to trigger
ack-based detection. To ameliorate this weakness of tail packets, ack-based detection. To ameliorate this weakness of tail packets,
the sender schedules an alarm when the last retransmittable packet the sender schedules a timer when the last retransmittable packet
before quiescence is transmitted. When this alarm fires, a Tail Loss before quiescence is transmitted. Upon timeout, a Tail Loss Probe
Probe (TLP) packet is sent to evoke an acknowledgement from the (TLP) packet is sent to evoke an acknowledgement from the receiver.
receiver.
The alarm duration, or Probe Timeout (PTO), is set based on the The timer duration, or Probe Timeout (PTO), is set based on the
following conditions: following conditions:
o PTO SHOULD be scheduled for max(1.5*SRTT+MaxAckDelay, o PTO SHOULD be scheduled for max(1.5*SRTT+MaxAckDelay,
kMinTLPTimeout) kMinTLPTimeout)
o If RTO (Section 3.3.3) is earlier, schedule a TLP alarm in its o If RTO (Section 4.3.3) is earlier, schedule a TLP in its place.
place. That is, PTO SHOULD be scheduled for min(RTO, PTO). That is, PTO SHOULD be scheduled for min(RTO, PTO).
MaxAckDelay is the maximum ack delay supplied in an incoming ACK
frame. MaxAckDelay excludes ack delays that aren't included in an
RTT sample because they're too large and excludes those which
reference an ack-only packet.
QUIC diverges from TCP by calculating MaxAckDelay dynamically, QUIC includes MaxAckDelay in all probe timeouts, because it assumes
instead of assuming a constant delayed ack timeout for all the ack delay may come into play, regardless of the number of packets
connections. QUIC includes this in all probe timeouts, because it outstanding. TCP's TLP assumes if at least 2 packets are
assume 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. outstanding, acks will not be delayed.
A PTO value of at least 1.5*SRTT ensures that the ACK is overdue. A PTO value of at least 1.5*SRTT ensures that the ACK is overdue.
The 1.5 is based on [TLP], but implementations MAY experiment with The 1.5 is based on [TLP], but implementations MAY experiment with
other constants. other constants.
To reduce latency, it is RECOMMENDED that the sender set and allow To reduce latency, it is RECOMMENDED that the sender set and allow
the TLP alarm to fire twice before setting an RTO alarm. In other the TLP timer to fire twice before setting an RTO timer. In other
words, when the TLP alarm fires the first time, a TLP packet is sent, words, when the TLP timer expires the first time, a TLP packet is
and it is RECOMMENDED that the TLP alarm be scheduled for a second sent, and it is RECOMMENDED that the TLP timer be scheduled for a
time. When the TLP alarm fires the second time, a second TLP packet second time. When the TLP timer expires the second time, a second
is sent, and an RTO alarm SHOULD be scheduled Section 3.3.3. 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 A TLP packet SHOULD carry new data when possible. If new data is
unavailable or new data cannot be sent due to flow control, a TLP unavailable or new data cannot be sent due to flow control, a TLP
packet MAY retransmit unacknowledged data to potentially reduce packet MAY retransmit unacknowledged data to potentially reduce
recovery time. Since a TLP alarm is used to send a probe into the recovery time. Since a TLP timer is used to send a probe into the
network prior to establishing any packet loss, prior unacknowledged network prior to establishing any packet loss, prior unacknowledged
packets SHOULD NOT be marked as lost when a TLP alarm fires. 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 sender may not know that a packet being sent is a tail packet.
Consequently, a sender may have to arm or adjust the TLP alarm on Consequently, a sender may have to arm or adjust the TLP timer on
every sent retransmittable packet. every sent retransmittable packet.
3.3.3. Retransmission Timeout 4.3.3. Retransmission Timeout
A Retransmission Timeout (RTO) alarm is the final backstop for loss A Retransmission Timeout (RTO) timer is the final backstop for loss
detection. The algorithm used in QUIC is based on the RTO algorithm detection. The algorithm used in QUIC is based on the RTO algorithm
for TCP [RFC5681] and is additionally resilient to spurious RTO for TCP [RFC5681] and is additionally resilient to spurious RTO
events [RFC5682]. events [RFC5682].
When the last TLP packet is sent, an alarm is scheduled for the RTO When the last TLP packet is sent, a timer is set for the RTO period.
period. When this alarm fires, the sender sends two packets, to When this timer expires, the sender sends two packets, to evoke
evoke acknowledgements from the receiver, and restarts the RTO alarm. acknowledgements from the receiver, and restarts the RTO timer.
Similar to TCP [RFC6298], the RTO period is set based on the Similar to TCP [RFC6298], the RTO period is set based on the
following conditions: following conditions:
o When the final TLP packet is sent, the RTO period is set to o When the final TLP packet is sent, the RTO period is set to
max(SRTT + 4*RTTVAR + MaxAckDelay, kMinRTOTimeout) max(SRTT + 4*RTTVAR + MaxAckDelay, kMinRTOTimeout)
o When an RTO alarm fires, the RTO period is doubled. o When an RTO timer expires, the RTO period is doubled.
The sender typically has incurred a high latency penalty by the time The sender typically has incurred a high latency penalty by the time
an RTO alarm fires, and this penalty increases exponentially in an RTO timer expires, and this penalty increases exponentially in
subsequent consecutive RTO events. Sending a single packet on an RTO subsequent consecutive RTO events. Sending a single packet on an RTO
event therefore makes the connection very sensitive to single packet event therefore makes the connection very sensitive to single packet
loss. Sending two packets instead of one significantly increases loss. Sending two packets instead of one significantly increases
resilience to packet drop in both directions, thus reducing the resilience to packet drop in both directions, thus reducing the
probability of consecutive RTO events. probability of consecutive RTO events.
QUIC's RTO algorithm differs from TCP in that the firing of an RTO QUIC's RTO algorithm differs from TCP in that the firing of an RTO
alarm is not considered a strong enough signal of packet loss, so timer is not considered a strong enough signal of packet loss, so
does not result in an immediate change to congestion window or does not result in an immediate change to congestion window or
recovery state. An RTO alarm fires only when there's a prolonged recovery state. An RTO timer expires only when there's a prolonged
period of network silence, which could be caused by a change in the period of network silence, which could be caused by a change in the
underlying network RTT. underlying network RTT.
QUIC also diverges from TCP by including MaxAckDelay in the RTO QUIC also diverges from TCP by including MaxAckDelay in the RTO
period. QUIC is able to explicitly model delay at the receiver via period. Since QUIC corrects for this delay in its SRTT and RTTVAR
the ack delay field in the ACK frame. Since QUIC corrects for this computations, it is necessary to add this delay explicitly in the TLP
delay in its SRTT and RTTVAR computations, it is necessary to add and RTO computation.
this delay explicitly in the TLP and RTO computation.
When an acknowledgment is received for a packet sent on an RTO event, When an acknowledgment is received for a packet sent on an RTO event,
any unacknowledged packets with lower packet numbers than those any unacknowledged packets with lower packet numbers than those
acknowledged MUST be marked as lost. acknowledged MUST be marked as lost.
A packet sent when an RTO alarm fires MAY carry new data if available A packet sent when an RTO timer expires MAY carry new data if
or unacknowledged data to potentially reduce recovery time. Since available or unacknowledged data to potentially reduce recovery time.
this packet is sent as a probe into the network prior to establishing Since this packet is sent as a probe into the network prior to
any packet loss, prior unacknowledged packets SHOULD NOT be marked as establishing any packet loss, prior unacknowledged packets SHOULD NOT
lost. be marked as lost.
A packet sent on an RTO alarm MUST NOT be blocked by the sender's A packet sent on an RTO timer MUST NOT be blocked by the sender's
congestion controller. A sender MUST however count these bytes as congestion controller. A sender MUST however count these bytes as
additional bytes in flight, since this packet adds network load additional bytes in flight, since this packet adds network load
without establishing packet loss. without establishing packet loss.
3.4. Generating Acknowledgements 4.4. Generating Acknowledgements
QUIC SHOULD delay sending acknowledgements in response to packets, QUIC SHOULD delay sending acknowledgements in response to packets,
but MUST NOT excessively delay acknowledgements of packets containing but MUST NOT excessively delay acknowledgements of packets containing
frames other than ACK or ACN_ECN. Specifically, implementaions MUST frames other than ACK or ACN_ECN. Specifically, implementaions MUST
attempt to enforce a maximum ack delay to avoid causing the peer attempt to enforce a maximum ack delay to avoid causing the peer
spurious timeouts. The RECOMMENDED maximum ack delay in QUIC is spurious timeouts. The RECOMMENDED maximum ack delay in QUIC is
25ms. 25ms.
An acknowledgement MAY be sent for every second full-sized packet, as An acknowledgement MAY be sent for every second full-sized packet, as
TCP does [RFC5681], or may be sent less frequently, as long as the TCP does [RFC5681], or may be sent less frequently, as long as the
skipping to change at page 12, line 34 skipping to change at page 13, line 16
Similarly, packets marked with the ECN Congestion Experienced (CE) Similarly, packets marked with the ECN Congestion Experienced (CE)
codepoint in the IP header SHOULD be acknowledged immediately, to codepoint in the IP header SHOULD be acknowledged immediately, to
reduce the peer's response time to congestion events. reduce the peer's response time to congestion events.
As an optimization, a receiver MAY process multiple packets before As an optimization, a receiver MAY process multiple packets before
sending any ACK frames in response. In this case they can determine sending any ACK frames in response. In this case they can determine
whether an immediate or delayed acknowledgement should be generated whether an immediate or delayed acknowledgement should be generated
after processing incoming packets. after processing incoming packets.
3.4.1. Crypto Handshake Data 4.4.1. Crypto Handshake Data
In order to quickly complete the handshake and avoid spurious In order to quickly complete the handshake and avoid spurious
retransmissions due to handshake alarm timeouts, handshake packets retransmissions due to handshake timeouts, handshake packets SHOULD
SHOULD use a very short ack delay, such as 1ms. ACK frames MAY be use a very short ack delay, such as 1ms. ACK frames MAY be sent
sent immediately when the crypto stack indicates all data for that immediately when the crypto stack indicates all data for that
encryption level has been received. encryption level has been received.
3.4.2. ACK Ranges 4.4.2. ACK Ranges
When an ACK frame is sent, one or more ranges of acknowledged packets When an ACK frame is sent, one or more ranges of acknowledged packets
are included. Including older packets reduces the chance of spurious are included. Including older packets reduces the chance of spurious
retransmits caused by losing previously sent ACK frames, at the cost retransmits caused by losing previously sent ACK frames, at the cost
of larger ACK frames. of larger ACK frames.
ACK frames SHOULD always acknowledge the most recently received ACK frames SHOULD always acknowledge the most recently received
packets, and the more out-of-order the packets are, the more packets, and the more out-of-order the packets are, the more
important it is to send an updated ACK frame quickly, to prevent the important it is to send an updated ACK frame quickly, to prevent the
peer from declaring a packet as lost and spuriusly retransmitting the peer from declaring a packet as lost and spuriusly retransmitting the
frames it contains. frames it contains.
Below is one recommended approach for determining what packets to Below is one recommended approach for determining what packets to
include in an ACK frame. include in an ACK frame.
3.4.3. Receiver Tracking of ACK Frames 4.4.3. Receiver Tracking of ACK Frames
When a packet containing an ACK frame is sent, the largest When a packet containing an ACK frame is sent, the largest
acknowledged in that frame may be saved. When a packet containing an acknowledged in that frame may be saved. When a packet containing an
ACK frame is acknowledged, the receiver can stop acknowledging ACK frame is acknowledged, the receiver can stop acknowledging
packets less than or equal to the largest acknowledged in the sent packets less than or equal to the largest acknowledged in the sent
ACK frame. ACK frame.
In cases without ACK frame loss, this algorithm allows for a minimum In cases without ACK frame loss, this algorithm allows for a minimum
of 1 RTT of reordering. In cases with ACK frame loss, this approach of 1 RTT of reordering. In cases with ACK frame loss, this approach
does not guarantee that every acknowledgement is seen by the sender does not guarantee that every acknowledgement is seen by the sender
before it is no longer included in the ACK frame. Packets could be before it is no longer included in the ACK frame. Packets could be
received out of order and all subsequent ACK frames containing them received out of order and all subsequent ACK frames containing them
could be lost. In this case, the loss recovery algorithm may cause could be lost. In this case, the loss recovery algorithm may cause
spurious retransmits, but the sender will continue making forward spurious retransmits, but the sender will continue making forward
progress. progress.
3.5. Pseudocode 4.5. Pseudocode
3.5.1. Constants of interest 4.5.1. 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 (RECOMMENDED 2): Maximum number of tail loss probes before kMaxTLPs (RECOMMENDED 2): Maximum number of tail loss probes before
an RTO fires. an RTO expires.
kReorderingThreshold (RECOMMENDED 3): Maximum reordering in packet kReorderingThreshold (RECOMMENDED 3): Maximum reordering in packet
number space before FACK style loss detection considers a packet number space before FACK style loss detection considers a packet
lost. lost.
kTimeReorderingFraction (RECOMMENDED 1/8): Maximum reordering in kTimeReorderingFraction (RECOMMENDED 1/8): Maximum reordering in
time space before time based loss detection considers a packet time space before time based loss detection considers a packet
lost. In fraction of an RTT. lost. In fraction of an RTT.
kUsingTimeLossDetection (RECOMMENDED false): Whether time based loss kUsingTimeLossDetection (RECOMMENDED false): Whether time based loss
detection is in use. If false, uses FACK style loss detection. detection is in use. If false, uses FACK style loss detection.
kMinTLPTimeout (RECOMMENDED 10ms): Minimum time in the future a tail kMinTLPTimeout (RECOMMENDED 10ms): Minimum time in the future a tail
loss probe alarm may be set for. loss probe timer may be set for.
kMinRTOTimeout (RECOMMENDED 200ms): Minimum time in the future an kMinRTOTimeout (RECOMMENDED 200ms): Minimum time in the future an
RTO alarm may be set for. RTO timer may be set for.
kDelayedAckTimeout (RECOMMENDED 25ms): The length of the peer's kDelayedAckTimeout (RECOMMENDED 25ms): The length of the peer's
delayed ack timer. delayed ack timer.
kInitialRtt (RECOMMENDED 100ms): The RTT used before an RTT sample kInitialRtt (RECOMMENDED 100ms): The RTT used before an RTT sample
is taken. is taken.
3.5.2. Variables of interest 4.5.2. 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_alarm: Multi-modal alarm used for loss detection. loss_detection_timer: Multi-modal timer used for loss detection.
handshake_count: The number of times all unacknowledged handshake handshake_count: The number of times all unacknowledged handshake
data has been retransmitted without receiving an ack. data has been retransmitted without receiving an ack.
tlp_count: The number of times a tail loss probe has been sent tlp_count: The number of times a tail loss probe has been sent
without receiving an ack. without receiving an ack.
rto_count: The number of times an rto has been sent without rto_count: The number of times an rto has been sent without
receiving an ack. receiving an ack.
skipping to change at page 15, line 4 skipping to change at page 15, line 33
largest_acked_packet: The largest packet number acknowledged in an largest_acked_packet: The largest packet number acknowledged in an
ACK frame. ACK frame.
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 ack delay in an incoming ACK frame for max_ack_delay: The maximum ack delay in an incoming ACK frame for
this connection. Excludes ack delays for ack only packets and this connection. Excludes ack delays for non-retransmittable
those that create an RTT sample less than min_rtt. packets and those that create an RTT sample less than min_rtt.
reordering_threshold: The largest packet number gap between the reordering_threshold: The largest packet number gap between the
largest acked retransmittable packet and an unacknowledged largest acknowledged retransmittable packet and an unacknowledged
retransmittable packet before it is declared lost. retransmittable packet before it is declared lost.
time_reordering_fraction: The reordering window as a fraction of time_reordering_fraction: The reordering window as a fraction of
max(smoothed_rtt, latest_rtt). 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, including a number field indicating the packet number, a
time field indicating the time a packet was sent, a boolean time field indicating the time a packet was sent, a boolean
indicating whether the packet is ack only, and a bytes field 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 indicating the packet's size. sent_packets is ordered by packet
number, and packets remain in sent_packets until acknowledged or number, and packets remain in sent_packets until acknowledged or
lost. A sent_packets data structure is maintained per packet lost. A sent_packets data structure is maintained per packet
number space, and ACK processing only applies to a single space. number space, and ACK processing only applies to a single space.
3.5.3. Initialization 4.5.3. 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_alarm.reset() loss_detection_timer.reset()
handshake_count = 0 handshake_count = 0
tlp_count = 0 tlp_count = 0
rto_count = 0 rto_count = 0
if (kUsingTimeLossDetection) if (kUsingTimeLossDetection)
reordering_threshold = infinite reordering_threshold = infinite
time_reordering_fraction = kTimeReorderingFraction time_reordering_fraction = kTimeReorderingFraction
else: else:
reordering_threshold = kReorderingThreshold reordering_threshold = kReorderingThreshold
time_reordering_fraction = infinite 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
max_ack_delay = 0 max_ack_delay = 0
largest_sent_before_rto = 0 largest_sent_before_rto = 0
time_of_last_sent_retransmittable_packet = 0 time_of_last_sent_retransmittable_packet = 0
time_of_last_sent_handshake_packet = 0 time_of_last_sent_handshake_packet = 0
largest_sent_packet = 0 largest_sent_packet = 0
3.5.4. On Sending a Packet 4.5.4. On Sending a Packet
After any packet is sent, be it a new transmission or a rebundled After any packet is sent, be it a new transmission or a rebundled
transmission, the following OnPacketSent function is called. The transmission, the following OnPacketSent function is called. The
parameters to OnPacketSent are as follows: parameters to OnPacketSent are as follows:
o packet_number: The packet number of the sent packet. o packet_number: The packet number of the sent packet.
o is_ack_only: A boolean that indicates whether a packet only o ack_only: A boolean that indicates whether a packet contains only
contains an ACK frame. If true, it is still expected an ack will ACK or PADDING frame(s). If true, it is still expected an ack
be received for this packet, but it is not retransmittable. will be received for this packet, but it is not retransmittable.
o is_handshake_packet: A boolean that indicates whether a packet o in_flight: A boolean that indicates whether the packet counts
contains handshake data. towards bytes in flight.
o is_handshake_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 o sent_bytes: The number of bytes sent in the packet, not including
UDP or IP overhead, but including QUIC framing overhead. UDP or IP overhead, but including QUIC framing overhead.
Pseudocode for OnPacketSent follows: Pseudocode for OnPacketSent follows:
OnPacketSent(packet_number, is_ack_only, is_handshake_packet, OnPacketSent(packet_number, ack_only, in_flight,
sent_bytes): is_handshake_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 = now
sent_packets[packet_number].ack_only = is_ack_only sent_packets[packet_number].ack_only = ack_only
if !is_ack_only: sent_packets[packet_number].in_flight = in_flight
if !ack_only:
if is_handshake_packet: if is_handshake_packet:
time_of_last_sent_handshake_packet = now time_of_last_sent_handshake_packet = now
time_of_last_sent_retransmittable_packet = now time_of_last_sent_retransmittable_packet = now
OnPacketSentCC(sent_bytes) OnPacketSentCC(sent_bytes)
sent_packets[packet_number].bytes = sent_bytes sent_packets[packet_number].bytes = sent_bytes
SetLossDetectionAlarm() SetLossDetectionTimer()
3.5.5. On Receiving an Acknowledgment 4.5.5. On Receiving an Acknowledgment
When an ACK frame is received, it may acknowledge 0 or more packets. When an ACK frame is received, it may newly acknowledge any number of
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 = ack.largest_acked
// If the largest acked is newly acked, update the RTT. // If the largest acknowledged is newly acked,
// update the RTT.
if (sent_packets[ack.largest_acked]): if (sent_packets[ack.largest_acked]):
latest_rtt = now - sent_packets[ack.largest_acked].time latest_rtt = now - sent_packets[ack.largest_acked].time
UpdateRtt(latest_rtt, ack.ack_delay) UpdateRtt(latest_rtt, ack.ack_delay)
// Find all newly acked packets. // Find all newly acked packets.
for acked_packet in DetermineNewlyAckedPackets(): for acked_packet in DetermineNewlyAckedPackets():
OnPacketAcked(acked_packet.packet_number) OnPacketAcked(acked_packet.packet_number)
DetectLostPackets(ack.largest_acked_packet) DetectLostPackets(ack.largest_acked_packet)
SetLossDetectionAlarm() SetLossDetectionTimer()
// Process ECN information if present. // Process ECN information if present.
if (ACK frame contains ECN information): if (ACK frame contains ECN information):
ProcessECN(ack) ProcessECN(ack)
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)
// 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
// Only save into max ack delay if it's used // Only save into max ack delay if it's used
// for rtt calculation and is not ack only. // for rtt calculation and is not ack-only.
if (!sent_packets[ack.largest_acked].ack_only) if (!sent_packets[ack.largest_acked].ack_only)
max_ack_delay = max(max_ack_delay, ack_delay) max_ack_delay = max(max_ack_delay, 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
3.5.6. On Packet Acknowledgment 4.5.6. On Packet Acknowledgment
When a packet is acked for the first time, the following When a packet is acked 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 acked 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. of the newly acked packet.
If this is the first acknowledgement following RTO, check if the If this is the first acknowledgement following RTO, check if the
skipping to change at page 19, line 18 skipping to change at page 19, line 18
[RFC5682]. [RFC5682].
Pseudocode for OnPacketAcked follows: Pseudocode for OnPacketAcked follows:
OnPacketAcked(acked_packet): OnPacketAcked(acked_packet):
if (!acked_packet.is_ack_only): if (!acked_packet.is_ack_only):
OnPacketAckedCC(acked_packet) OnPacketAckedCC(acked_packet)
// If a packet sent prior to RTO was acked, then the RTO // If a packet sent prior to RTO was acked, then the RTO
// was spurious. Otherwise, inform congestion control. // was spurious. Otherwise, inform congestion control.
if (rto_count > 0 && if (rto_count > 0 &&
acked_packet.packet_number > largest_sent_before_rto) acked_packet.packet_number > largest_sent_before_rto):
OnRetransmissionTimeoutVerified() OnRetransmissionTimeoutVerified(
acket_packet.packet_number)
handshake_count = 0 handshake_count = 0
tlp_count = 0 tlp_count = 0
rto_count = 0 rto_count = 0
sent_packets.remove(acked_packet.packet_number) sent_packets.remove(acked_packet.packet_number)
3.5.7. Setting the Loss Detection Alarm 4.5.7. Setting the Loss Detection Timer
QUIC loss detection uses a single alarm for all timer-based loss QUIC loss detection uses a single timer for all timer-based loss
detection. The duration of the alarm is based on the alarm'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 SetLossDetectionAlarm defined below shows how the single function SetLossDetectionTimer defined below shows how the single
timer is set based on the alarm mode. timer is set.
3.5.7.1. Handshake Alarm 4.5.7.1. Handshake Timer
When a connection has unacknowledged handshake data, the handshake When a connection has unacknowledged handshake data, the handshake
alarm is set and when it expires, all unacknowledgedd handshake data timer is set and when it expires, all unacknowledgedd handshake data
is retransmitted. is retransmitted.
When stateless rejects are in use, the connection is considered When stateless rejects are in use, the connection is considered
immediately closed once a reject is sent, so no timer is set to immediately closed once a reject is sent, so no timer is set to
retransmit the reject. retransmit the reject.
Version negotiation packets are always stateless, and MUST be sent Version negotiation packets are always stateless, and MUST be sent
once per handshake packet that uses an unsupported QUIC version, and once per handshake packet that uses an unsupported QUIC version, and
MAY be sent in response to 0-RTT packets. MAY be sent in response to 0-RTT packets.
3.5.7.2. Tail Loss Probe and Retransmission Alarm 4.5.7.2. Tail Loss Probe and Retransmission Timer
Tail loss probes [TLP] and retransmission timeouts [RFC6298] are an Tail loss probes [TLP] and retransmission timeouts [RFC6298] are a
alarm based mechanism to recover from cases when there are timer based mechanism to recover from cases when there are
outstanding retransmittable packets, but an acknowledgement has not outstanding retransmittable packets, but an acknowledgement has not
been received in a timely manner. been received in a timely manner.
The TLP and RTO timers are armed when there is not unacknowledged The TLP and RTO timers are armed when there is not unacknowledged
handshake data. The TLP alarm is set until the max number of TLP handshake data. The TLP timer is set until the max number of TLP
packets have been sent, and then the RTO timer is set. packets have been sent, and then the RTO timer is set.
3.5.7.3. Early Retransmit Alarm 4.5.7.3. Early Retransmit Timer
Early retransmit [RFC5827] is implemented with a 1/4 RTT timer. It Early retransmit [RFC5827] is implemented with a 1/4 RTT timer. It
is part of QUIC's time based loss detection, but is always enabled, is part of QUIC's time based loss detection, but is always enabled,
even when only packet reordering loss detection is enabled. even when only packet reordering loss detection is enabled.
3.5.7.4. Pseudocode 4.5.7.4. Pseudocode
Pseudocode for SetLossDetectionAlarm follows: Pseudocode for SetLossDetectionTimer follows:
SetLossDetectionAlarm(): SetLossDetectionTimer():
// Don't arm the alarm if there are no packets with // Don't arm timer if there are no retransmittable packets
// retransmittable data in flight. // in flight.
if (bytes_in_flight == 0): if (bytes_in_flight == 0):
loss_detection_alarm.cancel() loss_detection_timer.cancel()
return return
if (handshake packets are outstanding): if (handshake packets are outstanding):
// Handshake retransmission alarm. // Handshake retransmission timer.
if (smoothed_rtt == 0): if (smoothed_rtt == 0):
alarm_duration = 2 * kInitialRtt timeout = 2 * kInitialRtt
else: else:
alarm_duration = 2 * smoothed_rtt timeout = 2 * smoothed_rtt
alarm_duration = max(alarm_duration + max_ack_delay, timeout = max(timeout + max_ack_delay, kMinTLPTimeout)
kMinTLPTimeout) timeout = timeout * (2 ^ handshake_count)
alarm_duration = alarm_duration * (2 ^ handshake_count) loss_detection_timer.set(
loss_detection_alarm.set( time_of_last_sent_handshake_packet + timeout)
time_of_last_sent_handshake_packet + alarm_duration)
return; return;
else if (loss_time != 0): else if (loss_time != 0):
// Early retransmit timer or time loss detection. // Early retransmit timer or time loss detection.
alarm_duration = loss_time - timeout = loss_time -
time_of_last_sent_retransmittable_packet time_of_last_sent_retransmittable_packet
else: else:
// RTO or TLP alarm // RTO or TLP timer
// Calculate RTO duration // Calculate RTO duration
alarm_duration = timeout =
smoothed_rtt + 4 * rttvar + max_ack_delay smoothed_rtt + 4 * rttvar + max_ack_delay
alarm_duration = max(alarm_duration, kMinRTOTimeout) timeout = max(timeout, kMinRTOTimeout)
alarm_duration = alarm_duration * (2 ^ rto_count) timeout = timeout * (2 ^ rto_count)
if (tlp_count < kMaxTLPs): if (tlp_count < kMaxTLPs):
// Tail Loss Probe // Tail Loss Probe
tlp_alarm_duration = max(1.5 * smoothed_rtt tlp_timeout = max(1.5 * smoothed_rtt
+ max_ack_delay, kMinTLPTimeout) + max_ack_delay, kMinTLPTimeout)
alarm_duration = min(tlp_alarm_duration, alarm_duration) timeout = min(tlp_timeout, timeout)
loss_detection_alarm.set( loss_detection_timer.set(
time_of_last_sent_retransmittable_packet + alarm_duration) time_of_last_sent_retransmittable_packet + timeout)
3.5.8. On Alarm Firing 4.5.8. On Timeout
QUIC uses one loss recovery alarm, which when set, can be in one of QUIC uses one loss recovery timer, which when set, can be in one of
several modes. When the alarm fires, the mode determines the action several modes. When the timer expires, the mode determines the
to be performed. action to be performed.
Pseudocode for OnLossDetectionAlarm follows: Pseudocode for OnLossDetectionTimeout follows:
OnLossDetectionAlarm(): OnLossDetectionTimeout():
if (handshake packets are outstanding): if (handshake packets are outstanding):
// Handshake retransmission alarm. // Handshake timeout.
RetransmitAllUnackedHandshakeData() RetransmitAllUnackedHandshakeData()
handshake_count++ handshake_count++
else if (loss_time != 0): else if (loss_time != 0):
// Early retransmit or Time Loss Detection // Early retransmit or Time Loss Detection
DetectLostPackets(largest_acked_packet) DetectLostPackets(largest_acked_packet)
else if (tlp_count < kMaxTLPs): else if (tlp_count < kMaxTLPs):
// Tail Loss Probe. // Tail Loss Probe.
SendOnePacket() SendOnePacket()
tlp_count++ tlp_count++
else: else:
// RTO. // RTO.
if (rto_count == 0) if (rto_count == 0)
largest_sent_before_rto = largest_sent_packet largest_sent_before_rto = largest_sent_packet
SendTwoPackets() SendTwoPackets()
rto_count++ rto_count++
SetLossDetectionAlarm() SetLossDetectionTimer()
3.5.9. Detecting Lost Packets 4.5.9. Detecting Lost Packets
Packets in QUIC are only considered lost once a larger packet number Packets in QUIC are only considered lost once a larger packet number
in the same packet number space is acknowledged. DetectLostPackets in the same packet number space is acknowledged. DetectLostPackets
is called every time an ack is received and operates on the is called every time an ack is received and operates on the
sent_packets for that packet number space. If the loss detection sent_packets for that packet number space. If the loss detection
alarm fires and the loss_time is set, the previous largest acked timer expires and the loss_time is set, the previous largest acked
packet is supplied. packet is supplied.
3.5.9.1. Pseudocode 4.5.9.1. Pseudocode
DetectLostPackets takes one parameter, acked, which is the largest DetectLostPackets takes one parameter, acked, which is the largest
acked packet. acked packet.
Pseudocode for DetectLostPackets follows: Pseudocode for DetectLostPackets follows:
DetectLostPackets(largest_acked): DetectLostPackets(largest_acked):
loss_time = 0 loss_time = 0
lost_packets = {} lost_packets = {}
delay_until_lost = infinite delay_until_lost = infinite
if (kUsingTimeLossDetection): if (kUsingTimeLossDetection):
delay_until_lost = delay_until_lost =
(1 + time_reordering_fraction) * (1 + time_reordering_fraction) *
max(latest_rtt, smoothed_rtt) max(latest_rtt, smoothed_rtt)
else if (largest_acked.packet_number == largest_sent_packet): else if (largest_acked.packet_number == largest_sent_packet):
// Early retransmit alarm. // Early retransmit timer.
delay_until_lost = 5/4 * max(latest_rtt, smoothed_rtt) delay_until_lost = 9/8 * max(latest_rtt, smoothed_rtt)
foreach (unacked < largest_acked.packet_number): foreach (unacked < largest_acked.packet_number):
time_since_sent = now() - unacked.time_sent time_since_sent = now() - unacked.time_sent
delta = largest_acked.packet_number - unacked.packet_number delta = largest_acked.packet_number - unacked.packet_number
if (time_since_sent > delay_until_lost || if (time_since_sent > delay_until_lost ||
delta > reordering_threshold): delta > reordering_threshold):
sent_packets.remove(unacked.packet_number) sent_packets.remove(unacked.packet_number)
if (!unacked.is_ack_only): if (!unacked.is_ack_only):
lost_packets.insert(unacked) lost_packets.insert(unacked)
else if (loss_time == 0 && delay_until_lost != infinite): else if (loss_time == 0 && delay_until_lost != infinite):
loss_time = now() + delay_until_lost - time_since_sent loss_time = now() + delay_until_lost - time_since_sent
// Inform the congestion controller of lost packets and // Inform the congestion controller of lost packets and
// lets it decide whether to retransmit immediately. // lets it decide whether to retransmit immediately.
if (!lost_packets.empty()): if (!lost_packets.empty()):
OnPacketsLost(lost_packets) OnPacketsLost(lost_packets)
3.6. Discussion 4.6. Discussion
The majority of constants were derived from best common practices The majority of constants were derived from best common practices
among widely deployed TCP implementations on the internet. among widely deployed TCP implementations on the internet.
Exceptions follow. Exceptions follow.
A shorter delayed ack time of 25ms was chosen because longer delayed A shorter delayed ack time of 25ms was chosen because longer delayed
acks can delay loss recovery and for the small number of connections acks can delay loss recovery and for the small number of connections
where less than packet per 25ms is delivered, acking every packet is where less than packet per 25ms is delivered, acking every packet is
beneficial to congestion control and loss recovery. beneficial to congestion control and loss recovery.
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.
4. Congestion Control 5. Congestion Control
QUIC's congestion control is based on TCP NewReno [RFC6582] QUIC's congestion control is based on TCP NewReno [RFC6582]. NewReno
congestion control to determine the congestion window. QUIC is a congestion window based congestion control. QUIC specifies the
congestion control is specified in bytes due to finer control and the congestion window in bytes rather than packets due to finer control
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 4.8.2) beyond the available bytes_in_flight (defined in Section 5.8.2) 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 the
TLP or RTO alarm fires, as described in Section 3.3.2 and TLP or RTO timer expires, as described in Section 4.3.2 and
Section 3.3.3. Section 4.3.3.
4.1. Explicit Congestion Notification Implementations MAY use other congestion control algorithms, and
endpoints MAY use different algorithms from one another. The signals
QUIC provides for congestion control are generic and are designed to
support different algorithms.
5.1. 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.
4.2. Slow Start 5.2. 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 sshthresh, 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 RTO. 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 ack is processed.
4.3. Congestion Avoidance 5.3. 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 MSS of bytes per approach that increases the congestion window by one maximum packet
congestion window acknowledged. When a loss is detected, NewReno size per congestion window acknowledged. When a loss is detected,
halves the congestion window and sets the slow start threshold to the NewReno halves the congestion window and sets the slow start
new congestion window. threshold to the new congestion window.
4.4. Recovery Period 5.4. 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
retransmits stream data and control frames, not packets, it defines retransmits stream data and control frames, not packets, it defines
the end of recovery as a packet sent after the start of recovery the end of recovery as a packet sent after the start of recovery
being acknowledged. This is slightly different from TCP's definition being acknowledged. This is slightly different from TCP's definition
of recovery, which ends when the lost packet that started recovery is of recovery, which ends when the lost 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.
4.5. Tail Loss Probe 5.5. Tail Loss Probe
A TLP packet MUST NOT be blocked by the sender's congestion A TLP packet MUST NOT be blocked by the sender's congestion
controller. The sender MUST however count these bytes as additional controller. The sender MUST however count these bytes as additional
bytes-in-flight, since a TLP adds network load without establishing bytes-in-flight, since a TLP adds network load without establishing
packet loss. packet loss.
Acknowledgement or loss of tail loss probes are treated like any Acknowledgement or loss of tail loss probes are treated like any
other packet. other packet.
4.6. Retransmission Timeout 5.6. Retransmission Timeout
When retransmissions are sent due to a retransmission timeout alarm, When retransmissions are sent due to a retransmission timeout timer,
no change is made to the congestion window until the next no change is made to the congestion window until the next
acknowledgement arrives. The retransmission timeout is considered acknowledgement arrives. The retransmission timeout is considered
spurious when this acknowledgement acknowledges packets sent prior to spurious when this acknowledgement acknowledges packets sent prior to
the first retransmission timeout. The retransmission timeout is the first retransmission timeout. The retransmission timeout is
considered valid when this acknowledgement acknowledges no packets considered valid when this acknowledgement acknowledges no packets
sent prior to the first retransmission timeout. In this case, the sent prior to the first retransmission timeout. In this case, the
congestion window MUST be reduced to the minimum congestion window congestion window MUST be reduced to the minimum congestion window
and slow start is re-entered. and slow start is re-entered.
4.7. Pacing 5.7. 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 retransmittable packets based on input sender pace sending of all in-flight packets based on input from the
from the congestion controller. For example, a pacer might congestion controller. For example, a pacer might distribute the
distribute the congestion window over the SRTT when used with a congestion window over the SRTT when used with a window-based
window-based controller, and a pacer might use the rate estimate of a controller, and a pacer might use the rate estimate of a rate-based
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
wrap the congestion controller and control the availability of the wrap the congestion controller and control the availability of the
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).
4.8. Pseudocode 5.8. Pseudocode
4.8.1. Constants of interest 5.8.1. 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.
kInitialMss (RECOMMENDED 1460 bytes): The max packet size is used kMaxDatagramSize (RECOMMENDED 1200 bytes): The sender's maximum
for calculating initial and minimum congestion windows. payload size. Does not include UDP or IP overhead. The max
packet size is used for calculating initial and minimum congestion
windows.
kInitialWindow (RECOMMENDED 10 * kInitialMss): Limit on the initial kInitialWindow (RECOMMENDED min(10 * kMaxDatagramSize,
amount of outstanding data in bytes.
kMinimumWindow (RECOMMENDED 2 * kInitialMss): Minimum congestion max(2* kMaxDatagramSize, 14600))):
window in bytes. Default limit on the initial amount of outstanding data in bytes.
Taken from [RFC6928].
kMinimumWindow (RECOMMENDED 2 * kMaxDatagramSize): Minimum
congestion window in bytes.
kLossReductionFactor (RECOMMENDED 0.5): Reduction in congestion kLossReductionFactor (RECOMMENDED 0.5): Reduction in congestion
window when a new loss event is detected. window when a new loss event is detected.
4.8.2. Variables of interest 5.8.2. 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_ECN frame. This variable is used to detect the peer in an ACK_ECN 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 frame, and have not been that contain at least one retransmittable or PADDING frame, and
acked or declared lost. The size does not include IP or UDP have not been acked or declared lost. The size does not include
IP or UDP overhead, but does include the QUIC header and AEAD
overhead. Packets only containing ACK frames do not count towards overhead. 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 end_of_recovery: The largest packet number sent when QUIC detects a
loss. When a larger packet is acknowledged, QUIC exits recovery. loss. When a larger packet 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.
4.8.3. Initialization 5.8.3. 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 end_of_recovery = 0
ssthresh = infinite ssthresh = infinite
ecn_ce_counter = 0 ecn_ce_counter = 0
4.8.4. On Packet Sent 5.8.4. 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
4.8.5. On Packet Acknowledgement 5.8.5. On Packet Acknowledgement
Invoked from loss detection's OnPacketAcked and is supplied with Invoked from loss detection's OnPacketAcked and is supplied with
acked_packet from sent_packets. acked_packet from sent_packets.
InRecovery(packet_number): InRecovery(packet_number):
return packet_number <= end_of_recovery return packet_number <= end_of_recovery
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.bytes
if (InRecovery(acked_packet.packet_number)): if (InRecovery(acked_packet.packet_number)):
// 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.bytes
else: else:
// Congestion avoidance. // Congestion avoidance.
congestion_window += congestion_window += kMaxDatagramSize * acked_packet.bytes
kInitialMss * acked_packet.bytes / congestion_window / congestion_window
4.8.6. On New Congestion Event 5.8.6. On New Congestion Event
Invoked from ProcessECN and OnPacketLost 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. Starts a new recovery period and reduces the congestion
window. window.
CongestionEvent(packet_number): CongestionEvent(packet_number):
// Start a new congestion event if packet_number // Start a new congestion event if packet_number
// is larger than the end of the previous recovery epoch. // is larger than the end of the previous recovery epoch.
if (!InRecovery(packet_number)): if (!InRecovery(packet_number)):
end_of_recovery = largest_sent_packet end_of_recovery = largest_sent_packet
congestion_window *= kMarkReductionFactor congestion_window *= kLossReductionFactor
congestion_window = max(congestion_window, kMinimumWindow) congestion_window = max(congestion_window, kMinimumWindow)
ssthresh = congestion_window
4.8.7. Process ECN Information 5.8.7. Process ECN Information
Invoked when an ACK_ECN frame is received from the peer. Invoked when an ACK_ECN frame is received from the 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 is past the end of the previous recovery epoch.
CongestionEvent(ack.largest_acked_packet) CongestionEvent(ack.largest_acked_packet)
4.8.8. On Packets Lost 5.8.8. 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.bytes
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.packet_number)
4.8.9. On Retransmission Timeout Verified 5.8.9. On Retransmission Timeout Verified
QUIC decreases the congestion window to the minimum value once the QUIC decreases the congestion window to the minimum value once the
retransmission timeout has been verified. retransmission timeout has been verified and removes any packets sent
before the newly acknowledged RTO packet.
OnRetransmissionTimeoutVerified() OnRetransmissionTimeoutVerified(packet_number)
congestion_window = kMinimumWindow 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 -= lost_packet.bytes
sent_packets.remove(sent_packet.packet_number)
5. IANA Considerations 6. Security Considerations
6.1. Congestion Signals
Congestion control fundamentally involves the consumption of signals
- both loss and ECN codepoints - from unauthenticated entities. On-
path attackers can spoof or alter these signals. An attacker can
cause endpoints to reduce their sending rate by dropping packets, or
alter send rate by changing ECN codepoints.
6.2. Traffic Analysis
Packets that carry only ACK frames can be heuristically identified by
observing packet size. Acknowledgement patterns may expose
information about link characteristics or application behavior.
Endpoints can use PADDING frames or bundle acknowledgments with other
frames to reduce leaked information.
6.3. Misreporting ECN Markings
A receiver can misreport ECN markings to alter the congestion
response of a sender. Suppressing reports of ECN-CE markings could
cause a sender to increase their send rate. This increase could
result in congestion and loss.
A sender MAY attempt to detect suppression of reports by marking
occasional packets that they send with ECN-CE. If a packet marked
with ECN-CE is not reported as having been marked when the packet is
acknowledged, the sender SHOULD then disable ECN for that path.
Reporting additional ECN-CE markings will cause a sender to reduce
their sending rate, which is similar in effect to advertising reduced
connection flow control limits and so no advantage is gained by doing
so.
Endpoints choose the congestion controller that they use. Though
congestion controllers generally treat reports of ECN-CE markings as
equivalent to loss [RFC8311], the exact response for each controller
could be different. Failure to correctly respond to information
about ECN markings is therefore difficult to detect.
7. IANA Considerations
This document has no IANA actions. Yet. This document has no IANA actions. Yet.
6. References 8. References
6.1. 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-13 (work in progress), June 2018. transport-14 (work in progress), August 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>.
6.2. Informative References 8.2. Informative References
[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 27 skipping to change at page 31, line 27
NewReno Modification to TCP's Fast Recovery Algorithm", NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, DOI 10.17487/RFC6582, April 2012, RFC 6582, DOI 10.17487/RFC6582, April 2012,
<https://www.rfc-editor.org/info/rfc6582>. <https://www.rfc-editor.org/info/rfc6582>.
[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M., [RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
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,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>.
[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.
6.3. URIs 8.3. 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-12 A.1. Since draft-ietf-quic-recovery-13
o Corrected the lack of ssthresh reduction in CongestionEvent
pseudocode (#1598)
o Considerations for ECN spoofing (#1426, #1626)
o Clarifications for PADDING and congestion control (#837, #838,
#1517, #1531, #1540)
o Reduce early retransmission timer to RTT/8 (#945, #1581)
o Packets are declared lost after an RTO is verified (#935, #1582)
A.2. 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.2. Since draft-ietf-quic-recovery-11 A.3. Since draft-ietf-quic-recovery-11
No significant changes. No significant changes.
A.3. Since draft-ietf-quic-recovery-10 A.4. 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.4. Since draft-ietf-quic-recovery-09 A.5. Since draft-ietf-quic-recovery-09
No significant changes. No significant changes.
A.5. Since draft-ietf-quic-recovery-08 A.6. Since draft-ietf-quic-recovery-08
o Clarified pacing and RTO (#967, #977) o Clarified pacing and RTO (#967, #977)
A.6. Since draft-ietf-quic-recovery-07 A.7. 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.7. Since draft-ietf-quic-recovery-06 A.8. Since draft-ietf-quic-recovery-06
No significant changes. No significant changes.
A.8. Since draft-ietf-quic-recovery-05 A.9. Since draft-ietf-quic-recovery-05
o Add more congestion control text (#776) o Add more congestion control text (#776)
A.9. Since draft-ietf-quic-recovery-04 A.10. Since draft-ietf-quic-recovery-04
No significant changes. No significant changes.
A.10. Since draft-ietf-quic-recovery-03 A.11. Since draft-ietf-quic-recovery-03
No significant changes. No significant changes.
A.11. Since draft-ietf-quic-recovery-02 A.12. 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.12. Since draft-ietf-quic-recovery-01 A.13. 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.13. Since draft-ietf-quic-recovery-00 A.14. Since draft-ietf-quic-recovery-00
o Improved description of constants and ACK behavior o Improved description of constants and ACK behavior
A.14. Since draft-iyengar-quic-loss-recovery-01 A.15. Since draft-iyengar-quic-loss-recovery-01
o Adopted as base for draft-ietf-quic-recovery o Adopted as base for draft-ietf-quic-recovery
o Updated authors/editors list o Updated authors/editors list
o Added table of contents o Added table of contents
Acknowledgments Acknowledgments
Authors' Addresses Authors' Addresses
Jana Iyengar (editor) Jana Iyengar (editor)
Fastly Fastly
Email: jri.ietf@gmail.com Email: jri.ietf@gmail.com
Ian Swett (editor) Ian Swett (editor)
Google Google
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