draft-ietf-quic-recovery-07.txt   draft-ietf-quic-recovery-08.txt 
QUIC J. Iyengar, Ed. QUIC J. Iyengar, Ed.
Internet-Draft I. Swett, Ed. Internet-Draft I. Swett, Ed.
Intended status: Standards Track Google Intended status: Standards Track Google
Expires: May 18, 2018 November 14, 2017 Expires: June 8, 2018 December 5, 2017
QUIC Loss Detection and Congestion Control QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-07 draft-ietf-quic-recovery-08
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
https://mailarchive.ietf.org/arch/search/?email_list=quic [1]. https://mailarchive.ietf.org/arch/search/?email_list=quic [1].
Working Group information can be found at https://github.com/quicwg Working Group information can be found at https://github.com/quicwg
[2]; source code and issues list for this draft can be found at [2]; source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/recovery [3]. https://github.com/quicwg/base-drafts/labels/-recovery [3].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 18, 2018. This Internet-Draft will expire on June 8, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Design of the QUIC Transmission Machinery . . . . . . . . . . 3 2. Design of the QUIC Transmission Machinery . . . . . . . . . . 4
2.1. Relevant Differences Between QUIC and TCP . . . . . . . . 4 2.1. Relevant Differences Between QUIC and TCP . . . . . . . . 4
2.1.1. Monotonically Increasing Packet Numbers . . . . . . . 4 2.1.1. Monotonically Increasing Packet Numbers . . . . . . . 4
2.1.2. No Reneging . . . . . . . . . . . . . . . . . . . . . 5 2.1.2. No Reneging . . . . . . . . . . . . . . . . . . . . . 5
2.1.3. More ACK Ranges . . . . . . . . . . . . . . . . . . . 5 2.1.3. More ACK Ranges . . . . . . . . . . . . . . . . . . . 5
2.1.4. Explicit Correction For Delayed Acks . . . . . . . . 5 2.1.4. Explicit Correction For Delayed Acks . . . . . . . . 5
3. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 5 3. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Computing the RTT estimate . . . . . . . . . . . . . . . 5 3.1. Computing the RTT estimate . . . . . . . . . . . . . . . 6
3.2. Ack-based Detection . . . . . . . . . . . . . . . . . . . 5 3.2. Ack-based Detection . . . . . . . . . . . . . . . . . . . 6
3.2.1. Fast Retransmit . . . . . . . . . . . . . . . . . . . 6 3.2.1. Fast Retransmit . . . . . . . . . . . . . . . . . . . 6
3.2.2. Early Retransmit . . . . . . . . . . . . . . . . . . 6 3.2.2. Early Retransmit . . . . . . . . . . . . . . . . . . 7
3.3. Timer-based Detection . . . . . . . . . . . . . . . . . . 7 3.3. Timer-based Detection . . . . . . . . . . . . . . . . . . 8
3.3.1. Tail Loss Probe . . . . . . . . . . . . . . . . . . . 7 3.3.1. Tail Loss Probe . . . . . . . . . . . . . . . . . . . 8
3.3.2. Retransmission Timeout . . . . . . . . . . . . . . . 9 3.3.2. Retransmission Timeout . . . . . . . . . . . . . . . 9
3.3.3. Handshake Timeout . . . . . . . . . . . . . . . . . . 10 3.3.3. Handshake Timeout . . . . . . . . . . . . . . . . . . 10
3.4. Algorithm Details . . . . . . . . . . . . . . . . . . . . 10 3.4. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.1. Constants of interest . . . . . . . . . . . . . . . . 10 3.4.1. Constants of interest . . . . . . . . . . . . . . . . 11
3.4.2. Variables of interest . . . . . . . . . . . . . . . . 11 3.4.2. Variables of interest . . . . . . . . . . . . . . . . 12
3.4.3. Initialization . . . . . . . . . . . . . . . . . . . 12 3.4.3. Initialization . . . . . . . . . . . . . . . . . . . 13
3.4.4. On Sending a Packet . . . . . . . . . . . . . . . . . 13 3.4.4. On Sending a Packet . . . . . . . . . . . . . . . . . 13
3.4.5. On Ack Receipt . . . . . . . . . . . . . . . . . . . 13 3.4.5. On Ack Receipt . . . . . . . . . . . . . . . . . . . 14
3.4.6. On Packet Acknowledgment . . . . . . . . . . . . . . 14 3.4.6. On Packet Acknowledgment . . . . . . . . . . . . . . 15
3.4.7. Setting the Loss Detection Alarm . . . . . . . . . . 15 3.4.7. Setting the Loss Detection Alarm . . . . . . . . . . 16
3.4.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . 17 3.4.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . 17
3.4.9. Detecting Lost Packets . . . . . . . . . . . . . . . 17 3.4.9. Detecting Lost Packets . . . . . . . . . . . . . . . 18
3.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 18 3.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 19
4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 19 4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 19
4.1. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 19 4.1. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 20
4.2. Congestion Avoidance . . . . . . . . . . . . . . . . . . 19 4.2. Congestion Avoidance . . . . . . . . . . . . . . . . . . 20
4.3. Recovery Period . . . . . . . . . . . . . . . . . . . . . 19 4.3. Recovery Period . . . . . . . . . . . . . . . . . . . . . 20
4.4. Tail Loss Probe . . . . . . . . . . . . . . . . . . . . . 19 4.4. Tail Loss Probe . . . . . . . . . . . . . . . . . . . . . 20
4.5. Retransmission Timeout . . . . . . . . . . . . . . . . . 20 4.5. Retransmission Timeout . . . . . . . . . . . . . . . . . 20
4.6. Pacing Rate . . . . . . . . . . . . . . . . . . . . . . . 20 4.6. Pacing Rate . . . . . . . . . . . . . . . . . . . . . . . 21
4.7. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 20 4.7. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 21
4.7.1. Constants of interest . . . . . . . . . . . . . . . . 20 4.7.1. Constants of interest . . . . . . . . . . . . . . . . 21
4.7.2. Variables of interest . . . . . . . . . . . . . . . . 20 4.7.2. Variables of interest . . . . . . . . . . . . . . . . 21
4.7.3. Initialization . . . . . . . . . . . . . . . . . . . 21 4.7.3. Initialization . . . . . . . . . . . . . . . . . . . 22
4.7.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 21 4.7.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 22
4.7.5. On Packet Acknowledgement . . . . . . . . . . . . . . 21 4.7.5. On Packet Acknowledgement . . . . . . . . . . . . . . 22
4.7.6. On Packets Lost . . . . . . . . . . . . . . . . . . . 22 4.7.6. On Packets Lost . . . . . . . . . . . . . . . . . . . 23
4.7.7. On Retransmission Timeout Verified . . . . . . . . . 22 4.7.7. On Retransmission Timeout Verified . . . . . . . . . 23
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1. Normative References . . . . . . . . . . . . . . . . . . 23 6.1. Normative References . . . . . . . . . . . . . . . . . . 23
6.2. Informative References . . . . . . . . . . . . . . . . . 24 6.2. Informative References . . . . . . . . . . . . . . . . . 24
6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 24 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 25
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 24 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 25
B.1. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 24 B.1. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 25
B.2. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 24 B.2. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 25
B.3. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 25 B.3. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 25
B.4. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 25 B.4. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 25
B.5. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 25 B.5. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 25
B.6. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 25 B.6. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 26
B.7. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 25 B.7. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 26
B.8. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 25 B.8. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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 1.1. Notational Conventions
The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document. It's not shouting; when they are capitalized, they have "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
the special meaning defined in [RFC2119]. "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Design of the QUIC Transmission Machinery 2. 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
includes a packet sequence number (referred to below as a packet includes a packet sequence number (referred to below as a packet
number). These packet numbers never repeat in the lifetime of a number). These packet numbers never repeat in the lifetime of a
connection, and are monotonically increasing, which makes duplicate connection, and are monotonically increasing, which prevents
detection trivial. This fundamental design decision obviates the ambiguity. This fundamental design decision obviates the need for
need for disambiguating between transmissions and retransmissions and disambiguating between transmissions and retransmissions and
eliminates significant complexity from QUIC's interpretation of TCP eliminates significant complexity from QUIC's interpretation of TCP
loss detection mechanisms. loss detection mechanisms.
Every packet may contain several frames. We outline the frames that Every packet may contain several frames. We outline the frames that
are important to the loss detection and congestion control machinery are important to the loss detection and congestion control machinery
below. below.
o Retransmittable frames are frames requiring reliable delivery. o Retransmittable frames are frames requiring reliable delivery.
The most common are STREAM frames, which typically contain The most common are STREAM frames, which typically contain
application data. application data.
o Crypto handshake data is sent on stream 0, and uses the o Crypto handshake data is sent on stream 0, and uses the
reliability machinery of QUIC underneath. reliability machinery of QUIC underneath.
o ACK frames contain acknowledgment information. QUIC uses a SACK- o ACK frames contain acknowledgment information. ACK frames contain
based scheme, where acks express up to 256 ranges. one or more ranges of acknowledged packets.
2.1. Relevant Differences Between QUIC and TCP 2.1. 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. Monotonically Increasing Packet Numbers 2.1.1. 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 sequence number (referred QUIC separates the two: QUIC uses a packet number for transmissions,
to as the "packet number") for transmissions, and any data that is to and any data that is to be delivered to the receiving application(s)
be delivered to the receiving application(s) is sent in one or more is sent in one or more streams, with delivery order determined by
streams, with stream offsets encoded within STREAM frames inside of stream offsets encoded within STREAM frames.
packets that determine delivery order.
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.
2.1.2. No Reneging 2.1.2. No Reneging
QUIC ACKs contain information that is equivalent to TCP SACK, but QUIC ACKs contain information that is similar to TCP SACK, but QUIC
QUIC does not allow any acked packet to be reneged, greatly does not allow any acked packet to be reneged, greatly simplifying
simplifying implementations on both sides and reducing memory implementations on both sides and reducing memory pressure on the
pressure on the sender. sender.
2.1.3. More ACK Ranges 2.1.3. More ACK Ranges
QUIC supports up to 256 ACK ranges, opposed to TCP's 3 SACK ranges. QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In
In high loss environments, this speeds recovery. high loss environments, this speeds recovery, reduces spurious
retransmits, and ensures forward progress without relying on
timeouts.
2.1.4. Explicit Correction For Delayed Acks 2.1.4. 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
skipping to change at page 5, line 44 skipping to change at page 6, line 7
3. Loss Detection 3. 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 3.1. Computing the RTT estimate
(To be filled) RTT is calculated when an ACK frame arrives by computing the
difference between the current time and the time the largest newly
acked packet was sent. If no packets are newly acknowledged, RTT
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
result is larger than the Min RTT. If the result is smaller than the
min_rtt, the RTT should be used, but the ack delay field should be
ignored.
Like TCP, QUIC calculates both smoothed RTT and RTT variance as
specified in [RFC6298].
Min RTT is the minimum RTT measured over the connection, prior to
adjusting by ack delay. Ignoring ack delay for min RTT prevents
intentional or unintentional underestimation of min RTT, which in
turn prevents underestimating smoothed RTT.
3.2. Ack-based Detection 3.2. 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.
(TODO: Define unacknowledged packet, ackable packet, outstanding (TODO: Define unacknowledged packet, ackable packet, outstanding
bytes.) bytes.)
skipping to change at page 7, line 46 skipping to change at page 8, line 25
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 ackable packet before the sender schedules an alarm when the last ackable packet before
quiescence is transmitted. When this alarm fires, a Tail Loss Probe quiescence is transmitted. When this alarm fires, a Tail Loss Probe
(TLP) packet is sent to evoke an acknowledgement from the receiver. (TLP) packet is sent to evoke an acknowledgement from the receiver.
The alarm duration, or Probe Timeout (PTO), is set based on the The alarm duration, or Probe Timeout (PTO), is set based on the
following conditions: following conditions:
o If there is exactly one unacknowledged packet, PTO SHOULD be o PTO SHOULD be scheduled for max(1.5*SRTT+MaxAckDelay, 10ms)
scheduled for max(2_SRTT, 1.5_SRTT+kDelayedAckTimeout)
o If there are more than one unacknowledged packets, PTO SHOULD be
scheduled for max(2*SRTT, 10ms).
o If RTO is earlier, schedule a TLP alarm in its place. That is,
PTO SHOULD be scheduled for min(RTO, PTO).
kDelayedAckTimeout is the expected delayed ACK timer. When there is
exactly one unacknowledged packet, the alarm duration includes time
for an acknowledgment to be received, and additionally, a
kDelayedAckTimeout period to compensate for the delayed
acknowledgment timer at the receiver.
The RECOMMENDED value for kDelayedAckTimeout is 25ms.
(TODO: Add negotiability of delayed ack timeout.) o If RTO (Section 3.3.2) is earlier, schedule a TLP alarm in its
place. That is, PTO SHOULD be scheduled for min(RTO, PTO).
A PTO value of at least 2_SRTT ensures that the ACK is overdue. MaxAckDelay is the maximum ack delay supplied in an incoming ACK
Using a PTO of exactly 1_SRTT may generate spurious probes, and frame. MaxAckDelay excludes ack delays that aren't included in an
2*SRTT is simply the next integral value of RTT. RTT sample because they're too large and excludes those which
reference an ack-only packet.
(TODO: These values of 2 and 1.5 are a bit arbitrary. Reconsider QUIC diverges from TCP by calculating MaxAckDelay dynamically,
these.) instead of assuming a constant delayed ack timeout for all
connections. QUIC includes this in all probe timeouts, because it
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.
If the Retransmission Timeout (RTO, Section 3.3.2) period is smaller A PTO value of at least 1.5*SRTT ensures that the ACK is overdue.
than the computed PTO, then a PTO is scheduled for the smaller RTO The 1.5 is based on [LOSS-PROBE], but implementations MAY experiment
period. with 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 alarm to fire twice before setting an RTO alarm. In other
words, when the TLP alarm fires the first time, a TLP packet is sent, words, when the TLP alarm fires the first time, a TLP packet is sent,
and it is RECOMMENDED that the TLP alarm be scheduled for a second and it is RECOMMENDED that the TLP alarm be scheduled for a second
time. When the TLP alarm fires the second time, a second TLP packet time. When the TLP alarm fires the second time, a second TLP packet
is sent, and an RTO alarm SHOULD be scheduled Section 3.3.2. is sent, and an RTO alarm SHOULD be scheduled Section 3.3.2.
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 alarm 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 alarm fires.
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.
A sender will commonly not know that a packet being sent is a tail A sender may not know that a packet being sent is a tail packet.
packet. Consequently, a sender may have to arm or adjust the TLP Consequently, a sender may have to arm or adjust the TLP alarm on
alarm on every sent ackable packet. every sent ackable packet.
3.3.2. Retransmission Timeout 3.3.2. Retransmission Timeout
A Retransmission Timeout (RTO) alarm is the final backstop for loss A Retransmission Timeout (RTO) alarm 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, an alarm is scheduled for the RTO
period. When this alarm fires, the sender sends two packets, to period. When this alarm fires, the sender sends two packets, to
evoke acknowledgements from the receiver, and restarts the RTO alarm. evoke acknowledgements from the receiver, and restarts the RTO alarm.
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, minRTO) max(SRTT + 4*RTTVAR + MaxAckDelay, minRTO)
o When an RTO alarm fires, the RTO period is doubled. o When an RTO alarm fires, 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 alarm fires, 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. An alarm is not considered a strong enough signal of packet loss, so
RTO alarm fires only when there's a prolonged period of network does not result in an immediate change to congestion window or
silence, which could be caused by a change in the underlying network recovery state. An RTO alarm fires only when there's a prolonged
RTT. period of network silence, which could be caused by a change in the
underlying network RTT.
QUIC also diverges from TCP by including MaxAckDelay in the RTO
period. QUIC is able to explicitly model delay at the receiver via
the ack delay field in the ACK frame. Since QUIC corrects for this
delay in its SRTT and RTTVAR computations, it is necessary to add
this delay explicitly in the TLP and RTO computation.
When an acknowledgment is received for a packet sent on an RTO event, When 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 alarm fires MAY carry new data if available
or unacknowledged data to potentially reduce recovery time. Since or unacknowledged data to potentially reduce recovery time. Since
this packet is sent as a probe into the network prior to establishing this packet is sent as a probe into the network prior to establishing
any packet loss, prior unacknowledged packets SHOULD NOT be marked as any packet loss, prior unacknowledged packets SHOULD NOT be marked as
lost. lost.
skipping to change at page 10, line 11 skipping to change at page 10, line 34
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.
3.3.3. Handshake Timeout 3.3.3. Handshake Timeout
Handshake packets, which contain STREAM frames for stream 0, are Handshake packets, which contain STREAM frames for stream 0, are
critical to QUIC transport and crypto negotiation, so a separate critical to QUIC transport and crypto negotiation, so a separate
alarm is used for them. alarm is used for them.
The handshake timeout SHOULD be set to twice the initial RTT. The initial handshake timeout SHOULD be set to twice the initial RTT.
There are no prior RTT samples within this connection. However, this At the beginning, there are no prior RTT samples within a connection.
may be a resumed connection over the same network, in which case, a Resumed connections over the same network SHOULD use the previous
client SHOULD use the previous connection's final smoothed RTT value connection's final smoothed RTT value as the resumed connection's
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 the first handshake packet is sent, the sender SHOULD set an When the first handshake packet is sent, the sender SHOULD set an
alarm for the handshake timeout period. alarm for the handshake timeout period.
When the alarm fires, the sender MUST retransmit all unacknowledged When the alarm fires, the sender MUST retransmit all unacknowledged
handshake frames. The sender SHOULD double the handshake timeout and handshake data. On each consecutive firing of the handshake alarm,
set an alarm for this period. the sender SHOULD double the handshake timeout and set an alarm for
this period.
On each consecutive firing of the handshake alarm, the sender SHOULD
double the handshake timeout period.
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 alarm SHOULD be set for twice the newly
computed smoothed RTT. computed smoothed RTT.
Handshake frames may be cancelled by handshake state transitions. In Handshake data may be cancelled by handshake state transitions. In
particular, all non-protected frames SHOULD no longer be transmitted particular, all non-protected data SHOULD no longer be transmitted
once packet protection is available. once packet protection is available.
(TODO: Work this section some more. Add text on client vs. server, (TODO: Work this section some more. Add text on client vs. server,
and on stateless retry.) and on stateless retry.)
3.4. Algorithm Details 3.4. Pseudocode
3.4.1. Constants of interest 3.4.1. Constants of interest
Constants used in loss recovery are based on a combination of RFCs, Constants used in loss recovery are based on a combination of RFCs,
papers, and common practice. Some may need to be changed or papers, and common practice. Some may need to be changed or
negotiated in order to better suit a variety of environments. negotiated in order to better suit a variety of environments.
kMaxTLPs (default 2): Maximum number of tail loss probes before an kMaxTLPs (default 2): Maximum number of tail loss probes before an
RTO fires. RTO fires.
kReorderingThreshold (default 3): Maximum reordering in packet kReorderingThreshold (default 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 (default 1/8): Maximum reordering in time kTimeReorderingFraction (default 1/8): Maximum reordering in time
space before time based loss detection considers a packet lost. space before time based loss detection considers a packet lost.
In fraction of an RTT. In fraction of an RTT.
kUsingTimeLossDetection (default false): Whether time based loss
detection is in use. If false, uses FACK style loss detection.
kMinTLPTimeout (default 10ms): Minimum time in the future a tail kMinTLPTimeout (default 10ms): Minimum time in the future a tail
loss probe alarm may be set for. loss probe alarm may be set for.
kMinRTOTimeout (default 200ms): Minimum time in the future an RTO kMinRTOTimeout (default 200ms): Minimum time in the future an RTO
alarm may be set for. alarm may be set for.
kDelayedAckTimeout (default 25ms): The length of the peer's delayed kDelayedAckTimeout (default 25ms): The length of the peer's delayed
ack timer. ack timer.
kDefaultInitialRtt (default 100ms): The default RTT used before an kDefaultInitialRtt (default 100ms): The default RTT used before an
skipping to change at page 11, line 50 skipping to change at page 12, line 30
largest_sent_before_rto: The last packet number sent prior to the largest_sent_before_rto: The last packet number sent prior to the
first retransmission timeout. first retransmission timeout.
time_of_last_sent_packet: The time the most recent packet was sent. time_of_last_sent_packet: The time the most recent packet was sent.
largest_sent_packet: The packet number of the most recently sent largest_sent_packet: The packet number of the most recently sent
packet. packet.
largest_acked_packet: The largest packet number acknowledged in an largest_acked_packet: The largest packet number acknowledged in 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.
max_ack_delay: The maximum ack delay in an incoming ACK frame for
this connection. Excludes ack delays for ack only packets and
those that create an RTT sample less than min_rtt.
reordering_threshold: The largest delta between the largest acked reordering_threshold: The largest delta between the largest acked
retransmittable packet and a packet containing retransmittable retransmittable packet and a packet containing retransmittable
frames before it's declared lost. frames before it's 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, and a bytes time field indicating the time a packet was sent, a boolean
field indicating the packet's size. sent_packets is ordered by indicating whether the packet is ack only, and a bytes field
packet number, and packets remain in sent_packets until indicating the packet's size. sent_packets is ordered by packet
acknowledged or lost. number, and packets remain in sent_packets until acknowledged or
lost.
3.4.3. Initialization 3.4.3. Initialization
At the beginning of the connection, initialize the loss detection At the beginning of the connection, initialize the loss detection
variables as follows: variables as follows:
loss_detection_alarm.reset() loss_detection_alarm.reset()
handshake_count = 0 handshake_count = 0
tlp_count = 0 tlp_count = 0
rto_count = 0 rto_count = 0
if (UsingTimeLossDetection()) 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 = 0
max_ack_delay = 0
largest_sent_before_rto = 0 largest_sent_before_rto = 0
time_of_last_sent_packet = 0 time_of_last_sent_packet = 0
largest_sent_packet = 0 largest_sent_packet = 0
3.4.4. On Sending a Packet 3.4.4. 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:
skipping to change at page 13, line 27 skipping to change at page 14, line 15
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, sent_bytes): OnPacketSent(packet_number, is_ack_only, sent_bytes):
time_of_last_sent_packet = now time_of_last_sent_packet = now
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
if !is_ack_only: if !is_ack_only:
OnPacketSentCC(sent_bytes) OnPacketSentCC(sent_bytes)
sent_packets[packet_number].bytes = sent_bytes sent_packets[packet_number].bytes = sent_bytes
SetLossDetectionAlarm() SetLossDetectionAlarm()
3.4.5. On Ack Receipt 3.4.5. On Ack Receipt
When an ack is received, it may acknowledge 0 or more packets. When an ack is received, it may acknowledge 0 or more 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 acked 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
if (latest_rtt > ack.ack_delay): UpdateRtt(latest_rtt, ack.ack_delay)
latest_rtt -= ack.delay
UpdateRtt(latest_rtt)
// 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() SetLossDetectionAlarm()
UpdateRtt(latest_rtt): UpdateRtt(latest_rtt, ack_delay):
// min_rtt ignores ack delay.
min_rtt = min(min_rtt, latest_rtt)
// Adjust for ack delay if it's plausible.
if (latest_rtt - min_rtt > ack_delay):
latest_rtt -= ack_delay
// Only save into max ack delay if it's used
// for rtt calculation and is not ack only.
if (!sent_packets[ack.largest_acked].ack_only)
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 = 3/4 * rttvar + 1/4 * abs(smoothed_rtt - latest_rtt) rttvar = 3/4 * rttvar + 1/4 * abs(smoothed_rtt - latest_rtt)
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt
3.4.6. On Packet Acknowledgment 3.4.6. On Packet Acknowledgment
skipping to change at page 15, line 25 skipping to change at page 16, line 26
sent_packets.remove(acked_packet_number) sent_packets.remove(acked_packet_number)
3.4.7. Setting the Loss Detection Alarm 3.4.7. Setting the Loss Detection Alarm
QUIC loss detection uses a single alarm for all timer-based loss QUIC loss detection uses a single alarm for all timer-based loss
detection. The duration of the alarm is based on the alarm's mode, detection. The duration of the alarm is based on the alarm'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 SetLossDetectionAlarm defined below shows how the single
timer is set based on the alarm mode. timer is set based on the alarm mode.
3.4.7.1. Handshake Packets 3.4.7.1. Handshake Alarm
The initial flight has no prior RTT sample. A client SHOULD remember
the previous RTT it observed when resumption is attempted and use
that for an initial RTT value. If no previous RTT is available, the
initial RTT defaults to 100ms.
Endpoints MUST retransmit handshake frames if not acknowledged within
a time limit. This time limit will start as the largest of twice the
RTT value and MinTLPTimeout. Each consecutive handshake
retransmission doubles the time limit, until an acknowledgement is
received.
Handshake frames may be cancelled by handshake state transitions. In When a connection has unacknowledged handshake data, the handshake
particular, all non-protected frames SHOULD be no longer be alarm is set and when it expires, all unacknowledgedd handshake data
transmitted once packet protection is available. 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 0RTT packets. MAY be sent in response to 0RTT packets.
3.4.7.2. Tail Loss Probe and Retransmission Timeout 3.4.7.2. Tail Loss Probe and Retransmission Alarm
Tail loss probes [LOSS-PROBE] and retransmission timeouts [RFC6298] Tail loss probes [LOSS-PROBE] and retransmission timeouts [RFC6298]
are an alarm based mechanism to recover from cases when there are are an alarm 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.
3.4.7.3. Early Retransmit 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
packets have been sent, and then the RTO tiemr is set.
3.4.7.3. Early Retransmit Alarm
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.
3.4.7.4. Pseudocode 3.4.7.4. Pseudocode
Pseudocode for SetLossDetectionAlarm follows: Pseudocode for SetLossDetectionAlarm follows:
SetLossDetectionAlarm(): SetLossDetectionAlarm():
if (retransmittable packets are not outstanding): // Don't arm the alarm if there are no packets with
// retransmittable data in flight.
if (num_retransmittable_packets_outstanding == 0):
loss_detection_alarm.cancel() loss_detection_alarm.cancel()
return return
if (handshake packets are outstanding): if (handshake packets are outstanding):
// Handshake retransmission alarm. // Handshake retransmission alarm.
if (smoothed_rtt == 0): if (smoothed_rtt == 0):
alarm_duration = 2 * kDefaultInitialRtt alarm_duration = 2 * kDefaultInitialRtt
else: else:
alarm_duration = 2 * smoothed_rtt alarm_duration = 2 * smoothed_rtt
alarm_duration = max(alarm_duration, kMinTLPTimeout) alarm_duration = max(alarm_duration, kMinTLPTimeout)
alarm_duration = alarm_duration * (2 ^ handshake_count) alarm_duration = alarm_duration * (2 ^ handshake_count)
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 - now alarm_duration = loss_time - time_of_last_sent_packet
else if (tlp_count < kMaxTLPs): else if (tlp_count < kMaxTLPs):
// Tail Loss Probe // Tail Loss Probe
if (retransmittable_packets_outstanding == 1): alarm_duration = max(1.5 * smoothed_rtt + max_ack_delay,
alarm_duration = 1.5 * smoothed_rtt + kDelayedAckTimeout kMinTLPTimeout)
else:
alarm_duration = kMinTLPTimeout
alarm_duration = max(alarm_duration, 2 * smoothed_rtt)
else: else:
// RTO alarm // RTO alarm
alarm_duration = smoothed_rtt + 4 * rttvar alarm_duration = smoothed_rtt + 4 * rttvar
alarm_duration = max(alarm_duration, kMinRTOTimeout) alarm_duration = max(alarm_duration, kMinRTOTimeout)
alarm_duration = alarm_duration * (2 ^ rto_count) alarm_duration = alarm_duration * (2 ^ rto_count)
loss_detection_alarm.set(now + alarm_duration) loss_detection_alarm.set(time_of_last_sent_packet
+ alarm_duration)
3.4.8. On Alarm Firing 3.4.8. On Alarm Firing
QUIC uses one loss recovery alarm, which when set, can be in one of QUIC uses one loss recovery alarm, which when set, can be in one of
several modes. When the alarm fires, the mode determines the action several modes. When the alarm fires, the mode determines the action
to be performed. to be performed.
Pseudocode for OnLossDetectionAlarm follows: Pseudocode for OnLossDetectionAlarm follows:
OnLossDetectionAlarm(): OnLossDetectionAlarm():
skipping to change at page 18, line 16 skipping to change at page 19, line 9
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 (time_reordering_fraction != infinite): if (kUsingTimeLossDetection):
delay_until_lost = delay_until_lost =
(1 + time_reordering_fraction) * max(latest_rtt, smoothed_rtt) (1 + time_reordering_fraction) * 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 alarm.
delay_until_lost = 9/8 * max(latest_rtt, smoothed_rtt) delay_until_lost = 5/4 * 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):
lost_packets.insert(unacked) lost_packets.insert(unacked)
else if (delta > reordering_threshold) else if (delta > reordering_threshold)
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
skipping to change at page 19, line 15 skipping to change at page 20, line 8
4. Congestion Control 4. Congestion Control
QUIC's congestion control is based on TCP NewReno[RFC6582] congestion QUIC's congestion control is based on TCP NewReno[RFC6582] congestion
control to determine the congestion window and pacing rate. QUIC control to determine the congestion window and pacing rate. QUIC
congestion control is specified in bytes due to finer control and the congestion control is specified in bytes due to finer control and the
ease of appropriate byte counting[RFC3465]. ease of appropriate byte counting[RFC3465].
4.1. Slow Start 4.1. 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. QUIC re-enters slow start after a retransmission timeout. loss. QUIC re-enters slow start anytime the congestion window is
While in slow start, QUIC increases the congestion window by the less than sshthresh, which typically only occurs after an RTO. While
number of acknowledged bytes when each ack is processed. in slow start, QUIC increases the congestion window by the number of
acknowledged bytes when each ack is processed.
4.2. Congestion Avoidance 4.2. 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 MSS of bytes per
congestion window acknowledged. When a loss is detected, NewReno congestion window acknowledged. When a loss is detected, NewReno
halves the congestion window and sets the slow start threshold to the halves the congestion window and sets the slow start threshold to the
new congestion window. new congestion window.
skipping to change at page 21, line 5 skipping to change at page 21, line 45
when a new loss event is detected. when a new loss event is detected.
4.7.2. Variables of interest 4.7.2. 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.
bytes_in_flight: The sum of the size in bytes of all sent packets bytes_in_flight: The sum of the size in bytes of all sent packets
that contain at least one retransmittable or PADDING frame, and that contain at least one retransmittable or PADDING frame, and
have not been acked or declared lost. The size does not include have not been acked or declared lost. The size does not include
IP or UDP overhead. Packets only containing ack frames do not IP or UDP overhead. Packets only containing ACK frames do not
count towards byte_in_flight to ensure congestion control does not count towards byte_in_flight to ensure congestion control does not
impede congestion feedback. impede 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.
4.7.3. Initialization 4.7.3. 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
skipping to change at page 23, line 12 skipping to change at page 23, line 42
This document has no IANA actions. Yet. This document has no IANA actions. Yet.
6. References 6. References
6.1. Normative References 6.1. 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-07 (work in progress), November 2017. transport-00 (work in progress), December 2017.
[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>.
[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 24, line 5 skipping to change at page 24, line 32
"Computing TCP's Retransmission Timer", RFC 6298, "Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011, DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>. <https://www.rfc-editor.org/info/rfc6298>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References 6.2. Informative References
[LOSS-PROBE] [LOSS-PROBE]
Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis, 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.
[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
skipping to change at page 24, line 33 skipping to change at page 25, line 16
"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 6.3. 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. Acknowledgments Appendix A. Acknowledgments
Appendix B. Change Log Appendix B. 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.
B.1. Since draft-ietf-quic-recovery-06 B.1. Since draft-ietf-quic-recovery-06
 End of changes. 59 change blocks. 
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