Internet DRAFT - draft-ietf-quic-recovery
draft-ietf-quic-recovery
QUIC J. Iyengar, Ed.
Internet-Draft Fastly
Intended status: Standards Track I. Swett, Ed.
Expires: 19 July 2021 Google
15 January 2021
QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-34
Abstract
This document describes loss detection and congestion control
mechanisms for QUIC.
Note to Readers
Discussion of this draft takes place on the QUIC working group
mailing list (quic@ietf.org (mailto:quic@ietf.org)), which is
archived at https://mailarchive.ietf.org/arch/
search/?email_list=quic.
Working Group information can be found at https://github.com/quicwg;
source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/-recovery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 July 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Design of the QUIC Transmission Machinery . . . . . . . . . . 5
4. Relevant Differences Between QUIC and TCP . . . . . . . . . . 6
4.1. Separate Packet Number Spaces . . . . . . . . . . . . . . 6
4.2. Monotonically Increasing Packet Numbers . . . . . . . . . 6
4.3. Clearer Loss Epoch . . . . . . . . . . . . . . . . . . . 7
4.4. No Reneging . . . . . . . . . . . . . . . . . . . . . . . 7
4.5. More ACK Ranges . . . . . . . . . . . . . . . . . . . . . 7
4.6. Explicit Correction For Delayed Acknowledgments . . . . . 7
4.7. Probe Timeout Replaces RTO and TLP . . . . . . . . . . . 7
4.8. The Minimum Congestion Window is Two Packets . . . . . . 8
5. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 8
5.1. Generating RTT samples . . . . . . . . . . . . . . . . . 8
5.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 9
5.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 10
6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Acknowledgment-Based Detection . . . . . . . . . . . . . 12
6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 13
6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 13
6.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 14
6.2.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 14
6.2.2. Handshakes and New Paths . . . . . . . . . . . . . . 16
6.2.3. Speeding Up Handshake Completion . . . . . . . . . . 17
6.2.4. Sending Probe Packets . . . . . . . . . . . . . . . . 18
6.3. Handling Retry Packets . . . . . . . . . . . . . . . . . 19
6.4. Discarding Keys and Packet State . . . . . . . . . . . . 19
7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 20
7.1. Explicit Congestion Notification . . . . . . . . . . . . 20
7.2. Initial and Minimum Congestion Window . . . . . . . . . . 21
7.3. Congestion Control States . . . . . . . . . . . . . . . . 21
7.3.1. Slow Start . . . . . . . . . . . . . . . . . . . . . 22
7.3.2. Recovery . . . . . . . . . . . . . . . . . . . . . . 22
7.3.3. Congestion Avoidance . . . . . . . . . . . . . . . . 23
7.4. Ignoring Loss of Undecryptable Packets . . . . . . . . . 23
7.5. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 24
7.6. Persistent Congestion . . . . . . . . . . . . . . . . . . 24
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7.6.1. Duration . . . . . . . . . . . . . . . . . . . . . . 24
7.6.2. Establishing Persistent Congestion . . . . . . . . . 25
7.6.3. Example . . . . . . . . . . . . . . . . . . . . . . . 26
7.7. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.8. Under-utilizing the Congestion Window . . . . . . . . . . 28
8. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8.1. Loss and Congestion Signals . . . . . . . . . . . . . . . 28
8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 28
8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 28
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Normative References . . . . . . . . . . . . . . . . . . 29
10.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 32
A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 32
A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 32
A.2. Constants of Interest . . . . . . . . . . . . . . . . . . 33
A.3. Variables of interest . . . . . . . . . . . . . . . . . . 33
A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 34
A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 34
A.6. On Receiving a Datagram . . . . . . . . . . . . . . . . . 35
A.7. On Receiving an Acknowledgment . . . . . . . . . . . . . 35
A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 37
A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 39
A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 39
A.11. Upon Dropping Initial or Handshake Keys . . . . . . . . . 40
Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 41
B.1. Constants of interest . . . . . . . . . . . . . . . . . . 41
B.2. Variables of interest . . . . . . . . . . . . . . . . . . 41
B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 42
B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 42
B.5. On Packet Acknowledgment . . . . . . . . . . . . . . . . 42
B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 43
B.7. Process ECN Information . . . . . . . . . . . . . . . . . 44
B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 44
B.9. Removing Discarded Packets From Bytes In Flight . . . . . 44
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 45
C.1. Since draft-ietf-quic-recovery-32 . . . . . . . . . . . . 45
C.2. Since draft-ietf-quic-recovery-31 . . . . . . . . . . . . 45
C.3. Since draft-ietf-quic-recovery-30 . . . . . . . . . . . . 45
C.4. Since draft-ietf-quic-recovery-29 . . . . . . . . . . . . 45
C.5. Since draft-ietf-quic-recovery-28 . . . . . . . . . . . . 46
C.6. Since draft-ietf-quic-recovery-27 . . . . . . . . . . . . 46
C.7. Since draft-ietf-quic-recovery-26 . . . . . . . . . . . . 46
C.8. Since draft-ietf-quic-recovery-25 . . . . . . . . . . . . 46
C.9. Since draft-ietf-quic-recovery-24 . . . . . . . . . . . . 46
C.10. Since draft-ietf-quic-recovery-23 . . . . . . . . . . . . 46
C.11. Since draft-ietf-quic-recovery-22 . . . . . . . . . . . . 47
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C.12. Since draft-ietf-quic-recovery-21 . . . . . . . . . . . . 47
C.13. Since draft-ietf-quic-recovery-20 . . . . . . . . . . . . 47
C.14. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 47
C.15. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 48
C.16. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 48
C.17. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 49
C.18. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 49
C.19. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 49
C.20. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 50
C.21. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 50
C.22. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 50
C.23. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 50
C.24. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 50
C.25. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 50
C.26. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 51
C.27. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 51
C.28. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 51
C.29. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 51
C.30. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 51
C.31. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 51
C.32. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 51
C.33. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 51
Appendix D. Contributors . . . . . . . . . . . . . . . . . . . . 52
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 52
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction
QUIC is a secure general-purpose transport protocol, described in
[QUIC-TRANSPORT]). This document describes loss detection and
congestion control mechanisms for QUIC.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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.
Definitions of terms that are used in this document:
Ack-eliciting frames: All frames other than ACK, PADDING, and
CONNECTION_CLOSE are considered ack-eliciting.
Ack-eliciting packets: Packets that contain ack-eliciting frames
elicit an ACK from the receiver within the maximum acknowledgment
delay and are called ack-eliciting packets.
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In-flight packets: Packets are considered in-flight when they are
ack-eliciting or contain a PADDING frame, and they have been sent
but are not acknowledged, declared lost, or discarded along with
old keys.
3. Design of the QUIC Transmission Machinery
All transmissions in QUIC are sent with a packet-level header, which
indicates the encryption level and includes a packet sequence number
(referred to below as a packet number). The encryption level
indicates the packet number space, as described in Section 12.3 in
[QUIC-TRANSPORT]. Packet numbers never repeat within a packet number
space for the lifetime of a connection. Packet numbers are sent in
monotonically increasing order within a space, preventing ambiguity.
It is permitted for some packet numbers to never be used, leaving
intentional gaps.
This design obviates the need for disambiguating between
transmissions and retransmissions; this eliminates significant
complexity from QUIC's interpretation of TCP loss detection
mechanisms.
QUIC packets can contain multiple frames of different types. The
recovery mechanisms ensure that data and frames that need reliable
delivery are acknowledged or declared lost and sent in new packets as
necessary. The types of frames contained in a packet affect recovery
and congestion control logic:
* All packets are acknowledged, though packets that contain no ack-
eliciting frames are only acknowledged along with ack-eliciting
packets.
* Long header packets that contain CRYPTO frames are critical to the
performance of the QUIC handshake and use shorter timers for
acknowledgment.
* Packets containing frames besides ACK or CONNECTION_CLOSE frames
count toward congestion control limits and are considered in-
flight.
* PADDING frames cause packets to contribute toward bytes in flight
without directly causing an acknowledgment to be sent.
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4. Relevant Differences Between QUIC and TCP
Readers familiar with TCP's loss detection and congestion control
will find algorithms here that parallel well-known TCP ones.
However, protocol differences between QUIC and TCP contribute to
algorithmic differences. These protocol differences are briefly
described below.
4.1. Separate Packet Number Spaces
QUIC uses separate packet number spaces for each encryption level,
except 0-RTT and all generations of 1-RTT keys use the same packet
number space. Separate packet number spaces ensures acknowledgment
of packets sent with one level of encryption will not cause spurious
retransmission of packets sent with a different encryption level.
Congestion control and round-trip time (RTT) measurement are unified
across packet number spaces.
4.2. Monotonically Increasing Packet Numbers
TCP conflates transmission order at the sender with delivery order at
the receiver, resulting in the retransmission ambiguity problem
([RETRANSMISSION]). QUIC separates transmission order from delivery
order: packet numbers indicate transmission order, and delivery order
is determined by the stream offsets in STREAM frames.
QUIC's packet number is strictly increasing within a packet number
space, and directly encodes transmission order. A higher packet
number signifies that the packet was sent later, and a lower packet
number signifies that the packet was sent earlier. When a packet
containing ack-eliciting frames is detected lost, QUIC includes
necessary frames in a new packet with a new packet number, removing
ambiguity about which packet is acknowledged when an ACK is received.
Consequently, more accurate RTT measurements can be made, spurious
retransmissions are trivially detected, and mechanisms such as Fast
Retransmit can be applied universally, based only on packet number.
This design point significantly simplifies loss detection mechanisms
for QUIC. Most TCP mechanisms implicitly attempt to infer
transmission ordering based on TCP sequence numbers - a non-trivial
task, especially when TCP timestamps are not available.
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4.3. Clearer Loss Epoch
QUIC starts a loss epoch when a packet is lost. The loss epoch ends
when any packet sent after the start of the epoch is acknowledged.
TCP waits for the gap in the sequence number space to be filled, and
so if a segment is lost multiple times in a row, the loss epoch may
not end for several round trips. Because both should reduce their
congestion windows only once per epoch, QUIC will do it once for
every round trip that experiences loss, while TCP may only do it once
across multiple round trips.
4.4. No Reneging
QUIC ACK frames contain information similar to that in TCP Selective
Acknowledgements (SACKs, [RFC2018]). However, QUIC does not allow a
packet acknowledgement to be reneged, greatly simplifying
implementations on both sides and reducing memory pressure on the
sender.
4.5. More ACK Ranges
QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In
high loss environments, this speeds recovery, reduces spurious
retransmits, and ensures forward progress without relying on
timeouts.
4.6. Explicit Correction For Delayed Acknowledgments
QUIC endpoints measure the delay incurred between when a packet is
received and when the corresponding acknowledgment is sent, allowing
a peer to maintain a more accurate round-trip time estimate; see
Section 13.2 of [QUIC-TRANSPORT].
4.7. Probe Timeout Replaces RTO and TLP
QUIC uses a probe timeout (PTO; see Section 6.2), with a timer based
on TCP's RTO computation; see [RFC6297]. QUIC's PTO includes the
peer's maximum expected acknowledgment delay instead of using a fixed
minimum timeout.
Similar to the RACK-TLP loss detection algorithm for TCP ([RACK]),
QUIC does not collapse the congestion window when the PTO expires,
since a single packet loss at the tail does not indicate persistent
congestion. Instead, QUIC collapses the congestion window when
persistent congestion is declared; see Section 7.6. In doing this,
QUIC avoids unnecessary congestion window reductions, obviating the
need for correcting mechanisms such as F-RTO ([RFC5682]). Since QUIC
does not collapse the congestion window on a PTO expiration, a QUIC
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sender is not limited from sending more in-flight packets after a PTO
expiration if it still has available congestion window. This occurs
when a sender is application-limited and the PTO timer expires. This
is more aggressive than TCP's RTO mechanism when application-limited,
but identical when not application-limited.
QUIC allows probe packets to temporarily exceed the congestion window
whenever the timer expires.
4.8. The Minimum Congestion Window is Two Packets
TCP uses a minimum congestion window of one packet. However, loss of
that single packet means that the sender needs to waiting for a PTO
(Section 6.2) to recover, which can be much longer than a round-trip
time. Sending a single ack-eliciting packet also increases the
chances of incurring additional latency when a receiver delays its
acknowledgment.
QUIC therefore recommends that the minimum congestion window be two
packets. While this increases network load, it is considered safe,
since the sender will still reduce its sending rate exponentially
under persistent congestion (Section 6.2).
5. Estimating the Round-Trip Time
At a high level, an endpoint measures the time from when a packet was
sent to when it is acknowledged as a round-trip time (RTT) sample.
The endpoint uses RTT samples and peer-reported host delays (see
Section 13.2 of [QUIC-TRANSPORT]) to generate a statistical
description of the network path's RTT. An endpoint computes the
following three values for each path: the minimum value over a period
of time (min_rtt), an exponentially-weighted moving average
(smoothed_rtt), and the mean deviation (referred to as "variation" in
the rest of this document) in the observed RTT samples (rttvar).
5.1. Generating RTT samples
An endpoint generates an RTT sample on receiving an ACK frame that
meets the following two conditions:
* the largest acknowledged packet number is newly acknowledged, and
* at least one of the newly acknowledged packets was ack-eliciting.
The RTT sample, latest_rtt, is generated as the time elapsed since
the largest acknowledged packet was sent:
latest_rtt = ack_time - send_time_of_largest_acked
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An RTT sample is generated using only the largest acknowledged packet
in the received ACK frame. This is because a peer reports
acknowledgment delays for only the largest acknowledged packet in an
ACK frame. While the reported acknowledgment delay is not used by
the RTT sample measurement, it is used to adjust the RTT sample in
subsequent computations of smoothed_rtt and rttvar (Section 5.3).
To avoid generating multiple RTT samples for a single packet, an ACK
frame SHOULD NOT be used to update RTT estimates if it does not newly
acknowledge the largest acknowledged packet.
An RTT sample MUST NOT be generated on receiving an ACK frame that
does not newly acknowledge at least one ack-eliciting packet. A peer
usually does not send an ACK frame when only non-ack-eliciting
packets are received. Therefore an ACK frame that contains
acknowledgments for only non-ack-eliciting packets could include an
arbitrarily large ACK Delay value. Ignoring such ACK frames avoids
complications in subsequent smoothed_rtt and rttvar computations.
A sender might generate multiple RTT samples per RTT when multiple
ACK frames are received within an RTT. As suggested in [RFC6298],
doing so might result in inadequate history in smoothed_rtt and
rttvar. Ensuring that RTT estimates retain sufficient history is an
open research question.
5.2. Estimating min_rtt
min_rtt is the sender's estimate of the minimum RTT observed for a
given network path over a period of time. In this document, min_rtt
is used by loss detection to reject implausibly small rtt samples.
min_rtt MUST be set to the latest_rtt on the first RTT sample.
min_rtt MUST be set to the lesser of min_rtt and latest_rtt
(Section 5.1) on all other samples.
An endpoint uses only locally observed times in computing the min_rtt
and does not adjust for acknowledgment delays reported by the peer.
Doing so allows the endpoint to set a lower bound for the
smoothed_rtt based entirely on what it observes (see Section 5.3),
and limits potential underestimation due to erroneously-reported
delays by the peer.
The RTT for a network path may change over time. If a path's actual
RTT decreases, the min_rtt will adapt immediately on the first low
sample. If the path's actual RTT increases however, the min_rtt will
not adapt to it, allowing future RTT samples that are smaller than
the new RTT to be included in smoothed_rtt.
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Endpoints SHOULD set the min_rtt to the newest RTT sample after
persistent congestion is established. This is to allow a connection
to reset its estimate of min_rtt and smoothed_rtt (Section 5.3) after
a disruptive network event, and because it is possible that an
increase in path delay resulted in persistent congestion being
incorrectly declared.
Endpoints MAY re-establish the min_rtt at other times in the
connection, such as when traffic volume is low and an acknowledgment
is received with a low acknowledgment delay. Implementations SHOULD
NOT refresh the min_rtt value too often, since the actual minimum RTT
of the path is not frequently observable.
5.3. Estimating smoothed_rtt and rttvar
smoothed_rtt is an exponentially-weighted moving average of an
endpoint's RTT samples, and rttvar estimates the variation in the RTT
samples using a mean variation.
The calculation of smoothed_rtt uses RTT samples after adjusting them
for acknowledgment delays. These delays are decoded from the ACK
Delay field of ACK frames as described in Section 19.3 of
[QUIC-TRANSPORT].
The peer might report acknowledgment delays that are larger than the
peer's max_ack_delay during the handshake (Section 13.2.1 of
[QUIC-TRANSPORT]). To account for this, the endpoint SHOULD ignore
max_ack_delay until the handshake is confirmed, as defined in
Section 4.1.2 of [QUIC-TLS]. When they occur, these large
acknowledgment delays are likely to be non-repeating and limited to
the handshake. The endpoint can therefore use them without limiting
them to the max_ack_delay, avoiding unnecessary inflation of the RTT
estimate.
Note that a large acknowledgment delay can result in a substantially
inflated smoothed_rtt, if there is either an error in the peer's
reporting of the acknowledgment delay or in the endpoint's min_rtt
estimate. Therefore, prior to handshake confirmation, an endpoint
MAY ignore RTT samples if adjusting the RTT sample for acknowledgment
delay causes the sample to be less than the min_rtt.
After the handshake is confirmed, any acknowledgment delays reported
by the peer that are greater than the peer's max_ack_delay are
attributed to unintentional but potentially repeating delays, such as
scheduler latency at the peer or loss of previous acknowledgments.
Excess delays could also be due to a non-compliant receiver.
Therefore, these extra delays are considered effectively part of path
delay and incorporated into the RTT estimate.
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Therefore, when adjusting an RTT sample using peer-reported
acknowledgment delays, an endpoint:
* MAY ignore the acknowledgment delay for Initial packets, since
these acknowledgments are not delayed by the peer (Section 13.2.1
of [QUIC-TRANSPORT]);
* SHOULD ignore the peer's max_ack_delay until the handshake is
confirmed;
* MUST use the lesser of the acknowledgment delay and the peer's
max_ack_delay after the handshake is confirmed; and
* MUST NOT subtract the acknowledgment delay from the RTT sample if
the resulting value is smaller than the min_rtt. This limits the
underestimation of the smoothed_rtt due to a misreporting peer.
Additionally, an endpoint might postpone the processing of
acknowledgments when the corresponding decryption keys are not
immediately available. For example, a client might receive an
acknowledgment for a 0-RTT packet that it cannot decrypt because
1-RTT packet protection keys are not yet available to it. In such
cases, an endpoint SHOULD subtract such local delays from its RTT
sample until the handshake is confirmed.
Similar to [RFC6298], smoothed_rtt and rttvar are computed as
follows.
An endpoint initializes the RTT estimator during connection
establishment and when the estimator is reset during connection
migration; see Section 9.4 of [QUIC-TRANSPORT]. Before any RTT
samples are available for a new path or when the estimator is reset,
the estimator is initialized using the initial RTT; see
Section 6.2.2.
smoothed_rtt and rttvar are initialized as follows, where kInitialRtt
contains the initial RTT value:
smoothed_rtt = kInitialRtt
rttvar = kInitialRtt / 2
RTT samples for the network path are recorded in latest_rtt; see
Section 5.1. On the first RTT sample after initialization, the
estimator is reset using that sample. This ensures that the
estimator retains no history of past samples.
On the first RTT sample after initialization, smoothed_rtt and rttvar
are set as follows:
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smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2
On subsequent RTT samples, smoothed_rtt and rttvar evolve as follows:
ack_delay = decoded acknowledgment delay from ACK frame
if (handshake confirmed):
ack_delay = min(ack_delay, max_ack_delay)
adjusted_rtt = latest_rtt
if (min_rtt + ack_delay < latest_rtt):
adjusted_rtt = latest_rtt - ack_delay
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
rttvar_sample = abs(smoothed_rtt - adjusted_rtt)
rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
6. Loss Detection
QUIC senders use acknowledgments to detect lost packets, and a probe
time out (see Section 6.2) to ensure acknowledgments are received.
This section provides a description of these algorithms.
If a packet is lost, the QUIC transport needs to recover from that
loss, such as by retransmitting the data, sending an updated frame,
or discarding the frame. For more information, see Section 13.3 of
[QUIC-TRANSPORT].
Loss detection is separate per packet number space, unlike RTT
measurement and congestion control, because RTT and congestion
control are properties of the path, whereas loss detection also
relies upon key availability.
6.1. Acknowledgment-Based Detection
Acknowledgment-based loss detection implements the spirit of TCP's
Fast Retransmit ([RFC5681]), Early Retransmit ([RFC5827]), FACK
([FACK]), SACK loss recovery ([RFC6675]), and RACK-TLP ([RACK]).
This section provides an overview of how these algorithms are
implemented in QUIC.
A packet is declared lost if it meets all the following conditions:
* The packet is unacknowledged, in-flight, and was sent prior to an
acknowledged packet.
* The packet was sent kPacketThreshold packets before an
acknowledged packet (Section 6.1.1), or it was sent long enough in
the past (Section 6.1.2).
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The acknowledgment indicates that a packet sent later was delivered,
and the packet and time thresholds provide some tolerance for packet
reordering.
Spuriously declaring packets as lost leads to unnecessary
retransmissions and may result in degraded performance due to the
actions of the congestion controller upon detecting loss.
Implementations can detect spurious retransmissions and increase the
reordering threshold in packets or time to reduce future spurious
retransmissions and loss events. Implementations with adaptive time
thresholds MAY choose to start with smaller initial reordering
thresholds to minimize recovery latency.
6.1.1. Packet Threshold
The RECOMMENDED initial value for the packet reordering threshold
(kPacketThreshold) is 3, based on best practices for TCP loss
detection ([RFC5681], [RFC6675]). In order to remain similar to TCP,
implementations SHOULD NOT use a packet threshold less than 3; see
[RFC5681].
Some networks may exhibit higher degrees of packet reordering,
causing a sender to detect spurious losses. Additionally, packet
reordering could be more common with QUIC than TCP, because network
elements that could observe and reorder TCP packets cannot do that
for QUIC, because QUIC packet numbers are encrypted. Algorithms that
increase the reordering threshold after spuriously detecting losses,
such as RACK [RACK], have proven to be useful in TCP and are expected
to be at least as useful in QUIC.
6.1.2. Time Threshold
Once a later packet within the same packet number space has been
acknowledged, an endpoint SHOULD declare an earlier packet lost if it
was sent a threshold amount of time in the past. To avoid declaring
packets as lost too early, this time threshold MUST be set to at
least the local timer granularity, as indicated by the kGranularity
constant. The time threshold is:
max(kTimeThreshold * max(smoothed_rtt, latest_rtt), kGranularity)
If packets sent prior to the largest acknowledged packet cannot yet
be declared lost, then a timer SHOULD be set for the remaining time.
Using max(smoothed_rtt, latest_rtt) protects from the two following
cases:
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* the latest RTT sample is lower than the smoothed RTT, perhaps due
to reordering where the acknowledgment encountered a shorter path;
* the latest RTT sample is higher than the smoothed RTT, perhaps due
to a sustained increase in the actual RTT, but the smoothed RTT
has not yet caught up.
The RECOMMENDED time threshold (kTimeThreshold), expressed as a
round-trip time multiplier, is 9/8. The RECOMMENDED value of the
timer granularity (kGranularity) is 1ms.
Note: TCP's RACK ([RACK]) specifies a slightly larger threshold,
equivalent to 5/4, for a similar purpose. Experience with QUIC
shows that 9/8 works well.
Implementations MAY experiment with absolute thresholds, thresholds
from previous connections, adaptive thresholds, or including RTT
variation. Smaller thresholds reduce reordering resilience and
increase spurious retransmissions, and larger thresholds increase
loss detection delay.
6.2. Probe Timeout
A Probe Timeout (PTO) triggers sending one or two probe datagrams
when ack-eliciting packets are not acknowledged within the expected
period of time or the server may not have validated the client's
address. A PTO enables a connection to recover from loss of tail
packets or acknowledgments.
As with loss detection, the probe timeout is per packet number space.
That is, a PTO value is computed per packet number space.
A PTO timer expiration event does not indicate packet loss and MUST
NOT cause prior unacknowledged packets to be marked as lost. When an
acknowledgment is received that newly acknowledges packets, loss
detection proceeds as dictated by packet and time threshold
mechanisms; see Section 6.1.
The PTO algorithm used in QUIC implements the reliability functions
of Tail Loss Probe [RACK], RTO [RFC5681], and F-RTO algorithms for
TCP [RFC5682]. The timeout computation is based on TCP's
retransmission timeout period [RFC6298].
6.2.1. Computing PTO
When an ack-eliciting packet is transmitted, the sender schedules a
timer for the PTO period as follows:
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PTO = smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay
The PTO period is the amount of time that a sender ought to wait for
an acknowledgment of a sent packet. This time period includes the
estimated network roundtrip-time (smoothed_rtt), the variation in the
estimate (4*rttvar), and max_ack_delay, to account for the maximum
time by which a receiver might delay sending an acknowledgment.
When the PTO is armed for Initial or Handshake packet number spaces,
the max_ack_delay in the PTO period computation is set to 0, since
the peer is expected to not delay these packets intentionally; see
13.2.1 of [QUIC-TRANSPORT].
The PTO period MUST be at least kGranularity, to avoid the timer
expiring immediately.
When ack-eliciting packets in multiple packet number spaces are in
flight, the timer MUST be set to the earlier value of the Initial and
Handshake packet number spaces.
An endpoint MUST NOT set its PTO timer for the application data
packet number space until the handshake is confirmed. Doing so
prevents the endpoint from retransmitting information in packets when
either the peer does not yet have the keys to process them or the
endpoint does not yet have the keys to process their acknowledgments.
For example, this can happen when a client sends 0-RTT packets to the
server; it does so without knowing whether the server will be able to
decrypt them. Similarly, this can happen when a server sends 1-RTT
packets before confirming that the client has verified the server's
certificate and can therefore read these 1-RTT packets.
A sender SHOULD restart its PTO timer every time an ack-eliciting
packet is sent or acknowledged, or when Initial or Handshake keys are
discarded (Section 4.9 of [QUIC-TLS]). This ensures the PTO is
always set based on the latest estimate of the round-trip time and
for the correct packet across packet number spaces.
When a PTO timer expires, the PTO backoff MUST be increased,
resulting in the PTO period being set to twice its current value.
The PTO backoff factor is reset when an acknowledgment is received,
except in the following case. A server might take longer to respond
to packets during the handshake than otherwise. To protect such a
server from repeated client probes, the PTO backoff is not reset at a
client that is not yet certain that the server has finished
validating the client's address. That is, a client does not reset
the PTO backoff factor on receiving acknowledgments in Initial
packets.
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This exponential reduction in the sender's rate is important because
consecutive PTOs might be caused by loss of packets or
acknowledgments due to severe congestion. Even when there are ack-
eliciting packets in-flight in multiple packet number spaces, the
exponential increase in probe timeout occurs across all spaces to
prevent excess load on the network. For example, a timeout in the
Initial packet number space doubles the length of the timeout in the
Handshake packet number space.
The total length of time over which consecutive PTOs expire is
limited by the idle timeout.
The PTO timer MUST NOT be set if a timer is set for time threshold
loss detection; see Section 6.1.2. A timer that is set for time
threshold loss detection will expire earlier than the PTO timer in
most cases and is less likely to spuriously retransmit data.
6.2.2. Handshakes and New Paths
Resumed connections over the same network MAY use the previous
connection's final smoothed RTT value as the resumed connection's
initial RTT. When no previous RTT is available, the initial RTT
SHOULD be set to 333ms. This results in handshakes starting with a
PTO of 1 second, as recommended for TCP's initial retransmission
timeout; see Section 2 of [RFC6298].
A connection MAY use the delay between sending a PATH_CHALLENGE and
receiving a PATH_RESPONSE to set the initial RTT (see kInitialRtt in
Appendix A.2) for a new path, but the delay SHOULD NOT be considered
an RTT sample.
Initial packets and Handshake packets could be never acknowledged,
but they are removed from bytes in flight when the Initial and
Handshake keys are discarded, as described below in Section 6.4.
When Initial or Handshake keys are discarded, the PTO and loss
detection timers MUST be reset, because discarding keys indicates
forward progress and the loss detection timer might have been set for
a now discarded packet number space.
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6.2.2.1. Before Address Validation
Until the server has validated the client's address on the path, the
amount of data it can send is limited to three times the amount of
data received, as specified in Section 8.1 of [QUIC-TRANSPORT]. If
no additional data can be sent, the server's PTO timer MUST NOT be
armed until datagrams have been received from the client, because
packets sent on PTO count against the anti-amplification limit. Note
that the server could fail to validate the client's address even if
0-RTT is accepted.
Since the server could be blocked until more datagrams are received
from the client, it is the client's responsibility to send packets to
unblock the server until it is certain that the server has finished
its address validation (see Section 8 of [QUIC-TRANSPORT]). That is,
the client MUST set the probe timer if the client has not received an
acknowledgment for any of its Handshake packets and the handshake is
not confirmed (see Section 4.1.2 of [QUIC-TLS]), even if there are no
packets in flight. When the PTO fires, the client MUST send a
Handshake packet if it has Handshake keys, otherwise it MUST send an
Initial packet in a UDP datagram with a payload of at least 1200
bytes.
6.2.3. Speeding Up Handshake Completion
When a server receives an Initial packet containing duplicate CRYPTO
data, it can assume the client did not receive all of the server's
CRYPTO data sent in Initial packets, or the client's estimated RTT is
too small. When a client receives Handshake or 1-RTT packets prior
to obtaining Handshake keys, it may assume some or all of the
server's Initial packets were lost.
To speed up handshake completion under these conditions, an endpoint
MAY, for a limited number of times per connection, send a packet
containing unacknowledged CRYPTO data earlier than the PTO expiry,
subject to the address validation limits in Section 8.1 of
[QUIC-TRANSPORT]. Doing so at most once for each connection is
adequate to quickly recover from a single packet loss. An endpoint
that always retransmits packets in response to receiving packets that
it cannot process risks creating an infinite exchange of packets.
Endpoints can also use coalesced packets (see Section 12.2 of
[QUIC-TRANSPORT]) to ensure that each datagram elicits at least one
acknowledgment. For example, a client can coalesce an Initial packet
containing PING and PADDING frames with a 0-RTT data packet and a
server can coalesce an Initial packet containing a PING frame with
one or more packets in its first flight.
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6.2.4. Sending Probe Packets
When a PTO timer expires, a sender MUST send at least one ack-
eliciting packet in the packet number space as a probe. An endpoint
MAY send up to two full-sized datagrams containing ack-eliciting
packets, to avoid an expensive consecutive PTO expiration due to a
single lost datagram, or transmit data from multiple packet number
spaces. All probe packets sent on a PTO MUST be ack-eliciting.
In addition to sending data in the packet number space for which the
timer expired, the sender SHOULD send ack-eliciting packets from
other packet number spaces with in-flight data, coalescing packets if
possible. This is particularly valuable when the server has both
Initial and Handshake data in-flight or the client has both Handshake
and Application Data in-flight, because the peer might only have
receive keys for one of the two packet number spaces.
If the sender wants to elicit a faster acknowledgment on PTO, it can
skip a packet number to eliminate the acknowledgment delay.
An endpoint SHOULD include new data in packets that are sent on PTO
expiration. Previously sent data MAY be sent if no new data can be
sent. Implementations MAY use alternative strategies for determining
the content of probe packets, including sending new or retransmitted
data based on the application's priorities.
It is possible the sender has no new or previously-sent data to send.
As an example, consider the following sequence of events: new
application data is sent in a STREAM frame, deemed lost, then
retransmitted in a new packet, and then the original transmission is
acknowledged. When there is no data to send, the sender SHOULD send
a PING or other ack-eliciting frame in a single packet, re-arming the
PTO timer.
Alternatively, instead of sending an ack-eliciting packet, the sender
MAY mark any packets still in flight as lost. Doing so avoids
sending an additional packet, but increases the risk that loss is
declared too aggressively, resulting in an unnecessary rate reduction
by the congestion controller.
Consecutive PTO periods increase exponentially, and as a result,
connection recovery latency increases exponentially as packets
continue to be dropped in the network. Sending two packets on PTO
expiration increases resilience to packet drops, thus reducing the
probability of consecutive PTO events.
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When the PTO timer expires multiple times and new data cannot be
sent, implementations must choose between sending the same payload
every time or sending different payloads. Sending the same payload
may be simpler and ensures the highest priority frames arrive first.
Sending different payloads each time reduces the chances of spurious
retransmission.
6.3. Handling Retry Packets
A Retry packet causes a client to send another Initial packet,
effectively restarting the connection process. A Retry packet
indicates that the Initial was received, but not processed. A Retry
packet cannot be treated as an acknowledgment, because it does not
indicate that a packet was processed or specify the packet number.
Clients that receive a Retry packet reset congestion control and loss
recovery state, including resetting any pending timers. Other
connection state, in particular cryptographic handshake messages, is
retained; see Section 17.2.5 of [QUIC-TRANSPORT].
The client MAY compute an RTT estimate to the server as the time
period from when the first Initial was sent to when a Retry or a
Version Negotiation packet is received. The client MAY use this
value in place of its default for the initial RTT estimate.
6.4. Discarding Keys and Packet State
When Initial and Handshake packet protection keys are discarded (see
Section 4.9 of [QUIC-TLS]), all packets that were sent with those
keys can no longer be acknowledged because their acknowledgments
cannot be processed. The sender MUST discard all recovery state
associated with those packets and MUST remove them from the count of
bytes in flight.
Endpoints stop sending and receiving Initial packets once they start
exchanging Handshake packets; see Section 17.2.2.1 of
[QUIC-TRANSPORT]. At this point, recovery state for all in-flight
Initial packets is discarded.
When 0-RTT is rejected, recovery state for all in-flight 0-RTT
packets is discarded.
If a server accepts 0-RTT, but does not buffer 0-RTT packets that
arrive before Initial packets, early 0-RTT packets will be declared
lost, but that is expected to be infrequent.
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It is expected that keys are discarded after packets encrypted with
them would be acknowledged or declared lost. However, Initial and
Handshake secrets are discarded as soon as handshake and 1-RTT keys
are proven to be available to both client and server; see
Section 4.9.1 of [QUIC-TLS].
7. Congestion Control
This document specifies a sender-side congestion controller for QUIC
similar to TCP NewReno ([RFC6582]).
The signals QUIC provides for congestion control are generic and are
designed to support different sender-side algorithms. A sender can
unilaterally choose a different algorithm to use, such as Cubic
([RFC8312]).
If a sender uses a different controller than that specified in this
document, the chosen controller MUST conform to the congestion
control guidelines specified in Section 3.1 of [RFC8085].
Similar to TCP, packets containing only ACK frames do not count
towards bytes in flight and are not congestion controlled. Unlike
TCP, QUIC can detect the loss of these packets and MAY use that
information to adjust the congestion controller or the rate of ACK-
only packets being sent, but this document does not describe a
mechanism for doing so.
The algorithm in this document specifies and uses the controller's
congestion window in bytes.
An endpoint MUST NOT send a packet if it would cause bytes_in_flight
(see Appendix B.2) to be larger than the congestion window, unless
the packet is sent on a PTO timer expiration (see Section 6.2) or
when entering recovery (see Section 7.3.2).
7.1. Explicit Congestion Notification
If a path has been validated to support ECN ([RFC3168], [RFC8311]),
QUIC treats a Congestion Experienced (CE) codepoint in the IP header
as a signal of congestion. This document specifies an endpoint's
response when the peer-reported ECN-CE count increases; see
Section 13.4.2 of [QUIC-TRANSPORT].
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7.2. Initial and Minimum Congestion Window
QUIC begins every connection in slow start with the congestion window
set to an initial value. Endpoints SHOULD use an initial congestion
window of 10 times the maximum datagram size (max_datagram_size),
while limiting the window to the larger of 14720 bytes or twice the
maximum datagram size. This follows the analysis and recommendations
in [RFC6928], increasing the byte limit to account for the smaller
8-byte overhead of UDP compared to the 20-byte overhead for TCP.
If the maximum datagram size changes during the connection, the
initial congestion window SHOULD be recalculated with the new size.
If the maximum datagram size is decreased in order to complete the
handshake, the congestion window SHOULD be set to the new initial
congestion window.
Prior to validating the client's address, the server can be further
limited by the anti-amplification limit as specified in Section 8.1
of [QUIC-TRANSPORT]. Though the anti-amplification limit can prevent
the congestion window from being fully utilized and therefore slow
down the increase in congestion window, it does not directly affect
the congestion window.
The minimum congestion window is the smallest value the congestion
window can decrease to as a response to loss, increase in the peer-
reported ECN-CE count, or persistent congestion. The RECOMMENDED
value is 2 * max_datagram_size.
7.3. Congestion Control States
The NewReno congestion controller described in this document has
three distinct states, as shown in Figure 1.
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New Path or +------------+
persistent congestion | Slow |
(O)---------------------->| Start |
+------------+
|
Loss or |
ECN-CE increase |
v
+------------+ Loss or +------------+
| Congestion | ECN-CE increase | Recovery |
| Avoidance |------------------>| Period |
+------------+ +------------+
^ |
| |
+----------------------------+
Acknowledgment of packet
sent during recovery
Figure 1: Congestion Control States and Transitions
These states and the transitions between them are described in
subsequent sections.
7.3.1. Slow Start
A NewReno sender is in slow start any time the congestion window is
below the slow start threshold. A sender begins in slow start
because the slow start threshold is initialized to an infinite value.
While a sender is in slow start, the congestion window increases by
the number of bytes acknowledged when each acknowledgment is
processed. This results in exponential growth of the congestion
window.
The sender MUST exit slow start and enter a recovery period when a
packet is lost or when the ECN-CE count reported by its peer
increases.
A sender re-enters slow start any time the congestion window is less
than the slow start threshold, which only occurs after persistent
congestion is declared.
7.3.2. Recovery
A NewReno sender enters a recovery period when it detects the loss of
a packet or the ECN-CE count reported by its peer increases. A
sender that is already in a recovery period stays in it and does not
re-enter it.
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On entering a recovery period, a sender MUST set the slow start
threshold to half the value of the congestion window when loss is
detected. The congestion window MUST be set to the reduced value of
the slow start threshold before exiting the recovery period.
Implementations MAY reduce the congestion window immediately upon
entering a recovery period or use other mechanisms, such as
Proportional Rate Reduction ([PRR]), to reduce the congestion window
more gradually. If the congestion window is reduced immediately, a
single packet can be sent prior to reduction. This speeds up loss
recovery if the data in the lost packet is retransmitted and is
similar to TCP as described in Section 5 of [RFC6675].
The recovery period aims to limit congestion window reduction to once
per round trip. Therefore during a recovery period, the congestion
window does not change in response to new losses or increases in the
ECN-CE count.
A recovery period ends and the sender enters congestion avoidance
when a packet sent during the recovery period is acknowledged. This
is slightly different from TCP's definition of recovery, which ends
when the lost segment that started recovery is acknowledged
([RFC5681]).
7.3.3. Congestion Avoidance
A NewReno sender is in congestion avoidance any time the congestion
window is at or above the slow start threshold and not in a recovery
period.
A sender in congestion avoidance uses an Additive Increase
Multiplicative Decrease (AIMD) approach that MUST limit the increase
to the congestion window to at most one maximum datagram size for
each congestion window that is acknowledged.
The sender exits congestion avoidance and enters a recovery period
when a packet is lost or when the ECN-CE count reported by its peer
increases.
7.4. Ignoring Loss of Undecryptable Packets
During the handshake, some packet protection keys might not be
available when a packet arrives and the receiver can choose to drop
the packet. In particular, Handshake and 0-RTT packets cannot be
processed until the Initial packets arrive and 1-RTT packets cannot
be processed until the handshake completes. Endpoints MAY ignore the
loss of Handshake, 0-RTT, and 1-RTT packets that might have arrived
before the peer had packet protection keys to process those packets.
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Endpoints MUST NOT ignore the loss of packets that were sent after
the earliest acknowledged packet in a given packet number space.
7.5. Probe Timeout
Probe packets MUST NOT be blocked by the congestion controller. A
sender MUST however count these packets as being additionally in
flight, since these packets add network load without establishing
packet loss. Note that sending probe packets might cause the
sender's bytes in flight to exceed the congestion window until an
acknowledgment is received that establishes loss or delivery of
packets.
7.6. Persistent Congestion
When a sender establishes loss of all packets sent over a long enough
duration, the network is considered to be experiencing persistent
congestion.
7.6.1. Duration
The persistent congestion duration is computed as follows:
(smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay) *
kPersistentCongestionThreshold
Unlike the PTO computation in Section 6.2, this duration includes the
max_ack_delay irrespective of the packet number spaces in which
losses are established.
This duration allows a sender to send as many packets before
establishing persistent congestion, including some in response to PTO
expiration, as TCP does with Tail Loss Probes ([RACK]) and a
Retransmission Timeout ([RFC5681]).
Larger values of kPersistentCongestionThreshold cause the sender to
become less responsive to persistent congestion in the network, which
can result in aggressive sending into a congested network. Too small
a value can result in a sender declaring persistent congestion
unnecessarily, resulting in reduced throughput for the sender.
The RECOMMENDED value for kPersistentCongestionThreshold is 3, which
results in behavior that is approximately equivalent to a TCP sender
declaring an RTO after two TLPs.
This design does not use consecutive PTO events to establish
persistent congestion, since application patterns impact PTO
expirations. For example, a sender that sends small amounts of data
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with silence periods between them restarts the PTO timer every time
it sends, potentially preventing the PTO timer from expiring for a
long period of time, even when no acknowledgments are being received.
The use of a duration enables a sender to establish persistent
congestion without depending on PTO expiration.
7.6.2. Establishing Persistent Congestion
A sender establishes persistent congestion after the receipt of an
acknowledgment if two packets that are ack-eliciting are declared
lost, and:
* across all packet number spaces, none of the packets sent between
the send times of these two packets are acknowledged;
* the duration between the send times of these two packets exceeds
the persistent congestion duration (Section 7.6.1); and
* a prior RTT sample existed when these two packets were sent.
These two packets MUST be ack-eliciting, since a receiver is required
to acknowledge only ack-eliciting packets within its maximum ack
delay; see Section 13.2 of [QUIC-TRANSPORT].
The persistent congestion period SHOULD NOT start until there is at
least one RTT sample. Before the first RTT sample, a sender arms its
PTO timer based on the initial RTT (Section 6.2.2), which could be
substantially larger than the actual RTT. Requiring a prior RTT
sample prevents a sender from establishing persistent congestion with
potentially too few probes.
Since network congestion is not affected by packet number spaces,
persistent congestion SHOULD consider packets sent across packet
number spaces. A sender that does not have state for all packet
number spaces or an implementation that cannot compare send times
across packet number spaces MAY use state for just the packet number
space that was acknowledged. This might result in erroneously
declaring persistent congestion, but it will not lead to a failure to
detect persistent congestion.
When persistent congestion is declared, the sender's congestion
window MUST be reduced to the minimum congestion window
(kMinimumWindow), similar to a TCP sender's response on an RTO
([RFC5681]).
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7.6.3. Example
The following example illustrates how a sender might establish
persistent congestion. Assume:
smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay = 2
kPersistentCongestionThreshold = 3
Consider the following sequence of events:
+========+===========================+
| Time | Action |
+========+===========================+
| t=0 | Send packet #1 (app data) |
+--------+---------------------------+
| t=1 | Send packet #2 (app data) |
+--------+---------------------------+
| t=1.2 | Recv acknowledgment of #1 |
+--------+---------------------------+
| t=2 | Send packet #3 (app data) |
+--------+---------------------------+
| t=3 | Send packet #4 (app data) |
+--------+---------------------------+
| t=4 | Send packet #5 (app data) |
+--------+---------------------------+
| t=5 | Send packet #6 (app data) |
+--------+---------------------------+
| t=6 | Send packet #7 (app data) |
+--------+---------------------------+
| t=8 | Send packet #8 (PTO 1) |
+--------+---------------------------+
| t=12 | Send packet #9 (PTO 2) |
+--------+---------------------------+
| t=12.2 | Recv acknowledgment of #9 |
+--------+---------------------------+
Table 1
Packets 2 through 8 are declared lost when the acknowledgment for
packet 9 is received at t = 12.2.
The congestion period is calculated as the time between the oldest
and newest lost packets: 8 - 1 = 7. The persistent congestion
duration is: 2 * 3 = 6. Because the threshold was reached and
because none of the packets between the oldest and the newest lost
packets were acknowledged, the network is considered to have
experienced persistent congestion.
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While this example shows PTO expiration, they are not required for
persistent congestion to be established.
7.7. Pacing
A sender SHOULD pace sending of all in-flight packets based on input
from the congestion controller.
Sending multiple packets into the network without any delay between
them creates a packet burst that might cause short-term congestion
and losses. Senders MUST either use pacing or limit such bursts.
Senders SHOULD limit bursts to the initial congestion window; see
Section 7.2. A sender with knowledge that the network path to the
receiver can absorb larger bursts MAY use a higher limit.
An implementation should take care to architect its congestion
controller to work well with a pacer. For instance, a pacer might
wrap the congestion controller and control the availability of the
congestion window, or a pacer might pace out packets handed to it by
the congestion controller.
Timely delivery of ACK frames is important for efficient loss
recovery. Packets containing only ACK frames SHOULD therefore not be
paced, to avoid delaying their delivery to the peer.
Endpoints can implement pacing as they choose. A perfectly paced
sender spreads packets exactly evenly over time. For a window-based
congestion controller, such as the one in this document, that rate
can be computed by averaging the congestion window over the round-
trip time. Expressed as a rate in units of bytes per time, where
congestion_window is in bytes:
rate = N * congestion_window / smoothed_rtt
Or, expressed as an inter-packet interval in units of time:
interval = ( smoothed_rtt * packet_size / congestion_window ) / N
Using a value for "N" that is small, but at least 1 (for example,
1.25) ensures that variations in round-trip time do not result in
under-utilization of the congestion window.
Practical considerations, such as packetization, scheduling delays,
and computational efficiency, can cause a sender to deviate from this
rate over time periods that are much shorter than a round-trip time.
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One possible implementation strategy for pacing uses a leaky bucket
algorithm, where the capacity of the "bucket" is limited to the
maximum burst size and the rate the "bucket" fills is determined by
the above function.
7.8. Under-utilizing the Congestion Window
When bytes in flight is smaller than the congestion window and
sending is not pacing limited, the congestion window is under-
utilized. When this occurs, the congestion window SHOULD NOT be
increased in either slow start or congestion avoidance. This can
happen due to insufficient application data or flow control limits.
A sender that paces packets (see Section 7.7) might delay sending
packets and not fully utilize the congestion window due to this
delay. A sender SHOULD NOT consider itself application limited if it
would have fully utilized the congestion window without pacing delay.
A sender MAY implement alternative mechanisms to update its
congestion window after periods of under-utilization, such as those
proposed for TCP in [RFC7661].
8. Security Considerations
8.1. Loss and Congestion Signals
Loss detection and congestion control fundamentally involve
consumption of signals, such as delay, loss, and ECN markings, from
unauthenticated entities. An attacker can cause endpoints to reduce
their sending rate by manipulating these signals; by dropping
packets, by altering path delay strategically, or by changing ECN
codepoints.
8.2. Traffic Analysis
Packets that carry only ACK frames can be heuristically identified by
observing packet size. Acknowledgment patterns may expose
information about link characteristics or application behavior. To
reduce leaked information, endpoints can bundle acknowledgments with
other frames, or they can use PADDING frames at a potential cost to
performance.
8.3. 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.
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A sender can detect suppression of reports by marking occasional
packets that it sends with an ECN-CE marking. If a packet sent with
an ECN-CE marking is not reported as having been CE marked when the
packet is acknowledged, then the sender can disable ECN for that path
by not setting ECT codepoints in subsequent packets sent on that path
[RFC3168].
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. Congestion
controllers respond to reports of ECN-CE by reducing their rate, but
the response may vary. Markings can be treated as equivalent to loss
([RFC3168]), but other responses can be specified, such as
([RFC8511]) or ([RFC8311]).
9. IANA Considerations
This document has no IANA actions.
10. References
10.1. Normative References
[QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", Work in Progress, Internet-Draft, draft-ietf-quic-
tls-34, 15 January 2021,
<https://tools.ietf.org/html/draft-ietf-quic-tls-34>.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", Work in Progress,
Internet-Draft, draft-ietf-quic-transport-34, 15 January
2021, <https://tools.ietf.org/html/draft-ietf-quic-
transport-34>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
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[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[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>.
10.2. Informative References
[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
Refining TCP Congestion Control", ACM SIGCOMM , August
1996.
[PRR] Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional
Rate Reduction for TCP", RFC 6937, DOI 10.17487/RFC6937,
May 2013, <https://www.rfc-editor.org/info/rfc6937>.
[RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
RACK-TLP loss detection algorithm for TCP", Work in
Progress, Internet-Draft, draft-ietf-tcpm-rack-15, 22
December 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-tcpm-rack-15.txt>.
[RETRANSMISSION]
Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", ACM SIGCOMM
CCR , January 1995.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP", RFC 5682,
DOI 10.17487/RFC5682, September 2009,
<https://www.rfc-editor.org/info/rfc5682>.
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[RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
P. Hurtig, "Early Retransmit for TCP and Stream Control
Transmission Protocol (SCTP)", RFC 5827,
DOI 10.17487/RFC5827, May 2010,
<https://www.rfc-editor.org/info/rfc5827>.
[RFC6297] Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort
Transport Protocols", RFC 6297, DOI 10.17487/RFC6297, June
2011, <https://www.rfc-editor.org/info/rfc6297>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.
[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, DOI 10.17487/RFC6582, April 2012,
<https://www.rfc-editor.org/info/rfc6582>.
[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP",
RFC 6675, DOI 10.17487/RFC6675, August 2012,
<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>.
[RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
TCP to Support Rate-Limited Traffic", RFC 7661,
DOI 10.17487/RFC7661, October 2015,
<https://www.rfc-editor.org/info/rfc7661>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>.
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[RFC8511] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", RFC 8511,
DOI 10.17487/RFC8511, December 2018,
<https://www.rfc-editor.org/info/rfc8511>.
Appendix A. Loss Recovery Pseudocode
We now describe an example implementation of the loss detection
mechanisms described in Section 6.
The pseudocode segments in this section are licensed as Code
Components; see the copyright notice.
A.1. Tracking Sent Packets
To correctly implement congestion control, a QUIC sender tracks every
ack-eliciting packet until the packet is acknowledged or lost. It is
expected that implementations will be able to access this information
by packet number and crypto context and store the per-packet fields
(Appendix A.1.1) for loss recovery and congestion control.
After a packet is declared lost, the endpoint can still maintain
state for it for an amount of time to allow for packet reordering;
see Section 13.3 of [QUIC-TRANSPORT]. This enables a sender to
detect spurious retransmissions.
Sent packets are tracked for each packet number space, and ACK
processing only applies to a single space.
A.1.1. Sent Packet Fields
packet_number: The packet number of the sent packet.
ack_eliciting: A boolean that indicates whether a packet is ack-
eliciting. If true, it is expected that an acknowledgment will be
received, though the peer could delay sending the ACK frame
containing it by up to the max_ack_delay.
in_flight: A boolean that indicates whether the packet counts
towards bytes in flight.
sent_bytes: The number of bytes sent in the packet, not including
UDP or IP overhead, but including QUIC framing overhead.
time_sent: The time the packet was sent.
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A.2. Constants of Interest
Constants used in loss recovery are based on a combination of RFCs,
papers, and common practice.
kPacketThreshold: Maximum reordering in packets before packet
threshold loss detection considers a packet lost. The value
recommended in Section 6.1.1 is 3.
kTimeThreshold: Maximum reordering in time before time threshold
loss detection considers a packet lost. Specified as an RTT
multiplier. The value recommended in Section 6.1.2 is 9/8.
kGranularity: Timer granularity. This is a system-dependent value,
and Section 6.1.2 recommends a value of 1ms.
kInitialRtt: The RTT used before an RTT sample is taken. The value
recommended in Section 6.2.2 is 333ms.
kPacketNumberSpace: An enum to enumerate the three packet number
spaces.
enum kPacketNumberSpace {
Initial,
Handshake,
ApplicationData,
}
A.3. Variables of interest
Variables required to implement the congestion control mechanisms are
described in this section.
latest_rtt: The most recent RTT measurement made when receiving an
ack for a previously unacked packet.
smoothed_rtt: The smoothed RTT of the connection, computed as
described in Section 5.3.
rttvar: The RTT variation, computed as described in Section 5.3.
min_rtt: The minimum RTT seen over a period of time, ignoring
acknowledgment delay, as described in Section 5.2.
first_rtt_sample: The time that the first RTT sample was obtained.
max_ack_delay: The maximum amount of time by which the receiver
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intends to delay acknowledgments for packets in the Application
Data packet number space, as defined by the eponymous transport
parameter (Section 18.2 of [QUIC-TRANSPORT]). Note that the
actual ack_delay in a received ACK frame may be larger due to late
timers, reordering, or loss.
loss_detection_timer: Multi-modal timer used for loss detection.
pto_count: The number of times a PTO has been sent without receiving
an ack.
time_of_last_ack_eliciting_packet[kPacketNumberSpace]: The time the
most recent ack-eliciting packet was sent.
largest_acked_packet[kPacketNumberSpace]: The largest packet number
acknowledged in the packet number space so far.
loss_time[kPacketNumberSpace]: The time at which the next packet in
that packet number space can be considered lost based on exceeding
the reordering window in time.
sent_packets[kPacketNumberSpace]: An association of packet numbers
in a packet number space to information about them. Described in
detail above in Appendix A.1.
A.4. Initialization
At the beginning of the connection, initialize the loss detection
variables as follows:
loss_detection_timer.reset()
pto_count = 0
latest_rtt = 0
smoothed_rtt = kInitialRtt
rttvar = kInitialRtt / 2
min_rtt = 0
first_rtt_sample = 0
for pn_space in [ Initial, Handshake, ApplicationData ]:
largest_acked_packet[pn_space] = infinite
time_of_last_ack_eliciting_packet[pn_space] = 0
loss_time[pn_space] = 0
A.5. On Sending a Packet
After a packet is sent, information about the packet is stored. The
parameters to OnPacketSent are described in detail above in
Appendix A.1.1.
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Pseudocode for OnPacketSent follows:
OnPacketSent(packet_number, pn_space, ack_eliciting,
in_flight, sent_bytes):
sent_packets[pn_space][packet_number].packet_number =
packet_number
sent_packets[pn_space][packet_number].time_sent = now()
sent_packets[pn_space][packet_number].ack_eliciting =
ack_eliciting
sent_packets[pn_space][packet_number].in_flight = in_flight
sent_packets[pn_space][packet_number].sent_bytes = sent_bytes
if (in_flight):
if (ack_eliciting):
time_of_last_ack_eliciting_packet[pn_space] = now()
OnPacketSentCC(sent_bytes)
SetLossDetectionTimer()
A.6. On Receiving a Datagram
When a server is blocked by anti-amplification limits, receiving a
datagram unblocks it, even if none of the packets in the datagram are
successfully processed. In such a case, the PTO timer will need to
be re-armed.
Pseudocode for OnDatagramReceived follows:
OnDatagramReceived(datagram):
// If this datagram unblocks the server, arm the
// PTO timer to avoid deadlock.
if (server was at anti-amplification limit):
SetLossDetectionTimer()
A.7. On Receiving an Acknowledgment
When an ACK frame is received, it may newly acknowledge any number of
packets.
Pseudocode for OnAckReceived and UpdateRtt follow:
IncludesAckEliciting(packets):
for packet in packets:
if (packet.ack_eliciting):
return true
return false
OnAckReceived(ack, pn_space):
if (largest_acked_packet[pn_space] == infinite):
largest_acked_packet[pn_space] = ack.largest_acked
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else:
largest_acked_packet[pn_space] =
max(largest_acked_packet[pn_space], ack.largest_acked)
// DetectAndRemoveAckedPackets finds packets that are newly
// acknowledged and removes them from sent_packets.
newly_acked_packets =
DetectAndRemoveAckedPackets(ack, pn_space)
// Nothing to do if there are no newly acked packets.
if (newly_acked_packets.empty()):
return
// Update the RTT if the largest acknowledged is newly acked
// and at least one ack-eliciting was newly acked.
if (newly_acked_packets.largest().packet_number ==
ack.largest_acked &&
IncludesAckEliciting(newly_acked_packets)):
latest_rtt =
now() - newly_acked_packets.largest().time_sent
UpdateRtt(ack.ack_delay)
// Process ECN information if present.
if (ACK frame contains ECN information):
ProcessECN(ack, pn_space)
lost_packets = DetectAndRemoveLostPackets(pn_space)
if (!lost_packets.empty()):
OnPacketsLost(lost_packets)
OnPacketsAcked(newly_acked_packets)
// Reset pto_count unless the client is unsure if
// the server has validated the client's address.
if (PeerCompletedAddressValidation()):
pto_count = 0
SetLossDetectionTimer()
UpdateRtt(ack_delay):
if (first_rtt_sample == 0):
min_rtt = latest_rtt
smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2
first_rtt_sample = now()
return
// min_rtt ignores acknowledgment delay.
min_rtt = min(min_rtt, latest_rtt)
// Limit ack_delay by max_ack_delay after handshake
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// confirmation.
if (handshake confirmed):
ack_delay = min(ack_delay, max_ack_delay)
// Adjust for acknowledgment delay if plausible.
adjusted_rtt = latest_rtt
if (latest_rtt > min_rtt + ack_delay):
adjusted_rtt = latest_rtt - ack_delay
rttvar = 3/4 * rttvar + 1/4 * abs(smoothed_rtt - adjusted_rtt)
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
A.8. Setting the Loss Detection Timer
QUIC loss detection uses a single timer for all timeout loss
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
function SetLossDetectionTimer defined below shows how the single
timer is set.
This algorithm may result in the timer being set in the past,
particularly if timers wake up late. Timers set in the past fire
immediately.
Pseudocode for SetLossDetectionTimer follows (where the "^" operator
represents exponentiation):
GetLossTimeAndSpace():
time = loss_time[Initial]
space = Initial
for pn_space in [ Handshake, ApplicationData ]:
if (time == 0 || loss_time[pn_space] < time):
time = loss_time[pn_space];
space = pn_space
return time, space
GetPtoTimeAndSpace():
duration = (smoothed_rtt + max(4 * rttvar, kGranularity))
* (2 ^ pto_count)
// Arm PTO from now when there are no inflight packets.
if (no in-flight packets):
assert(!PeerCompletedAddressValidation())
if (has handshake keys):
return (now() + duration), Handshake
else:
return (now() + duration), Initial
pto_timeout = infinite
pto_space = Initial
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for space in [ Initial, Handshake, ApplicationData ]:
if (no in-flight packets in space):
continue;
if (space == ApplicationData):
// Skip Application Data until handshake confirmed.
if (handshake is not confirmed):
return pto_timeout, pto_space
// Include max_ack_delay and backoff for Application Data.
duration += max_ack_delay * (2 ^ pto_count)
t = time_of_last_ack_eliciting_packet[space] + duration
if (t < pto_timeout):
pto_timeout = t
pto_space = space
return pto_timeout, pto_space
PeerCompletedAddressValidation():
// Assume clients validate the server's address implicitly.
if (endpoint is server):
return true
// Servers complete address validation when a
// protected packet is received.
return has received Handshake ACK ||
handshake confirmed
SetLossDetectionTimer():
earliest_loss_time, _ = GetLossTimeAndSpace()
if (earliest_loss_time != 0):
// Time threshold loss detection.
loss_detection_timer.update(earliest_loss_time)
return
if (server is at anti-amplification limit):
// The server's timer is not set if nothing can be sent.
loss_detection_timer.cancel()
return
if (no ack-eliciting packets in flight &&
PeerCompletedAddressValidation()):
// There is nothing to detect lost, so no timer is set.
// However, the client needs to arm the timer if the
// server might be blocked by the anti-amplification limit.
loss_detection_timer.cancel()
return
timeout, _ = GetPtoTimeAndSpace()
loss_detection_timer.update(timeout)
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A.9. On Timeout
When the loss detection timer expires, the timer's mode determines
the action to be performed.
Pseudocode for OnLossDetectionTimeout follows:
OnLossDetectionTimeout():
earliest_loss_time, pn_space = GetLossTimeAndSpace()
if (earliest_loss_time != 0):
// Time threshold loss Detection
lost_packets = DetectAndRemoveLostPackets(pn_space)
assert(!lost_packets.empty())
OnPacketsLost(lost_packets)
SetLossDetectionTimer()
return
if (bytes_in_flight > 0):
// PTO. Send new data if available, else retransmit old data.
// If neither is available, send a single PING frame.
_, pn_space = GetPtoTimeAndSpace()
SendOneOrTwoAckElicitingPackets(pn_space)
else:
assert(!PeerCompletedAddressValidation())
// Client sends an anti-deadlock packet: Initial is padded
// to earn more anti-amplification credit,
// a Handshake packet proves address ownership.
if (has Handshake keys):
SendOneAckElicitingHandshakePacket()
else:
SendOneAckElicitingPaddedInitialPacket()
pto_count++
SetLossDetectionTimer()
A.10. Detecting Lost Packets
DetectAndRemoveLostPackets is called every time an ACK is received or
the time threshold loss detection timer expires. This function
operates on the sent_packets for that packet number space and returns
a list of packets newly detected as lost.
Pseudocode for DetectAndRemoveLostPackets follows:
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DetectAndRemoveLostPackets(pn_space):
assert(largest_acked_packet[pn_space] != infinite)
loss_time[pn_space] = 0
lost_packets = []
loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)
// Minimum time of kGranularity before packets are deemed lost.
loss_delay = max(loss_delay, kGranularity)
// Packets sent before this time are deemed lost.
lost_send_time = now() - loss_delay
foreach unacked in sent_packets[pn_space]:
if (unacked.packet_number > largest_acked_packet[pn_space]):
continue
// Mark packet as lost, or set time when it should be marked.
// Note: The use of kPacketThreshold here assumes that there
// were no sender-induced gaps in the packet number space.
if (unacked.time_sent <= lost_send_time ||
largest_acked_packet[pn_space] >=
unacked.packet_number + kPacketThreshold):
sent_packets[pn_space].remove(unacked.packet_number)
lost_packets.insert(unacked)
else:
if (loss_time[pn_space] == 0):
loss_time[pn_space] = unacked.time_sent + loss_delay
else:
loss_time[pn_space] = min(loss_time[pn_space],
unacked.time_sent + loss_delay)
return lost_packets
A.11. Upon Dropping Initial or Handshake Keys
When Initial or Handshake keys are discarded, packets from the space
are discarded and loss detection state is updated.
Pseudocode for OnPacketNumberSpaceDiscarded follows:
OnPacketNumberSpaceDiscarded(pn_space):
assert(pn_space != ApplicationData)
RemoveFromBytesInFlight(sent_packets[pn_space])
sent_packets[pn_space].clear()
// Reset the loss detection and PTO timer
time_of_last_ack_eliciting_packet[pn_space] = 0
loss_time[pn_space] = 0
pto_count = 0
SetLossDetectionTimer()
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Appendix B. Congestion Control Pseudocode
We now describe an example implementation of the congestion
controller described in Section 7.
The pseudocode segments in this section are licensed as Code
Components; see the copyright notice.
B.1. Constants of interest
Constants used in congestion control are based on a combination of
RFCs, papers, and common practice.
kInitialWindow: Default limit on the initial bytes in flight as
described in Section 7.2.
kMinimumWindow: Minimum congestion window in bytes as described in
Section 7.2.
kLossReductionFactor: Scaling factor applied to reduce the
congestion window when a new loss event is detected. Section 7
recommends a value is 0.5.
kPersistentCongestionThreshold: Period of time for persistent
congestion to be established, specified as a PTO multiplier.
Section 7.6 recommends a value of 3.
B.2. Variables of interest
Variables required to implement the congestion control mechanisms are
described in this section.
max_datagram_size: The sender's current maximum payload size. Does
not include UDP or IP overhead. The max datagram size is used for
congestion window computations. An endpoint sets the value of
this variable based on its Path Maximum Transmission Unit (PMTU;
see Section 14.2 of [QUIC-TRANSPORT]), with a minimum value of
1200 bytes.
ecn_ce_counters[kPacketNumberSpace]: The highest value reported for
the ECN-CE counter in the packet number space by the peer in an
ACK frame. This value is used to detect increases in the reported
ECN-CE counter.
bytes_in_flight: The sum of the size in bytes of all sent packets
that contain at least one ack-eliciting or PADDING frame, and have
not been acknowledged or declared lost. The size does not include
IP or UDP overhead, but does include the QUIC header and AEAD
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overhead. Packets only containing ACK frames do not count towards
bytes_in_flight to ensure congestion control does not impede
congestion feedback.
congestion_window: Maximum number of bytes allowed to be in flight.
congestion_recovery_start_time: The time the current recovery period
started due to the detection of loss or ECN. When a packet sent
after this time is acknowledged, QUIC exits congestion recovery.
ssthresh: Slow start threshold in bytes. When the congestion window
is below ssthresh, the mode is slow start and the window grows by
the number of bytes acknowledged.
The congestion control pseudocode also accesses some of the variables
from the loss recovery pseudocode.
B.3. Initialization
At the beginning of the connection, initialize the congestion control
variables as follows:
congestion_window = kInitialWindow
bytes_in_flight = 0
congestion_recovery_start_time = 0
ssthresh = infinite
for pn_space in [ Initial, Handshake, ApplicationData ]:
ecn_ce_counters[pn_space] = 0
B.4. On Packet Sent
Whenever a packet is sent, and it contains non-ACK frames, the packet
increases bytes_in_flight.
OnPacketSentCC(sent_bytes):
bytes_in_flight += sent_bytes
B.5. On Packet Acknowledgment
Invoked from loss detection's OnAckReceived and is supplied with the
newly acked_packets from sent_packets.
In congestion avoidance, implementers that use an integer
representation for congestion_window should be careful with division,
and can use the alternative approach suggested in Section 2.1 of
[RFC3465].
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InCongestionRecovery(sent_time):
return sent_time <= congestion_recovery_start_time
OnPacketsAcked(acked_packets):
for acked_packet in acked_packets:
OnPacketAcked(acked_packet)
OnPacketAcked(acked_packet):
if (!acked_packet.in_flight):
return;
// Remove from bytes_in_flight.
bytes_in_flight -= acked_packet.sent_bytes
// Do not increase congestion_window if application
// limited or flow control limited.
if (IsAppOrFlowControlLimited())
return
// Do not increase congestion window in recovery period.
if (InCongestionRecovery(acked_packet.time_sent)):
return
if (congestion_window < ssthresh):
// Slow start.
congestion_window += acked_packet.sent_bytes
else:
// Congestion avoidance.
congestion_window +=
max_datagram_size * acked_packet.sent_bytes
/ congestion_window
B.6. On New Congestion Event
Invoked from ProcessECN and OnPacketsLost when a new congestion event
is detected. If not already in recovery, this starts a recovery
period and reduces the slow start threshold and congestion window
immediately.
OnCongestionEvent(sent_time):
// No reaction if already in a recovery period.
if (InCongestionRecovery(sent_time)):
return
// Enter recovery period.
congestion_recovery_start_time = now()
ssthresh = congestion_window * kLossReductionFactor
congestion_window = max(ssthresh, kMinimumWindow)
// A packet can be sent to speed up loss recovery.
MaybeSendOnePacket()
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B.7. Process ECN Information
Invoked when an ACK frame with an ECN section is received from the
peer.
ProcessECN(ack, pn_space):
// If the ECN-CE counter reported by the peer has increased,
// this could be a new congestion event.
if (ack.ce_counter > ecn_ce_counters[pn_space]):
ecn_ce_counters[pn_space] = ack.ce_counter
sent_time = sent_packets[ack.largest_acked].time_sent
OnCongestionEvent(sent_time)
B.8. On Packets Lost
Invoked when DetectAndRemoveLostPackets deems packets lost.
OnPacketsLost(lost_packets):
sent_time_of_last_loss = 0
// Remove lost packets from bytes_in_flight.
for lost_packet in lost_packets:
if lost_packet.in_flight:
bytes_in_flight -= lost_packet.sent_bytes
sent_time_of_last_loss =
max(sent_time_of_last_loss, lost_packet.time_sent)
// Congestion event if in-flight packets were lost
if (sent_time_of_last_loss != 0):
OnCongestionEvent(sent_time_of_last_loss)
// Reset the congestion window if the loss of these
// packets indicates persistent congestion.
// Only consider packets sent after getting an RTT sample.
if (first_rtt_sample == 0):
return
pc_lost = []
for lost in lost_packets:
if lost.time_sent > first_rtt_sample:
pc_lost.insert(lost)
if (InPersistentCongestion(pc_lost)):
congestion_window = kMinimumWindow
congestion_recovery_start_time = 0
B.9. Removing Discarded Packets From Bytes In Flight
When Initial or Handshake keys are discarded, packets sent in that
space no longer count toward bytes in flight.
Pseudocode for RemoveFromBytesInFlight follows:
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RemoveFromBytesInFlight(discarded_packets):
// Remove any unacknowledged packets from flight.
foreach packet in discarded_packets:
if packet.in_flight
bytes_in_flight -= size
Appendix C. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
Issue and pull request numbers are listed with a leading octothorp.
C.1. Since draft-ietf-quic-recovery-32
* Clarifications to definition of persistent congestion (#4413,
#4414, #4421, #4429, #4437)
C.2. Since draft-ietf-quic-recovery-31
* Limit the number of Initial packets sent in response to
unauthenticated packets (#4183, #4188)
C.3. Since draft-ietf-quic-recovery-30
Editorial changes only.
C.4. Since draft-ietf-quic-recovery-29
* Allow caching of packets that can't be decrypted, by allowing the
reported acknowledgment delay to exceed max_ack_delay prior to
confirming the handshake (#3821, #3980, #4035, #3874)
* Persistent congestion cannot include packets sent before the first
RTT sample for the path (#3875, #3889)
* Recommend reset of min_rtt in persistent congestion (#3927, #3975)
* Persistent congestion is independent of packet number space
(#3939, #3961)
* Only limit bursts to the initial window without information about
the path (#3892, #3936)
* Add normative requirements for increasing and reducing the
congestion window (#3944, #3978, #3997, #3998)
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C.5. Since draft-ietf-quic-recovery-28
* Refactored pseudocode to correct PTO calculation (#3564, #3674,
#3681)
C.6. Since draft-ietf-quic-recovery-27
* Added recommendations for speeding up handshake under some loss
conditions (#3078, #3080)
* PTO count is reset when handshake progress is made (#3272, #3415)
* PTO count is not reset by a client when the server might be
awaiting address validation (#3546, #3551)
* Recommend repairing losses immediately after entering the recovery
period (#3335, #3443)
* Clarified what loss conditions can be ignored during the handshake
(#3456, #3450)
* Allow, but don't recommend, using RTT from previous connection to
seed RTT (#3464, #3496)
* Recommend use of adaptive loss detection thresholds (#3571, #3572)
C.7. Since draft-ietf-quic-recovery-26
No changes.
C.8. Since draft-ietf-quic-recovery-25
No significant changes.
C.9. Since draft-ietf-quic-recovery-24
* Require congestion control of some sort (#3247, #3244, #3248)
* Set a minimum reordering threshold (#3256, #3240)
* PTO is specific to a packet number space (#3067, #3074, #3066)
C.10. Since draft-ietf-quic-recovery-23
* Define under-utilizing the congestion window (#2630, #2686, #2675)
* PTO MUST send data if possible (#3056, #3057)
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* Connection Close is not ack-eliciting (#3097, #3098)
* MUST limit bursts to the initial congestion window (#3160)
* Define the current max_datagram_size for congestion control
(#3041, #3167)
C.11. Since draft-ietf-quic-recovery-22
* PTO should always send an ack-eliciting packet (#2895)
* Unify the Handshake Timer with the PTO timer (#2648, #2658, #2886)
* Move ACK generation text to transport draft (#1860, #2916)
C.12. Since draft-ietf-quic-recovery-21
* No changes
C.13. Since draft-ietf-quic-recovery-20
* Path validation can be used as initial RTT value (#2644, #2687)
* max_ack_delay transport parameter defaults to 0 (#2638, #2646)
* ACK delay only measures intentional delays induced by the
implementation (#2596, #2786)
C.14. Since draft-ietf-quic-recovery-19
* Change kPersistentThreshold from an exponent to a multiplier
(#2557)
* Send a PING if the PTO timer fires and there's nothing to send
(#2624)
* Set loss delay to at least kGranularity (#2617)
* Merge application limited and sending after idle sections. Always
limit burst size instead of requiring resetting CWND to initial
CWND after idle (#2605)
* Rewrite RTT estimation, allow RTT samples where a newly acked
packet is ack-eliciting but the largest_acked is not (#2592)
* Don't arm the handshake timer if there is no handshake data
(#2590)
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* Clarify that the time threshold loss alarm takes precedence over
the crypto handshake timer (#2590, #2620)
* Change initial RTT to 500ms to align with RFC6298 (#2184)
C.15. Since draft-ietf-quic-recovery-18
* Change IW byte limit to 14720 from 14600 (#2494)
* Update PTO calculation to match RFC6298 (#2480, #2489, #2490)
* Improve loss detection's description of multiple packet number
spaces and pseudocode (#2485, #2451, #2417)
* Declare persistent congestion even if non-probe packets are sent
and don't make persistent congestion more aggressive than RTO
verified was (#2365, #2244)
* Move pseudocode to the appendices (#2408)
* What to send on multiple PTOs (#2380)
C.16. Since draft-ietf-quic-recovery-17
* After Probe Timeout discard in-flight packets or send another
(#2212, #1965)
* Endpoints discard initial keys as soon as handshake keys are
available (#1951, #2045)
* 0-RTT state is discarded when 0-RTT is rejected (#2300)
* Loss detection timer is cancelled when ack-eliciting frames are in
flight (#2117, #2093)
* Packets are declared lost if they are in flight (#2104)
* After becoming idle, either pace packets or reset the congestion
controller (#2138, 2187)
* Process ECN counts before marking packets lost (#2142)
* Mark packets lost before resetting crypto_count and pto_count
(#2208, #2209)
* Congestion and loss recovery state are discarded when keys are
discarded (#2327)
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C.17. Since draft-ietf-quic-recovery-16
* Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP
and min crypto timeouts; eliminate timeout validation (#2114,
#2166, #2168, #1017)
* Redefine how congestion avoidance in terms of when the period
starts (#1928, #1930)
* Document what needs to be tracked for packets that are in flight
(#765, #1724, #1939)
* Integrate both time and packet thresholds into loss detection
(#1969, #1212, #934, #1974)
* Reduce congestion window after idle, unless pacing is used (#2007,
#2023)
* Disable RTT calculation for packets that don't elicit
acknowledgment (#2060, #2078)
* Limit ack_delay by max_ack_delay (#2060, #2099)
* Initial keys are discarded once Handshake keys are available
(#1951, #2045)
* Reorder ECN and loss detection in pseudocode (#2142)
* Only cancel loss detection timer if ack-eliciting packets are in
flight (#2093, #2117)
C.18. Since draft-ietf-quic-recovery-14
* Used max_ack_delay from transport params (#1796, #1782)
* Merge ACK and ACK_ECN (#1783)
C.19. Since draft-ietf-quic-recovery-13
* Corrected the lack of ssthresh reduction in CongestionEvent
pseudocode (#1598)
* Considerations for ECN spoofing (#1426, #1626)
* Clarifications for PADDING and congestion control (#837, #838,
#1517, #1531, #1540)
* Reduce early retransmission timer to RTT/8 (#945, #1581)
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* Packets are declared lost after an RTO is verified (#935, #1582)
C.20. Since draft-ietf-quic-recovery-12
* Changes to manage separate packet number spaces and encryption
levels (#1190, #1242, #1413, #1450)
* Added ECN feedback mechanisms and handling; new ACK_ECN frame
(#804, #805, #1372)
C.21. Since draft-ietf-quic-recovery-11
No significant changes.
C.22. Since draft-ietf-quic-recovery-10
* Improved text on ack generation (#1139, #1159)
* Make references to TCP recovery mechanisms informational (#1195)
* Define time_of_last_sent_handshake_packet (#1171)
* Added signal from TLS the data it includes needs to be sent in a
Retry packet (#1061, #1199)
* Minimum RTT (min_rtt) is initialized with an infinite value
(#1169)
C.23. Since draft-ietf-quic-recovery-09
No significant changes.
C.24. Since draft-ietf-quic-recovery-08
* Clarified pacing and RTO (#967, #977)
C.25. Since draft-ietf-quic-recovery-07
* Include ACK delay in RTO(and TLP) computations (#981)
* ACK delay in SRTT computation (#961)
* Default RTT and Slow Start (#590)
* Many editorial fixes.
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C.26. Since draft-ietf-quic-recovery-06
No significant changes.
C.27. Since draft-ietf-quic-recovery-05
* Add more congestion control text (#776)
C.28. Since draft-ietf-quic-recovery-04
No significant changes.
C.29. Since draft-ietf-quic-recovery-03
No significant changes.
C.30. Since draft-ietf-quic-recovery-02
* Integrate F-RTO (#544, #409)
* Add congestion control (#545, #395)
* Require connection abort if a skipped packet was acknowledged
(#415)
* Simplify RTO calculations (#142, #417)
C.31. Since draft-ietf-quic-recovery-01
* Overview added to loss detection
* Changes initial default RTT to 100ms
* Added time-based loss detection and fixes early retransmit
* Clarified loss recovery for handshake packets
* Fixed references and made TCP references informative
C.32. Since draft-ietf-quic-recovery-00
* Improved description of constants and ACK behavior
C.33. Since draft-iyengar-quic-loss-recovery-01
* Adopted as base for draft-ietf-quic-recovery
* Updated authors/editors list
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* Added table of contents
Appendix D. Contributors
The IETF QUIC Working Group received an enormous amount of support
from many people. The following people provided substantive
contributions to this document:
* Alessandro Ghedini
* Benjamin Saunders
* Gorry Fairhurst
* 山本和彦 (Kazu Yamamoto)
* 奥 一穂 (Kazuho Oku)
* Lars Eggert
* Magnus Westerlund
* Marten Seemann
* Martin Duke
* Martin Thomson
* Mirja Kühlewind
* Nick Banks
* Praveen Balasubramanian
Acknowledgments
Authors' Addresses
Jana Iyengar (editor)
Fastly
Email: jri.ietf@gmail.com
Ian Swett (editor)
Google
Email: ianswett@google.com
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