draft-ietf-quic-recovery.txt

QUIC                                                     J. Iyengar, Ed.
Internet-Draft                                                    Fastly
Intended status: Standards Track                           I. Swett, Ed.
Expires: 10 July 2021                                             Google
                                                          6 January 2021


               QUIC Loss Detection and Congestion Control
                        draft-ietf-quic-recovery

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 10 July 2021.

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   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
   2.  Conventions and Definitions
   3.  Design of the QUIC Transmission Machinery
   4.  Relevant Differences Between QUIC and TCP
     4.1.  Separate Packet Number Spaces
     4.2.  Monotonically Increasing Packet Numbers
     4.3.  Clearer Loss Epoch
     4.4.  No Reneging
     4.5.  More ACK Ranges
     4.6.  Explicit Correction For Delayed Acknowledgments
     4.7.  Probe Timeout Replaces RTO and TLP
     4.8.  The Minimum Congestion Window is Two Packets
   5.  Estimating the Round-Trip Time
     5.1.  Generating RTT samples
     5.2.  Estimating min_rtt
     5.3.  Estimating smoothed_rtt and rttvar
   6.  Loss Detection
     6.1.  Acknowledgment-Based Detection
       6.1.1.  Packet Threshold
       6.1.2.  Time Threshold
     6.2.  Probe Timeout
       6.2.1.  Computing PTO
       6.2.2.  Handshakes and New Paths
       6.2.3.  Speeding Up Handshake Completion
       6.2.4.  Sending Probe Packets
     6.3.  Handling Retry Packets
     6.4.  Discarding Keys and Packet State
   7.  Congestion Control
     7.1.  Explicit Congestion Notification
     7.2.  Initial and Minimum Congestion Window
     7.3.  Congestion Control States
       7.3.1.  Slow Start
       7.3.2.  Recovery
       7.3.3.  Congestion Avoidance
     7.4.  Ignoring Loss of Undecryptable Packets
     7.5.  Probe Timeout
     7.6.  Persistent Congestion
       7.6.1.  Duration
       7.6.2.  Establishing Persistent Congestion
       7.6.3.  Example
     7.7.  Pacing
     7.8.  Under-utilizing the Congestion Window
   8.  Security Considerations
     8.1.  Congestion Signals
     8.2.  Traffic Analysis
     8.3.  Misreporting ECN Markings
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  Loss Recovery Pseudocode
     A.1.  Tracking Sent Packets
       A.1.1.  Sent Packet Fields
     A.2.  Constants of Interest
     A.3.  Variables of interest
     A.4.  Initialization
     A.5.  On Sending a Packet
     A.6.  On Receiving a Datagram
     A.7.  On Receiving an Acknowledgment
     A.8.  Setting the Loss Detection Timer
     A.9.  On Timeout
     A.10. Detecting Lost Packets
     A.11. Upon Dropping Initial or Handshake Keys
   Appendix B.  Congestion Control Pseudocode
     B.1.  Constants of interest
     B.2.  Variables of interest
     B.3.  Initialization
     B.4.  On Packet Sent
     B.5.  On Packet Acknowledgment
     B.6.  On New Congestion Event
     B.7.  Process ECN Information
     B.8.  On Packets Lost
     B.9.  Removing Discarded Packets From Bytes In Flight
   Appendix C.  Change Log
     C.1.  Since draft-ietf-quic-recovery-32
     C.2.  Since draft-ietf-quic-recovery-31
     C.3.  Since draft-ietf-quic-recovery-30
     C.4.  Since draft-ietf-quic-recovery-29
     C.5.  Since draft-ietf-quic-recovery-28
     C.6.  Since draft-ietf-quic-recovery-27
     C.7.  Since draft-ietf-quic-recovery-26
     C.8.  Since draft-ietf-quic-recovery-25
     C.9.  Since draft-ietf-quic-recovery-24
     C.10. Since draft-ietf-quic-recovery-23
     C.11. Since draft-ietf-quic-recovery-22
     C.12. Since draft-ietf-quic-recovery-21
     C.13. Since draft-ietf-quic-recovery-20
     C.14. Since draft-ietf-quic-recovery-19
     C.15. Since draft-ietf-quic-recovery-18
     C.16. Since draft-ietf-quic-recovery-17
     C.17. Since draft-ietf-quic-recovery-16
     C.18. Since draft-ietf-quic-recovery-14
     C.19. Since draft-ietf-quic-recovery-13
     C.20. Since draft-ietf-quic-recovery-12
     C.21. Since draft-ietf-quic-recovery-11
     C.22. Since draft-ietf-quic-recovery-10
     C.23. Since draft-ietf-quic-recovery-09
     C.24. Since draft-ietf-quic-recovery-08
     C.25. Since draft-ietf-quic-recovery-07
     C.26. Since draft-ietf-quic-recovery-06
     C.27. Since draft-ietf-quic-recovery-05
     C.28. Since draft-ietf-quic-recovery-04
     C.29. Since draft-ietf-quic-recovery-03
     C.30. Since draft-ietf-quic-recovery-02
     C.31. Since draft-ietf-quic-recovery-01
     C.32. Since draft-ietf-quic-recovery-00
     C.33. Since draft-iyengar-quic-loss-recovery-01
   Appendix D.  Contributors
   Acknowledgments
   Authors' Addresses

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.

   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 [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.

   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.

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.

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 ACKs contain information that is similar to TCP SACK, but QUIC
   does not allow any acknowledged packet 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.  QUIC's PTO includes the peer's maximum
   expected acknowledgment delay instead of using a fixed minimum
   timeout.  QUIC does not collapse the congestion window until
   persistent congestion (Section 7.6) is declared, unlike TCP, which
   collapses the congestion window upon expiry of an RTO.  Instead of
   collapsing the congestion window and declaring everything in-flight
   lost, QUIC allows probe packets to temporarily exceed the congestion
   window whenever the timer expires.

   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 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.

   A single packet loss at the tail does not indicate persistent
   congestion, so QUIC specifies a time-based definition to ensure one
   or more packets are sent prior to a dramatic decrease in congestion
   window; see Section 7.6.

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 observed over
   the lifetime of the path (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

   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.  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.

   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 is the variation in the RTT
   samples, estimated 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 (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 however 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.

   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:

   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 ([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).

   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:

   *  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:

   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, when the handshake is confirmed
   (Section 4.1.2 of [QUIC-TLS]), 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.

   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.

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 one 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 occasions per each 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.  Endpoints
   that do not cease retransmitting packets in response to
   unauthenticated data risk 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.

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.

   When the PTO timer expires, an ack-eliciting packet MUST be sent.  An
   endpoint SHOULD include new data in this packet.  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.

   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 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
   anymore.  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.

   It is expected that keys are discarded after packets encrypted with
   them would be acknowledged or declared lost.  However, Initial
   secrets are discarded as soon as handshake 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].

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),
   limited 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.

                    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.

   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.
   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
   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.

   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]).

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.

   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 bytes:

   rate = N * congestion_window / smoothed_rtt

   Or, expressed as an inter-packet interval:

   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.

   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.  Congestion Signals

   Congestion control fundamentally involves the consumption of signals
   -- both loss and ECN codepoints -- from unauthenticated entities.
   On-path attackers can spoof or alter these signals.  An attacker can
   cause endpoints to reduce their sending rate by dropping packets, or
   alter send rate by changing ECN codepoints.

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.

   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, 6 January 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-tls>.

   [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, 6 January 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-transport>.

   [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>.

   [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.

   [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>.

   [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>.

   [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>.

   [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.

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 in the connection, 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
      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 will 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.

   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
     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
     // confirmation. Note that ack_delay is 0 for
     // acknowledgments of Initial and Handshake packets.
     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:

   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
     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

     // Determine which PN space to arm PTO for.
     timeout, _ = GetPtoTimeAndSpace()
     loss_detection_timer.update(timeout)

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:

   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()

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:  Reduction in 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
      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-in-flight that may be
      sent.

   congestion_recovery_start_time:  The time when QUIC first detects
      congestion due to loss or ECN, causing it to enter congestion
      recovery.  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].

   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()

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:

   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)

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)

   *  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)

   *  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)

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)

   *  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.

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

   *  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