QUIC                                                     J. Iyengar, Ed.
Internet-Draft                                                    Fastly
Intended status: Standards Track                           I. Swett, Ed.
Expires: March 15, May 7, 2020                                              Google
                                                      September 12,
                                                       November 04, 2019

               QUIC Loss Detection and Congestion Control


   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), which is archived at
   https://mailarchive.ietf.org/arch/search/?email_list=quic [1].

   Working Group information can be found at https://github.com/quicwg
   [2]; source code and issues list for this draft can be found at
   https://github.com/quicwg/base-drafts/labels/-recovery [3].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 15, May 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Design of the QUIC Transmission Machinery . . . . . . . . . .   5
     3.1.  Relevant Differences Between QUIC and TCP . . . . . . . .   5
       3.1.1.  Separate Packet Number Spaces . . . . . . . . . . . .   6
       3.1.2.  Monotonically Increasing Packet Numbers . . . . . . .   6
       3.1.3.  Clearer Loss Epoch  . . . . . . . . . . . . . . . . .   6
       3.1.4.  No Reneging . . . . . . . . . . . . . . . . . . . . .   7
       3.1.5.  More ACK Ranges . . . . . . . . . . . . . . . . . . .   7
       3.1.6.  Explicit Correction For Delayed Acknowledgements  . .   7
   4.  Estimating the Round-Trip Time  . . . . . . . . . . . . . . .   7
     4.1.  Generating RTT samples  . . . . . . . . . . . . . . . . .   7
     4.2.  Estimating min_rtt  . . . . . . . . . . . . . . . . . . .   8
     4.3.  Estimating smoothed_rtt and rttvar  . . . . . . . . . . .   8
   5.  Loss Detection  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Acknowledgement-based Detection . . . . . . . . . . . . .  10
       5.1.1.  Packet Threshold  . . . . . . . . . . . . . . . . . .  10
       5.1.2.  Time Threshold  . . . . . . . . . . . . . . . . . . .  10
     5.2.  Probe Timeout . . . . . . . . . . . . . . . . . . . . . .  11
       5.2.1.  Computing PTO . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Handshakes and New Paths  . . . . . . . . . . . . . . . .  12
       5.3.1.  Sending Probe Packets . . . . . . . . . . . . . . . .  13
       5.3.2.  Loss Detection  . . . . . . . . . . . . . . . . . . .  14
     5.4.  Handling Retry and Version Negotiation Packets  . . . . . . . . . . . . . . . . .  14
     5.5.  Discarding Keys and Packet State  . . . . . . . . . . . .  14
     5.6.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Explicit Congestion Notification  . . . . . . . . . . . .  15
     6.2.  Slow Start  . . . . . . . . . . . . . . . . . . . . . . .  16
     6.3.  Congestion Avoidance  . . . . . . . . . . . . . . . . . .  16
     6.4.  Recovery Period . . . . . . . . . . . . . . . . . . . . .  16
     6.5.  Ignoring Loss of Undecryptable Packets  . . . . . . . . .  16
     6.6.  Probe Timeout . . . . . . . . . . . . . . . . . . . . . .  17  16
     6.7.  Persistent Congestion . . . . . . . . . . . . . . . . . .  17
     6.8.  Pacing  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.9.  Under-utilizing the Congestion Window . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     7.1.  Congestion Signals  . . . . . . . . . . . . . . . . . . .  19
     7.2.  Traffic Analysis  . . . . . . . . . . . . . . . . . . . .  19
     7.3.  Misreporting ECN Markings . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Appendix A.  Loss Recovery Pseudocode . . . . . . . . . . . . . .  22
     A.1.  Tracking Sent Packets . . . . . . . . . . . . . . . . . .  22
       A.1.1.  Sent Packet Fields  . . . . . . . . . . . . . . . . .  22
     A.2.  Constants of interest . . . . . . . . . . . . . . . . . .  23
     A.3.  Variables of interest . . . . . . . . . . . . . . . . . .  23
     A.4.  Initialization  . . . . . . . . . . . . . . . . . . . . .  24
     A.5.  On Sending a Packet . . . . . . . . . . . . . . . . . . .  25  24
     A.6.  On Receiving an Acknowledgment  . . . . . . . . . . . . .  25
     A.7.  On Packet Acknowledgment  . . . . . . . . . . . . . . . .  26
     A.8.  Setting the Loss Detection Timer  . . . . . . . . . . . .  27
     A.9.  On Timeout  . . . . . . . . . . . . . . . . . . . . . . .  29
     A.10. Detecting Lost Packets  . . . . . . . . . . . . . . . . .  29
   Appendix B.  Congestion Control Pseudocode  . . . . . . . . . . .  30
     B.1.  Constants of interest . . . . . . . . . . . . . . . . . .  30
     B.2.  Variables of interest . . . . . . . . . . . . . . . . . .  31
     B.3.  Initialization  . . . . . . . . . . . . . . . . . . . . .  32
     B.4.  On Packet Sent  . . . . . . . . . . . . . . . . . . . . .  32
     B.5.  On Packet Acknowledgement . . . . . . . . . . . . . . . .  32
     B.6.  On New Congestion Event . . . . . . . . . . . . . . . . .  33
     B.7.  Process ECN Information . . . . . . . . . . . . . . . . .  33
     B.8.  On Packets Lost . . . . . . . . . . . . . . . . . . . . .  34
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  34
     C.1.  Since draft-ietf-quic-recovery-22 draft-ietf-quic-recovery-23 . . . . . . . . . . . .  34
     C.2.  Since draft-ietf-quic-recovery-21 draft-ietf-quic-recovery-22 . . . . . . . . . . . .  34  35
     C.3.  Since draft-ietf-quic-recovery-20 draft-ietf-quic-recovery-21 . . . . . . . . . . . .  35
     C.4.  Since draft-ietf-quic-recovery-19 draft-ietf-quic-recovery-20 . . . . . . . . . . . .  35
     C.5.  Since draft-ietf-quic-recovery-18 draft-ietf-quic-recovery-19 . . . . . . . . . . . .  35
     C.6.  Since draft-ietf-quic-recovery-17 draft-ietf-quic-recovery-18 . . . . . . . . . . . .  36
     C.7.  Since draft-ietf-quic-recovery-16 draft-ietf-quic-recovery-17 . . . . . . . . . . . .  36
     C.8.  Since draft-ietf-quic-recovery-14 draft-ietf-quic-recovery-16 . . . . . . . . . . . .  37  36
     C.9.  Since draft-ietf-quic-recovery-13 draft-ietf-quic-recovery-14 . . . . . . . . . . . .  37
     C.10. Since draft-ietf-quic-recovery-12 draft-ietf-quic-recovery-13 . . . . . . . . . . . .  37
     C.11. Since draft-ietf-quic-recovery-11 draft-ietf-quic-recovery-12 . . . . . . . . . . . .  37  38
     C.12. Since draft-ietf-quic-recovery-10 draft-ietf-quic-recovery-11 . . . . . . . . . . . .  37  38
     C.13. Since draft-ietf-quic-recovery-09 draft-ietf-quic-recovery-10 . . . . . . . . . . . .  38
     C.14. Since draft-ietf-quic-recovery-08 draft-ietf-quic-recovery-09 . . . . . . . . . . . .  38
     C.15. Since draft-ietf-quic-recovery-07 draft-ietf-quic-recovery-08 . . . . . . . . . . . .  38
     C.16. Since draft-ietf-quic-recovery-06 draft-ietf-quic-recovery-07 . . . . . . . . . . . .  38
     C.17. Since draft-ietf-quic-recovery-05 draft-ietf-quic-recovery-06 . . . . . . . . . . . .  38  39
     C.18. Since draft-ietf-quic-recovery-04 draft-ietf-quic-recovery-05 . . . . . . . . . . . .  38  39
     C.19. Since draft-ietf-quic-recovery-03 draft-ietf-quic-recovery-04 . . . . . . . . . . . .  38  39
     C.20. Since draft-ietf-quic-recovery-02 draft-ietf-quic-recovery-03 . . . . . . . . . . . .  38  39
     C.21. Since draft-ietf-quic-recovery-01 draft-ietf-quic-recovery-02 . . . . . . . . . . . .  39
     C.22. Since draft-ietf-quic-recovery-00 draft-ietf-quic-recovery-01 . . . . . . . . . . . .  39
     C.23. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . .  39
     C.24. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . .  39
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  39  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39  40

1.  Introduction

   QUIC is a new multiplexed and secure transport atop UDP.  QUIC builds
   on decades of transport and security experience, and implements
   mechanisms that make it attractive as a modern general-purpose
   transport.  The QUIC protocol is described in [QUIC-TRANSPORT].

   QUIC implements the spirit of existing TCP loss recovery mechanisms,
   described in RFCs, various Internet-drafts, and also those prevalent
   in the Linux TCP implementation.  This document describes QUIC
   congestion control and loss recovery, and where applicable,
   attributes the TCP equivalent in RFCs, Internet-drafts, academic
   papers, and/or TCP implementations.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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-only:  Any packet containing only one or more ACK frame(s).

   In-flight:  Packets are considered in-flight when they have been sent
      and are not ACK-only, and they are not acknowledged, declared
      lost, or abandoned along with old keys.

   Ack-eliciting Frames:  All frames besides ACK or PADDING 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 ack delay and
      are called ack-eliciting packets.

   Crypto Packets:  Packets containing CRYPTO data sent in Initial or
      Handshake packets.

   Out-of-order Packets:  Packets that do not increase the largest
      received packet number for its packet number space by exactly one.
      Packets arrive out of order when earlier packets are lost or

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 monotonically increase
   within a space, preventing ambiguity.

   This design obviates the need for disambiguating between
   transmissions and retransmissions and eliminates significant
   complexity from QUIC's interpretation of TCP loss detection

   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:

   o  All packets are acknowledged, though packets that contain no ack-
      eliciting frames are only acknowledged along with ack-eliciting

   o  Long header packets that contain CRYPTO frames are critical to the
      performance of the QUIC handshake and use shorter timers for

   o  Packets that contain only containing frames besides ACK or CONNECTION_CLOSE frames do not
      count toward congestion control limits and are not considered in-flight. in-

   o  PADDING frames cause packets to contribute toward bytes in flight
      without directly causing an acknowledgment to be sent.

3.1.  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.
   Protocol differences between QUIC and TCP however contribute to
   algorithmic differences.  We briefly describe these protocol
   differences below.

3.1.1.  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 acknowledgement
   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.

3.1.2.  Monotonically Increasing Packet Numbers

   TCP conflates transmission order at the sender with delivery order at
   the receiver, which results in retransmissions of the same data
   carrying the same sequence number, and consequently leads to
   "retransmission ambiguity".  QUIC separates the two: QUIC uses a
   packet number to indicate transmission order, and any application
   data is sent in one or more streams, with delivery order determined
   by stream offsets encoded within 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 rebundles
   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.

3.1.3.  Clearer Loss Epoch

   QUIC ends a loss epoch when a packet sent after loss is declared 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
   correctly once for every round trip that experiences loss, while TCP
   may only do it once across multiple round trips.

3.1.4.  No Reneging

   QUIC ACKs contain information that is similar to TCP SACK, but QUIC
   does not allow any acked packet to be reneged, greatly simplifying
   implementations on both sides and reducing memory pressure on the

3.1.5.  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

3.1.6.  Explicit Correction For Delayed Acknowledgements

   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.  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 connection's RTT.  An endpoint computes the
   following three values: the minimum value observed over the lifetime
   of the connection (min_rtt), an exponentially-weighted moving average
   (smoothed_rtt), and the variance in the observed RTT samples

4.1.  Generating RTT samples

   An endpoint generates an RTT sample on receiving an ACK frame that
   meets the following two conditions:

   o  the largest acknowledged packet number is newly acknowledged, and

   o  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 host
   delays for only the largest acknowledged packet in an ACK frame.
   While the reported host 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 4.3.

   To avoid generating multiple RTT samples using the same 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
   does not send an ACK frame on receiving only non-ack-eliciting
   packets, so an ACK frame that is subsequently sent can include an
   arbitrarily large Ack Delay field.  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.

4.2.  Estimating min_rtt

   min_rtt is the minimum RTT observed over the lifetime of the
   connection.  min_rtt is set to the latest_rtt on the first sample in
   a connection, and to the lesser of min_rtt and latest_rtt on
   subsequent samples.

   An endpoint uses only locally observed times in computing the min_rtt
   and does not adjust for host 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 4.3), and limits potential
   underestimation due to erroneously-reported delays by the peer.

4.3.  Estimating smoothed_rtt and rttvar

   smoothed_rtt is an exponentially-weighted moving average of an
   endpoint's RTT samples, and rttvar is the endpoint's estimated
   variance in the RTT samples.

   The calculation of smoothed_rtt uses path latency after adjusting RTT
   samples for host delays.  For packets sent in the ApplicationData
   packet number space, a peer limits any delay in sending an
   acknowledgement for an ack-eliciting packet to no greater than the
   value it advertised in the max_ack_delay transport parameter.

   Consequently, when a peer reports an Ack Delay that is greater than
   its max_ack_delay, the delay is attributed to reasons out of the
   peer's control, such as scheduler latency at the peer or loss of
   previous ACK frames.  Any delays beyond the peer's max_ack_delay are
   therefore considered effectively part of path delay and incorporated
   into the smoothed_rtt estimate.

   When adjusting an RTT sample using peer-reported acknowledgement
   delays, an endpoint:

   o  MUST ignore the Ack Delay field of the ACK frame for packets sent
      in the Initial and Handshake packet number space.

   o  MUST use the lesser of the value reported in Ack Delay field of
      the ACK frame and the peer's max_ack_delay transport parameter.

   o  MUST NOT apply the adjustment if the resulting RTT sample is
      smaller than the min_rtt.  This limits the underestimation that a
      misreporting peer can cause to the smoothed_rtt.

   On the first RTT sample in a connection, the smoothed_rtt is set to
   the latest_rtt.

   smoothed_rtt and rttvar are computed as follows, similar to
   [RFC6298].  On the first RTT sample in a connection:

   smoothed_rtt = latest_rtt
   rttvar = latest_rtt / 2

   On subsequent RTT samples, smoothed_rtt and rttvar evolve as follows:

   ack_delay = min(Ack Delay in ACK Frame, 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

5.  Loss Detection

   QUIC senders use both ack information and timeouts to detect lost
   packets, and 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 abandoning the frame.  For more information, see Section 13.3 of

5.1.  Acknowledgement-based Detection

   Acknowledgement-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:

   o  The packet is unacknowledged, in-flight, and was sent prior to an
      acknowledged packet.

   o  Either its packet number is kPacketThreshold smaller than an
      acknowledged packet (Section 5.1.1), or it was sent long enough in
      the past (Section 5.1.2).

   The acknowledgement indicates that a packet sent later was delivered,
   while the packet and time thresholds provide some tolerance for
   packet reordering.

   Spuriously declaring packets as lost leads to unnecessary
   retransmissions and may result in degraded performance due to the
   actions of the congestion controller upon detecting loss.
   Implementations that detect spurious retransmissions and increase the
   reordering threshold in packets or time MAY choose to start with
   smaller initial reordering thresholds to minimize recovery latency.

5.1.1.  Packet Threshold

   The RECOMMENDED initial value for the packet reordering threshold
   (kPacketThreshold) is 3, based on best practices for TCP loss
   detection [RFC5681] [RFC6675].

   Some networks may exhibit higher degrees of reordering, causing a
   sender to detect spurious losses.  Implementers MAY use algorithms
   developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's
   reordering resilience.

5.1.2.  Time Threshold

   Once a later packet 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 kGranularity.  The time threshold is:

   kTimeThreshold * max(SRTT, latest_RTT, 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(SRTT, latest_RTT) max(smoothed_rtt, latest_rtt) protects from the two following

   o  the latest RTT sample is lower than the SRTT, smoothed RTT, perhaps due
      to reordering where the acknowledgement encountered a shorter

   o  the latest RTT sample is higher than the SRTT, smoothed RTT, perhaps due
      to a sustained increase in the actual RTT, but the smoothed SRTT RTT
      has not yet caught up.

   The RECOMMENDED time threshold (kTimeThreshold), expressed as a
   round-trip time multiplier, is 9/8.

   Implementations MAY experiment with absolute thresholds, thresholds
   from previous connections, adaptive thresholds, or including RTT
   variance.  Smaller thresholds reduce reordering resilience and
   increase spurious retransmissions, and larger thresholds increase
   loss detection delay.

5.2.  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 handshake has not been completed.  A PTO
   enables a connection to recover from loss of tail packets or
   acknowledgements.  The PTO algorithm used in QUIC implements the
   reliability functions of Tail Loss Probe [TLP] [RACK], RTO [RFC5681] and
   F-RTO algorithms for TCP [RFC5682], and the timeout computation is
   based on TCP's retransmission timeout period [RFC6298].

5.2.1.  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

   kGranularity, smoothed_rtt, rttvar, and max_ack_delay are defined in
   Appendix A.2 and Appendix A.3.

   The PTO period is the amount of time that a sender ought to wait for
   an acknowledgement of a sent packet.  This time period includes the
   estimated network roundtrip-time (smoothed_rtt), the variance in the
   estimate (4*rttvar), and max_ack_delay, to account for the maximum
   time by which a receiver might delay sending an acknowledgement.

   The PTO value MUST be set to at least kGranularity, to avoid the
   timer expiring immediately.

   When a PTO timer expires, the PTO period MUST be set to twice its
   current value.  This exponential reduction in the sender's rate is
   important because the PTOs might be caused by loss of packets or
   acknowledgements due to severe congestion.  The life of a connection
   that is experiencing consecutive PTOs is limited by the endpoint's
   idle timeout.

   A sender computes its PTO timer every time an ack-eliciting packet is
   sent.  A sender might choose to optimize this by setting the timer
   fewer times if it knows that more ack-eliciting packets will be sent
   within a short period of time.

   The probe timer is not set if the time threshold Section 5.1.2 loss
   detection timer is set.  The time threshold loss detection timer is
   expected to both expire earlier than the PTO and be less likely to
   spuriously retransmit data.

5.3.  Handshakes and New Paths

   The initial probe timeout for a new connection or new path SHOULD be
   set to twice the initial RTT.  Resumed connections over the same
   network SHOULD use the previous connection's final smoothed RTT value
   as the resumed connection's initial RTT.  If no previous RTT is
   available, the initial RTT SHOULD be set to 500ms, resulting in a 1
   second initial timeout as recommended in [RFC6298].

   A connection MAY use the delay between sending a PATH_CHALLENGE and
   receiving a PATH_RESPONSE to seed initial_rtt for a new path, but the
   delay SHOULD NOT be considered an RTT sample.

   Until the server has validated the client's address on the path, the
   amount of data it can send is limited, limited to three times the amount of
   data received, as specified in Section 8.1 of [QUIC-TRANSPORT].  Data at Initial encryption MUST be retransmitted
   before Handshake data and data at Handshake encryption MUST be
   retransmitted before any ApplicationData data.  If
   no data can be sent, then the PTO alarm MUST NOT be armed until data has been
   received from the client. armed.

   Since the server could be blocked until more packets 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
   acknowledgement for one of its Handshake or 1-RTT packets.

   Prior to handshake completion, when few to none RTT samples have been
   generated, it is possible that the probe timer expiration is due to
   an incorrect RTT estimate at the client.  To allow the client to
   improve its RTT estimate, the new packet that it sends MUST be ack-
   eliciting.  If Handshake keys are available to the client, it MUST
   send a Handshake packet, and otherwise it MUST send an Initial packet
   in a UDP datagram of at least 1200 bytes.

   Initial packets and Handshake packets may never be acknowledged, but
   they are removed from bytes in flight when the Initial and Handshake
   keys are discarded.

5.3.1.  Sending Probe Packets

   When a PTO timer expires, a sender MUST send at least one ack-
   eliciting packet as a probe, unless there is no data available to
   send.  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.

   When the PTO timer expires, and there is new or previously sent
   unacknowledged data, it MUST be sent.  Data that was previously sent
   with Initial encryption MUST be sent before Handshake data and data
   previously sent at Handshake encryption MUST be sent before any
   ApplicationData data.

   It is possible that 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.  In the absence of any new application data, a PTO
   timer expiration now would find the sender with no new or previously-
   sent data to send.  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.

   Probe packets sent on a PTO MUST be ack-eliciting.  A probe packet
   SHOULD carry new data when possible.  A probe packet MAY carry
   retransmitted unacknowledged data when new data is unavailable, when
   flow control does not permit new data to be sent, or to
   opportunistically reduce loss recovery delay.  Implementations MAY
   use alternate alternative strategies for determining the content of probe
   packets, including sending new or retransmitted data based on the
   application's priorities.

   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

5.3.2.  Loss Detection

   Delivery or loss of packets in flight is established when an ACK
   frame is received that newly acknowledges one or more packets.

   A PTO timer expiration event does not indicate packet loss and MUST
   NOT cause prior unacknowledged packets to be marked as lost.  When an
   acknowledgement is received that newly acknowledges packets, loss
   detection proceeds as dictated by packet and time threshold
   mechanisms; see Section 5.1.

5.4.  Handling Retry and Version Negotiation Packets

   A Retry or Version Negotiation packet causes a client to send another Initial packet,
   effectively restarting the connection process and
   resetting congestion control and loss recovery state, including
   resetting any pending timers.  Either process.  A Retry packet
   indicates that the Initial was received received, but not processed.  Neither  A Retry
   packet can cannot be treated as an acknowledgment for acknowledgment, because it does not
   indicate that a packet was processed or specify the Initial. 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 however 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 to seed the RTT estimator in place of its default for a subsequent connection attempt
   to the server. initial RTT estimate.

5.5.  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 acknowledgements 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 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.  Initial secrets however
   might be destroyed sooner, as soon as handshake keys are available
   (see Section 4.9.1 of [QUIC-TLS]).

5.6.  Discussion

   The majority of constants were derived from best common practices
   among widely deployed TCP implementations on the internet.
   Exceptions follow.

   A shorter delayed ack time of 25ms was chosen because longer delayed
   acks can delay loss recovery and for the small number of connections
   where less than packet per 25ms is delivered, acking every packet is
   beneficial to congestion control and loss recovery.

6.  Congestion Control

   QUIC's congestion control is based on TCP NewReno [RFC6582].  NewReno
   is a congestion window based congestion control.  QUIC specifies the
   congestion window in bytes rather than packets due to finer control
   and the ease of appropriate byte counting [RFC3465].

   QUIC hosts MUST NOT send packets if they would increase
   bytes_in_flight (defined in Appendix B.2) beyond the available
   congestion window, unless the packet is a probe packet sent after a
   PTO timer expires, as described in Section 5.2.

   Implementations MAY use other congestion control algorithms, such as
   Cubic [RFC8312], and endpoints MAY use different algorithms from one
   another.  The signals QUIC provides for congestion control are
   generic and are designed to support different algorithms.

6.1.  Explicit Congestion Notification

   If a path has been verified to support ECN, QUIC treats a Congestion
   Experienced codepoint in the IP header as a signal of congestion.
   This document specifies an endpoint's response when its peer receives
   packets with the Congestion Experienced codepoint.  As discussed in
   [RFC8311], endpoints are permitted to experiment with other response

6.2.  Slow Start

   QUIC begins every connection in slow start and exits slow start upon
   loss or upon increase in the ECN-CE counter.  QUIC re-enters slow
   start anytime the congestion window is less than ssthresh, which only
   occurs after persistent congestion is declared.  While in slow start,
   QUIC increases the congestion window by the number of bytes
   acknowledged when each acknowledgment is processed.

6.3.  Congestion Avoidance

   Slow start exits to congestion avoidance.  Congestion avoidance in
   NewReno uses an additive increase multiplicative decrease (AIMD)
   approach that increases the congestion window by one maximum packet
   size per congestion window acknowledged.  When a loss is detected,
   NewReno halves the congestion window and sets the slow start
   threshold to the new congestion window.

6.4.  Recovery Period

   Recovery is a period of time beginning with detection of a lost
   packet or an increase in the ECN-CE counter.  Because QUIC does not
   retransmit packets, it defines the end of recovery as a packet sent
   after the start of recovery being acknowledged.  This is slightly
   different from TCP's definition of recovery, which ends when the lost
   packet that started recovery is acknowledged.

   The recovery period limits congestion window reduction to once per
   round trip.  During recovery, the congestion window remains unchanged
   irrespective of new losses or increases in the ECN-CE counter.

6.5.  Ignoring Loss of Undecryptable Packets

   During the handshake, some packet protection keys might not be
   available when a packet arrives.  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 arrive before the peer has packet protection keys to
   process those packets.

6.6.  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
   acknowledgement is received that establishes loss or delivery of

6.7.  Persistent Congestion

   When an ACK frame is received that establishes loss of all in-flight
   packets sent over a long enough period of time, the network is
   considered to be experiencing persistent congestion.  Commonly, this
   can be established by consecutive PTOs, but since the PTO timer is
   reset when a new ack-eliciting packet is sent, an explicit duration
   must be used to account for those cases where PTOs do not occur or
   are substantially delayed.  This duration is computed as follows:

   (smoothed_rtt + 4 * rttvar + max_ack_delay) *

   For example, assume:

   smoothed_rtt = 1 rttvar = 0 max_ack_delay = 0
   kPersistentCongestionThreshold = 3

   If an eck-eliciting ack-eliciting packet is sent at time = 0, the following
   scenario would illustrate persistent congestion:

                     | t=0 | Send Pkt #1 (App Data) |
                     | t=1 | Send Pkt #2 (PTO 1)    |
                     |     |                        |
                     | t=3 | Send Pkt #3 (PTO 2)    |
                     |     |                        |
                     | t=7 | Send Pkt #4 (PTO 3)    |
                     |     |                        |
                     | t=8 | Recv ACK of Pkt #4     |

   The first three packets are determined to be lost when the ACK of
   packet 4 is received at t=8.  The congestion period is calculated as
   the time between the oldest and newest lost packets: (3 - 0) = 3.
   The duration for persistent congestion is equal to: (1 *
   kPersistentCongestionThreshold) = 3.  Because the threshold was
   reached and because none of the packets between the oldest and the
   newest packets are acknowledged, the network is considered to have
   experienced persistent congestion.

   When persistent congestion is established, the sender's congestion
   window MUST be reduced to the minimum congestion window
   (kMinimumWindow).  This response of collapsing the congestion window
   on persistent congestion is functionally similar to a sender's
   response on a Retransmission Timeout (RTO) in TCP [RFC5681] after
   Tail Loss Probes (TLP) [TLP]. [RACK].

6.8.  Pacing

   This document does not specify a pacer, but it is RECOMMENDED that a
   sender pace sending of all in-flight packets based on input from the
   congestion controller.  For example, a pacer might distribute the
   congestion window over the SRTT smoothed RTT when used with a window-based
   controller, and a pacer might use the rate estimate of a rate-based

   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.

   Sending multiple packets into the network without any delay between
   them creates a packet burst that might cause short-term congestion
   and losses.  Implementations MUST either use pacing or limit such
   bursts to the initial congestion window, which is recommended to be
   the minimum of 10 * max_datagram_size and max(2* max_datagram_size,
   14720)), where max_datagram_size is the current maximum size of a
   datagram for the connection, not including UDP or IP overhead.

   As an example of a well-known and publicly available implementation
   of a flow pacer, implementers are referred to the Fair Queue packet
   scheduler (fq qdisc) in Linux (3.11 onwards).

6.9.  Under-utilizing the Congestion Window


   When bytes in flight is smaller than the congestion window that and
   sending is under-utilized 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 credit.

   A sender MAY use the pipeACK method described in section 4.3 of
   [RFC7661] to determine if the congestion window is sufficiently

   A sender that paces packets (see Section 6.8) 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.

   Bursting more than an initial window's worth of data into the network
   might cause short-term congestion and losses.  Implemementations
   SHOULD either use pacing or reduce their congestion window to limit
   such bursts.

   A sender MAY implement alternate alternative mechanisms to update its
   congestion window after periods of under-utilization, such as those
   proposed for TCP in [RFC7661].

7.  Security Considerations

7.1.  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.

7.2.  Traffic Analysis

   Packets that carry only ACK frames can be heuristically identified by
   observing packet size.  Acknowledgement patterns may expose
   information about link characteristics or application behavior.
   Endpoints can use PADDING frames or bundle acknowledgments with other
   frames to reduce leaked information.

7.3.  Misreporting ECN Markings

   A receiver can misreport ECN markings to alter the congestion
   response of a sender.  Suppressing reports of ECN-CE markings could
   cause a sender to increase their send rate.  This increase could
   result in congestion and loss.

   A sender MAY attempt to detect suppression of reports by marking
   occasional packets that they send with ECN-CE.  If a packet marked
   with ECN-CE is not reported as having been marked when the packet is
   acknowledged, the sender SHOULD then disable ECN for that path.

   Reporting additional ECN-CE markings will cause a sender to reduce
   their sending rate, which is similar in effect to advertising reduced
   connection flow control limits and so no advantage is gained by doing

   Endpoints choose the congestion controller that they use.  Though
   congestion controllers generally treat reports of ECN-CE markings as
   equivalent to loss [RFC8311], the exact response for each controller
   could be different.  Failure to correctly respond to information
   about ECN markings is therefore difficult to detect.

8.  IANA Considerations

   This document has no IANA actions.  Yet.

9.  References

9.1.  Normative References

              Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", draft-ietf-quic-tls-23 draft-ietf-quic-tls-24 (work in progress),
              September November

              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport-24 (work in progress), September November 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,

9.2.  Informative References

   [FACK]     Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
              Refining TCP Congestion Control", ACM SIGCOMM , August

   [RACK]     Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
              a time-based fast loss detection algorithm for TCP",
              draft-ietf-tcpm-rack-05 (work in progress), April 2019.

   [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>.

   [RFC4653]  Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
              "Improving the Robustness of TCP to Non-Congestion
              Events", RFC 4653, DOI 10.17487/RFC4653, August 2006,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [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,

   [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,

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [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,

   [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,

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,

   [RFC7661]  Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
              TCP to Support Rate-Limited Traffic", RFC 7661,
              DOI 10.17487/RFC7661, October 2015,

   [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,

   [TLP]      Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
              "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
              Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
              in progress), February 2013.

9.3.  URIs

   [1] https://mailarchive.ietf.org/arch/search/?email_list=quic

   [2] https://github.com/quicwg

   [3] https://github.com/quicwg/base-drafts/labels/-recovery

Appendix A.  Loss Recovery Pseudocode

   We now describe an example implementation of the loss detection
   mechanisms described in Section 5.

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 track it for an
   amount of time comparable to the maximum expected packet reordering,
   such as 1 RTT.  This allows for detection of spurious

   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 acknowledgement will
      be received, though the peer could delay sending the ACK frame
      containing it by up to the MaxAckDelay.

   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.  Some may need to be changed or
   negotiated in order to better suit a variety of environments.

   kPacketThreshold:  Maximum reordering in packets before packet
      threshold loss detection considers a packet lost.  The RECOMMENDED
      value is 3.

   kTimeThreshold:  Maximum reordering in time before time threshold
      loss detection considers a packet lost.  Specified as an RTT
      multiplier.  The RECOMMENDED value is 9/8.

   kGranularity:  Timer granularity.  This is a system-dependent value.
      However, implementations SHOULD use a value no smaller than 1ms.

   kInitialRtt:  The RTT used before an RTT sample is taken.  The
      RECOMMENDED value is 500ms.

   kPacketNumberSpace:  An enum to enumerate the three packet number

     enum kPacketNumberSpace {

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 [RFC6298]

   rttvar:  The RTT variance, computed as described in [RFC6298]

   min_rtt:  The minimum RTT seen in the connection, ignoring ack delay.

   max_ack_delay:  The maximum amount of time by which the receiver
      intends to delay acknowledgments for packets in the
      ApplicationData packet number space.  The actual ack_delay in a
      received ACK frame may be larger due to late timers, reordering,
      or lost ACKs.

   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_sent_ack_eliciting_packet:  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:

      pto_count = 0
      latest_rtt = 0
      smoothed_rtt = 0
      rttvar = 0
      min_rtt = 0
      max_ack_delay = 0
      time_of_last_sent_ack_eliciting_packet = 0
      for pn_space in [ Initial, Handshake, ApplicationData ]:
        largest_acked_packet[pn_space] = infinite
        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 =
      sent_packets[pn_space][packet_number].time_sent = now
      sent_packets[pn_space][packet_number].ack_eliciting =
      sent_packets[pn_space][packet_number].in_flight = in_flight
      if (in_flight):
        if (ack_eliciting):
          time_of_last_sent_ack_eliciting_packet = now
        sent_packets[pn_space][packet_number].size = sent_bytes

A.6.  On Receiving an Acknowledgment

   When an ACK frame is received, it may newly acknowledge any number of

   Pseudocode for OnAckReceived and UpdateRtt follow:

   OnAckReceived(ack, pn_space):
     if (largest_acked_packet[pn_space] == infinite):
       largest_acked_packet[pn_space] = ack.largest_acked
       largest_acked_packet[pn_space] =
           max(largest_acked_packet[pn_space], ack.largest_acked)

     // Nothing to do if there are no newly acked packets.
     newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)
     if (newly_acked_packets.empty()):

     // If the largest acknowledged is newly acked and
     // at least one ack-eliciting was newly acked, update the RTT.
     if (sent_packets[pn_space].contains(ack.largest_acked) &&
       latest_rtt =
         now - sent_packets[pn_space][ack.largest_acked].time_sent
       ack_delay = 0
       if (pn_space == ApplicationData):
         ack_delay = ack.ack_delay

     // Process ECN information if present.
     if (ACK frame contains ECN information):
         ProcessECN(ack, pn_space)

     for acked_packet in newly_acked_packets:
       OnPacketAcked(acked_packet.packet_number, pn_space)


     pto_count = 0


     // First RTT sample.
     if (smoothed_rtt == 0):
       min_rtt = latest_rtt
       smoothed_rtt = latest_rtt
       rttvar = latest_rtt / 2

     // min_rtt ignores ack delay.
     min_rtt = min(min_rtt, latest_rtt)
     // Limit ack_delay by max_ack_delay
     ack_delay = min(ack_delay, max_ack_delay)
     // Adjust for ack delay if 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.7.  On Packet Acknowledgment

   When a packet is acknowledged for the first time, the following
   OnPacketAcked function is called.  Note that a single ACK frame may
   newly acknowledge several packets.  OnPacketAcked must be called once
   for each of these newly acknowledged packets.

   OnPacketAcked takes two parameters: acked_packet, which is the struct
   detailed in Appendix A.1.1, and the packet number space that this ACK
   frame was sent for.

   Pseudocode for OnPacketAcked follows:

      OnPacketAcked(acked_packet, pn_space):
        if (acked_packet.in_flight):

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 SHOULD
   fire immediately.

   Pseudocode for SetLossDetectionTimer follows:

   // Returns the earliest loss_time and the packet number
   // space it's from.  Returns 0 if all times are 0.
     time = loss_time[Initial]
     space = Initial
     for pn_space in [ Handshake, ApplicationData ]:
       if (loss_time[pn_space] != 0 &&
           (time == 0 || loss_time[pn_space] < time)):
         time = loss_time[pn_space];
         space = pn_space
     return time, space

     # 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 ||
            has received 1-RTT ACK

     loss_time, _ = GetEarliestLossTime()
     if (loss_time != 0):
       // Time threshold loss detection.

     if (no ack-eliciting packets in flight &&

     // Use a default timeout if there are no RTT measurements
     if (smoothed_rtt == 0):
       timeout = 2 * kInitialRtt
       // Calculate PTO duration
       timeout = smoothed_rtt + max(4 * rttvar, kGranularity) +
     timeout = timeout * (2 ^ pto_count)

       time_of_last_sent_ack_eliciting_packet + 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:

     loss_time, pn_space = GetEarliestLossTime()
     if (loss_time != 0):
       // Time threshold loss Detection

     if (endpoint is client without 1-RTT keys):
       // 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):
       // PTO. Send new data if available, else retransmit old data.
       // If neither is available, send a single PING frame.


A.10.  Detecting Lost Packets

   DetectLostPackets is called every time an ACK is received and
   operates on the sent_packets for that packet number space.

   Pseudocode for DetectLostPackets follows:

     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]):

       // Mark packet as lost, or set time when it should be marked.
       if (unacked.time_sent <= lost_send_time ||
           largest_acked_packet[pn_space] >=
             unacked.packet_number + kPacketThreshold):
         if (unacked.in_flight):
         if (loss_time[pn_space] == 0):
           loss_time[pn_space] = unacked.time_sent + loss_delay
           loss_time[pn_space] = min(loss_time[pn_space],
                                     unacked.time_sent + loss_delay)

     // Inform the congestion controller of lost packets and
     // let it decide whether to retransmit immediately.
     if (!lost_packets.empty()):

Appendix B.  Congestion Control Pseudocode

   We now describe an example implementation of the congestion
   controller described in Section 6.

B.1.  Constants of interest

   Constants used in congestion control are based on a combination of
   RFCs, papers, and common practice.  Some may need to be changed or
   negotiated in order to better suit a variety of environments.

   kMaxDatagramSize:  The sender's maximum payload size.  Does not
      include UDP or IP overhead.  The max packet size is used for
      calculating initial and minimum congestion windows.  The
      RECOMMENDED value is 1200 bytes.

   kInitialWindow:  Default limit on the initial amount of data in
      flight, in bytes.  Taken from [RFC6928], but increased slightly to
      account for the smaller 8 byte overhead of UDP vs 20 bytes for
      TCP.  The RECOMMENDED value is the minimum of 10 *
      max_datagram_size and max(2* kMaxDatagramSize, max(2 * max_datagram_size, 14720)).

   kMinimumWindow:  Minimum congestion window in bytes.  The RECOMMENDED
      value is 2 * kMaxDatagramSize. max_datagram_size.

   kLossReductionFactor:  Reduction in congestion window when a new loss
      event is detected.  The RECOMMENDED value is 0.5.

   kPersistentCongestionThreshold:  Period of time for persistent
      congestion to be established, specified as a PTO multiplier.  The
      rationale for this threshold is to enable a sender to use initial
      PTOs for aggressive probing, as TCP does with Tail Loss Probe
      (TLP) [TLP] [RACK], before establishing persistent congestion, as TCP
      does with a Retransmission Timeout (RTO) [RFC5681].  The
      RECOMMENDED value for kPersistentCongestionThreshold is 3, which
      is approximately equivalent to having two TLPs before an RTO in

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 PMTU (see Section 14.1 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 acked or declared lost.  The size does not include IP or
      UDP overhead, but does include the QUIC header and AEAD overhead.
      Packets only containing ACK frames do not count towards
      bytes_in_flight to ensure congestion control does not impede
      congestion feedback.

   congestion_window:  Maximum number of bytes-in-flight that may be

   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.

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.

        bytes_in_flight += bytes_sent

B.5.  On Packet Acknowledgement

   Invoked from loss detection's OnPacketAcked and is supplied with the
   acked_packet from sent_packets.

        return sent_time <= congestion_recovery_start_time

        // Remove from bytes_in_flight.
        bytes_in_flight -= acked_packet.size
        if (InCongestionRecovery(acked_packet.time_sent)):
          // Do not increase congestion window in recovery period.
        if (IsAppLimited()):
          // Do not increase congestion_window if application
          // limited.
        if (congestion_window < ssthresh):
          // Slow start.
          congestion_window += acked_packet.size
          // Congestion avoidance.
          congestion_window += kMaxDatagramSize max_datagram_size * acked_packet.size
              / congestion_window

B.6.  On New Congestion Event

   Invoked from ProcessECN and OnPacketsLost when a new congestion event
   is detected.  May start a new recovery period and reduces the
   congestion window.

        // Start a new congestion event if packet was sent after the
        // start of the previous congestion recovery period.
        if (!InCongestionRecovery(sent_time)):
          congestion_recovery_start_time = Now()
          congestion_window *= kLossReductionFactor
          congestion_window = max(congestion_window, kMinimumWindow)
          ssthresh = congestion_window

B.7.  Process ECN Information

   Invoked when an ACK frame with an ECN section is received from the

      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

B.8.  On Packets Lost

   Invoked from DetectLostPackets when packets are deemed lost.

        pto = smoothed_rtt + max(4 * rttvar, kGranularity) +
        congestion_period = pto * kPersistentCongestionThreshold
        // Determine if all packets in the time period before the
        // newest lost packet, including the edges, are marked
        // lost
        return AreAllPacketsLost(largest_lost_packet,

        // Remove lost packets from bytes_in_flight.
        for (lost_packet : lost_packets):
          bytes_in_flight -= lost_packet.size
        largest_lost_packet = lost_packets.last()

        // Collapse congestion window if persistent congestion
        if (InPersistentCongestion(largest_lost_packet)):
          congestion_window = kMinimumWindow

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-23

   o  Define under-utilizing the congestion window (#2630, #2686, #2675)

   o  PTO MUST send data if possible (#3056, #3057)

   o  Connection Close is not ack-eliciting (#3097, #3098)

   o  MUST limit bursts to the initial congestion window (#3160)

   o  Define the current max_datagram_size for congestion control
      (#3041, #3167)

   o  Separate PTO by packet number space (#3067, #3074, #3066)

C.2.  Since draft-ietf-quic-recovery-22

   o  PTO should always send an ack-eliciting packet (#2895)

   o  Unify the Handshake Timer with the PTO timer (#2648, #2658, #2886)

   o  Move ACK generation text to transport draft (#1860, #2916)


C.3.  Since draft-ietf-quic-recovery-21

   o  No changes


C.4.  Since draft-ietf-quic-recovery-20

   o  Path validation can be used as initial RTT value (#2644, #2687)

   o  max_ack_delay transport parameter defaults to 0 (#2638, #2646)

   o  Ack Delay only measures intentional delays induced by the
      implementation (#2596, #2786)


C.5.  Since draft-ietf-quic-recovery-19

   o  Change kPersistentThreshold from an exponent to a multiplier

   o  Send a PING if the PTO timer fires and there's nothing to send

   o  Set loss delay to at least kGranularity (#2617)

   o  Merge application limited and sending after idle sections.  Always
      limit burst size instead of requiring resetting CWND to initial
      CWND after idle (#2605)

   o  Rewrite RTT estimation, allow RTT samples where a newly acked
      packet is ack-eliciting but the largest_acked is not (#2592)

   o  Don't arm the handshake timer if there is no handshake data

   o  Clarify that the time threshold loss alarm takes precedence over
      the crypto handshake timer (#2590, #2620)

   o  Change initial RTT to 500ms to align with RFC6298 (#2184)


C.6.  Since draft-ietf-quic-recovery-18

   o  Change IW byte limit to 14720 from 14600 (#2494)

   o  Update PTO calculation to match RFC6298 (#2480, #2489, #2490)

   o  Improve loss detection's description of multiple packet number
      spaces and pseudocode (#2485, #2451, #2417)

   o  Declare persistent congestion even if non-probe packets are sent
      and don't make persistent congestion more aggressive than RTO
      verified was (#2365, #2244)

   o  Move pseudocode to the appendices (#2408)

   o  What to send on multiple PTOs (#2380)


C.7.  Since draft-ietf-quic-recovery-17

   o  After Probe Timeout discard in-flight packets or send another
      (#2212, #1965)

   o  Endpoints discard initial keys as soon as handshake keys are
      available (#1951, #2045)

   o  0-RTT state is discarded when 0-RTT is rejected (#2300)

   o  Loss detection timer is cancelled when ack-eliciting frames are in
      flight (#2117, #2093)

   o  Packets are declared lost if they are in flight (#2104)

   o  After becoming idle, either pace packets or reset the congestion
      controller (#2138, 2187)

   o  Process ECN counts before marking packets lost (#2142)

   o  Mark packets lost before resetting crypto_count and pto_count
      (#2208, #2209)

   o  Congestion and loss recovery state are discarded when keys are
      discarded (#2327)


C.8.  Since draft-ietf-quic-recovery-16

   o  Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP
      and min crypto timeouts; eliminate timeout validation (#2114,
      #2166, #2168, #1017)

   o  Redefine how congestion avoidance in terms of when the period
      starts (#1928, #1930)

   o  Document what needs to be tracked for packets that are in flight
      (#765, #1724, #1939)

   o  Integrate both time and packet thresholds into loss detection
      (#1969, #1212, #934, #1974)

   o  Reduce congestion window after idle, unless pacing is used (#2007,

   o  Disable RTT calculation for packets that don't elicit
      acknowledgment (#2060, #2078)

   o  Limit ack_delay by max_ack_delay (#2060, #2099)

   o  Initial keys are discarded once Handshake keys are avaialble available
      (#1951, #2045)

   o  Reorder ECN and loss detection in pseudocode (#2142)

   o  Only cancel loss detection timer if ack-eliciting packets are in
      flight (#2093, #2117)


C.9.  Since draft-ietf-quic-recovery-14

   o  Used max_ack_delay from transport params (#1796, #1782)

   o  Merge ACK and ACK_ECN (#1783)


C.10.  Since draft-ietf-quic-recovery-13

   o  Corrected the lack of ssthresh reduction in CongestionEvent
      pseudocode (#1598)

   o  Considerations for ECN spoofing (#1426, #1626)

   o  Clarifications for PADDING and congestion control (#837, #838,
      #1517, #1531, #1540)

   o  Reduce early retransmission timer to RTT/8 (#945, #1581)

   o  Packets are declared lost after an RTO is verified (#935, #1582)


C.11.  Since draft-ietf-quic-recovery-12

   o  Changes to manage separate packet number spaces and encryption
      levels (#1190, #1242, #1413, #1450)

   o  Added ECN feedback mechanisms and handling; new ACK_ECN frame
      (#804, #805, #1372)


C.12.  Since draft-ietf-quic-recovery-11

   No significant changes.


C.13.  Since draft-ietf-quic-recovery-10

   o  Improved text on ack generation (#1139, #1159)

   o  Make references to TCP recovery mechanisms informational (#1195)

   o  Define time_of_last_sent_handshake_packet (#1171)

   o  Added signal from TLS the data it includes needs to be sent in a
      Retry packet (#1061, #1199)

   o  Minimum RTT (min_rtt) is initialized with an infinite value


C.14.  Since draft-ietf-quic-recovery-09

   No significant changes.


C.15.  Since draft-ietf-quic-recovery-08

   o  Clarified pacing and RTO (#967, #977)


C.16.  Since draft-ietf-quic-recovery-07

   o  Include Ack Delay in RTO(and TLP) computations (#981)

   o  Ack Delay in SRTT computation (#961)

   o  Default RTT and Slow Start (#590)

   o  Many editorial fixes.


C.17.  Since draft-ietf-quic-recovery-06

   No significant changes.


C.18.  Since draft-ietf-quic-recovery-05

   o  Add more congestion control text (#776)


C.19.  Since draft-ietf-quic-recovery-04

   No significant changes.


C.20.  Since draft-ietf-quic-recovery-03

   No significant changes.


C.21.  Since draft-ietf-quic-recovery-02

   o  Integrate F-RTO (#544, #409)

   o  Add congestion control (#545, #395)

   o  Require connection abort if a skipped packet was acknowledged

   o  Simplify RTO calculations (#142, #417)


C.22.  Since draft-ietf-quic-recovery-01

   o  Overview added to loss detection

   o  Changes initial default RTT to 100ms

   o  Added time-based loss detection and fixes early retransmit

   o  Clarified loss recovery for handshake packets

   o  Fixed references and made TCP references informative


C.23.  Since draft-ietf-quic-recovery-00

   o  Improved description of constants and ACK behavior


C.24.  Since draft-iyengar-quic-loss-recovery-01

   o  Adopted as base for draft-ietf-quic-recovery

   o  Updated authors/editors list
   o  Added table of contents


Authors' Addresses

   Jana Iyengar (editor)

   Email: jri.ietf@gmail.com

   Ian Swett (editor)

   Email: ianswett@google.com