QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-06

QUIC                                                     J. Iyengar, Ed.
Internet-Draft                                             I. Swett, Ed.
Intended status: Standards Track                                  Google
Expires: March 26, 2018                               September 22, 2017


               QUIC Loss Detection and Congestion Control
                      draft-ietf-quic-recovery-06

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

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 26, 2018.

Copyright Notice

   Copyright (c) 2017 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



Iyengar & Swett          Expires March 26, 2018                 [Page 1]


Internet-Draft             QUIC Loss Detection            September 2017


   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Design of the QUIC Transmission Machinery . . . . . . . . . .   3
     2.1.  Relevant Differences Between QUIC and TCP . . . . . . . .   4
       2.1.1.  Monotonically Increasing Packet Numbers . . . . . . .   4
       2.1.2.  No Reneging . . . . . . . . . . . . . . . . . . . . .   5
       2.1.3.  More ACK Ranges . . . . . . . . . . . . . . . . . . .   5
       2.1.4.  Explicit Correction For Delayed Acks  . . . . . . . .   5
   3.  Loss Detection  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Algorithm Details . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Constants of interest . . . . . . . . . . . . . . . .   6
       3.2.2.  Variables of interest . . . . . . . . . . . . . . . .   7
       3.2.3.  Initialization  . . . . . . . . . . . . . . . . . . .   8
       3.2.4.  On Sending a Packet . . . . . . . . . . . . . . . . .   8
       3.2.5.  On Ack Receipt  . . . . . . . . . . . . . . . . . . .   9
       3.2.6.  On Packet Acknowledgment  . . . . . . . . . . . . . .   9
       3.2.7.  Setting the Loss Detection Alarm  . . . . . . . . . .  10
       3.2.8.  On Alarm Firing . . . . . . . . . . . . . . . . . . .  12
       3.2.9.  Detecting Lost Packets  . . . . . . . . . . . . . . .  13
     3.3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Slow Start  . . . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Congestion Avoidance  . . . . . . . . . . . . . . . . . .  15
     4.3.  Recovery Period . . . . . . . . . . . . . . . . . . . . .  15
     4.4.  Tail Loss Probe . . . . . . . . . . . . . . . . . . . . .  15
     4.5.  Retransmission Timeout  . . . . . . . . . . . . . . . . .  15
     4.6.  Pacing Rate . . . . . . . . . . . . . . . . . . . . . . .  16
     4.7.  Pseudocode  . . . . . . . . . . . . . . . . . . . . . . .  16
       4.7.1.  Constants of interest . . . . . . . . . . . . . . . .  16
       4.7.2.  Variables of interest . . . . . . . . . . . . . . . .  16
       4.7.3.  Initialization  . . . . . . . . . . . . . . . . . . .  17
       4.7.4.  On Packet Sent  . . . . . . . . . . . . . . . . . . .  17
       4.7.5.  On Packet Acknowledgement . . . . . . . . . . . . . .  17
       4.7.6.  On Packets Lost . . . . . . . . . . . . . . . . . . .  17
       4.7.7.  On Retransmission Timeout Verified  . . . . . . . . .  18
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18



Iyengar & Swett          Expires March 26, 2018                 [Page 2]


Internet-Draft             QUIC Loss Detection            September 2017


     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  19
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  19
     B.1.  Since draft-ietf-quic-recovery-05 . . . . . . . . . . . .  19
     B.2.  Since draft-ietf-quic-recovery-04 . . . . . . . . . . . .  19
     B.3.  Since draft-ietf-quic-recovery-03 . . . . . . . . . . . .  19
     B.4.  Since draft-ietf-quic-recovery-02 . . . . . . . . . . . .  20
     B.5.  Since draft-ietf-quic-recovery-01 . . . . . . . . . . . .  20
     B.6.  Since draft-ietf-quic-recovery-00 . . . . . . . . . . . .  20
     B.7.  Since draft-iyengar-quic-loss-recovery-01 . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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 known 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.

1.1.  Notational Conventions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
   the special meaning defined in [RFC2119].

2.  Design of the QUIC Transmission Machinery

   All transmissions in QUIC are sent with a packet-level header, which
   includes a packet sequence number (referred to below as a packet
   number).  These packet numbers never repeat in the lifetime of a
   connection, and are monotonically increasing, which makes duplicate
   detection trivial.  This fundamental design decision obviates the
   need for disambiguating between transmissions and retransmissions and
   eliminates significant complexity from QUIC's interpretation of TCP
   loss detection mechanisms.

   Every packet may contain several frames.  We outline the frames that
   are important to the loss detection and congestion control machinery
   below.




Iyengar & Swett          Expires March 26, 2018                 [Page 3]


Internet-Draft             QUIC Loss Detection            September 2017


   o  Retransmittable frames are frames requiring reliable delivery.
      The most common are STREAM frames, which typically contain
      application data.

   o  Crypto handshake data is sent on stream 0, and uses the
      reliability machinery of QUIC underneath.

   o  ACK frames contain acknowledgment information.  QUIC uses a SACK-
      based scheme, where acks express up to 256 ranges.  The ACK frame
      also includes a receive timestamp for each packet newly acked.

2.1.  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.

2.1.1.  Monotonically Increasing Packet Numbers

   TCP conflates transmission sequence number at the sender with
   delivery sequence number at the receiver, which results in
   retransmissions of the same data carrying the same sequence number,
   and consequently to problems caused by "retransmission ambiguity".
   QUIC separates the two: QUIC uses a packet sequence number (referred
   to as the "packet number") for transmissions, and any data that is to
   be delivered to the receiving application(s) is sent in one or more
   streams, with stream offsets encoded within STREAM frames inside of
   packets that determine delivery order.

   QUIC's packet number is strictly increasing, and directly encodes
   transmission order.  A higher QUIC packet number signifies that the
   packet was sent later, and a lower QUIC packet number signifies that
   the packet was sent earlier.  When a packet containing frames is
   deemed 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.





Iyengar & Swett          Expires March 26, 2018                 [Page 4]


Internet-Draft             QUIC Loss Detection            September 2017


2.1.2.  No Reneging

   QUIC ACKs contain information that is equivalent 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 sender.

2.1.3.  More ACK Ranges

   QUIC supports up to 256 ACK ranges, opposed to TCP's 3 SACK ranges.
   In high loss environments, this speeds recovery.

2.1.4.  Explicit Correction For Delayed Acks

   QUIC ACKs explicitly encode the delay incurred at the receiver
   between when a packet is received and when the corresponding ACK is
   sent.  This allows the receiver of the ACK to adjust for receiver
   delays, specifically the delayed ack timer, when estimating the path
   RTT.  This mechanism also allows a receiver to measure and report the
   delay from when a packet was received by the OS kernel, which is
   useful in receivers which may incur delays such as context-switch
   latency before a userspace QUIC receiver processes a received packet.

3.  Loss Detection

3.1.  Overview

   QUIC uses a combination of ack information and alarms to detect lost
   packets.  An unacknowledged QUIC packet is marked as lost in one of
   the following ways:

   o  A packet is marked as lost if at least one packet that was sent a
      threshold number of packets (kReorderingThreshold) after it has
      been acknowledged.  This indicates that the unacknowledged packet
      is either lost or reordered beyond the specified threshold.  This
      mechanism combines both TCP's FastRetransmit and FACK mechanisms.

   o  If a packet is near the tail, where fewer than
      kReorderingThreshold packets are sent after it, the sender cannot
      expect to detect loss based on the previous mechanism.  In this
      case, a sender uses both ack information and an alarm to detect
      loss.  Specifically, when the last sent packet is acknowledged,
      the sender waits a short period of time to allow for reordering
      and then marks any unacknowledged packets as lost.  This mechanism
      is based on the Linux implementation of TCP Early Retransmit.

   o  If a packet is sent at the tail, there are no packets sent after
      it, and the sender cannot use ack information to detect its loss.



Iyengar & Swett          Expires March 26, 2018                 [Page 5]


Internet-Draft             QUIC Loss Detection            September 2017


      The sender therefore relies on an alarm to detect such tail
      losses.  This mechanism is based on TCP's Tail Loss Probe.

   o  If all else fails, a Retransmission Timeout (RTO) alarm is always
      set when any retransmittable packet is outstanding.  When this
      alarm fires, all unacknowledged packets are marked as lost.

   o  Instead of a packet threshold to tolerate reordering, a QUIC
      sender may use a time threshold.  This allows for senders to be
      tolerant of short periods of significant reordering.  In this
      mechanism, a QUIC sender marks a packet as lost when a larger
      packet number is acknowledged and a threshold amount of time has
      passed since the packet was sent.

   o  Handshake packets, which contain STREAM frames for stream 0, are
      critical to QUIC transport and crypto negotiation, so a separate
      alarm period is used for them.

3.2.  Algorithm Details

3.2.1.  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.

   kMaxTLPs (default 2):  Maximum number of tail loss probes before an
      RTO fires.

   kReorderingThreshold (default 3):  Maximum reordering in packet
      number space before FACK style loss detection considers a packet
      lost.

   kTimeReorderingFraction (default 1/8):  Maximum reordering in time
      space before time based loss detection considers a packet lost.
      In fraction of an RTT.

   kMinTLPTimeout (default 10ms):  Minimum time in the future a tail
      loss probe alarm may be set for.

   kMinRTOTimeout (default 200ms):  Minimum time in the future an RTO
      alarm may be set for.

   kDelayedAckTimeout (default 25ms):  The length of the peer's delayed
      ack timer.

   kDefaultInitialRtt (default 100ms):  The default RTT used before an
      RTT sample is taken.



Iyengar & Swett          Expires March 26, 2018                 [Page 6]


Internet-Draft             QUIC Loss Detection            September 2017


3.2.2.  Variables of interest

   Variables required to implement the congestion control mechanisms are
   described in this section.

   loss_detection_alarm:  Multi-modal alarm used for loss detection.

   handshake_count:  The number of times the handshake packets have been
      retransmitted without receiving an ack.

   tlp_count:  The number of times a tail loss probe has been sent
      without receiving an ack.

   rto_count:  The number of times an rto has been sent without
      receiving an ack.

   largest_sent_before_rto:  The last packet number sent prior to the
      first retransmission timeout.

   time_of_last_sent_packet:  The time the most recent packet was sent.

   largest_sent_packet:  The packet number of the most recently sent
      packet.

   largest_acked_packet:  The largest packet number acknowledged in an
      ack frame.

   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]

   reordering_threshold:  The largest delta between the largest acked
      retransmittable packet and a packet containing retransmittable
      frames before it's declared lost.

   time_reordering_fraction:  The reordering window as a fraction of
      max(smoothed_rtt, latest_rtt).

   loss_time:  The time at which the next packet will be considered lost
      based on early transmit or exceeding the reordering window in
      time.

   sent_packets:  An association of packet numbers to information about
      them, including a number field indicating the packet number, a



Iyengar & Swett          Expires March 26, 2018                 [Page 7]


Internet-Draft             QUIC Loss Detection            September 2017


      time field indicating the time a packet was sent, and a bytes
      field indicating the packet's size.  sent_packets is ordered by
      packet number, and packets remain in sent_packets until
      acknowledged or lost.

3.2.3.  Initialization

   At the beginning of the connection, initialize the loss detection
   variables as follows:

      loss_detection_alarm.reset()
      handshake_count = 0
      tlp_count = 0
      rto_count = 0
      if (UsingTimeLossDetection())
        reordering_threshold = infinite
        time_reordering_fraction = kTimeReorderingFraction
      else:
        reordering_threshold = kReorderingThreshold
        time_reordering_fraction = infinite
      loss_time = 0
      smoothed_rtt = 0
      rttvar = 0
      largest_sent_before_rto = 0
      time_of_last_sent_packet = 0
      largest_sent_packet = 0

3.2.4.  On Sending a Packet

   After any packet is sent, be it a new transmission or a rebundled
   transmission, the following OnPacketSent function is called.  The
   parameters to OnPacketSent are as follows:

   o  packet_number: The packet number of the sent packet.

   o  is_ack_only: A boolean that indicates whether a packet only
      contains an ACK frame.  If true, it is still expected an ack will
      be received for this packet, but it is not congestion controlled.

   o  sent_bytes: The number of bytes sent in the packet, not including
      UDP or IP overhead, but including QUIC framing overhead.

   Pseudocode for OnPacketSent follows:








Iyengar & Swett          Expires March 26, 2018                 [Page 8]


Internet-Draft             QUIC Loss Detection            September 2017


    OnPacketSent(packet_number, is_ack_only, sent_bytes):
      time_of_last_sent_packet = now
      largest_sent_packet = packet_number
      sent_packets[packet_number].packet_number = packet_number
      sent_packets[packet_number].time = now
      if !is_ack_only:
        OnPacketSentCC(sent_bytes)
        sent_packets[packet_number].bytes = sent_bytes
        SetLossDetectionAlarm()

3.2.5.  On Ack Receipt

   When an ack is received, it may acknowledge 0 or more packets.

   Pseudocode for OnAckReceived and UpdateRtt follow:

      OnAckReceived(ack):
        largest_acked_packet = ack.largest_acked
        // If the largest acked is newly acked, update the RTT.
        if (sent_packets[ack.largest_acked]):
          latest_rtt = now - sent_packets[ack.largest_acked].time
          if (latest_rtt > ack.ack_delay):
            latest_rtt -= ack.delay
          UpdateRtt(latest_rtt)
        // Find all newly acked packets.
        for acked_packet in DetermineNewlyAckedPackets():
          OnPacketAcked(acked_packet.packet_number)

        DetectLostPackets(ack.largest_acked_packet)
        SetLossDetectionAlarm()


      UpdateRtt(latest_rtt):
        // Based on {{RFC6298}}.
        if (smoothed_rtt == 0):
          smoothed_rtt = latest_rtt
          rttvar = latest_rtt / 2
        else:
          rttvar = 3/4 * rttvar + 1/4 * abs(smoothed_rtt - latest_rtt)
          smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt

3.2.6.  On Packet Acknowledgment

   When a packet is acked 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 acked packets.




Iyengar & Swett          Expires March 26, 2018                 [Page 9]


Internet-Draft             QUIC Loss Detection            September 2017


   OnPacketAcked takes one parameter, acked_packet, which is the packet
   number of the newly acked packet, and returns a list of packet
   numbers that are detected as lost.

   If this is the first acknowledgement following RTO, check if the
   smallest newly acknowledged packet is one sent by the RTO, and if so,
   inform congestion control of a verified RTO, similar to F-RTO
   [RFC5682]

   Pseudocode for OnPacketAcked follows:

      OnPacketAcked(acked_packet_number):
        OnPacketAckedCC(acked_packet_number)
        // If a packet sent prior to RTO was acked, then the RTO
        // was spurious.  Otherwise, inform congestion control.
        if (rto_count > 0 &&
            acked_packet_number > largest_sent_before_rto)
          OnRetransmissionTimeoutVerified()
        handshake_count = 0
        tlp_count = 0
        rto_count = 0
        sent_packets.remove(acked_packet_number)

3.2.7.  Setting the Loss Detection Alarm

   QUIC loss detection uses a single alarm for all timer-based loss
   detection.  The duration of the alarm is based on the alarm's mode,
   which is set in the packet and timer events further below.  The
   function SetLossDetectionAlarm defined below shows how the single
   timer is set based on the alarm mode.

3.2.7.1.  Handshake Packets

   The initial flight has no prior RTT sample.  A client SHOULD remember
   the previous RTT it observed when resumption is attempted and use
   that for an initial RTT value.  If no previous RTT is available, the
   initial RTT defaults to 100ms.

   Endpoints MUST retransmit handshake frames if not acknowledged within
   a time limit.  This time limit will start as the largest of twice the
   RTT value and MinTLPTimeout.  Each consecutive handshake
   retransmission doubles the time limit, until an acknowledgement is
   received.

   Handshake frames may be cancelled by handshake state transitions.  In
   particular, all non-protected frames SHOULD be no longer be
   transmitted once packet protection is available.




Iyengar & Swett          Expires March 26, 2018                [Page 10]


Internet-Draft             QUIC Loss Detection            September 2017


   When stateless rejects are in use, the connection is considered
   immediately closed once a reject is sent, so no timer is set to
   retransmit the reject.

   Version negotiation packets are always stateless, and MUST be sent
   once per handshake packet that uses an unsupported QUIC version, and
   MAY be sent in response to 0RTT packets.

3.2.7.2.  Tail Loss Probe and Retransmission Timeout

   Tail loss probes [LOSS-PROBE] and retransmission timeouts [RFC6298]
   are an alarm based mechanism to recover from cases when there are
   outstanding retransmittable packets, but an acknowledgement has not
   been received in a timely manner.

3.2.7.3.  Early Retransmit

   Early retransmit [RFC5827] is implemented with a 1/4 RTT timer.  It
   is part of QUIC's time based loss detection, but is always enabled,
   even when only packet reordering loss detection is enabled.

3.2.7.4.  Pseudocode

   Pseudocode for SetLossDetectionAlarm follows:



























Iyengar & Swett          Expires March 26, 2018                [Page 11]


Internet-Draft             QUIC Loss Detection            September 2017


    SetLossDetectionAlarm():
       if (retransmittable packets are not outstanding):
         loss_detection_alarm.cancel()
         return

       if (handshake packets are outstanding):
         // Handshake retransmission alarm.
         if (smoothed_rtt == 0):
           alarm_duration = 2 * kDefaultInitialRtt
         else:
           alarm_duration = 2 * smoothed_rtt
         alarm_duration = max(alarm_duration, kMinTLPTimeout)
         alarm_duration = alarm_duration * (2 ^ handshake_count)
       else if (loss_time != 0):
         // Early retransmit timer or time loss detection.
         alarm_duration = loss_time - now
       else if (tlp_count < kMaxTLPs):
         // Tail Loss Probe
         if (retransmittable_packets_outstanding == 1):
           alarm_duration = 1.5 * smoothed_rtt + kDelayedAckTimeout
         else:
           alarm_duration = kMinTLPTimeout
         alarm_duration = max(alarm_duration, 2 * smoothed_rtt)
       else:
         // RTO alarm
         alarm_duration = smoothed_rtt + 4 * rttvar
         alarm_duration = max(alarm_duration, kMinRTOTimeout)
         alarm_duration = alarm_duration * (2 ^ rto_count)

       loss_detection_alarm.set(now + alarm_duration)

3.2.8.  On Alarm Firing

   QUIC uses one loss recovery alarm, which when set, can be in one of
   several modes.  When the alarm fires, the mode determines the action
   to be performed.

   Pseudocode for OnLossDetectionAlarm follows:













Iyengar & Swett          Expires March 26, 2018                [Page 12]


Internet-Draft             QUIC Loss Detection            September 2017


      OnLossDetectionAlarm():
        if (handshake packets are outstanding):
          // Handshake retransmission alarm.
          RetransmitAllHandshakePackets()
          handshake_count++
        else if (loss_time != 0):
          // Early retransmit or Time Loss Detection
          DetectLostPackets(largest_acked_packet)
        else if (tlp_count < kMaxTLPs):
          // Tail Loss Probe.
          SendOnePacket()
          tlp_count++
        else:
          // RTO.
          if (rto_count == 0)
            largest_sent_before_rto = largest_sent_packet
          SendTwoPackets()
          rto_count++

        SetLossDetectionAlarm()

3.2.9.  Detecting Lost Packets

   Packets in QUIC are only considered lost once a larger packet number
   is acknowledged.  DetectLostPackets is called every time an ack is
   received.  If the loss detection alarm fires and the loss_time is
   set, the previous largest acked packet is supplied.

3.2.9.1.  Handshake Packets

   The receiver MUST ignore unprotected packets that ack protected
   packets.  The receiver MUST trust protected acks for unprotected
   packets, however.  Aside from this, loss detection for handshake
   packets when an ack is processed is identical to other packets.

3.2.9.2.  Pseudocode

   DetectLostPackets takes one parameter, acked, which is the largest
   acked packet.

   Pseudocode for DetectLostPackets follows:










Iyengar & Swett          Expires March 26, 2018                [Page 13]


Internet-Draft             QUIC Loss Detection            September 2017


   DetectLostPackets(largest_acked):
     loss_time = 0
     lost_packets = {}
     delay_until_lost = infinite
     if (time_reordering_fraction != infinite):
       delay_until_lost =
         (1 + time_reordering_fraction) * max(latest_rtt, smoothed_rtt)
     else if (largest_acked.packet_number == largest_sent_packet):
       // Early retransmit alarm.
       delay_until_lost = 9/8 * max(latest_rtt, smoothed_rtt)
     foreach (unacked < largest_acked.packet_number):
       time_since_sent = now() - unacked.time_sent
       packet_delta = largest_acked.packet_number - unacked.packet_number
       if (time_since_sent > delay_until_lost):
         lost_packets.insert(unacked)
       else if (packet_delta > reordering_threshold)
         lost_packets.insert(unacked)
       else if (loss_time == 0 && delay_until_lost != infinite):
         loss_time = now() + delay_until_lost - time_since_sent

     // Inform the congestion controller of lost packets and
     // lets it decide whether to retransmit immediately.
     if (!lost_packets.empty())
       OnPacketsLost(lost_packets)
       foreach (packet in lost_packets)
         sent_packets.remove(packet.packet_number)

3.3.  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.

   The default initial RTT of 100ms was chosen because it is slightly
   higher than both the median and mean min_rtt typically observed on
   the public internet.

4.  Congestion Control

   QUIC's congestion control is based on TCP NewReno[RFC6582] congestion
   control to determine the congestion window and pacing rate.  QUIC
   congestion control is specified in bytes due to finer control and the
   ease of appropriate byte counting[RFC3465].



Iyengar & Swett          Expires March 26, 2018                [Page 14]


Internet-Draft             QUIC Loss Detection            September 2017


4.1.  Slow Start

   QUIC begins every connection in slow start and exits slow start upon
   loss.  QUIC re-enters slow start after a retransmission timeout.
   While in slow start, QUIC increases the congestion window by the
   number of acknowledged bytes when each ack is processed.

4.2.  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 MSS of bytes 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.

4.3.  Recovery Period

   Recovery is a period of time beginning with detection of a lost
   packet.  Because QUIC retransmits frames, not packets, it defines the
   end of recovery as all packets outstanding at the start of recovery
   being acknowledged or lost.  This is slightly different from TCP's
   definition of recovery ending when the lost packet that started
   recovery is acknowledged.  During recovery, the congestion window is
   not increased or decreased.  As such, multiple lost packets only
   decrease the congestion window once as long as they're lost before
   exiting recovery.  This causes QUIC to decrease the congestion window
   multiple times if retransmisions are lost, but limits the reduction
   to once per round trip.

4.4.  Tail Loss Probe

   If recovery sends a tail loss probe, no change is made to the
   congestion window or pacing rate.  Acknowledgement or loss of tail
   loss probes are treated like any other packet.

4.5.  Retransmission Timeout

   When retransmissions are sent due to a retransmission timeout alarm,
   no change is made to the congestion window or pacing rate until the
   next acknowledgement arrives.  When an ack arrives, if packets prior
   to the first retransmission timeout are acknowledged, then the
   congestion window remains the same.  If no packets prior to the first
   retransmission timeout are acknowledged, the retransmission timeout
   has been validated and the congestion window must be reduced to the
   minimum congestion window and slow start is begun.





Iyengar & Swett          Expires March 26, 2018                [Page 15]


Internet-Draft             QUIC Loss Detection            September 2017


4.6.  Pacing Rate

   The pacing rate is a function of the mode, the congestion window, and
   the smoothed rtt.  Specifically, the pacing rate is 2 times the
   congestion window divided by the smoothed RTT during slow start and
   1.25 times the congestion window divided by the smoothed RTT during
   slow start.  In order to fairly compete with flows that are not
   pacing, it is recommended to not pace the first 10 sent packets when
   exiting quiescence.

4.7.  Pseudocode

4.7.1.  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.

   kDefaultMss (default 1460 bytes):  The default max packet size used
      for calculating default and minimum congestion windows.

   kInitialWindow (default 10 * kDefaultMss):  Default limit on the
      amount of outstanding data in bytes.

   kMinimumWindow (default 2 * kDefaultMss):  Default minimum congestion
      window.

   kLossReductionFactor (default 0.5):  Reduction in congestion window
      when a new loss event is detected.

4.7.2.  Variables of interest

   Variables required to implement the congestion control mechanisms are
   described in this section.

   bytes_in_flight:  The sum of the size in bytes of all sent packets
      that contain at least one retransmittable or PADDING frame, and
      have not been acked or declared lost.  The size does not include
      IP or UDP overhead.  Packets only containing ack frames do not
      count towards byte_in_flight to ensure congestion control does not
      impede congestion feedback.

   congestion_window:  Maximum number of bytes in flight that may be
      sent.

   end_of_recovery:  The largest packet number sent when QUIC detects a
      loss.  When a larger packet is acknowledged, QUIC exits recovery.




Iyengar & Swett          Expires March 26, 2018                [Page 16]


Internet-Draft             QUIC Loss Detection            September 2017


   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.

4.7.3.  Initialization

   At the beginning of the connection, initialize the congestion control
   variables as follows:

      congestion_window = kInitialWindow
      bytes_in_flight = 0
      end_of_recovery = 0
      ssthresh = infinite

4.7.4.  On Packet Sent

   Whenever a packet is sent, and it contains non-ACK frames, the packet
   increases bytes_in_flight.

      OnPacketSentCC(bytes_sent):
        bytes_in_flight += bytes_sent

4.7.5.  On Packet Acknowledgement

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

      OnPacketAckedCC(acked_packet):
        // Remove from bytes_in_flight.
        bytes_in_flight -= acked_packet.bytes
        if (acked_packet.packet_number < end_of_recovery):
          // Do not increase congestion window in recovery period.
          return
        if (congestion_window < ssthresh):
          // Slow start.
          congestion_window += acked_packets.bytes
        else:
          // Congestion avoidance.
          congestion_window +=
            kDefaultMss * acked_packets.bytes / congestion_window

4.7.6.  On Packets Lost

   Invoked by loss detection from DetectLostPackets when new packets are
   detected lost.






Iyengar & Swett          Expires March 26, 2018                [Page 17]


Internet-Draft             QUIC Loss Detection            September 2017


      OnPacketsLost(lost_packets):
        // Remove lost packets from bytes_in_flight.
        for (lost_packet : lost_packets):
          bytes_in_flight -= lost_packet.bytes
        largest_lost_packet = lost_packets.last()
        // Start a new recovery epoch if the lost packet is larger
        // than the end of the previous recovery epoch.
        if (end_of_recovery < largest_lost_packet.packet_number):
          end_of_recovery = largest_sent_packet
          congestion_window *= kLossReductionFactor
          congestion_window = max(congestion_window, kMinimumWindow)
          ssthresh = congestion_window

4.7.7.  On Retransmission Timeout Verified

   QUIC decreases the congestion window to the minimum value once the
   retransmission timeout has been verified.

      OnRetransmissionTimeoutVerified()
        congestion_window = kMinimumWindow

5.  IANA Considerations

   This document has no IANA actions.  Yet.

6.  References

6.1.  Normative References

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport (work in progress), September 2017.

   [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>.

6.2.  Informative References

   [LOSS-PROBE]
              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.





Iyengar & Swett          Expires March 26, 2018                [Page 18]


Internet-Draft             QUIC Loss Detection            September 2017


   [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>.

   [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>.

Appendix A.  Acknowledgments

Appendix B.  Change Log

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

B.1.  Since draft-ietf-quic-recovery-05

   o  Add more congestion control text (#776)

B.2.  Since draft-ietf-quic-recovery-04

   No significant changes.

B.3.  Since draft-ietf-quic-recovery-03

   No significant changes.







Iyengar & Swett          Expires March 26, 2018                [Page 19]


Internet-Draft             QUIC Loss Detection            September 2017


B.4.  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
      (#415)

   o  Simplify RTO calculations (#142, #417)

B.5.  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

B.6.  Since draft-ietf-quic-recovery-00

   o  Improved description of constants and ACK behavior

B.7.  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)
   Google

   Email: jri@google.com


   Ian Swett (editor)
   Google

   Email: ianswett@google.com




Iyengar & Swett          Expires March 26, 2018                [Page 20]