Network Working Group                                         J. Iyengar
Internet-Draft                                                  I. Swett
Intended status: Experimental                                     Google
Expires: May 4, 2017                                    October 31, 2016

               QUIC Congestion Control And Loss Recovery


   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.  QUIC implements the spirit of known TCP loss detection
   mechanisms, described in RFCs, various Internet-drafts, and also
   those prevalent in the Linux TCP implementation.  This document
   describes QUIC loss detection and congestion control, and attributes
   the TCP equivalent in RFCs, Internet-drafts, academic papers, and TCP

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   This Internet-Draft will expire on May 4, 2017.

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   to this document.  Code Components extracted from this document must
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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 [draft-hamilton-quic-

   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.

   This document first describes pre-requisite parts of the QUIC
   transmission machinery, then discusses QUIC's default congestion
   control and loss detection mechanisms, and finally lists the various
   TCP mechanisms that QUIC loss detection implements (in spirit.)

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

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

   o  Crypto handshake data is also sent as STREAM data, and uses the
      reliability machinery of QUIC underneath.

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   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

   There are some notable differences between QUIC and TCP which are
   important for reasoning about the differences between the loss
   recovery mechanisms employed by the two protocols.  We briefly
   describe these 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.

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.

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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

   We now describe QUIC's loss detection as functions that should be
   called on packet transmission, when a packet is acked, and timer
   expiration events.

3.1.  Variables of interest

   We first describe the variables required to implement the loss
   detection mechanisms described in this section.

   o  loss_detection_alarm: Multi-modal alarm used for loss detection.

   o  alarm_mode: QUIC maintains a single loss detection alarm, which
      switches between various modes.  This mode is used to determine
      the duration of the alarm.

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

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

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

   o  smoothed_rtt: The smoothed RTT of the connection, computed as
      described in [RFC 6298].  TODO: Describe RTT computations.

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

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   o  time_loss: When true, loss detection operates solely based on
      reordering threshold in time, rather than in packet number gaps.

3.2.  Initialization

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

   handshake_count = 0;
   tlp_count = 0;
   rto_count = 0;
   smoothed_rtt = 0;
   reordering_threshold = 3;
   time_loss = false;

3.3.  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.

   Pseudocode for SetLossDetectionAlarm follows.

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    if (retransmittable packets are outstanding):

    if (handshake packets are outstanding):

      alarm_duration = max(1.5 * smoothed_rtt, 10ms) << handshake_count;
    else if (largest sent packet is acked):
      // Set alarm based on short timer for early retransmit.
      alarm_duration = 0.25 x smoothed_rtt;
    else if (tlp_count < 2):

      if (retransmittable_packets_outstanding = 1):
        alarm_duration = max(1.5 x smoothed_rtt + delayed_ack_timer,
                             2 x smoothed_rtt);
        alarm_duration = max (10ms, 2 x smoothed_rtt);

      if (rto_count = 0):
        alarm_duration = max(200ms, smoothed_rtt + 4 x rttvar);
        alarm_duration = loss_detection_alarm.get_delay() << 1;


    loss_detecton_alarm.set(now + alarm_duration);

3.4.  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_retransmittble: A boolean that indicates whether the packet
      contains at least one frame requiring reliable deliver.  The
      retransmittability of various QUIC frames is described in [draft-
      hamilton-quic-protocol].  If false, it is still acceptable for an
      ack to be received for this packet.  However, a caller MUST NOT
      set is_retransmittable to true if an ack is not expected.

   Pseudocode for OnPacketSent follows.

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   OnPacketSent(packet_number, is_retransmittable):
     if is_retransmittable:

3.5.  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.

   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.

   Pseudocode for OnPacketAcked follows.

     handshake_count = 0;
     tlp_count = 0;
     rto_count = 0;
     UpdateRtt(); // TODO: document RTT estimator.

3.6.  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.  OnAlarm returns a list of packet numbers that are
   detected as lost.

   Pseudocode for OnAlarm follows.

     lost_packets = DetectLostPackets(acked_packet);

3.7.  Detecting Lost Packets

   Packets in QUIC are only considered lost once a larger packet number
   is acknowledged.  DetectLostPackets is called every time there is a
   new largest packet or if the loss detection alarm fires the previous
   largest acked packet is supplied.

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   DetectLostPackets takes one parameter, acked_packet, which is the
   packet number of the largest acked packet, and returns a list of
   packet numbers detected as lost.

   Pseudocode for DetectLostPackets follows.

     lost_packets = {};
     foreach (unacked_packet less than acked_packet):
         if (unacked_packet.time_sent <
             acked_packet.time_sent - 1/8 * smoothed_rtt):
       else if (unacked_packet.packet_number <
                acked_packet.packet_number - reordering_threshold)
     return lost_packets;

4.  Congestion Control

   (describe NewReno-style congestion control for QUIC.)

5.  TCP mechanisms in QUIC

   QUIC implements the spirit of a variety of RFCs, Internet drafts, and
   other well-known TCP loss recovery mechanisms, though the
   implementation details differ from the TCP implementations.

5.1.  RFC 6298 (RTO computation)

   QUIC calculates SRTT and RTTVAR according to the standard formulas.
   An RTT sample is only taken if the delayed ack correction is smaller
   than the measured RTT (otherwise a negative RTT would result), and
   the ack's contains a new, larger largest observed packet number.
   min_rtt is only based on the observed RTT, but SRTT uses the delayed
   ack correction delta.

   As described above, QUIC implements RTO with the standard timeout and
   CWND reduction.  However, QUIC retransmits the earliest outstanding
   packets rather than the latest, because QUIC doesn't have
   retransmission ambiguity.  QUIC uses the commonly accepted min RTO of
   200ms instead of the 1s the RFC specifies.

5.2.  FACK Loss Recovery (paper)

   QUIC implements the algorithm for early loss recovery described in
   the FACK paper (and implemented in the Linux kernel.)  QUIC uses the
   packet number to measure the FACK reordering threshold.  Currently

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   QUIC does not implement an adaptive threshold as many TCP
   implementations(ie: the Linux kernel) do.

5.3.  RFC 3782, RFC 6582 (NewReno Fast Recovery)

   QUIC only reduces its CWND once per congestion window, in keeping
   with the NewReno RFC.  It tracks the largest outstanding packet at
   the time the loss is declared and any losses which occur before that
   packet number are considered part of the same loss event.  It's worth
   noting that some TCP implementations may do this on a sequence number
   basis, and hence consider multiple losses of the same packet a single
   loss event.

5.4.  TLP (draft)

   QUIC always sends two tail loss probes before RTO is triggered.  QUIC
   invokes tail loss probe even when a loss is outstanding, which is
   different than some TCP implementations.

5.5.  RFC 5827 (Early Retransmit) with Delay Timer

   QUIC implements early retransmit with a timer in order to minimize
   spurious retransmits.  The timer is set to 1/4 SRTT after the final
   outstanding packet is acked.

5.6.  RFC 5827 (F-RTO)

   QUIC implements F-RTO by not reducing the CWND and SSThresh until a
   subsequent ack is received and it's sure the RTO was not spurious.
   Conceptually this is similar, but it makes for a much cleaner
   implementation with fewer edge cases.

5.7.  RFC 6937 (Proportional Rate Reduction)

   PRR-SSRB is implemented by QUIC in the epoch when recovering from a

5.8.  TCP Cubic (draft) with optional RFC 5681 (Reno)

   TCP Cubic is the default congestion control algorithm in QUIC.  Reno
   is also an easily available option which may be requested via
   connection options and is fully implemented.

5.9.  Hybrid Slow Start (paper)

   QUIC implements hybrid slow start, but disables ack train detection,
   because it has shown to falsely trigger when coupled with packet
   pacing, which is also on by default in QUIC.  Currently the minimum

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   delay increase is 4ms, the maximum is 16ms, and within that range
   QUIC exits slow start if the min_rtt within a round increases by more
   than &#8539; of the connection mi

5.10.  RACK (draft)

   QUIC's loss detection is by it's time-ordered nature, very similar to
   RACK.  Though QUIC defaults to loss detection based on reordering
   threshold in packets, it could just as easily be based on fractions
   of an rtt, as RACK does.



6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key Words for use in RFCs to Indicate
              Requirement Levels", March 1997.

6.2.  Informative References

              Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
              A UDP-Based Multiplexed and Secure Transport", July 2016.

Authors' Addresses

   Janardhan Iyengar


   Ian Swett


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