QUIC Congestion Control And Loss Recovery
draft-iyengar-quic-loss-recovery-00

Network Working Group                                         J. Iyengar
Internet-Draft                                                  I. Swett
Intended status: Informational                                    Google
Expires: January 9, 2017                                    July 8, 2016

               QUIC Congestion Control And Loss Recovery
                  draft-iyengar-quic-loss-recovery-00

Abstract

   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 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.

Status of This Memo

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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-
   transport-protocol].

   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 parts of the QUIC transmission
   machinery that are necessary to describe the congestion control and
   loss recovery mechanisms.  The document then describes QUIC's default
   congestion control and default loss recovery, followed by a list of
   the various TCP mechanisms that QUIC implements (in spirit) in its
   loss recovery mechanisms.

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 can contain several frames; we outline the frames that
   are important to the loss detection and congestion control machinery
   below.

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

   o  ACK frames contain acknowledgment information.  QUIC uses a SACK-
      based scheme, where the largest_acked packet number is explicitly

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      reported in the ACK frame, and packets with sequence numbers
      lesser than the largest_acked are reported as ACK ranges.  The ACK
      frame also includes a receive timestamp for each packet newly
      acked.

   o  To limit the ACK blocks to the ones that haven't yet been received
      by the sender, the sender periodically sends STOP_WAITING frames
      that signal the receiver to stop acking packets below a specified
      sequence number, raising the "least unacked" packet number at the
      receiver.  A sender of an ACK frame thus reports only those ACK
      blocks between the received least unacked and the reported largest
      observed packet numbers.  It is recommended for the sender to send
      the most recent largest acked packet it has received in an ack as
      the STOP_WAITING frame's least unacked value.

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 Sequence 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 transmission 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 sequence number is strictly increasing, and directly
   encodes transmission order.  A higher QUIC sequence number signifies
   that the packet was sent later, and a lower QUIC sequence number
   signifies that the packet was sent earlier.

   This design point significantly simplifies loss detection mechanisms
   for QUIC.  Most TCP mechanisms implicitly attempt to infer
   transmission ordering based on the TCP sequence numbers; a non-
   trivial task, especially when TCP timestamps are not available.

   QUIC resends lost packets with new packet sequence numbers when
   retransmission is necessary, removing ambiguity about which packet is
   acknowledged when an ACK is received.  Consequently, more accurate
   RTT measurements can be made, spurious retransmissions are trivially

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   detected, and mechanisms such as Fast Retransmit can be applied
   universally, based only on sequence number.

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 255 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.  An Overview of QUIC Loss Recovery

   We briefly describe QUIC's actions on packet transmission, ack
   reception, and timer expiration events.

3.1.  On Sending a Packet

   A retransmission timer may be set based on the mode:

   o  If the handshake has not completed, start a handshake timer.

      *  1.5x the SRTT, with exponential backoff.

   o  If there are outstanding packets which have not been ACKed,
      possibly set the loss timer

      *  Depends on the loss detection implementation, default is
         0.25RTT in the case of Early Retransmit.

   o  If fewer than 2 TLPs have been sent, compute and restart TLP
      timer.

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      *  Timer is set for max(10ms, 2*SRTT) if there are multiple
         packets in flight

      *  Timer is set to max(1.5*SRTT + delayed ack timer, 2*SRTT) if
         there is only one packet in flight.

   o  If 2 TLPs have been sent, set the RTO timer.

      *  Timer is set to max(200ms, SRTT+4*RTTVAR) with exponential
         backoff after the first RTO.

3.2.  On Receiving an ACK

   The following steps are performed when an ACK is received:

   o  Validate the ack, including ignoring any out of order acks.

   o  Update RTT measurements.

   o  Sender marks unacked packets lower than the largest_observed and
      acked in this ACK frame as ACKED.

   o  Packets with packet number lesser than the largest_observed that
      are not yet acked have missing_reports incremented based on
      FACK.(largest_observed - missing packet number)

   o  Threshold is set to 3 by default.

   o  Packets with missing_reports > threshold are marked for
      retransmission.  This logic implements Fast Retransmission and
      FACK-based retransmission together.

   o  If packets are outstanding and the largest observed is the largest
      sent packet, the retransmission timer will be set to 0.25SRTT,
      implementing Early Retransmit with timer.

   o  Stop timers if no packets are outstanding.

3.3.  On Timer Expiration

   QUIC uses one loss recovery timer, which when set, can be in one of
   several states.  When the timer expires, the state determines the
   action to be performed.  (TODO: describe when the timers are set)

   o  Handshake state:

      *  Retransmit any outstanding handshake packets.

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   o  Loss timer state:

      *  Lose the outstanding packets which have not yet been ACKed so
         far.

      *  Report the loss to the congestion controller.

      *  Retransmit as many as the congestion controller allows.

   o  TLP state:

      *  Retransmit the smallest outstanding packet which is
         retransmittable.

      *  Do not mark any packets as lost until an ACK arrives.

      *  Restart timer for a TLP or RTO.

   o  RTO state:

      *  Retransmit the two smallest outstanding packets which are
         retransmittable.

      *  Do not collapse the congestion window (ie: set to 1 packet)
         until an ack arrives and confirms that the RTO was not
         spurious.  Note that this step obviates the need to implement
         FRTO.

      *  Restart the timer for next RTO (with exponential backoff.)

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.

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   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 sequence number to measure the FACK reordering threshold.
   Currently 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
   loss.

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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
   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 ⅛ of the connection min_rtt.

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

   [draft-hamilton-quic-transport-protocol]
              Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
              A UDP-Based Multiplexed and Secure Transport", July 2016.

Authors' Addresses

   Janardhan Iyengar
   Google

   Email: jri@google.com

   Ian Swett
   Google

   Email: ianswett@google.com

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