Network Working Group                                 P. Balasubramanian
Internet-Draft                                                 Confluent
Intended status: Standards Track                                Y. Huang
Expires: 18 December 2022                                       M. Olson
                                                            16 June 2022

                 HyStart++: Modified Slow Start for TCP


   This doument describes HyStart++, a simple modification to the slow
   start phase of congestion control algorithms.  Traditional slow start
   can overshoot the ideal send rate in many cases, causing high packet
   loss and poor performance.  HyStart++ uses a delay increase heuristic
   to find an exit point before possible overshoot.  It also adds a
   mitigation to prevent jitter from causing premature slow start exit.

Status of This Memo

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   This Internet-Draft will expire on 18 December 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  HyStart++ Algorithm . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Algorithm Details . . . . . . . . . . . . . . . . . . . .   4
     4.3.  Tuning constants and other considerations . . . . . . . .   6
   5.  Deployments and Performance Evaluations . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC5681] describes the slow start congestion control algorithm for
   TCP.  The slow start algorithm is used when the congestion window
   (cwnd) is less than the slow start threshold (ssthresh).  During slow
   start, in absence of packet loss signals, TCP increases cwnd
   exponentially to probe the network capacity.  This fast growth can
   overshoot the ideal sending rate and cause significant packet loss
   which cannot always be recovered efficiently.

   HyStart++ uses delay increase as a signal to exit slow start before
   potential packet loss occurs as a result of overshoot.  This is one
   of two algorithms specified in [HyStart].  After the slow start exit,
   a novel Conservative Slow Start (CSS) phase is used to determine
   whether the slow start exit was premature and to resume slow start.
   This mitigation improves performance in presence of jitter.
   HyStart++ reduces packet loss and retransmissions, and improves
   goodput in lab measurements and real world deployments.

   While this document describes Hystart++ for TCP, it can also be used
   for other transport protocols which use slow start such as QUIC

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Definitions

   We repeat here some definition from [RFC5681] to aid the reader.

   SENDER MAXIMUM SEGMENT SIZE (SMSS): The SMSS is the size of the
   largest segment that the sender can transmit.  This value can be
   based on the maximum transmission unit of the network, the path MTU
   discovery [RFC1191, RFC4821] algorithm, RMSS (see next item), or
   other factors.  The size does not include the TCP/IP headers and

   largest segment the receiver is willing to accept.  This is the value
   specified in the MSS option sent by the receiver during connection
   startup.  Or, if the MSS option is not used, it is 536 bytes
   [RFC1122].  The size does not include the TCP/IP headers and options.

   RECEIVER WINDOW (rwnd): The most recently advertised receiver window.

   CONGESTION WINDOW (cwnd): A TCP state variable that limits the amount
   of data a TCP can send.  At any given time, a TCP MUST NOT send data
   with a sequence number higher than the sum of the highest
   acknowledged sequence number and the minimum of cwnd and rwnd.

4.  HyStart++ Algorithm

4.1.  Summary

   [HyStart] specifies two algorithms (a "Delay Increase" algorithm and
   an "Inter-Packet Arrival" algorithm) to be run in parallel to detect
   that the sending rate has reached capacity.  In practice, the Inter-
   Packet Arrival algorithm does not perform well and is not able to
   detect congestion early, primarily due to ACK compression.  The idea
   of the Delay Increase algorithm is to look for spikes in RTT (round-
   trip time), which suggest that the bottleneck buffer is filling up.

   In HyStart++, a TCP sender uses traditional slow start and then uses
   the "Delay Increase" algorithm to trigger an exit from slow start.
   But instead of going straight from slow start to congestion
   avoidance, the sender spends a number of RTTs in a Conservative Slow
   Start (CSS) phase to determine whether the exit from slow start was
   premature.  During CSS, the congestion window is grown exponentially

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   like in regular slow start, but with a smaller exponential base,
   resulting in less aggressive growth.  If the RTT reduces during CSS,
   it's concluded that the RTT spike was not related to congestion
   caused by the connection sending at a rate greater than the ideal
   send rate, and the connection resumes slow start.  If the RTT
   inflation persists throughout CSS, the connection enters congestion

4.2.  Algorithm Details

   For the pseudocode, we assume that Appropriate Byte Counting (as
   described in [RFC3465]) is in use and L is the cwnd increase limit as
   discussed in RFC 3465.

   lastRoundMinRTT and currentRoundMinRTT are initialized to infinity at
   the initialization time

   Hystart++ measures rounds using sequence numbers, as follows:

      Define windowEnd as a sequence number initialized to SND.NXT

      When windowEnd is ACKed, the current round ends and windowEnd is
      set to SND.NXT

   At the start of each round during standard slow start ([RFC5681]) and

      lastRoundMinRTT = currentRoundMinRTT

      currentRoundMinRTT = infinity

      rttSampleCount = 0

   For each arriving ACK in slow start, where N is the number of
   previously unacknowledged bytes acknowledged in the arriving ACK:

      Update the cwnd

      -  cwnd = cwnd + min (N, L * SMSS)

      Keep track of minimum observed RTT

      -  currentRoundMinRTT = min(currentRoundMinRTT, currRTT)

      -  where currRTT is the RTT sampled from the latest incoming ACK

      -  rttSampleCount += 1

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      For rounds where at least N_RTT_SAMPLE RTT samples have been
      obtained and currentRoundMinRTT and lastRoundMinRTT are valid,
      check if delay increase triggers slow start exit

      -  if (rttSampleCount >= N_RTT_SAMPLE AND currentRoundMinRTT !=
         infinity AND lastRoundMinRTT != infinity)

         o  RttThresh = clamp(MIN_RTT_THRESH, lastRoundMinRTT / 8,

         o  if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))

            +  cssBaselineMinRtt = currentRoundMinRTT

            +  exit slow start and enter CSS

   CSS lasts at most CSS_ROUNDS rounds.  If the transition into CSS
   happens in the middle of a round, that partial round counts towards
   the limit.

   For each arriving ACK in CSS, where N is the number of previously
   unacknowledged bytes acknowledged in the arriving ACK:

      Update the cwnd

      -  cwnd = cwnd + (min (N, L * SMSS) / CSS_GROWTH_DIVISOR)

      Keep track of minimum observed RTT

      -  currentRoundMinRTT = min(currentRoundMinRTT, currRTT)

      -  where currRTT is the sampled RTT from the incoming ACK

      -  rttSampleCount += 1

      For CSS rounds where N_RTT_SAMPLE RTT samples have been obtained,
      check if current round's minRTT drops below baseline indicating
      that HyStart exit was spurious.

      -  if (currentRoundMinRTT < cssBaselineMinRtt)

         o  cssBaselineMinRtt = infinity

         o  resume slow start including HyStart++

   If CSS_ROUNDS rounds are complete, enter congestion avoidance.

   *  ssthresh = cwnd

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   If loss or ECN-marking is observed anytime during standard slow start
   or CSS, enter congestion avoidance.

   *  ssthresh = cwnd

4.3.  Tuning constants and other considerations

   It is RECOMMENDED that a HyStart++ implementation use the following

   *  MIN_RTT_THRESH = 4 msec

   *  MAX_RTT_THRESH = 16 msec

   *  N_RTT_SAMPLE = 8


   *  CSS_ROUNDS = 5

   These constants have been determined with lab measurements and real
   world deployments.  An implementation MAY tune them for different
   network characteristics.

   The delay increase sensitivity is determined by MIN_RTT_THRESH and
   MAX_RTT_THRESH.  Smaller values of MIN_RTT_THRESH may cause spurious
   exits from slow start.  Larger values of MAX_RTT_THRESH may result in
   slow start not exiting until loss is encountered for connections on
   large RTT paths.

   A TCP implementation is required to take at least one RTT sample each
   round.  Using lower values of N_RTT_SAMPLE will lower the accuracy of
   the measured RTT for the round; higher values will improve accuracy
   at the cost of more processing.

   The minimum value of CSS_GROWTH_DIVISOR MUST be at least 2.  A value
   of 1 results in the same aggressive behavior as regular slow start.
   Values larger than 4 will cause the algorithm to be less aggressive
   and maybe less performant.

   Smaller values of CSS_ROUNDS may miss detecting jitter and larger
   values may limit performance.

   An implementation SHOULD use HyStart++ only for the initial slow
   start (when ssthresh is at its initial value of arbitrarily high per
   [RFC5681]) and fall back to using traditional slow start for the
   remainder of the connection lifetime.  This is acceptable because
   subsequent slow starts will use the discovered ssthresh value to exit

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   slow start and avoid the overshoot problem.  An implementation MAY
   use HyStart++ to grow the restart window ([RFC5681]) after a long
   idle period.

   In application limited scenarios, the amount of data in flight could
   fall below the BDP and result in smaller RTT samples which can
   trigger an exit back to slow start.  It is expected that a connection
   might oscillate between CSS and slow start in such scenarios.  But
   this behavior will neither result in a connection prematurely
   entering congestion avoidance nor cause overshooting compared to slow

5.  Deployments and Performance Evaluations

   As of the time of writing, HyStart++ as described in draft versions
   01 through 04 was default enabled for all TCP connections in the
   Windows operating system for over three years with an actual L = 8.
   The original Hystart has been default-enabled for all TCP connections
   in the Linux operating system using the default congestion control
   module CUBIC ([RFC8312]) for a decade with an infinite L.

   In lab measurements with Windows TCP, HyStart++ shows both goodput
   improvements as well as reductions in packet loss and
   retransmissions.  For example across a variety of tests on a 100 Mbps
   link with a bottleneck buffer size of bandwidth-delay product,
   HyStart++ reduces bytes retransmitted by 50% and retransmission
   timeouts by 36%.

   In an A/B test for HyStart++ draft 01 across a large Windows device
   population, out of 52 billion TCP connections, 0.7% of connections
   move from 1 RTO to 0 RTOs and another 0.7% connections move from 2
   RTOs to 1 RTO with HyStart++. This test did not focus on send heavy
   connections and the impact on send heavy connections is likely much
   higher.  We plan to conduct more such production experiments to
   gather more data in the future.

6.  Security Considerations

   HyStart++ enhances slow start and inherits the general security
   considerations discussed in [RFC5681].

7.  IANA Considerations

   This document has no actions for IANA.

8.  References

8.1.  Normative References

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

   [RFC3465]  Allman, M., "TCP Congestion Control with Appropriate Byte
              Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
              2003, <>.

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

8.2.  Informative References

   [HyStart]  Ha, S. and I. Ree, "Hybrid Slow Start for High-Bandwidth
              and Long-Distance Networks",
              DOI 10.1145/1851182.1851192,  International Workshop on
              Protocols for Fast Long-Distance Networks, 2008,

   [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,

   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <>.

Authors' Addresses

   Praveen Balasubramanian
   899 West Evelyn Ave
   Mountain View, CA 94041
   United States of America

   Yi Huang
   One Microsoft Way
   Redmond, WA 94052
   United States of America
   Phone: +1 425 703 0447

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   Matt Olson
   Phone: +1 425 538 8598

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