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HyStart++: Modified Slow Start for TCP

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9406.
Authors Praveen Balasubramanian , Yi Huang , Matt Olson
Last updated 2023-02-16 (Latest revision 2023-01-30)
Replaces draft-balasubramanian-tcpm-hystartplusplus
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestone
Nov 2022
Submit a document on modifying slow start to avoid large tail drops as a Proposed Standard RFC
Document shepherd Michael Tüxen
Shepherd write-up Show Last changed 2022-08-30
IESG IESG state Approved-announcement to be sent::Revised I-D Needed
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Martin Duke
Send notices to
IANA IANA review state IANA OK - No Actions Needed
Network Working Group                                 P. Balasubramanian
Internet-Draft                                                 Confluent
Intended status: Standards Track                                Y. Huang
Expires: 3 August 2023                                          M. Olson
                                                         30 January 2023

                 HyStart++: Modified Slow Start for TCP


   This document 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

   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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 3 August 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   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 . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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.

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   While this document describes Hystart++ for TCP, it can also be used
   for other transport protocols which use slow start such as QUIC
   [RFC9002] or SCTP [RFC9260].

2.  Terminology

   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.

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.

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   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
   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

   The following pseudocode uses a limit, L, to control the
   aggressiveness of the cwnd increase during both standard slow start
   and CSS.  While an arriving ACK may newly acknowledge an arbitrary
   number of bytes, the Hystart++ algorithm limits the number of those
   bytes applied to increase the cwnd to L*SMSS bytes.

   lastRoundMinRTT and currentRoundMinRTT are initialized to infinity at
   the initialization time. currRTT is the RTT sampled from the latest
   incoming ACK and initialized to infinity.

   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

   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:

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     currentRoundMinRTT = min(currentRoundMinRTT, currRTT)
     rttSampleCount += 1

   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)
     RttThresh = clamp(MIN_RTT_THRESH,
                       lastRoundMinRTT / 8,
     if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))
       cssBaselineMinRtt = currentRoundMinRTT
       exit slow start and enter CSS

   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)
   rttSampleCount += 1

   For CSS rounds where at least 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)
     cssBaselineMinRtt = infinity
     resume slow start including HyStart++

   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.

   If CSS_ROUNDS rounds are complete, enter congestion avoidance.

   ssthresh = cwnd

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

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   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
   L = infinity if paced, L = 8 if non-paced

   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.

   A paced TCP implementation SHOULD use L = infinity.  Burst concerns
   are mitigated by pacing and this setting allows for optimal cwnd
   growth on modern networks.

   For TCP implementations that pace to mitigate burst concerns, L
   values smaller than INFINITY may suffer performance problems due to
   slow cwnd growth in high speed networks.  For non-paced TCP
   implementations, L values smaller than 8 may suffer performance
   problems due to slow cwnd growth in high speed networks; L values
   larger than 8 may cause an increase in burstiness and thereby loss
   rates, and result in poor performance.

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   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
   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 this document
   has been default enabled for all TCP connections in the Windows
   operating system for over two years with pacing disabled and an
   actual L = 8.

   In lab measurements with Windows TCP, HyStart++ shows both goodput
   improvements as well as reductions in packet loss and retransmissions
   compared to traditional slow start.  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 (RTOs) by 36%.

   In an A/B test where we compare HyStart++ draft 01 to traditional
   slow start 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

6.  Security Considerations

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

   An attacker can cause Hystart++ to exit slow start prematurely and
   impair the performance of a TCP connection by, for example, dropping
   data packets or their acknowledgments.

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   The ACK division attack outlined in [SCWA99] does not affect
   Hystart++ because the congestion window increase in Hystart++ is
   based on the number of bytes newly acknowledged in each arriving ACK
   rather than by a particular constant on each arriving ACK.

7.  IANA Considerations

   This document has no actions for IANA.

8.  Acknowledgements

   During the discussions of this work on the TCPM mailing list, in
   working group meetings, helpful comments, critiques, and reviews were
   received from (listed alphabetically by last name): Mark Allman, Bob
   Briscoe, Neal Cardwell, Yuchung Cheng, Junho Choi, Martin Duke, Reese
   Enghardt, Christian Huitema, Ilpo Järvinen, Yoshifumi Nishida,
   Randall Stewart, and Michael Tuexen.

9.  References

9.1.  Normative References

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

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

9.2.  Informative References

   [HyStart]  Ha, S. and I. Ree, "Taming the elephants: New TCP slow
              start",  Computer Networks vol. 55, no. 9, pp. 2092-2110,
              DOI 10.1016/j.comnet.2011.01.014, 2011,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

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   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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

   [RFC9260]  Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
              Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
              June 2022, <>.

   [SCWA99]   Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
              "TCP congestion control with a misbehaving receiver",  ACM
              Computer Communication Review, 29(5),
              DOI 10.1145/505696.505704, 1999,

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

   Matt Olson
   Phone: +1 425 538 8598

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