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HyStart++: Modified Slow Start for TCP
RFC 9406

Document Type RFC - Proposed Standard (May 2023)
Authors Praveen Balasubramanian , Yi Huang , Matt Olson
Last updated 2023-05-15
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 RFC 9406 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Martin Duke
Send notices to
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IANA Actions
RFC 9406

Internet Engineering Task Force (IETF)                P. Balasubramanian
Request for Comments: 9406                                     Confluent
Category: Standards Track                                       Y. Huang
ISSN: 2070-1721                                                 M. Olson
                                                                May 2023

                 HyStart++: Modified Slow Start for TCP


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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2023 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
   ( 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.  Terminology
   3.  Definitions
   4.  HyStart++ Algorithm
     4.1.  Summary
     4.2.  Algorithm Details
     4.3.  Tuning Constants and Other Considerations
   5.  Deployments and Performance Evaluations
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

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 the absence of packet loss signals, TCP increases the cwnd
   exponentially to probe the network capacity.  This fast growth can
   overshoot the ideal sending rate and cause significant packet loss
   that cannot always be recovered efficiently.

   HyStart++ builds upon Hybrid Start (HyStart), originally described in
   [HyStart].  HyStart++ uses increase in round-trip delay 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] for
   finding a safe exit point for slow start.  After the slow start exit,
   a new 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 the 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 that use slow start, such as QUIC
   [RFC9002] or the Stream Control Transmission Protocol (SCTP)

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

   To aid the reader, we repeat some definitions from [RFC5681]:

   SENDER MAXIMUM SEGMENT SIZE (SMSS):  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
      algorithm [RFC1191] [RFC4821], RMSS (see next item), or other
      factors.  The size does not include the TCP/IP headers and

   RECEIVER MAXIMUM SEGMENT SIZE (RMSS):  The size of the largest
      segment that 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

   RECEIVER WINDOW (rwnd):  The most recently advertised receiver

   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 the 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 standard 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 in a fashion
   similar to 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.

   lastRoundMinRTT = infinity
   currentRoundMinRTT = infinity
   currRTT = 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 SND.NXT.

   At the start of each round during standard slow start [RFC5681] and
   CSS, initialize the variables used to compute the last round's and
   current round's minimum RTT:

   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 the minimum observed RTT:

     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 to see if
   delay increase triggers slow start exit:

   if ((rttSampleCount >= N_RTT_SAMPLE) AND
       (currentRoundMinRTT != infinity) AND
       (lastRoundMinRTT != infinity))
     RttThresh = max(MIN_RTT_THRESH,
       min(lastRoundMinRTT / MIN_RTT_DIVISOR, MAX_RTT_THRESH))
     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 the minimum observed RTT:

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

   For CSS rounds where at least N_RTT_SAMPLE RTT samples have been
   obtained, check to see if the current round's minRTT drops below
   baseline (cssBaselineMinRtt) indicating that slow start exit was

   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 by
   setting the ssthresh to the current cwnd.

   ssthresh = cwnd

   If loss or Explicit Congestion Notification (ECN) marking is observed
   at any time during standard slow start or CSS, enter congestion
   avoidance by setting the ssthresh to the current cwnd.

   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.

   MIN_RTT_DIVISOR is a fraction of RTT to compute the delay threshold.
   A smaller value would mean a larger threshold and thus less
   sensitivity to delay increase, and vice versa.

   While all TCP implementations are REQUIRED to take at least one RTT
   sample each round, implementations of HyStart++ are RECOMMENDED to
   take at least N_RTT_SAMPLE RTT samples.  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

   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.

   Packet pacing [ASA00] is a possible mechanism to avoid large bursts
   and their associated harm.  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.

   An implementation SHOULD use HyStart++ only for the initial slow
   start (when the ssthresh is at its initial value of arbitrarily high
   per [RFC5681]) and fall back to using standard 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

   In application-limited scenarios, the amount of data in flight could
   fall below the bandwidth-delay product (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 start.

5.  Deployments and Performance Evaluations

   At the time of this 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 goodput
   improvements as well as reductions in packet loss and retransmissions
   compared to standard 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 compared an implementation of HyStart++
   (based on an earlier draft version of this document) to standard 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% of 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].

   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.

   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 IANA actions.

8.  References

8.1.  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,

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

8.2.  Informative References

   [ASA00]    Aggarwal, A., Savage, S., and T. Anderson, "Understanding
              the performance of TCP pacing", Proceedings IEEE INFOCOM
              2000, DOI 10.1109/INFCOM.2000.832483, March 2000,

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

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

   [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
              SIGCOMM Computer Communication Review, vol. 29, issue 5,
              pp. 71-78, DOI 10.1145/505696.505704, October 1999,


   During the discussions of this work on the TCPM mailing list and 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 Tüxen.

Authors' Addresses

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

   Yi Huang
   One Microsoft Way
   Redmond, WA 98052
   United States of America
   Phone: +1 425 703 0447

   Matt Olson
   One Microsoft Way
   Redmond, WA 98052
   United States of America
   Phone: +1 425 538 8598