Network Working Group                                 P. Balasubramanian
Internet-Draft                                                  Y. Huang
Intended status: Standards Track                                M. Olson
Expires: January 14, 2021                                      Microsoft
                                                           July 13, 2020

                 HyStart++: Modified Slow Start for TCP


   This doument describes HyStart++, a simple modification to the slow
   start phase of TCP congestion control algorithms.  Traditional slow
   start can cause overshotting of the ideal send rate and cause large
   packet loss within a round-trip time which results in poor
   performance.  HyStart++ combines the use of one variant of HyStart
   and Limited Slow Start (LSS) to prevent overshooting of the ideal
   sending rate, while also mitigating poor performance which can result
   from false positives when HyStart is used alone.

Status of This Memo

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  HyStart++ Algorithm . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Use of HyStart Delay Increase and Limited Slow Start  . .   3
     4.2.  Algorithm Details . . . . . . . . . . . . . . . . . . . .   4
     4.3.  Tuning constants  . . . . . . . . . . . . . . . . . . . .   5
   5.  Deployments and Performance Evaluations . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   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 sender increases cwnd
   exponentially to probe the network capacity.  Such a fast growth can
   lead to overshooting the ideal sending rate and cause significant
   packet loss.  This is counter-productive for the TCP flow itself, and
   also impacts the rest of the traffic sharing the bottleneck link.
   TCP has several mechanisms for loss recovery, but they are only
   effective for moderate loss.  When these techniques are unable to
   recover lost packets, a last-resort retransmission timeout (RTO) is
   used to trigger packet recovery.  In most operating systems, the
   minimum RTO is set to a large value (200 msec or 300 msec) to prevent
   spurious timeouts.  This results in a long idle time which
   drastically impairs flow completion times.

   HyStart++ adds delay increase as a signal to exit slow start before
   any packet loss occurs.  This is one of two algorithms specified in
   [HyStart].  After the HyStart delay algorithm finds an exit point,
   LSS is used in conjunction with congestion avoidance for further
   congestion window increases until the first packet loss is detected.
   HyStart++ reduces packet loss and retransmissions, and improves
   goodput in lab measurements as well as real world deployments.

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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.  Use of HyStart Delay Increase and Limited Slow Start

   [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 RTT spikes, which
   suggest that the bottleneck buffer is filling up.

   After the HyStart "Delay Increase" algorithm triggers an exit from
   slow start, LSS (described in [RFC3742]) is used to increase Cwnd
   until congestion is observed.  LSS is used because the HyStart exit
   is often premature as a result of RTT fluctuations or transient queue
   buildup.  LSS grows the cwnd fast but much slower than traditional

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   slow start.  LSS helps avoid massive packet losses and subsequent
   time spent in loss recovery or retransmission timeout.

4.2.  Algorithm Details

   We assume that Appropriate Byte Counting (as described in [RFC3465])
   is in use and L is the cwnd increase limit.  The choice of value of L
   is up to the implementation.

   A round is chosen to be approximately the Round-Trip Time (RTT).
   Round can be approximated using sequence numbers as follows:

      Define windowEnd as a sequence number initialize to SND.UNA

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

   At the start of each round during slow start:

      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 and

      Update the cwnd

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

      Keep track of minimum observed RTT

         currentRoundMinRTT = min(currentRoundMinRTT, currRTT)

         where currRTT is the measured RTT based on the incoming ACK

         rttSampleCount += 1

      For rounds where cwnd is at or higher than LOW_CWND and
      N_RTT_SAMPLE RTT samples have been obtained, check if delay
      increase triggers slow start exit

         if (cwnd >= (LOW_CWND * SMSS) AND rttSampleCount >=

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            RttThresh = clamp(MIN_RTT_THRESH, lastRoundMinRTT / 8,

            if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))

               ssthresh = cwnd

               exit slow start and enter LSS

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

      K = cwnd / (LSS_DIVISOR * ssthresh)

      cwnd = max(cwnd + (min (N, L * SMSS) / K), CA_cwnd())

   CA_cwnd() denotes the cwnd that a congestion control algorithm would
   have increased to if congestion avoidance started instead of LSS.
   LSS grows cwnd very fast but for long-lived flows in high BDP
   networks, the congestion avoidance algorithm could increase cwnd much
   faster.  For example, CUBIC congestion avoidance [RFC8312] in convex
   region can ramp up cwnd rapidly.  Taking the max can help improve
   performance when exiting slow start prematurely.

   HyStart++ ends when congestion is observed.

4.3.  Tuning constants

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

      LOW_CWND = 16

      MIN_RTT_THRESH = 4 msec

      MAX_RTT_THRESH = 16 msec

      LSS_DIVISOR = 0.25

      N_RTT_SAMPLE = 8

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

   Using smaller values of LOW_CWND will cause the algorithm to kick in
   before the last round RTT can be measured, particularly if the
   implementation uses an initial cwnd of 10 MSS.  Higher values will

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   delay the detection of delay increase and reduce the ability of
   HyStart++ to prevent overshoot problems.

   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 maximum value of LSS_DIVISOR SHOULD NOT exceed 0.5, which is the
   value recommended in [RFC3742].  Otherwise the cwnd growth could
   again become too aggressive and cause ideal send rate overshoot.
   Smaller values will cause the algorithm to be less aggressive and may
   leave some cwnd growth on the table.

   An implementation SHOULD use HyStart++ only for the initial slow
   start 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.  An implementation MAY use HyStart++ to grow the restart
   window ([RFC5681]) after a long idle period.

5.  Deployments and Performance Evaluations

   As of the time of writing, HyStart++ has been default enabled for all
   TCP connections in Windows for two years.  The original Hystart has
   been default-enabled for all TCP connections in Linux TCP for a

   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 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

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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.  Acknowledgements

   Neal Cardwell suggested the idea of using the maximum of cwnd value
   computed by LSS and congestion avoidance after exiting slow start.

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,

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

   [RFC3742]  Floyd, S., "Limited Slow-Start for TCP with Large
              Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March
              2004, <>.

   [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, "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,

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Authors' Addresses

   Praveen Balasubramanian
   One Microsoft Way
   Redmond, WA  98052

   Phone: +1 425 538 2782

   Yi Huang

   Phone: +1 425 703 0447

   Matt Olson

   Phone: +1 425 538 8598

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