Internet DRAFT - draft-balasubramanian-tcpm-hystartplusplus

draft-balasubramanian-tcpm-hystartplusplus







Network Working Group                                 P. Balasubramanian
Internet-Draft                                                  Y. Huang
Intended status: Informational                                  M. Olson
Expires: January 23, 2020                                      Microsoft
                                                           July 22, 2019


                 HyStart++: Modified Slow Start for TCP
             draft-balasubramanian-tcpm-hystartplusplus-01

Abstract

   This informational memo describes HyStart++, a simple modification to
   the slow start phase of TCP congestion control algorithms.  HyStart++
   combines the use of one variant of HyStart and Limited Slow Start
   (LSS) to prevent overshooting of the ideal sending rate value, while
   also mitigating poor performance which can result from false
   positives when HyStart is used alone.  This memo also describes the
   details of the current implementation in the Windows operating
   system.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 23, 2020.

Copyright Notice

   Copyright (c) 2019 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
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   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  HyStart++ Algorithm . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Use of HyStart Delay Increase and Limited Slow Start  . .   3
     3.2.  Algorithm Details . . . . . . . . . . . . . . . . . . . .   3
     3.3.  Constant used and tuning  . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   [RFC0793] and [RFC5681] describe the slow start mechanism for TCP.
   The slow start algorithm is used when congestion window (cwnd) is
   less than the slow start threshold (ssthresh).  During slow start, a
   TCP increments cwnd by at most SMSS bytes per ACK.  In absence of
   packet loss signals, slow start effectively doubles the congestion
   window each round trip time.

   While traditional TCP slow start can ramp up very quickly, it
   frequently overshoots the ideal sending rate and causes a lot of
   unnecessary packet drops.  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
   ms or 300ms) 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 for further congestion window increases until the first
   packet loss occurs.

   This document describes HyStart++ as implemented in the Microsoft
   Windows operating system.  HyStart++ is widely deployed on the public



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   Internet.  A precise documentation of running code enables follow-up
   IETF Experimental or Standards Track RFCs.  It also enables other
   implementations and sharing of results for various workloads.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  HyStart++ Algorithm

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

3.2.  Algorithm Details

   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




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   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, 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 MIN_SSTHRESH and
      N_RTT_SAMPLE RTT samples have been obtained, check if delay
      increase triggers slow start exit

         if (cwnd >= MIN_SSTHRESH AND rttSampleCount >= N_RTT_SAMPLE)

            Eta = clamp(MIN_ETA, lastRoundMinRTT / 8, MAX_ETA)

            if (currentRoundMinRTT >= (lastRoundMinRTT + Eta))

               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 + N / 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 cwnd exceeds ssthresh or when congestion is
   observed.





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3.3.  Constant used and tuning

   The Windows operating system implementation of HyStart++ uses the
   following constants:

      MIN_SSTHRESH = 16

      MIN_ETA = 4 msec

      MAX_ETA = 16 msec

      LSS_DIVISOR = 0.25

      N_RTT_SAMPLE = 8

   An implementation MAY experiment with these constants and tune them
   for different network characteristics.  Windows operating system
   implementation uses the same values for all connections.

   An implementation MAY choose to use HyStart++ for all slow starts
   including the ones post a retransmission timeout, or a long idle
   period.  The Windows operating system implementation uses HyStart++
   only for the initial slow start and uses traditional slow start for
   subsequent ones.  This is acceptable because subsequent slow starts
   will use the discovered ssthresh value to exit slow start.

4.  Security Considerations

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

5.  IANA Considerations

   This document has no actions for IANA.

6.  Acknowledgements

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

7.  References

7.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.




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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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3742]  Floyd, S., "Limited Slow-Start for TCP with Large
              Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March
              2004, <https://www.rfc-editor.org/info/rfc3742>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

7.2.  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, March 2010,
              <https://doi.org/10.1016/j.comnet.2011.01.014>.

   [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,
              <https://www.rfc-editor.org/info/rfc8312>.

Authors' Addresses

   Praveen Balasubramanian
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: +1 425 538 2782
   Email: pravb@microsoft.com


   Yi Huang
   Microsoft

   Phone: +1 425 703 0447
   Email: huanyi@microsoft.com








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   Matt Olson
   Microsoft

   Phone: +1 425 538 8598
   Email: maolson@microsoft.com














































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