HyStart++: Modified Slow Start for TCP
draft-ietf-tcpm-hystartplusplus-00

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Last updated 2020-07-13
Replaces draft-balasubramanian-tcpm-hystartplusplus
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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
                   draft-ietf-tcpm-hystartplusplus-00

Abstract

   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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   This Internet-Draft will expire on January 14, 2021.

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Balasubramanian, et al. Expires January 14, 2021                [Page 1]
Internet-Draft                  HyStart++                      July 2020

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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
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