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

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Network Working Group                                 P. Balasubramanian
Internet-Draft                                                  Y. Huang
Intended status: Informational                                  M. Olson
Expires: January 9, 2020                                       Microsoft
                                                            July 8, 2019

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

Abstract

   This experimental 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 9, 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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   (https://trustee.ietf.org/license-info) in effect on the date of
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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
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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  . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   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.  In the 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
   Internet.  A precise documentation of running code enables follow-up

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