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Internet Engineering Task Force                              Sally Floyd
INTERNET DRAFT                                                      ICSI
draft-floyd-tcp-slowstart-00.txt                              June, 2002



        Limited Slow-Start for TCP with Large Congestion Windows



                          Status of this Memo


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   This note proposes a modification for TCP's slow-start for use with
   TCP connections with large congestion windows.  For TCP connections
   that are able to use congestion windows of thousands (or tens of
   thousands) of MSS-sized segments (for MSS the sender's MAXIMUM
   SEGMENT SIZE), the current slow-start procedure can result in
   increasing the congestion window by thousands of segments in a single
   round-trip time.  Such an increase can easily result in thousands of
   packets being dropped in one round-trip time.  This is often counter-
   productive for the TCP flow itself, and is also hard on the rest of
   the traffic sharing the congested link.  This note proposes Limited
   Slow-Start, limiting the number of segments by which the congestion
   window is increased for one window of data during slow-start, in
   order to improve performance for TCP connections with large
   congestion windows.



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1.  Introduction

   This note proposes a modification for TCP's slow-start for use with
   TCP connections with large congestion windows.  For TCP connections
   that are able to use congestion windows of thousands (or tens of
   thousands) of MSS-sized segments (for MSS the sender's MAXIMUM
   SEGMENT SIZE), the current slow-start procedure can result in
   increasing the congestion window by thousands of segments in a single
   round-trip time.  Such an increase can easily result in thousands of
   packets being dropped in one round-trip time.  This is often counter-
   productive for the TCP flow itself, and is also hard on the rest of
   the traffic sharing the congested link.  This note proposes Limited
   Slow-Start, limiting the number of segments by which the congestion
   window is increased for one window of data during slow-start, in
   order to improve performance for TCP connections with large
   congestion windows.

   When slow-start results in a large increase in the congestion window
   in one round-trip time, the subsequent drop of a large number of
   packets in the network can result in unnecessary retransmit timeouts
   for the TCP connection.  The TCP connection could end up in
   congestion avoidance phase with a very small congestion window, and
   could take a large number of round-trip times to recover its old
   congestion window.  This poor performance is illustrated in [F02].

2.  The Proposal for Limited Slow-Start

   Limited Slow-Start introduces a parameter, "max_ssthresh", and the
   slow-start is only modified for values of the congestion window
   "cwnd" greater than "max_ssthresh".  That is, for:

     cwnd <= max_ssthresh

   cwnd is increased by one MSS (MAXIMUM SEGMENT SIZE) for every
   arriving ACK (acknowledgement) during slow-start, as is always the
   case.  For:

     cwnd > max_ssthresh

   the invariant is maintained that the congestion window is increased
   during slow-start by at most max_ssthresh/2 MSS per round-trip time.

   This is done as follows:








Floyd                         Experimental                      [Page 2]

draft-floyd-tcp-slowstart-00                                   June 2002


   For each arriving ACK in slow-start:
     If (cwnd <= max_ssthresh)
        cwnd += MSS;
     else
        K = int(cwnd/(0.5 max_ssthresh));
        cwnd += int(MSS/K);

   Thus during Limited Slow-Start the window is increased by 1/K MSS for
   each arriving ACK, for K = int(cwnd/(0.5 max_ssthresh)), instead of
   by 1 MSS as in the standard slow-start [RFC2581].

   Our current recommendation would be for max_ssthresh to be set to 100
   MSS.  (This is illustrated in the NS simulator, for snapshots after
   May 1, 2002, in the tests "./test-all-tcpHighspeed tcp1A" and
   "./test-all-tcpHighspeed tcpHighspeed1" in the subdirectory
   "tcl/lib".)

   With Limited Slow-Start, when the congestion window is greater than
   max_ssthresh the window is increased by at most 1/2 MSS for each
   arriving ACK, when the congestion window is greater than 1.5
   max_ssthresh the window is increased by at most 1/3 MSS for each
   arriving ACK, and so on.

   With Limited Slow-Start it takes:

     log(max_ssthresh)

   round-trip times to reach a congestion window of max_ssthresh, and it
   takes:

     log(max_ssthresh) + (cwnd - max_ssthresh)/(max_ssthresh/2)

   round-trip times to reach a congestion window of cwnd, for a
   congestion window greater than max_ssthresh.

   By limiting the maximum increase in the congestion window in a round-
   trip time, Limited Slow-Start can reduce the number of drops during
   slow-start, and improve the performance of TCP connections with large
   congestion windows.

3.  Related Proposals

   There has been considerable research on mechanisms for the TCP sender
   to learn about the limitations of the available bandwidth, and to
   exit slow-start before receiving a congestion indication from the
   network [VEGAS,H96].  This draft proposes a simple limitation on
   slow-start that can be effective in some cases even in the absence of
   such mechanisms.



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draft-floyd-tcp-slowstart-00                                   June 2002


   Rate-based pacing has also been proposed to improve the performance
   of TCP during slow-start [VH97,AD98,KCRP99,ASA00].  We believe that
   rate-based pacing could be of significant benefit, and could be used
   in addition to the Limited Slow-Start in this proposal.

   Appropriate Byte Counting [A01] proposes that TCP increase its
   congestion window as a function of the number of bytes acknowledged,
   rather than as a function of the number of ACKs received.
   Appropriate Byte Counting is largely orthogonal to this proposal for
   Limited Slow-Start.

   (If I have missed acknowledgements to other relevent proposals,
   including to other proposals to set the initial value of ssthresh,
   please let me know.)

4.  Acknowledgements

   This proposal is part of a larger proposal for HighSpeed TCP for TCP
   connections with large congestion windows, and resulted from
   simulations done by Evandro de Souza, in joint work with Deb Agarwal.
   This proposal for Limited Slow-Start drew in part from discussions
   with Tom Kelly, who has used a similar modified slow-start in his own
   research with congestion control for high-bandwidth connections.

5.  Normative References

   [A01] Mark Allman, "TCP Congestion Control with Appropriate Byte
   Counting", draft-allman-tcp-abc-02.txt, internet-draft, work-in-
   progress, November 2001.

   [RFC2581] M. Allman and V. Paxson, "TCP Congestion Control", RFC
   2581, April 1999.

6.  Informative References

   [AD98] Mohit Aron and Peter Druschel, "TCP: Improving Start-up
   Dynamics by Adaptive Timers and Congestion Control"", TR98-318, Rice
   University, 1998.  URL "http://cs-
   tr.cs.rice.edu/Dienst/UI/2.0/Describe/ncstrl.rice_cs/TR98-318/".

   [ASA00] A. Aggarwal, S. Savage, and T. Anderson, "Understanding the
   Performance of TCP Pacing", Proceedings of the 2000 IEEE Infocom
   Conference, Tel-Aviv, Israel, March, 2000.  URL
   "http://www.cs.ucsd.edu/~savage/".

   [F02] S. Floyd, "Performance Problems with TCP's Slow-Start", 2002.
   URL "http://www.icir.org/floyd/hstcp/slowstart/".




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   [H96] J. C. Hoe, "Improving the Start-up Behavior of a Congestion
   Control Scheme for TCP", SIGCOMM 96, 1996.  URL
   "http://www.acm.org/sigcomm/sigcomm96/program.html".

   [KCRP99] J. Kulik, R. Coulter, D. Rockwell, and C. Partridge, "A
   Simulation Study of Paced TCP", BBN Technical Memorandum No. 1218,
   1999.  URL "http://mimas.lcs.mit.edu/~jokulik/tcppacing.html".

   [VEGAS] Vegas Web Page, University of Arizona.  URL
   "http://www.cs.arizona.edu/protocols/".

   [VH97] Vikram Visweswaraiah and John Heidemann, "Rate Based Pacing
   for TCP", 1997.  URL
   "http://www.isi.edu/lsam/publications/rate_based_pacing/".

7.  Security Considerations

   This proposal makes no changes to the underlying security of TCP.

8.  IANA Considerations

   There are no IANA considerations regarding this document.

   AUTHORS' ADDRESSES


      Sally Floyd
      Phone: +1 (510) 666-2989
      ICIR (ICSI Center for Internet Research)
      Email: floyd@icir.org
      URL: http://www.icir.org/floyd/

      This draft was created in June 2002.


















Floyd                         Experimental                      [Page 5]


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