Network Working Group                                           S. Floyd
Request for Comments: 3742                                          ICSI
Category: Experimental                                        March 2004


        Limited Slow-Start for TCP with Large Congestion Windows

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document describes an optional 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 describes
   Limited Slow-Start as an optional mechanism for 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.

1.  Introduction

   This note describes an optional 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 describes
   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, a large number of packets might be dropped in
   the network (even with carefully-tuned active queue management
   mechanisms in the routers).  This 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 the 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
   modifies the slow-start mechanism for values of the congestion window
   where "cwnd" is greater than "max_ssthresh".  That is, during Slow-
   Start, when

      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.  During Limited Slow-Start, when

      max_ssthresh < cwnd <= ssthresh,

   the invariant is maintained so 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:

      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 standard slow-start [RFC2581].

   When

     ssthresh < cwnd,

   slow-start is exited, and the sender is in the Congestion Avoidance
   phase.

   Our recommendation would be for max_ssthresh to be set to 100 MSS.
   (This is illustrated in the NS [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".  Setting max_ssthresh to Infinity causes the TCP
   connection in NS not to use Limited Slow-Start.)

   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.

   Thus, with Limited Slow-Start with max_ssthresh set to 100 MSS, it
   would take 836 round-trip times to reach a congestion window of
   83,000 packets, compared to 16 round-trip times without Limited
   Slow-Start (assuming no packet drops).  With Limited Slow-Start, the
   largest transient queue during slow-start would be 100 packets;
   without Limited Slow-Start, the transient queue during Slow-Start
   would reach more than 32,000 packets.

   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.  Experimental Results

   Tom Dunigan has added Limited Slow-Start to the Linux 2.4.16 Web100
   kernel, and performed experiments comparing TCP with and without
   Limited Slow-Start [D02].  Results so far show improved performance
   for TCPs using Limited Slow-Start.  There are also several
   experiments comparing different values for max_ssthresh.

4.  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].  Other proposals set TCP's slow-start parameter
   ssthresh based on information from previous TCP connections to the
   same destination [WS95,G00].  This document proposes a simple
   limitation on slow-start that can be effective in some cases even in
   the absence of such mechanisms.  The max_ssthresh parameter does not
   replace ssthresh, but is an additional parameter.  Thus, Limited
   Slow-Start could be used in addition to mechanisms for setting
   ssthresh.

   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 [RFC3465] 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.

   Limited Slow-Start is also orthogonal to other proposals to change
   mechanisms for exiting slow-start.  For example, FACK TCP includes an
   overdamping mechanism to decrease the congestion window somewhat more
   aggressively when a loss occurs during slow-start [MM96].  It is also
   true that larger values for the MSS would reduce the size of the
   congestion window in units of MSS needed to fill a given pipe, and
   therefore would reduce the size of the transient queue in units of
   MSS.

5.  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 draws 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.  We
   also thank Tom Dunigan for his experiments with Limited Slow-Start.

   We thank Andrei Gurtov, Reiner Ludwig, members of the End-to-End
   Research Group, and members of the Transport Area Working Group, for
   feedback on this document.

6.  Security Considerations

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

7.  References

7.1.  Normative References

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

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

7.2.  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/".

   [D02]     T. Dunigan, "Floyd's TCP slow-start and AIMD mods", 2002.
             URL "http://www.csm.ornl.gov/~dunigan/net100/floyd.html".

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

   [G00]     A. Gurtov, "TCP Performance in the Presence of Congestion
             and Corruption Losses", Master's Thesis, University of
             Helsinki, Department of Computer Science, Helsinki,
             December 2000.  URL
             "http://www.cs.helsinki.fi/u/gurtov/papers/ms_thesis.html".

   [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://www.ir.bbn.com/documents/techmemos/index.html".

   [MM96]    M. Mathis and J. Mahdavi, "Forward Acknowledgment: Refining
             TCP Congestion Control", SIGCOMM, August 1996.

   [NS]      The Network Simulator (NS). URL
             "http://www.isi.edu/nsnam/ns/".

   [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/".

   [WS95]    G. Wright and W. Stevens, "TCP/IP Illustrated", Volume 2,
             Addison-Wesley Publishing Company, 1995.

Authors' Address

   Sally Floyd
   ICIR (ICSI Center for Internet Research)

   Phone: +1 (510) 666-2989
   EMail: floyd@icir.org
   URL: http://www.icir.org/floyd/

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