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+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
+
+
+
+Floyd                         Experimental                      [Page 1]
+
+RFC 3742     TCP's Slow-Start with Large Congestion Windows   March 2004
+
+
+   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].
+
+
+
+
+
+
+
+
+Floyd                         Experimental                      [Page 2]
+
+RFC 3742     TCP's Slow-Start with Large Congestion Windows   March 2004
+
+
+   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.
+
+
+
+
+
+
+
+Floyd                         Experimental                      [Page 3]
+
+RFC 3742     TCP's Slow-Start with Large Congestion Windows   March 2004
+
+
+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
+
+
+
+Floyd                         Experimental                      [Page 4]
+
+RFC 3742     TCP's Slow-Start with Large Congestion Windows   March 2004
+
+
+   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".
+
+
+
+
+
+Floyd                         Experimental                      [Page 5]
+
+RFC 3742     TCP's Slow-Start with Large Congestion Windows   March 2004
+
+
+   [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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+Floyd                         Experimental                      [Page 6]
+
+RFC 3742     TCP's Slow-Start with Large Congestion Windows   March 2004
+
+
+Full Copyright Statement
+
+   Copyright (C) The Internet Society (2004).  This document is subject
+   to the rights, licenses and restrictions contained in BCP 78 and
+   except as set forth therein, the authors retain all their rights.
+
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+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
+
+
+
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+Floyd                         Experimental                      [Page 7]
+