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Network Working Group                                           S. FloydRequest for Comments: 3742                                          ICSICategory: Experimental                                        March 2004Limited Slow-Start for TCP with Large Congestion WindowsStatus 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 theFloyd                         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 20043.  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 discussionsFloyd                         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.  References7.1.  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/Floyd                         Experimental                      [Page 6]

RFC 3742     TCP's Slow-Start with Large Congestion Windows   March 2004Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78 and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed   to pertain to the implementation or use of the technology   described in this document or the extent to which any license   under such rights might or might not be available; nor does it   represent that it has made any independent effort to identify any   such rights.  Information on the procedures with respect to   rights in RFC documents can be found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use   of such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository   athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention   any copyrights, patents or patent applications, or other   proprietary rights that may cover technology that may be required   to implement this standard.  Please address the information to the   IETF at ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Floyd                         Experimental                      [Page 7]