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
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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/
Floyd Experimental [Page 6]RFC 3742 TCP's Slow-Start with Large Congestion Windows March 2004
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