Increasing TCP's Initial Window

Current Status:

As the official minutes of the last TCP Implemention Working Group (tcpimpl) meeting show, at the December 1997 meeting at the Washington IETF, there was rough consensus to allow an initial window of two segments. At the March 1998 meeting at the L.A. IETF, there was a rough consensus to allow an initial window of three or four packets (depending on the segment size) for experimental purposes.

RFC 2581, the April 1999 Proposed Standard on TCP Congestion Control, says the following: ``IW, the initial value of cwnd, MUST be less than or equal to 2*SMSS bytes and MUST NOT be more than 2 segments.''

Allman, M., Floyd, S., and Partridge, C., Increasing TCP's Initial Window, RFC 2414, Experimental, September 1998.
This is the document proposing an increase in TCP's permitted initial window.

Simulations and Experiments:

Allman, M., Hayes, C., and Ostermann, S., An Evaluation of TCP with Larger Initial Windows, March 1998. Submitted to ACM Computer Communication Review.

Poduri, K., and Nichols, K., Simulation Studies of Increased Initial TCP Window Size, RFC 2415, Experimental, September 1998.
These simulations model both long-lived TCP connections (file transfers) and short-lived web-browsing style connections. Simulation scripts: tar file, compressed tar file. Additional files for modeling HTTP 1.1 (tar file).

Shepard, T., and Partridge, C., When TCP Starts Up With Four Packets Into Only Three Buffers . RFC 2415, Experimental, September 1998.
This paper explores TCP's behavior in a configuration with a 9600 bps modem and only three packet buffers before the modem. The simulations in this report show that a four-packet initial window does not degrade the performance of a long-lived TCP connection.

The proposal from the internet draft has been implemented in the ns simulator. The test suite for this (at the moment, this test suite only includes tests for one-way TCP) can be run with the command "./test-all-tcp-init-win" in the directory "tcl/test".

Talks:

Allman, M., An Evaluation of TCP with Larger Initial Windows. 40th IETF Meeting - TCP Implementations WG. December, 1997. Washington DC. Viewgraphs.
Experiments in the Internet show the larger initial windows proposed in [FAP97] improve the elapsed time for a 16 KB transfer.

Floyd, S., a talk on Increasing TCP's Initial Window (postscript, pdf). 40th IETF Meeting - TCP Implementations WG. December, 1997. Washington DC.
This talk shows that current TCP implementations routinely send bursts of three and (perhaps less routinely) four back-to-back packets.

Papers about other small changes that have been proposed to TCP:

Return to [ Sally Floyd].
Last modified: September 1998