RFC 2416 (rfc2416) - Page 1 of 7
When TCP Starts Up With Four Packets Into Only Three Buffers
Alternative Format: Original Text Document
Network Working Group T. Shepard
Request for Comments: 2416 C. Partridge
Category: Informational BBN Technologies
September 1998
When TCP Starts Up With Four Packets Into Only Three Buffers
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This memo is to document a simple experiment. The experiment showed
that in the case of a TCP receiver behind a 9600 bps modem link at
the edge of a fast Internet where there are only 3 buffers before the
modem (and the fourth packet of a four-packet start will surely be
dropped), no significant degradation in performance is experienced by
a TCP sending with a four-packet start when compared with a normal
slow start (which starts with just one packet).
Background
Sally Floyd has proposed that TCPs start their initial slow start by
sending as many as four packets (instead of the usual one packet) as
a means of getting TCP up-to-speed faster. (Slow starts instigated
due to timeouts would still start with just one packet.) Starting
with more than one packet might reduce the start-up latency over
long-fat pipes by two round-trip times. This proposal is documented
further in [1], [2], and in [3] and we assume the reader is familiar
with the details of this proposal.
On the end2end-interest mailing list, concern was raised that in the
(allegedly common) case where a slow modem is served by a router
which only allocates three buffers per modem (one buffer being
transmitted while two packets are waiting), that starting with four
packets would not be good because the fourth packet is sure to be
dropped.
Shepard & Partridge Informational