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