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When TCP Starts Up With Four Packets Into Only Three Buffers (RFC2416)

IP.com Disclosure Number: IPCOM000002992D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2000-Sep-13
Document File: 5 page(s) / 12K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

T. Shepard: AUTHOR [+2]

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).

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 26% of the total text.

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.

Vern Paxson replied with the comment (among other things) that the

four-packet start is no worse than what happens after two round trip

times in normal slow start, hence no new problem is introduced by

starting with as many as four packets. If there is a problem with a

four-packet start, then the problem already exists in a normal slow-

start startup after two round trip times when the slow-start

algorithm will release into the net four closely spaced packets.

The experiment reported here confirmed Vern Paxson's reasoning.

Scenario and experimental setup

+--------+ 100 Mbps +---+ 1.5 Mbps +---+ 9600 bps +----------+

| source +------------+ R +-------------+ R +--------------+ receiver |

+--------+ no delay +---+ 25 ms delay +---+ 150 ms delay +----------+

| |

| |

(we spy here) (this router has only 3 buffers

to hold packets going into t...