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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: 2019-Feb-11
Document File: 7 page(s) / 9K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

T. Shepard: AUTHOR [+1]

Related Documents

10.17487/RFC2416: DOI

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 memo provides information for the Internet community.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 28% 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.

Shepard & Partridge Informational [Page 1]

RFC 2416 TCP with Four Packets into Three Buffers September 1998

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 the 9600 bps link)

The scenario studied and simulated consists of three links between the source and sink. The first link is a 100 Mbps link with no delay. It connects the sender to a router. (It was included to have a means of logging the returning ACKs at the time they...

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