TCP.rar_share_tcp

Network Working Group M. Allman

Request for Comments: 2581 NASA Glenn/Sterling Software

Obsoletes: 2001 V. Paxson

Category: Standards Track ACIRI / ICSI

W. Stevens

Consultant

April 1999

TCP Congestion Control

Status of this Memo

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

This document defines TCP's four intertwined congestion control

algorithms: slow start, congestion avoidance, fast retransmit, and

fast recovery. In addition, the document specifies how TCP should

begin transmission after a relatively long idle period, as well as

discussing various acknowledgment generation methods.

1. Introduction

This document specifies four TCP [Pos81] congestion control

algorithms: slow start, congestion avoidance, fast retransmit and

generation.

Note that [Ste94] provides examples of these algorithms in action and

[WS95] provides an explanation of the source code for the BSD

implementation of these algorithms.

Allman, et. al. Standards Track [Page 1]

RFC 2581 TCP Congestion Control April 1999

This document is organized as follows. Section 2 provides various

definitions which will be used throughout the document. Section 3

provides a specification of the congestion control algorithms.

Section 4 outlines concerns related to the congestion control

algorithms and finally, section 5 outlines security considerations.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in [Bra97].

2. Definitions

This section provides the definition of several terms that will be

used throughout the remainder of this document.

largest segment the receiver is willing to accept. This is the

connection startup. Or, if the MSS option is not used, 536 bytes

[Bra89]. The size does not include the TCP/IP headers and

options.

FULL-SIZED SEGMENT: A segment that contains the maximum number of

data).

RECEIVER WINDOW (rwnd) The most recently advertised receiver window.

send data with a sequence number higher than the sum of the

congestion window after the three-way handshake is completed.

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RFC 2581 TCP Congestion Control April 1999

LOSS WINDOW (LW): The loss window is the size of the congestion

window after a TCP sender detects loss using its retransmission

timer.

RESTART WINDOW (RW): The restart window is the size of the

congestion window after a TCP restarts transmission after an idle

period (if the slow start algorithm is used; see section 4.1 for

more discussion).

FLIGHT SIZE: The amount of data that has been sent but not yet

acknowledged.

3. Congestion Control Algorithms

This section defines the four congestion control algorithms: slow

TCP sender to control the amount of outstanding data being injected

added to the TCP per-connection state. The congestion window (cwnd)

is a sender-side limit on the amount of data the sender can transmit

into the network before receiving an acknowledgment (ACK), while the

receiver's advertised window (rwnd) is a receiver-side limit on the

amount of outstanding data. The minimum of cwnd and rwnd governs

Another state variable, the slow start threshold (ssthresh), is used

to determine whether the slow start or congestion avoidance algorithm

is used to control data transmission, as discussed below.

requires TCP to slowly probe the network to determine the available