TCP Congestion Control
RFC 5681

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Network Working Group                                          M. Allman
Request for Comments: 5681                                     V. Paxson
Obsoletes: 2581                                                     ICSI
Category: Standards Track                                     E. Blanton
                                                       Purdue University
                                                          September 2009

                         TCP Congestion Control

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.  This document
   obsoletes RFC 2581.

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   This document may contain material from IETF Documents or IETF
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may

Allman, et al.              Standards Track                     [Page 1]
RFC 5681                 TCP Congestion Control           September 2009

   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table Of Contents

   1. Introduction ....................................................2
   2. Definitions .....................................................3
   3. Congestion Control Algorithms ...................................4
      3.1. Slow Start and Congestion Avoidance ........................4
      3.2. Fast Retransmit/Fast Recovery ..............................8
   4. Additional Considerations ......................................10
      4.1. Restarting Idle Connections ...............................10
      4.2. Generating Acknowledgments ................................11
      4.3. Loss Recovery Mechanisms ..................................12
   5. Security Considerations ........................................13
   6. Changes between RFC 2001 and RFC 2581 ..........................13
   7. Changes Relative to RFC 2581 ...................................14
   8. Acknowledgments ................................................15
   9. References .....................................................15
      9.1. Normative References ......................................15
      9.2. Informative References ....................................16

1.  Introduction

   This document specifies four TCP [RFC793] congestion control
   algorithms: slow start, congestion avoidance, fast retransmit and
   fast recovery.  These algorithms were devised in [Jac88] and [Jac90].
   Their use with TCP is standardized in [RFC1122].  Additional early
   work in additive-increase, multiplicative-decrease congestion control
   is given in [CJ89].

   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.

   In addition to specifying these congestion control algorithms, this
   document specifies what TCP connections should do after a relatively
   long idle period, as well as specifying and clarifying some of the
   issues pertaining to TCP ACK generation.

   This document obsoletes [RFC2581], which in turn obsoleted [RFC2001].

   This document is organized as follows.  Section 2 provides various
   definitions that 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.