rfc761.txt

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RFC: 761
IEN: 129
                                    
                                    
                                    
                                    
                                    
                                    
                                    
                              DOD STANDARD
                                    
                     TRANSMISSION CONTROL PROTOCOL
                                    
                                    
                                    
                              January 1980















                              prepared for
                                    
               Defense Advanced Research Projects Agency
                Information Processing Techniques Office
                         1400 Wilson Boulevard
                       Arlington, Virginia  22209







                                   by

                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291

January 1980                                                            
                                           Transmission Control Protocol



                           TABLE OF CONTENTS

    PREFACE ........................................................ iii

1.  INTRODUCTION ..................................................... 1

  1.1  Motivation .................................................... 1
  1.2  Scope ......................................................... 2
  1.3  About This Document ........................................... 2
  1.4  Interfaces .................................................... 3
  1.5  Operation ..................................................... 3

2.  PHILOSOPHY ....................................................... 7

  2.1  Elements of the Internetwork System ........................... 7
  2.2  Model of Operation ............................................ 7
  2.3  The Host Environment .......................................... 8
  2.4  Interfaces .................................................... 9
  2.5  Relation to Other Protocols ................................... 9
  2.6  Reliable Communication ....................................... 10
  2.7  Connection Establishment and Clearing ........................ 10
  2.8  Data Communication ........................................... 12
  2.9  Precedence and Security ...................................... 13
  2.10 Robustness Principle ......................................... 13

3.  FUNCTIONAL SPECIFICATION ........................................ 15

  3.1  Header Format ................................................ 15
  3.2  Terminology .................................................. 19
  3.3  Sequence Numbers ............................................. 24
  3.4  Establishing a connection .................................... 29
  3.5  Closing a Connection ......................................... 35
  3.6  Precedence and Security ...................................... 38
  3.7  Data Communication ........................................... 38
  3.8  Interfaces ................................................... 42
  3.9  Event Processing ............................................. 52

GLOSSARY ............................................................ 75

REFERENCES .......................................................... 83











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                                                            January 1980
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[Page ii]                                                               


January 1980                                                            
                                           Transmission Control Protocol



                                PREFACE



This document describes the DoD Standard Transmission Control Protocol
(TCP).  There have been eight earlier editions of the ARPA TCP
specification on which this standard is based, and the present text
draws heavily from them.  There have been many contributors to this work
both in terms of concepts and in terms of text.  This edition
incorporates the addition of security, compartmentation, and precedence
concepts into the TCP specification.

                                                           Jon Postel

                                                           Editor




































                                                              [Page iii]


January 1980 
RFC:761
IEN:129
Replaces:  IENs 124, 112,
81, 55, 44, 40, 27, 21, 5

                              DOD STANDARD

                     TRANSMISSION CONTROL PROTOCOL



                            1.  INTRODUCTION

The Transmission Control Protocol (TCP) is intended for use as a highly
reliable host-to-host protocol between hosts in packet-switched computer
communication networks, and especially in interconnected systems of such
networks.

This document describes the functions to be performed by the
Transmission Control Protocol, the program that implements it, and its
interface to programs or users that require its services.

1.1.  Motivation

  Computer communication systems are playing an increasingly important
  role in military, government, and civilian environments.  This
  document primarily focuses its attention on military computer
  communication requirements, especially robustness in the presence of
  communication unreliability and availability in the presence of
  congestion, but many of these problems are found in the civilian and
  government sector as well.

  As strategic and tactical computer communication networks are
  developed and deployed, it is essential to provide means of
  interconnecting them and to provide standard interprocess
  communication protocols which can support a broad range of
  applications.  In anticipation of the need for such standards, the
  Deputy Undersecretary of Defense for Research and Engineering has
  declared the Transmission Control Protocol (TCP) described herein to
  be a basis for DoD-wide inter-process communication protocol
  standardization.

  TCP is a connection-oriented, end-to-end reliable protocol designed to
  fit into a layered hierarchy of protocols which support multi-network
  applications.  The TCP provides for reliable inter-process
  communication between pairs of processes in host computers attached to
  distinct but interconnected computer communication networks.  Very few
  assumptions are made as to the reliability of the communication
  protocols below the TCP layer.  TCP assumes it can obtain a simple,
  potentially unreliable datagram service from the lower level
  protocols.  In principle, the TCP should be able to operate above a
  wide spectrum of communication systems ranging from hard-wired
  connections to packet-switched or circuit-switched networks.


                                                                [Page 1]


                                                            January 1980
Transmission Control Protocol
Introduction



  TCP is based on concepts first described by Cerf and Kahn in [1].  The
  TCP fits into a layered protocol architecture just above a basic
  Internet Protocol [2] which provides a way for the TCP to send and
  receive variable-length segments of information enclosed in internet
  datagram "envelopes".  The internet datagram provides a means for
  addressing source and destination TCPs in different networks.  The
  internet protocol also deals with any fragmentation or reassembly of
  the TCP segments required to achieve transport and delivery through
  multiple networks and interconnecting gateways.  The internet protocol
  also carries information on the precedence, security classification
  and compartmentation of the TCP segments, so this information can be
  communicated end-to-end across multiple networks.

                           Protocol Layering

                        +---------------------+
                        |     higher-level    |
                        +---------------------+
                        |        TCP          |
                        +---------------------+
                        |  internet protocol  |
                        +---------------------+
                        |communication network|
                        +---------------------+

                                Figure 1

  Much of this document is written in the context of TCP implementations
  which are co-resident with higher level protocols in the host
  computer.  As a practical matter, many computer systems will be
  connected to networks via front-end computers which house the TCP and
  internet protocol layers, as well as network specific software.  The
  TCP specification describes an interface to the higher level protocols
  which appears to be implementable even for the front-end case, as long
  as a suitable host-to-front end protocol is implemented.

1.2.  Scope

  The TCP is intended to provide a reliable process-to-process
  communication service in a multinetwork environment.  The TCP is
  intended to be a host-to-host protocol in common use in multiple
  networks.

1.3.  About this Document

  This document represents a specification of the behavior required of
  any TCP implementation, both in its interactions with higher level
  protocols and in its interactions with other TCPs.  The rest of this


[Page 2]                                                                


January 1980                                                            
                                           Transmission Control Protocol
                                                            Introduction



  section offers a very brief view of the protocol interfaces and
  operation.  Section 2 summarizes the philosophical basis for the TCP
  design.  Section 3 offers both a detailed description of the actions
  required of TCP when various events occur (arrival of new segments,
  user calls, errors, etc.) and the details of the formats of TCP
  segments.

1.4.  Interfaces

  The TCP interfaces on one side to user or application processes and on