RFC 793: Transmission Control Protocol | RFC Editor

Skip to content Primary navigationThe RFC SeriesFor AuthorsAbout UsHomeRFC 793 View as: Plain Text, Compact HTML Info RFC 793: Transmission Control ProtocolTCPSeptember 1981. J. PostelInternet StandardSubscribe4 subscribed Add to Set Added to 2 Sets Read aloud This RFC is now obsolete, see RFC 9293.This RFC was updated, see RFC 1122, RFC 3168, RFC 6093, RFC 6528.RFC: 793 TRANSMISSION CONTROL PROTOCOL DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION September 1981 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 September 1981 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 ........................................ 9 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 .................................... 30 3.5 Closing a Connection ......................................... 37 3.6 Precedence and Security ...................................... 40 3.7 Data Communication ........................................... 40 3.8 Interfaces ................................................... 44 3.9 Event Processing ............................................. 52 GLOSSARY ............................................................ 79 REFERENCES .......................................................... 85 [Page i] September 1981 Transmission Control Protocol [Page ii] September 1981 Transmission Control Protocol PREFACE This document describes the DoD Standard Transmission Control Protocol (TCP). There have been nine 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 clarifies several details and removes the end-of-letter buffer-size adjustments, and redescribes the letter mechanism as a push function. Jon Postel Editor [Page iii] RFC: 793 Replaces: RFC 761 IENs: 129, 124, 112, 81, 55, 44, 40, 27, 21, 5 TRANSMISSION CONTROL PROTOCOL DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION 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 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 focuses its attention primarily 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] September 1981 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] whi