RFC 791 (rfc791) - Page 1 of 45
Internet Protocol
Alternative Format: Original Text Document
RFC: 791
INTERNET 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
Internet Protocol
TABLE OF CONTENTS
PREFACE ........................................................ iii
1. INTRODUCTION ..................................................... 1
1.1 Motivation .................................................... 1
1.2 Scope ......................................................... 1
1.3 Interfaces .................................................... 1
1.4 Operation ..................................................... 2
2. OVERVIEW ......................................................... 5
2.1 Relation to Other Protocols ................................... 9
2.2 Model of Operation ............................................ 5
2.3 Function Description .......................................... 7
2.4 Gateways ...................................................... 9
3. SPECIFICATION ................................................... 11
3.1 Internet Header Format ....................................... 11
3.2 Discussion ................................................... 23
3.3 Interfaces ................................................... 31
APPENDIX A: Examples & Scenarios ................................... 34
APPENDIX B: Data Transmission Order ................................ 39
GLOSSARY ............................................................ 41
REFERENCES .......................................................... 45
[Page i]
September 1981
Internet Protocol
[Page ii]
September 1981
Internet Protocol
PREFACE
This document specifies the DoD Standard Internet Protocol. This
document is based on six earlier editions of the ARPA Internet Protocol
Specification, 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 revises aspects of addressing, error
handling, option codes, and the security, precedence, compartments, and
handling restriction features of the internet protocol.
Jon Postel
Editor
[Page iii]
September 1981
RFC: 791
Replaces: RFC 760
IENs 128, 123, 111,
80, 54, 44, 41, 28, 26
INTERNET PROTOCOL
DARPA INTERNET PROGRAM
PROTOCOL SPECIFICATION
1. INTRODUCTION
1.1. Motivation
The Internet Protocol is designed for use in interconnected systems of
packet-switched computer communication networks. Such a system has
been called a "catenet" [1]. The internet protocol provides for
transmitting blocks of data called datagrams from sources to
destinations, where sources and destinations are hosts identified by
fixed length addresses. The internet protocol also provides for
fragmentation and reassembly of long datagrams, if necessary, for
transmission through "small packet" networks.
1.2. Scope
The internet protocol is specifically limited in scope to provide the
functions necessary to deliver a package of bits (an internet
datagram) from a source to a destination over an interconnected system
of networks. There are no mechanisms to augment end-to-end data
reliability, flow control, sequencing, or other services commonly
found in host-to-host protocols. The internet protocol can capitalize
on the services of its supporting networks to provide various types
and qualities of service.
1.3. Interfaces
This protocol is called on by host-to-host protocols in an internet
environment. This protocol calls on local network protocols to carry
the internet datagram to the next gateway or destination host.
For example, a TCP module would call on the internet module to take a
TCP segment (including the TCP header and user data) as the data
portion of an internet datagram. The TCP module would provide the
addresses and other parameters in the internet header to the internet
module as arguments of the call. The internet module would then
create an internet datagram and call on the local network interface to
transmit the internet datagram.
In the ARPANET case, for example, the internet module would call on a
