[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



 

                                                          September 1981
Internet Protocol
Introduction



  local net module which would add the 1822 leader [2] to the internet
  datagram creating an ARPANET message to transmit to the IMP.  The