rfc753.txt

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                                                              March 1979

IEN: 85
RFC: 753
                                    
                                    
                                    
                                    
                                    
                                    
                       INTERNET MESSAGE PROTOCOL
                                    
                                    
                                    
                           Jonathan B. Postel

















                               March 1979
                                    
                                    
                                    
                                    
                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291
                                    
                             [phone]


< INC-PROJECT, MAIL-MAR-79.NLS.38, >, 31-Mar-79 19:50 JBP ;;;;






















































[Page 0]                                                          Postel


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                                               Internet Message Protocol



                           TABLE OF CONTENTS

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

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

  1.1.  Motivation ................................................... 1
  1.2.  Scope ........................................................ 1
  1.3.  The Internetwork Environment ................................. 2
  1.4.  Operation .................................................... 2
  1.5.  Interfaces ................................................... 3

2.  FUNCTIONAL DESCRIPTION ........................................... 5

  2.1.  Relation to Other Protocols .................................. 5
  2.2.  Terminology  ................................................. 5
  2.3.  Assumptions .................................................. 6
  2.4.  General Specification ........................................ 7
  2.5.  Mechanisms .................................................. 11

3.  DETAILED SPECIFICATION .......................................... 13

  3.1.  Overview of Message Structure ............................... 13
  3.2.  Data Elements ............................................... 13
  3.3.  Message Objects ............................................. 16
  3.4.  Command ..................................................... 23
  3.5.  Document .................................................... 31
  3.6.  Message Structure ........................................... 33
  3.7.  MPM Organization ............................................ 36
  3.8.  Interfaces .................................................. 39

4.  EXAMPLES & SCENARIOS ............................................ 41

  Example 1:  Message Format ........................................ 41
  Example 2:  Delivery and Acknowledgment ........................... 43

GLOSSARY ............................................................ 49

REFERENCES .......................................................... 51

APPENDICES .......................................................... 53

  








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[Page ii]                                                         Postel


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                                               Internet Message Protocol



                                PREFACE



This is the first edition of this specification and should be treated as
a request for comments, advice, and suggestions.  A great deal of prior
work has been done on computer aided message systems and some of this is
listed in the reference section.  This specification was shaped by many
discusions with members of the ARPA research community, and others
interested in the development of computer aided message systems.  This
document was prepared as part of the ARPA sponsored Internetwork
Concepts Research Project at ISI, with the assistance of Greg Finn, Alan
Katz, Paul Mockapetris, and Mamie Chew.

                                                              Jon Postel




































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[Page iv]                                                         Postel


March 1979                                                              
IEN: 85                                                        J. Postel
RFC: 753                                                         USC-ISI
                                                              March 1979




                       INTERNET MESSAGE PROTOCOL



                            1.  INTRODUCTION

This document describes an internetwork message system.  The system is
designed to transmit messages between message processing modules
according to formats and procedures specified in this document.  The
message processing modules are processes in host computers.  Message
processing modules are located in different networks and together
constitute an internetwork message delivery system.

This document is intended to provide all the information necessary to
implement a compatible cooperating module of this internetwork message
system.

1.1.  Motivation

  As computer supported message processing activities grow on individual
  host computers and in networks of computers, there is a natural desire
  to provide for the interconnection and interworking of such systems.
  This specification describes the formats and procedures of a general
  purpose internetwork message system, which can be used as a standard
  for the interconnection of individual message systems, or as a message
  system in its own right.

  We also provide for the communication of data items beyond the scope
  of contemporary message systems.  Messages can include typed segments
  which could represent drawings, or facsimile images, or digitized
  speech.  One can imagine message stations equipped with speakers and
  microphones (or telephone hand sets) where the body of a message or a
  portion of it is recorded digitized speech.  The output terminal could
  include a graphics display, and the message might present a drawing on
  the display, and verbally (via the speaker) describe certain features
  of the drawing.  This specification provides basic data elements for
  the transmission of structured binary data, as well as providing for
  text transmission.

1.2.  Scope

  The Internet Message Protocol is intended to be used for the
  transmission of messages between networks.  It may also be used for
  the local message system of a network or host.  This specification was
  developed in the context of the ARPA work on the interconnection of
  networks, but it is anticipated that it has a more general scope.


Postel                                                          [Page 1]


                                                              March 1979
Internet Message Protocol
Introduction



  The focus here is on the internal mechanisms to transmit messages,
  rather than the external interface to users.  It is assumed that a
  number of user interface programs will exist.  These will be both new
  programs designed to work with system and old programs designed to
  work with earlier systems.

1.3.  The Internetwork Environment

  The internetwork message environment consists of processes which run
  in hosts which are connected to networks which are interconnected by
  gateways.  Each individual network consists of many different hosts.
  The networks are tied together through gateways.  The gateways are
  essentially hosts on two (or more) networks and are not assumed to
  have much storage capacity or to "know" which hosts are on the
  networks to which they are attached [5].

1.4.  Operation

  The model of operation is that this protocol is implemented in a
  process.  Such a process is called a Message Processing Module or MPM.
  The MPMs exchange messages by establishing full duplex communication
  and sending the messages in a fixed format described in this document.
  The MPM may also communicate other information by means of commands
  described here.

  A message is formed by a user interacting with a User Interface
  Program or UIP.  The user may utilize several commands to create
  various fields of the message and may invoke an editor program to
  correct or format some or all of the message.  Once the user is
  satisfied with the messages it is "sent" by placing it in a data
  structure shared with the MPM.

  The MPM discovers the unprocessed input data (either by a specific
  request or by a general background search), examines it, and using
  routing tables determines which outgoing link to use.  The destination
  may be another user on this host, a user on another host in this
  network, or a user in another network.

  In the first case, another user on this host, the MPM places the
  message in a data structure shared with the destination user, where
  that user's UIP will look for incoming messages.

  In the second case, the user on another host in this network, the MPM
  transmits the message to the MPM on that host.  That MPM then repeats
  the routing decision, and discovering the destination is local to it,
  places the messages in the data structure shared with the destination
  user.



[Page 2]                                                          Postel


March 1979                                                              
                                               Internet Message Protocol
                                                            Introduction



  In the third case, the user on a host in another network, the MPM
  transmits the messages to an MPM in that network if it knows how to
  establish a connection directly to it, otherwise the MPM transmits the
  message to an MPM that is "closer" to the destination.  An MPM might
  not know of direct connections to MPMs in all other networks, but it
  must be able to select a next MPM to handle the message for each
  possible destination network.

  A MPM might know a way to establish direct connections to each of a
  few MPMs in other nearby networks, and send all other messages to a
  particular big brother MPM that has a wider knowledge of the internet
  environment.

  A individual network's message system may be quite different from the
  internet message system.  In this case, intranet messages will be
  delivered using the network's own message system.  If a message is
  addressed outside the network, it is given to a MPM which then sends
  it through the appropriate gateways via internet procedures and format
  to (or toward) the MPM in the destination network.  Eventually, the
  message gets to a MPM on the network of the recipient of the message.
  The message is then sent via the local message system to that host.

  When local message protocols are used, special conversion programs are
  required to transform local messages to internet format when they are