📄 rfc759.txt
字号:
IEN: 113
RFC: 759
INTERNET MESSAGE PROTOCOL
Jonathan B. Postel
August 1980
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, California 90291
(213) 822-1511
August 1980
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. Model of Operation ........................................... 2
1.5. Interfaces ................................................... 4
2. FUNCTIONAL DESCRIPTION ........................................... 5
2.1. Terminology .................................................. 5
2.2. Assumptions ................................................. 5
2.3. General Specification ........................................ 6
2.4. Mechanisms ................................................... 7
2.5. Relation to Other Protocols ................................. 10
3. DETAILED SPECIFICATION .......................................... 13
3.1. Overview of Message Structure ............................... 13
3.2. Message Structure ........................................... 14
3.3. Identification .............................................. 15
3.4. Command ..................................................... 15
3.5. Document .................................................... 19
3.6. Message Objects ............................................. 20
3.7. Data Elements ............................................... 27
4. OTHER ISSUES .................................................... 35
4.1. Accounting and Billing ...................................... 35
4.2. Addressing and Routing ...................................... 36
4.3. Encryption .................................................. 37
5. The MPM: A Possible Architecture ............................... 39
5.1. Interfaces .................................................. 39
5.2. MPM Organization ............................................ 40
6. EXAMPLES & SCENARIOS ............................................ 45
Example 1: Message Format ........................................ 45
Example 2: Delivery and Acknowledgment ........................... 47
Postel [Page i]
August 1980
Internet Message Protocol
Table Of Contents
7. SPECIFICATION SUMMARY ........................................... 55
7.1. Message Fields .............................................. 55
7.2. Deliver Message ............................................. 58
7.3. Acknowledge Message ......................................... 59
7.4. Probe Message ............................................... 61
7.5. Response Message ............................................ 62
7.6. Cancel Message .............................................. 64
7.7. Canceled Message ............................................ 66
7.8. Data Element Summary ........................................ 68
REFERENCES .......................................................... 69
[Page ii] Postel
August 1980
Internet Message Protocol
PREFACE
This is the second 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 discussions 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,
Suzanne Sluizer, Alan Katz, Paul Mockapetris, and Linda Sato.
Jon Postel
Postel [Page iii]
IEN: 113 J. Postel
RFC: 759 USC-ISI
August 1980
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
delivery 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
delivery system in its own right.
This system also provides for the communication of data items beyond
the scope of contemporary message systems. Messages can include data
objects 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
for the composition of complex data objects and their encoding in
machine independent basic data elements.
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
Postel [Page 1]
August 1980
Internet Message Protocol
Introduction
developed in the context of the ARPA work on the interconnection of
networks, but it is thought that it has a more general scope.
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 this 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 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 [1,2].
1.4. Model of Operation
This protocol is implemented in a process 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 message it is submitted for transmission by placing
it in a data structure read by 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 (or some other method), determines which outgoing link
to use. The destination may be another user on the same host, one on
another host on a network in common with the same host, or a user in
another network.
In the first case, another user on this host, the MPM places the
message in a data structure read by 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
[Page 2] Postel
August 1980
Internet Message Protocol
Introduction
the routing decision, and discovering the destination is local to it,
places the message in the data structure shared with the destination
user.
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.
An 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.
An 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 an MPM which then sends
it through the appropriate gateways to (or towards) the MPM in the
destination network. Eventually, the message gets to an 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
going out, and to transform internet messages to local format when
they come into the local environment. Such transformations
potentially lead to information loss. The internet message format
attempts to provide features to capture all the information any local
message system might use. However, a particular local message system
is unlikely to have features equivalent to all the possible features
of the internet message system. Thus, in some cases the
transformation of an internet message to a local message discards some
of the information. For example, if an internet message carrying
mixed text and speech data in the body is to be delivered in a local
system which only carries text, the speech data may be replaced by the
text string "There was some speech here". Such discarding of
information is to be avoided when at all possible, and to be deferred
as long as possible; still, the possibility remains that in some cases
it is the only reasonable thing to do.
Postel [Page 3]
August 1980
Internet Message Protocol
Introduction
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