    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

    1.5.  Interfaces

      The MPM calls on a reliable communication procedure to communicate
      with other MPMs.  This is a Transport Level protocol such as the
      Transmission Control Protocol (TCP) [3].  The interface to such a
      procedure conventionally provides calls to open and close connections,
      send and receive data on a connection, and some means to signal and be
      notified of special conditions (i.e., interrupts).

      The MPM receives input and produces output through data structures
      that are produced and consumed respectively by user interface (or
      other) programs.

    [Page 4]                                                          Postel

    August 1980
                                                   Internet Message Protocol

                           2.  FUNCTIONAL DESCRIPTION

    This section gives an overview of the Internet Message System and its
    environment.

    2.1.  Terminology

      The messages are routed by a process called the Message Processing
      Module or MPM.  Messages are created and consumed by User Interface
      Programs (UIPs) in conjunction with users.

      The basic unit transferred between MPMs is called a message.  A
      message is made up of a transaction identifier (which uniquely
      identifies the message), a command (which contains the necessary
      information for delivery), and document.  The document may have a
      header and a body.

      For a personal letter the document body corresponds to the contents of
      the letter; the document header corresponds to the date line,
      greeting, and signature.

      For an inter-office memo the document body corresponds to the text;
      the document header corresponds to the header of the memo.

      The commands correspond to the information used by the Post Office or
      the mail room to route the letter or memo.  Some of the information in
      the command is supplied by the UIP.

    2.2.  Assumptions

      The following assumptions are made about the internetwork environment:

      In general, it is not known what format intranet addresses will
      assume.  Since no standard addressing scheme would suit all networks,
      it is safe to assume there will be several and that they will change
      with time.  Thus, frequent software modification throughout all
      internet MPMs would be required if such MPMs were to know about the
      formats on many networks.  Therefore, each MPM which handles internet
      messages is required to know only the minimum necessary to deliver
      them.

      Each MPM is required to know completely only the addressing format of
      its own network(s).  In addition, the MPM must be able to select an
      output link for each message addressed to another network or host.
      This does not preclude more intelligent behavior on the part of a
      given MPM, but at least this minimum is necessary.  Each network has a
      unique name and numeric address.  Such names and addresses are

    Postel                                                          [Page 5]

                                                                 August 1980
    Internet Message Protocol
    Functional Description

      registered with a naming authority and may be listed in documents such
      as Assigned Numbers [4].

      Each MPM will have a unique internet address.  This feature will
      enable every MPM to place a unique "handling-stamp" on a message which
      passes through the MPM enroute to delivery.

    2.3.  General Specification

      There are several aspects to a distributed service to be specified.
      First, there is the service to be provided; that is, the
      characteristics of the service as seen by its users.  Second, there is
      the service it uses; that is, the characteristics it assumes to be
      provided by some lower level service.  And third, there is the
      protocol used between the modules of the distributed service.

           User                                          User
              \                                          /
              UIP                                      UIP
                \                                      /
             --+----------------------------------------+-- Service
               |   \                                /   | Interface
               |  +--------+                +--------+  |
               |  | Module | <--Protocol--> | Module |  |
               |  +--------+                +--------+  |
               |        \                       /       |
               |        +-----------------------+       |
               |        | Communication Service |       |
               |        +-----------------------+       |
               |                                        |
               +----------------------------------------+

                                Message Service

                                   Figure 1.

      The User/Message Service Interface

        The service the message delivery system provides is to accept
        messages conforming to a specified format, to attempt to deliver
        those messages, and to report on the success or failure of the
        delivery attempt.  This service is provided in the context of an
        interconnected system of networks and may involve relaying a message
        through several intermediate MPMs via different communication
        services.

    [Page 6]                                                          Postel

    August 1980
                                                   Internet Message Protocol
                                                      Functional Description

      The Message/Communication Service Interface

        The message delivery system calls on a communication service to
        transfer information from one MPM to another.  There may be
        different communication services used between different pairs of
        MPMs, though all communication services must meet the service
        characteristics described below.

        It is assumed that the communication service provides a reliable
        two-way data stream.  Such a data stream can usually be obtained in
        computer networks from the transport level protocol, for example,
        the Transmission Control Protocol (TCP) [3].  In any case, the
        properties the communication service must provide are:

          o  Logical connections for two way simultaneous data flow of
             arbitrary data (i.e., no forbidden codes).  All data sent is
             delivered in order.

          o  Simple commands to open and close the connections, and to send
             and receive data on the connections.

          o  Controlled flow of data so that data is not transmitted faster
             that the receiver chooses to consume it (on the average).

          o  Transmission errors are corrected without user notification or
             involvement of the sender or receiver.  Complete breakdown on
             communication is reported to the sender or receiver.

      The Message-Message Protocol

        The protocol used between the distributed modules of the message
        delivery system, that is, the MPMs, is a small set of commands which
        convey requests and replies.  These commands are encoded in a highly
        structured and rigidly specified format.

    2.4.  Mechanisms

      MPMs are processes which use some communication service.  A pair of
      MPMs which can communicate reside in a common interprocess
      communication environment.  An MPM might exist in two (or more)
      interprocess communication environments, and such an MPM might act to
      relay messages between MPMs.  Messages may be held for a time in an
      MPM; the total path required for delivery need not be available
      simultaneously.

      From the time a message is accepted from a UIP by an MPM until it is
      delivered to a UIP by an MPM and an acknowledgment is returned to the

    Postel                                                          [Page 7]

                                                                 August 1980
    Internet Message Protocol
    Functional Description

      originating UIP, the message is considered to be active in the message
      system.

         User                                                    User
           \                                                      /
           UIP                                                  UIP
             \                                                  /
          +---------------------------------------------------------+
          |    \                                              /     |
          |  +-----+                +-----+                +-----+  |
          |  | MPM | <--Protocol--> | MPM | <--Protocol--> | MPM |  |
          |  +-----+                +-----+                +-----+  |
          |     |                    /   \                    |     |
          |  +-----------------------+   +-----------------------+  |
          |  |Communication Service A|   |Communication Service B|  |
          |  +-----------------------+   +-----------------------+  |
          |                                                         |
          +---------------------------------------------------------+

                     Message Service with Internal Relaying

                                   Figure 2.

      It should be clear that there are two roles an MPM can play, an
      end-point MPM or a relay MPM.  Most MPMs will play both roles.  A
      relay MPM acts to relay messages from one communication environment to
      another.  An end-point MPM acts as a source or destination of
      messages.

      The transfer of data between UIPs and MPMs is viewed as the exchange
      of data structures which encode messages.  The transfer of data
      between MPMs is also in terms of the transmission of structured data.

    [Page 8]                                                          Postel

    August 1980
                                                   Internet Message Protocol
                                                      Functional Description

                        +-----+     DATA       +-----+
                 USER-->| UIP |-->STRUCTURES-->| MPM |-->other
                        +-----+    +-----+     +-----+    MPMs
                                   |     |
                                   |  +-----+
                                   +--|     |
                                      |  +-----+
                                      +--|     |
                                         |     |
                                         +-----+

                         +-----+     DATA       +-----+
                 other-->| MPM |-->STRUCTURES-->| UIP |-->USER
                 MPMs    +-----+    +-----+     +-----+
                                    |     |
                                    |  +-----+
                                    +--|     |
                                       |  +-----+
                                       +--|     |
                                          |     |
                                          +-----+

                                  Message Flow

                                   Figure 3.

      In the following, a message will be described as a structured data
      object represented in a particular kind of typed data elements.  This
      is how a message is presented when transmitted between MPMs or
      exchanged between an MPM and a UIP.  Internal to an MPM (or a UIP), a
      message may be represented in any convenient form.

    Postel                                                          [Page 9]

                                                                 August 1980
    Internet Message Protocol
    Functional Description

    2.5.  Relation to Other Protocols

      This protocol the benefited from the earlier work on message protocols
      in the ARPA Network [5,6,7,8,9], and the ideas of others about the
      design of computer message systems
      [10,11,12,13,14,15,16,17,18,19,20,21].

      Figure 4 illustrates the place of the message protocol in the ARPA
      internet protocol hierarchy:

       +------+ +-----+ +-------+ +-----+     +-----+
       |Telnet| | FTP | |Message| |Voice| ... |     | Application Level
       +------+ +-----+ +-------+ +-----+     +-----+
               \   |   /             |           |
                +-----+           +-----+     +-----+
                | TCP |           | RTP | ... |     | Host Level
                +-----+           +-----+     +-----+
                   |                 |           |
                  +-------------------------------+
                  |       Internet Protocol       |   Gateway Level
                  +-------------------------------+
                                  |
                    +---------------------------+
                    |   Local Network Protocol  |     Network Level
                    +---------------------------+
                                  |

                             Protocol Relationships

                                   Figure 4.

      Note that "local network" means an individual or specific network.
      For example, the ARPANET is a local network.

      The message protocol interfaces on one side to user interface programs
      and on the other side to a reliable transport protocol such as TCP.

      In this internet message system the MPMs communicate directly using
      the lower level transport protocol.  In the old ARPANET system,
      message transmission was part of the file transfer protocol.

    [Page 10]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                      Functional Description

            +------+   +-----+   +-------+
            |Telnet|   | FTP |---|Message|            Application Level
            +------+   +-----+   +-------+
                  \     /
        +-----+   +-----+
        |Voice|---| NCP |                             Host Level
        +-----+   +-----+
                     |
                     |
                     |                                Gateway Level
                     |
                     |
             +----------------+
             |    ARPA NET    |                       Network Level
             +----------------+

                             Old ARPANET Protocols

                                   Figure 5.

      Note that in the old ARPANET protocols one can't send messages (or
      communicate in any way) to other networks since it has no gateway
      level or internet protocol [5].

    Postel                                                         [Page 11]

                                                                 August 1980
    Internet Message Protocol

    [Page 12]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                           3.  DETAILED SPECIFICATION

    The presentation of the information in this section is difficult since
    everything depends on everything, and since this is a linear medium it
    has to come in some order.  In this attempt, a brief overview of the
    message structure is given, the detail of the message is presented in
    terms of data objects, the various data objects are defined, and finally
    the representation of the data elements is specified.  Several aspects
    of the message structure are based on the NSW Transaction Protocol [22],
    and similar (but more general) proposals [23,24].

    3.1.  Overview of Message Structure

      A message is normally composed of three parts:  the identification,
      the command, and the document.  Each part is in turn composed of data
      objects.

      The identification part is composed of a transaction number assigned
      by the originating MPM and the MPM identifier.

      The command part is composed of an operation type, an operation code,
      the arguments to the operation, error information, the destination
      mailbox, and a trace.  The trace is a list of the MPMs that have
      handled this message.

      The document part is a data structure.  The message delivery system
      does not depend on the contents of the document part.  A standard for
      the document part is defined in reference [25].

      The following sections define the representation of a message as a
      structured object composed of other objects.  Objects in turn are
      represented using a set of basic data elements.

      The basic data elements are defined in section 3.7.  In summary, these
      are exact forms for representing integers, strings, booleans, et
      cetera.  There are also two elements for building data structures:
      list and property list.  Lists are simple lists of elements, including
      lists.  Property lists are lists of pairs of elements, where the first
      element of each pair names the pair.  That is, a property list is a
      list of <name,value> pairs.  In general, when an object is composed of
      multiple instances of a simpler object it is represented as a list of
      the simpler objects.  When an object is composed of a variety of
      simpler objects it is represented as a property list of the simpler
      objects.  In most uses of the property list representation, the
      presence of <name,value> pairs in addition to those specifically
      required is permitted.

    Postel                                                         [Page 13]

                                                                 August 1980
    Internet Message Protocol
    Specification

    3.2.  Message Structure

      An internet message is composed of two or three parts.  The first is
      the Identification which identifies the transaction; the second is the
      Command; and the optional third part is the Document.

      When shipped between two MPMs, a message will take the form of a
      property list, with the <name,value> pairs in this order.

        MESSAGE is:

          ( Identification, Command [, Document ] )

        It is convenient to batch several messages together, shipping them
        as a unit from one MPM to another.  Such a group of messages is
        called a message-bag.

        A message-bag will be a list of Messages; each Message is of the
        form described above.

          MESSAGE-BAG is:

            ( Message, Message, ... )

      The Identification

        This is the transaction identifier.  It is assigned by the
        originating MPM.  The identification is composed of the MPM
        identifier, and a transaction number unique in that context for this
        message.

      The Command

        The command is composed of a mailbox, an operation code, the
        arguments to that operation, some error information, and a trace of
        the route of this message.  The command is implemented by a property
        list which contains <name,value> pairs, where the names are used to
        identify the associated argument values.

      The Document

        The document portion of an internet message is optional and when
        present is a data structure as defined in [25].

    [Page 14]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

    3.3.  Identification

      Each message must have a unique identifier while it exists in the
      message delivery system.  This is provided by the combination of the
      unique identifier of the MPM and a unique transaction number chosen
      for the message by this MPM.

        IDENTIFICATION is:

          ( mpm-identifier, transaction-number )

      The mpm-identifier is based on the host address of the computer in
      which the MPM resides.  If there is more than one MPM in a host the
      mpm-identifier must be extended to distinguish between the co-resident
      MPMs.

    3.4.  Command

      This section describes the commands MPMs use to communicate between
      themselves.  The commands come in pairs, with each request having a
      corresponding reply.

        COMMAND is:

          ( mailbox, operation, [arguments,]
                                        [error-class, error-string,] trace )

      The mailbox is the "To" specification of the message.  Mailbox is a
      property list of general information, some of which is the essential
      information for delivery, and some of which could be extra information
      which may be helpful for delivery.  Mailbox is different from address
      in that address is a very specific property list without extra
      information.  The mailbox includes a specification of the user,  when
      a command is addressed to the MPM itself (rather than a user it
      serves) the special user name "*MPM*" is specified.

      The operation is the name of the operation or procedure to be
      performed.

      The arguments to the operation vary from operation to operation.

      The error information is composed of a error class code and a
      character string, and indicates what, if any, error occurred.  The
      error information is normally present only in replies, and not present
      in requests.

    Postel                                                         [Page 15]

                                                                 August 1980
    Internet Message Protocol
    Specification

      The trace is a list of the MPMs that have handled the message.  Each
      MPM must add its handling-stamp to the list.

    [Page 16]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      3.4.1.  Command:  DELIVER

        function:  Sends a document to a mailbox.

        reply:  The reply is ACKNOWLEDGE.

        arguments:

          type-of-service:  one or more of the following:

            "REGULAR"  regular delivery
            "FORWARD"  message forwarding
            "GENDEL"   general delivery
            "PRIORITY" priority delivery

      3.4.2.  Command:  ACKNOWLEDGE

        function:  Reply to DELIVER.

        arguments:

          reference:  the identifier of the originating message.

          address:

            The address is the final mailbox the message was delivered to.
            This would be different from the original mailbox if the message
            was forwarded, and is limited to the essential information
            needed for delivery.

          type-of-service:  one of the following:

            "GENDEL"    message was accepted for general delivery
            "REGULAR"   message was accepted for normal delivery
            "PRIORITY"  message was accepted for priority delivery

          error-class:
          error-string:

            If the document was delivered successfully, the error
            information has class 0 and string "ok".  Otherwise, the error
            information has a non-zero class and the string would be one of
            "no such user", "no such host", "no such network", "address
            ambiguous", or a similar response.

          trail:   the trace from the DELIVER command.

    Postel                                                         [Page 17]

                                                                 August 1980
    Internet Message Protocol
    Specification

      3.4.3.  Command:  PROBE

        function:  Finds out if specified mailbox (specified in mailbox of
        the command) exists at a host.

        reply:  The reply is RESPONSE.

        arguments:  none.

      3.4.4.  Command:  RESPONSE

        function:  Reply to PROBE.

        arguments:

          reference:  the identification of the originating PROBE.

          address:  a specific address.

          error-class:
          error-string:

            If the mailbox was found the error class is 0 and the error
            string is "OK".  If the mailbox has moved and a forwarding
            address in known the error class is 1 and the error string is
            "Mailbox moved, see address".  Otherwise the error class is
            greater than 1 and the error string may be one of the following:
            "Mailbox doesn't exist", "Mailbox full", "Mailbox has moved, try
            the new location indicated in the address".

          trail:  the trace which came from the originating PROBE.

    [Page 18]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      3.4.5.  Command:  CANCEL

        function:  Abort request for specified transaction.

        reply:  The reply is CANCELED.

        arguments:

          reference:  identification of transaction to be canceled.

      3.4.6.  Command:  CANCELED

        function:  Reply to CANCEL.

        arguments:

          reference:  identification of canceled transaction.

          error-class:
          error-string:

            If the command was canceled the error class is 0 and the error
            string is "OK".  Otherwise the error class is positive and the
            error string may be one of the following: "No such transaction",
            or any error for an unreachable mailbox.

          trail:  the trace of the CANCEL command.

      To summarize again, a command generally consists of a property list of
      the following objects:

        name            value
        ----            -----
        mailbox         property list of address information
        operation       name of operation
        arguments       ---
        error-class     numeric class of the error
        error-string    text description of the error
        trace           list ( handling-stamp, ... )

    3.5.  Document

      The actual document follows the command.  The message delivery system
      does not depend on the document, examine it, or use it in any way.
      The standard for the contents of the document is reference [25].  The
      document must be the last <name,value> pair in the message property
      list.

    Postel                                                         [Page 19]

                                                                 August 1980
    Internet Message Protocol
    Specification

    3.6.  Message Objects

      In the composition of messages, we use a set of objects such as
      mailbox or date.  These objects are encoded in basic data elements.
      Some objects are simple things like integers or character strings,
      other objects are more complex things built up of lists or property
      lists.

      The following is a list of the objects used in messages.  The object
      descriptions are in alphabetical order.

      Action

        The type of handling action taken by the MPM when processing a
        message.  One of ORIGIN, RELAY, FORWARD, or DESTINATION.

      Address

        Address is intended to contain the minimum information necessary to
        deliver a message, and no more (compare with mailbox).

          An address is a property list.  An address contains the following
          <name,value> pairs:

            name    description
            ----    -----------
            NET     network name
            HOST    host name
            USER    user name

          or:

            name    description
            ----    -----------
            MPM     mpm-identifier
            USER    user name

      Answer

        A yes (true) or no (false) answer to a question.

      Arguments

        Many operations require arguments, which differ from command to
        command.  This "object" is a place holder for the actual arguments
        when commands are described in a general way.

    [Page 20]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      City

        The character string name of a city.

      Command

        (mailbox, operation [ ,arguments ]
                                      [ ,error-class, error-string ], trace)

      Country

        The character string name of a country.

      Date

        The date and time are represented according to the International
        Standards Organization (ISO) recommendations [26,27,28].  Taken
        together the ISO recommendations 2014, 3307, and 4031 result in the
        following representation of the date and time:

          yyyy-mm-dd-hh:mm:ss,fff+hh:mm

        Where yyyy is the four-digit year, mm is the two-digit month, dd is
        the two-digit day, hh is the two-digit hour in 24 hour time, mm is
        the two-digit minute, ss is the two-digit second, and fff is the
        decimal fraction of the second.  To this basic date and time is
        appended the offset from Greenwich as plus or minus hh hours and mm
        minutes.

        The time is local time and the offset is the difference between
        local time and Coordinated Universal Time (UTC).  To convert from
        local time to UTC algebraically subtract the offset from the local
        time.

        For example, when the time in
                  Los Angeles is  14:25:00-08:00
                  the UTC is      22:25:00

        or when the time in
                  Paris is        11:43:00+01:00
                  the UTC is      10:43:00

      Document

        The document is the user's composition and is not used by the
        message delivery system in any way.

    Postel                                                         [Page 21]

                                                                 August 1980
    Internet Message Protocol
    Specification

      Error-Class

        A numeric code for the class of the error.  The error classes are
        coded as follows:

          = 0: indicates success, no error.
            This is the normal case.
          = 1: failure, address changed.
            This error is used when forwarding is possible, but not allowed
            by the type of service specified.
          = 2: failure, resources unavailable.
            These errors are temporary and the command they respond to may
            work if attempted at a later time.
          = 3: failure, user error.
            For example, unknown operation, or bad arguments.
          = 4: failure, MPM error.  Recoverable.
            These errors are temporary and the command they respond to may
            work if attempted at a later time.
          = 5: failure, MPM error.  Permanent.
            These errors are permanent, there is no point in trying the same
            command again.
          = 6: Aborted as requested by user.
            The response to a successfully canceled command.

    [Page 22]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      Error-String

        This is a character string describing the error.  Possible errors:

                  error-string                  error-class

          No errors                                  0
          Ok                                         0
          Mailbox Moved, see address                 1
          Mailbox Full, try again later              2
          Syntax error, operation unrecognized       3
          Syntax error, in arguments                 3
          No Such User                               3
          No Such Host                               3
          No Such Network                            3
          No Such Transaction                        3
          Mailbox Does Not Exist                     3
          Ambiguous Address                          3
          Server error, try again later              4
          No service available                       5
          Command not implemented                    5
          Aborted as requested by user               6

      Handling-Stamp

        The handling-stamp indicates the MPM, the date (including the time)
        that a message was processed by an MPM, and the type of handling
        action taken.

        ( mpm-identifier, date, action )

      Host

        The character string name of a host.

      Identification

        This is the transaction identifier associated with a particular
        message.  It is the transaction number, and the MPM identifier of
        the originating MPM.

        ( mpm-identifier, transaction-number )

    Postel                                                         [Page 23]

                                                                 August 1980
    Internet Message Protocol
    Specification

      Internet Address

        This identifies a host in the ARPA internetwork environment.  When
        used as a part of identification, it identifies the originating host
        of a message.  The internet address is a 32 bit number, the higher
        order 8 bits identify the network, and the lower order 24 bits
        identify the host on that network [2].  For use in the MPMs the
        internet address is divided into eight bit fields and the value of
        each field is represented in decimal digits.  For example, the
        ARPANET address of ISIE is 167837748 and is represented as
        10,1,0,52.  Further, this representation may be extended to include
        an address within a host, such as the TCP port of the MPM, for
        example, 10,1,0,52,0,45.

      Mailbox

        This is the destination address of a user of the internetwork mail
        system.  Mailbox contains information such as network, host,
        location, and local user indentifier of the recipient of the
        message.  Some information contained in mailbox may not be necessary
        for delivery.

        As an example, when one sends a message to someone for the first
        time, he may include many items which are not necessary simply to
        insure delivery.  However, once he gets a reply to this message, the
        reply will contain an Address (as opposed to Mailbox) which may be
        used from then on.

          A mailbox is a property list.  A mailbox might contain the
          following <name,value> pairs:

            name    description
            ----    -----------
            MPM     mpm-identifier
            NET     network name
            HOST    host name
            PORT    address of MPM within the host
            USER    user name
            ORG     organization name
            CITY    city
            STATE   state
            COUNTRY country
            ZIP     zip code
            PHONE   phone number

        The minimum mail box is an Address.

    [Page 24]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      MPM-Identifier

        The internetwork address of the MPM.  This may be the ARPA Internet
        Address or an X.121 Public Data Network Address [29].  The
        mpm-identifier is a property list which has one <name,value> pair.
        This unusual structure is used so that it will be easy to determine
        the type of address used.

      Network

        This character string name of a network.

      Operation

        This names the operation or procedure to be performed.  It is a
        character string name.

      Organization

        This character string name of a organization.

      Phone

        This character string name representation of a phone number. For
        example the phone number of ISI is 1 (international region) + 213
        (area code) + 822 (central office) + 1511 (station number) =
        12138221511.

      Port

        This names the port or subaddress within a host of the MPM.  The
        default port for the MPM is 45 (55 octal) [4].

      Reference

        The reference is an identification from an earlier message.

      State

        The character string name of a state.

    Postel                                                         [Page 25]

                                                                 August 1980
    Internet Message Protocol
    Specification

      Trace

        Each MPM that handles the message must add its handling-stamp to
        this list.  This will allow detection of messages being sent in a
        loop within the internet mail system, and aid in fault isolation.

      Trail

        When a message is sent through the internetwork environment, it
        acquires this trace, a list of MPMs that have handled the message.
        This list is then carried as the trail in a reply or acknowledgment
        of that message.  Requests and replies always have a trace and each
        MPM adds its handling-stamp to this trace.  Replies, in addition,
        have a trail which is the complete trace of the original message.

      Transaction Number

        This is a number which is uniquely associated with this transaction
        by the originating MPM.  It identifies the transaction.  (A
        transaction is a message and acknowledgment.)  A transaction number
        must be unique during the time which the message (a request or
        reply) containing it could be active in the network.

      Type-of-Service

        A service parameter for the delivery of a message, for instance a
        message could be delivered (REGULAR), forwarded (FORWARD), turned
        over to general delivery (GENDEL) (i.e., allow a person to decide
        how to further attempt to deliver the message), or require priority
        handling (PRIORITY).

      User

        The character string name of a user.

      X121 Address

        This identifies a host in the Public Data Network environment.  When
        used as a part of identifier, it identifies the originating host of
        a message.  The X121 address is a sequence of up to 14 digits [29].
        For use in the MPMs the X121 address is represented in decimal
        digits.

    [Page 26]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      Zip Code

        The character string representation of a postal zip code.  The zip
        code of ISI is 90291.

    3.7.  Data Elements

      The data elements defined here are similar to the data structure and
      encoding used in NSW [30].

      Each of the diagrams which follow represent a sequence of octets.
      Field boundaries are denoted by the "|" character, octet boundaries by
      the "+" character.  Each element begins with a one-octet code.  The
      order of the information in each element is left-to-right.  In fields
      with numeric values the high-order (or most significant) bit is the
      left-most bit.  For transmission purposes, the leftmost octet is
      transmitted first.  Cohen has described some of the difficulties in
      mapping memory order to transmission order [31].

      Code  Type          Representation
      ----  ----          --------------

                          +------+
        0  No Operation   |  0   |
                          +------+

                          +------+------+------+------+------
        1  Padding        |  1   |     octet count    | Data ...
                          +------+------+------+------+------

                          +------+------+
        2  Boolean        |  2   | 1/0  |
                          +------+------+

                          +------+------+------+
        3  Index          |  3   |     Data    |
                          +------+------+------+

                          +------+------+------+------+------+
        4  Integer        |  4   |            Data           |
                          +------+------+------+------+------+

    Postel                                                         [Page 27]

                                                                 August 1980
    Internet Message Protocol
    Specification

           Extended       +------+------+------+------+------
        5  Precision      |  5   |    octet count     | Data ...
           Integer        +------+------+------+------+------

                          +------+------+------+------+------
        6  Bit String     |  6   |      bit count     | Data ...
                          +------+------+------+------+------

                          +------+------+------
        7  Name String    |  7   | count|  Data ...
                          +------+------+------

                          +------+------+------+------+------
        8  Text String    |  8   |     octet count    |  Data ...
                          +------+------+------+------+------

                          +------+------+------+------+-----
        9  List           |  9   |     octet count    | Data ...
                          +------+------+------+------+-----

                          +------+------+------+------+------
        10 Proplist       |  10  |     octet count    | Data ...
                          +------+------+------+------+------

                          +------+
        11 End of List    |  11  |
                          +------+

      Element code 0 (NOP) is an empty data element used for padding when it
      is necessary.  It is ignored.

      Element code 1 (PAD) is used to transmit large amounts of data with a
      message for test or padding purposes.  The type-octet is followed by a
      three-octet count of the number of octets to follow.  No action is
      taken with this data but the count of dummy octets must be correct to
      indicate the next element code.

      Element code 2 (BOOLEAN) is a boolean data element.  The octet
      following the type-octet has the value 1 for True and 0 for False.

    [Page 28]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      Element code 3 (INDEX) is a 16-bit unsigned integer datum.  Element
      code 3 occupies only 3 octets.

      Element code 4 (INTEGER) is a signed 32-bit integer datum.  This will
      always occupy five octets.  Representation is two's complement.

      Element code 5 (EPI) is an extended precision integer.  The type octet
      is followed by a three-octet count of the number of data octets to
      follow.  Representation is two's complement.

      Element code 6 (BITSTR) is a bit string element for binary data.  The
      bit string is padded on the right with zeros to fill out the last
      octet if the bit string does not end on an octet boundary.  This data
      type must have the bit-count in the three-octet count field instead of
      the number of octets.

      Element code 7 (NAME) is used for the representation of character
      string names (or other short strings).  The type octet is followed by
      a one-octet count of the number of characters (one per octet) to
      follow.  Seven bit ASCII characters are used, right justified in the
      octet.  The high order bit in the octet is zero.

      Element code 8 (TEXT) is used for the representation of text.  The
      type octet is followed by a three-octet count of the number of
      characters (one per octet) to follow.  Seven bit ASCII characters are
      used, right justified in the octet.  The high order bit in the octet
      is zero.

      Element code 9 (LIST) can be used to create structures composed of
      other elements.  The three-octet octet count specifies the number of
      octets in the whole list (i.e., the number of octets following this
      count field to the end of the list, not including the ENDLIST octet).
      The two-octet item count contains the number of elements which follow.
      Any element may be used including list itself.

        +------+------+------+------+------+------+
        |   9  |     octet count    |  item count |
        +------+------+------+------+------+------+
                                         +------+------/---+
                              repeated   |      element    |
                                         +------+------/---+
                                                         +-------+
                                                         |ENDLIST|
                                                         +-------+

      In some situations it may not be possible to know the length of a list

    Postel                                                         [Page 29]

                                                                 August 1980
    Internet Message Protocol
    Specification

      until the head of it has been transmitted.  To allow for this a
      special ENDLIST element is defined.  A list of undetermined length is
      transmitted with the octet count cleared to zero, and the item count
      cleared to zero.  A null or empty List, one with no elements, has an
      octet count of two (2) and an item count of zero (0).  The ENDLIST
      element always follows a LIST, even when the length is determined.

      Element code 10 (PROPLIST) is the Property List element.  It is a
      special case of the list element, in which the elements are in pairs
      and the first element of each pair is a name.  It has the following
      form:

        +------+------+------+------+------+
        |  10  |     octet count    | pair |
        +------+------+------+------+------+
                             +------+------/---+------+------/---+
                  repeated   | name element    | value element   |
                             +------+------/---+------+------/---+
                                                               +-------+
                                                               |ENDLIST|
                                                               +-------+

      The Property List structure consists of a set of unordered
      <name,value> pairs.  The pairs are composed of a name which must be a
      NAME element and a value which may be any kind of element.  Following
      the type code is a three-octet octet count of the following octets.
      Following the octet count is a one-octet pair count of the number of
      <name,value> pairs in the property list.

      The name of a <name,value> pair is to be unique within the property
      list, that is, there shall be at most one occurrence of any particular
      name in one property list.

      In some situations it may not be possible to know the length of a
      property list until the head of it has been transmitted.  To allow for
      this the special ENDLIST element is defined.  A property list of
      undetermined length is transmitted with the octet count cleared to
      zero, and the pair count cleared to zero.  A null or empty property
      list, one with no elements, has an octet count of one (1) and an pair
      count of zero (0).  The ENDLIST element always follows a property
      list, even when the length is determined.

      Element code 11 (ENDLIST) is the end of list element.  It marks the
      end of the corresponding list or property list.

    [Page 30]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

      Structure Sharing

        When messages are batched in message-bags for transmission, it may
        often be the case that the same document will be sent to more than
        one recipient.  Since the document portion can usually be expected
        to be the major part of the message, much repeated data would be
        sent if a copy of the document for each recipient were to be shipped
        in the message-bag.

        To avoid this redundancy, messages may be assembled in the
        message-bag so that actual data appears on its first occurrence and
        only references to it appear in later occurrences.  When data is
        shared, the first occurrence of the data will be tagged, and later
        locations where the data should appear will only reference the
        earlier tagged location.  All references to copied data point to
        earlier locations in the message-bag.  The data to be retrieved is
        indicated by the tag.

        This is a very general sharing mechanism.  PLEASE NOTE THAT THE MPM
        WILL NOT SUPPORT THE FULL USE OF THIS MECHANISM.  THE MPM WILL ONLY
        SUPPORT SHARING OF WHOLE DOCUMENTS.  No other level of sharing will
        be supported by the MPMs.

        This sharing mechanism may be used within a document as long as all
        references refer to tags within the same document.

        Sharing is implemented by placing a share-tag on the first
        occurrence of the data to be shared, and placing a share-reference
        at the locations where copies of that data should occur.

                               +------+------+------+
          12 Share Tag         |  12  | share-index |
                               +------+------+------+

                               +------+------+------+
          13 Share Reference   |  13  | share-index |
                               +------+------+------+

        Element code 12 (S-TAG) is a share tag element.  The two octets
        following the type-octet specify the shared data identification code
        for the following data element.  Note that s-tag is not a DATA
        element, in the sense that data elements encode higher level
        objects.

        Element code 13 (S-REF) is a share reference element.  The two

    Postel                                                         [Page 31]

                                                                 August 1980
    Internet Message Protocol
    Specification

        octets following the type-octet specify the referenced shared data
        identification code.

        An example of using this mechanism is

          ( ( <a>, <b> ) ( <c>, <b> ) )

        could be coded as follows to share <b>

          ( ( <a>, <s-tag-1><b> ) ( <c>, <s-ref-1> ) )

        To facilitate working with structures which may contain shared data,
        the two high-order bits of the list and property list element codes
        are reserved for indicating if the structure contains data to be
        shared or contains a reference to shared data.  That is, if the
        high-order bit of the list or property list element code octet is
        set to one then the property list contains a share-reference to
        shared data.  Or, if the second high-order bit is set to one the
        structure contains a share-tag for data to be shared.

        The example above is now repeated in detail showing the use of the
        high-order bits.

        +------+------+------+------+------+------+------+------+
        |11 - 9|01 - 9|  <a> |  12  |   0  |   1  |  <b> |  11  |
        +------+------+------+------+------+------+------+------+
                          +------+------+------+------+------+------+------+
                          |10 - 9|  <c> |  13  |   0  |   1  |  11  |  11  |
                          +------+------+------+------+------+------+------+

        It is not considered an error for an element to be tagged but not
        referenced.

        A substructure with internal sharing may be created.  If such a
        substructure is closed with respect to sharing -- that is, all
        references to its tagged elements are within the substructure --
        then there is no need for the knowledge of the sharing to propagate
        up the hierarchy of lists.  For example, if the substructure is:

          00-LIST ( a b c b )

        which with sharing is:

          11-LIST ( a T1:b c R1 )

        When this substructure is included in a large structure the high

    [Page 32]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                               Specification

        order bits can be reset since the substructure is closed with
        respect to sharing.  For example:

          00-LIST ( x 11-LIST ( a T1:b c R1 ) y )

        Note:  While sharing adds transmission and memory efficiency, it is
        costly in processing to separate shared elements.  This is the main
        reason for restricting the sharing supported by the MPM.  At some
        later time these restrictions may be eased.

        It is possible to create loops, "strange loops" and "tangled
        hierarchies" using this mechanism [32].  The MPM will not check for
        such improper structures within documents, and will not deliver
        messages involved in such structures between documents.

        If an encryption scheme is used to ensure the privacy of
        communication it is unlikely that any parts of the message can be
        shared.  This is due to the fact that in most case the encryption
        keys will be specific between two individuals.  There may be a few
        cases where encrypted data may be shared.  For example, all the
        members of a committee may use a common key when acting on committee
        business, or in a public key scheme a document may be "signed" using
        the private key of the sender and inspected by anyone using the
        public key of the sender.

    Postel                                                         [Page 33]

                                                                 August 1980
    Internet Message Protocol

    [Page 34]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                                4.  OTHER ISSUES

    This section discusses various other issues that need to be dealt with
    in a computer message system.

    4.1.  Accounting and Billing

      Accounting and billing must be performed by the MPM.  The charge to
      the user by the message delivery system must be predictable, and so
      cannot depend on the actual cost of sending a particular message which
      incurs random delays, handling and temporary storage charges.  Rather,
      these costs must be aggregated and charged back to the users on an
      average cost basis.  The user of the service may be charged based on
      the destination or distance, the length of the message, type of
      service, or other parameters selected as the message is entered into
      the delivery system, but must not depend on essentially random
      handling by the system of the particular message.

      This means it is pointless to have each message carry an accumulated
      charge (or list of charges).  Rather, the MPM will keep a log of
      messages handled and periodically bill the originators of those
      messages.

      It seems that the most reasonable scheme is to follow the practice of
      the international telephone authorities.  In such schemes the
      authority where the message originates bills the user of the service
      for the total charge.  The authorities assist each other in providing
      the international message transfer and the authorities periodically
      settle any differences in accounts due to an imbalance in
      international traffic.

      Thus the MPMs will keep logs of messages handled and will periodically
      charge their neighboring MPM for messages handled for them.  This
      settlement procedure is outside the message system and between the
      administrators of the MPMs.

      As traffic grows it will be impractical to log every message
      individually.  It will be necessary to establish categories of
      messages (e.g., short, medium, large) and only count the number in
      each category.

      The MPM at the source of the message will have a local means of
      identifying the user to charge for the message delivery service.  The
      relay and destination MPMs will know which neighbor MPMs to charge (or
      settle with) for delivery of their messages.

    Postel                                                         [Page 35]

                                                                 August 1980
    Internet Message Protocol
    Other Issues

    4.2.  Addressing and Routing

      The mailbox provides for many types of address information.  The MPMs
      in the ARPA internet can most effectively use the internet address
      [2].  The use of other address information is not yet very clear.
      Some thoughts on addressing issues may be found in the references
      [33,34,35].

      An MPM sometimes must make a routing decision when it is acting as a
      relay-MPM (or source MPM).  It must be able to use the information
      from the mailbox to determine to which of its neighbor MPMs to send
      the message.  One way this might be implemented is to have a table of
      destination networks with corresponding neighbor MPM identifiers to
      use for routing toward that network.

      It is not expected that such routing tables would be very dynamic.
      Changes would occur only when new MPMs came into existence or MPMs
      went out of service for periods of days.

      Even with relatively slowly changing routing information the MPMs need
      an automatic mechanism for adjusting their routing tables.  The
      routing problem here is quite similar to the problem of routing in a
      network of packet switches such as the ARPANET IMPs or a set of
      internet gateways.  A great deal of work has been done on such
      problems and many simple schemes have been found faulty.  There are
      details of these procedures which may become troublesome when the
      number of nodes grows beyond a certain point or the frequency of
      update exchanges gets large.

      A basic routing scheme is to have a table of <network-name,
      mpm-identifier> pairs.  The MPM could look up the network name found
      in the mailbox of the message and determine the internet
      mpm-identifier of the next MPM to which to route the message.  To
      permit automatic routing updates another column would be added to
      indicate the distance to the destination.  This could be measured in
      several ways, for example, the number of relay MPM (or hops) to the
      final destination.  In this case each entry in the table is a triple
      of <network-name, mpm-identifier, distance>.

      To update the routing information when changes occur an MPM updates
      its table.  It then sends to each next MPM in its table a table of
      pairs <network-name, distance>, which say in effect "I can get a
      message to each of these networks with "cost" distance."  An MPM which
      receives such an update will add to all the distances the distance to
      the MPM sending the update (e.g., one hop) and compare the information
      with its own table.

    [Page 36]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                                Other Issues

      If the update information shows that the distance to a destination
      network is now smaller via the MPM which sent the update, the MPM
      changes its own table to reflect the better route, and the new
      distance.  If the MPM has made changes in its table it sends update
      information to all the MPMs listed as next-MPMs in its table.

      One further feature is that when a new network comes into existence an
      entry must be added to the table in each MPM.  The MPMs should
      therefore expect the case that update information may contain entries
      which are new networks, and in such an event add these entries to
      their own tables.

      When a new MPM comes into existence it will have an initial table
      indicating that it is a good route (short distance) to the network it
      is in, and will have entries for a few neighbor networks.  It will
      send an initial "update" to those neighbor MPMs which will respond
      with more complete tables, thus informing the new MPM of routes to
      many networks.

      This routing update mechanism is a simple minded scheme and may have
      to be replaced as the system of MPMs grows.  In addition it ignores
      the opportunity for MPMs to use other information (besides destination
      network name) for routing.  MPMs may have tables that indicate
      next-MPMs based on city, telephone number, organization, or other
      categories of information.

    4.3.  Encryption

      It is straightforward to add the capability to have the document
      portion of messages either wholly or partially encrypted.  An
      additional basic data element is defined to carry encrypted data.  The
      data within this element may be composed of other elements, but that
      could only be perceived after the data was decrypted.

                          +------+------+------+------+
        14 Encrypt        |  14  |     octet count    |
                          +------+------+------+------+

                                 +------+------+------+-------
                                 |alg id|   key id    | Data ...
                                 +------+------+------+--------

      Element code 14 (ENCRYPT) is used to encapsulate encrypted data.  The
      format is the one-octet type code, the three-octet octet count, a
      one-octet algorithm identifier, a two-octet key identifier, and count
      octets of data.  Use of this element indicates that the data it

    Postel                                                         [Page 37]

                                                                 August 1980
    Internet Message Protocol
    Other Issues

      contains is encrypted.  The encryption scheme is indicated by the
      algorithm identifier, and the key used is indicated by the key
      identifier (this is not the key itself).  The NBS Data Encryption
      Standard (DES) [36], public key encryption [37,38,39], or other
      schemes may be used.

      To process this data element, the user is asked for the appropriate
      key and the data can then be decrypted.  The data thus revealed will
      be in the form of complete data element fields.  Encryption cannot
      occur over a partial field.  The revealed data is then processed
      normally.

      Note that there is no reason why all fields of a document could not be
      encrypted including all document header information such as From,
      Date, etc.

    [Page 38]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                     5.  THE MPM:  A POSSIBLE ARCHITECTURE

    The heart of the internet message system is the MPM which is responsible
    for routing and delivering messages.  Each network must have at least
    one MPM.  These MPMs are logically connected together, and internet mail
    is always transferred along logical channels between them.  The MPMs
    interface with existing local message systems.

    Since the local message system may be very different from the internet
    system, special programs may be necessary to convert incoming internet
    messages to the local format.  Likewise, messages outgoing to other
    networks may be converted to the internet format and sent via the MPMs.

    5.1.  Interfaces

      User Interface

        It is assumed that the interface between the MPM and the UIP
        provides for passing data structures which represent the document
        portion of the message.  In addition, this interface must pass the
        delivery address information (which becomes the information in the
        mailbox field of the command).  It is assumed that the information
        is passed between the UIP and the MPM via shared files, but this is
        not the only possible mechanism.  These two processes may be more
        strongly coupled (e.g., by sharing memory), or less strongly coupled
        (e.g., by communicating via logical channels).

        When a UIP passes a document and a destination address to the MPM,
        the MPM assigns a transaction-number and forms a message to send.
        The MPM must record the relationship between the transaction-number,
        the document, and the UIP, so that it can inform the UIP about the
        outcome of the delivery attempt for that document when the
        acknowledgment message is received at some later time.

        Assuming a file passing mode of communication between the UIP and
        the MPM the sending and receiving of mail might involve the
        following interactions:

          A user has an interactive session with a UIP to compose a document
          to send to a destination (or list of destinations).  When the user
          indicates to the UIP that the document is to be sent, the UIP
          places the information into a file for the MPM.  The UIP may then
          turn to the next request of the user.

          The MPM finds the file and extracts the the information.  It
          creates a message, assigning a transaction-number and forming a
          deliver command.  The MPM records the UIP associated with this
          message.  The MPM sends the message toward the destination.

    Postel                                                         [Page 39]

                                                                 August 1980
    Internet Message Protocol
    MPM Architecture

          When the MPM receives a deliver message from another MPM addressed
          to a user in its domain, it extracts the document and puts it into
          a file for the UIP associated with the destination user.  The MPM
          also sends an acknowledge message to the originating MPM.

          When the MPM receives an acknowledgment for a message it sent, the
          MPM creates a notification for the associated UIP and places it in
          a file for that UIP.

          The format of these files is up to each UIP/MPM interface pair.
          One reasonable choice is to use the same data structures used in
          the MPM-MPM communication.

      Communication Interface

        It is assumed here that the MPMs use an underlying communication
        system, and TCP [3] has been taken as the model.  In particular, the
        MPM is assumed to be listening for a TCP connection on a TCP port,
        i.e., it is a server process.  The port is either given explicitly
        in the mpm-identifier or takes the default vaule 45 (55 octal) [4].
        Again, this is not intended to limit the implementation choices;
        other forms of interprocess communication are allowed, and other
        types of physical interconnection are permitted.  One might even use
        dial telephone calls to interconnect MPMs (using suitable protocols
        to provide reliable communication) [12,19,20,21].

    5.2.  The MPM Organization

      Messages in the internet mail system are transmitted in lists called
      message-bags (or simply bags), each bag containing one or more
      messages.  Each MPM is expected to implement functions which will
      allow it to deliver local messages it receives and to forward
      non-local ones to other MPMs presumably closer to the message's
      destination.

      Loosely, each MPM can be separated into six components:

        1--Acceptor

          Receives incoming message-bags, from other MPMs, from UIPs, or
          from conversion programs.

        2--Message-Bag Processor

          Splits a bag into these three portions:

    [Page 40]                                                         Postel

    August 1980
                                                   Internet Message Protocol

            a.    Local Host Messages
            b.    Local Net Messages
            c.    Foreign Net Messages

        3--Local Host Delivery

          Delivers local host messages, may call on conversion program.

        4--Local Net Delivery

          Delivers local net messages, may call on conversion program.

        5--Foreign Net Router

          Forms message-bags for transmission to other MPMs and determines
          the first step in the route.

        6--Foreign Net Sender

          Activates transmission channels to other MPMs and sends
          message-bags to foreign MPMs.

      If the local net message system uses the protocol of the MPMs, then
      there need be no distinction between local net and foreign net
      delivery procedures.

      All of these components can be thought of as independent.  The
      function of the Acceptor is to await incoming message-bags and to
      insert them into the Bag-Input Queue.

      The Bag-Input queue is read by the message-bag Processor which will
      separate and deliver suitable portions of the message-bags it
      retrieves from the queue to one of three queues:

        a.    Local Host Queue
        b.    Local Net Queue
        c.    Foreign Net Queue

      When an MPM has a message to send to another MPM, it must add its own
      handling-stamp to the trace field of the command.  The trace then
      becomes a record of the route the message has taken.  An MPM should
      examine the trace field to see if the message is in a routing loop.
      All commands require the return of the trace as a trail in the
      matching reply command.

      All of these queues have as elements complete message-bags created by
      selecting messages from the input message-bags.

    Postel                                                         [Page 41]

                                                                 August 1980
    Internet Message Protocol

      The Local Host queue serves as input to the Local Host Delivery
      process.  This component is responsible for delivering messages to its
      local host.  It may call on a conversion program to reformat the
      messages into a form the local protocol will accept.  This will
      probably involve such things as copying shared information.

      The Local Net queue serves as input to the Local Net Delivery process.
      This component is responsible for delivering messages to other hosts
      on its local net.  It must be capable of handling whatever error
      conditions the local net might return, and should include the ability
      to retransmit.  It may call on a conversion program to reformat the
      messages into a form the local protocol will accept.  This will
      probably involve such things as copying shared information.

      The other two processes are more closely coupled.  The Foreign Net
      Router takes its input bags from the Foreign Net Queue.  From the
      internal information it contains, it determines which of the MPMs to
      which it is connected should receive the bag.

      It then places the bag along with the routing information into the
      Send Mail Queue.  The Foreign Net Sender retrieves it from that queue
      and transmits it across a channel to the intended foreign MPM.  The
      Sender aggregates messages to the same next MPM into a bag.

      The Foreign Net Router should be capable of receiving external input
      to its routing information table.  This may come from the Foreign Net
      Sender in the case of a channel going down, requiring a decision to
      either postpone delivery or to determine a new route.  The Router is
      responsible for maintaining sufficient information to determine where
      to send any incoming message-bag.

      Forwarding

        An MPM may have available information on the correct mailboxes of
        users which are not at its location.  This information, called a
        forwarding data base, may be used to return the correct address in
        response to a probe command, or to actually forward a deliver
        command (if allowed by the type of service).

        Because such forwarding may cause the route of a message to pass
        through an MPM already on the trace of this message, only the
        portion of the trace back to the most recent forward action should
        be used for loop detection by a relay relaying MPM, and only the
        forward action entries in the trace should be checked by a
        forwarding MPM.

    [Page 42]                                                         Postel

    August 1980
                                                   Internet Message Protocol

      Implementation Recommendations

        Transaction numbers can be assigned sequentially, with wrap around
        when the highest value is reached.  This should ensure that no
        message with a particular transaction number from this source is in
        the network when another instance of this transaction number is
        chosen.

        The processing to separate shared elements when the routes of the
        shared elements diverge while still preserving the sharing possible
        appears to be an O(N*M**2) operation where N is the number of
        distinct objects in a message which may be shared across message
        boundaries and M is the number of messages in the bag.

        Also note that share-tags may be copied into separate message bags
        which are not referenced.  These could be removed with another pass
        over the message bag.

    Postel                                                         [Page 43]

                                                                 August 1980
    Internet Message Protocol

    [Page 44]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                            6.  EXAMPLES & SCENARIOS

    Example 1:  Message Format

      Suppose we want to send the following message:

        Date: 1979-03-29-11:46-08:00
        From: Jon Postel <Postel@ISIE>
        Subject: Meeting Thursday
        To: Danny Cohen <Cohen@USC-ISIB>
        CC: Linda

        Danny:

        Please mark your calendar for our meeting Thursday at 3 pm.

        --jon.

      It will be encoded in the structured format.  The following will
      present successive steps in the top down generation of this message.
      The actual document above will not be shown in the coded form.

      1.  message

      2.  (identification, command, document)

      3.  (ID:(mpm-identifier, transaction-number),
           CMD:(MAILBOX:mailbox, OPERATION:operation,
                                                    arguments, TRACE:trace),
           DOC:<<document>>)

      4.  (ID:(mpm-identifier, transaction-number),
           CMD:(MAILBOX:mailbox, OPERATION:operation,
                                      TYPE-OF-SERVICE:regular, TRACE:trace),
           DOC:<<document>>)

      5.  (ID:(MPM:(IA:12,1,0,52,0,45), TRANSACTION:37),
           CMD:(MAILBOX:(MPM:(IA:12,3,0,52,0,45),
                         NET:ARPA,
                         HOST:ISIB,
                         PORT:45,
                         USER:Cohen),
                OPERATION:DELIVER,
                TYPE-OF-SERVICE:REGULAR,
                TRACE:(MPM:(IA:12,1,0,52,0,45)
                       DATE:1979-03-29-11:46-08:00,
                       ACTION:ORIGIN)),
           DOC:<<document>>)

    Postel                                                         [Page 45]

                                                                 August 1980
    Internet Message Protocol
    Examples & Scenarios

      6.  PROPLIST:(
            ID:PROPLIST:(
                 MPM:PROPLIST:(
                       IA:12,1,0,52,0,45),
                     ENDLIST
                 TRANSACTION:37)
               ENDLIST,

            CMD:PROPLIST(
                 MAILBOX:(PROPLIST:(
                            MPM:PROPLIST(
                                  IA:12,3,0,52,0,45),
                                ENDLIST
                            NET:ARPA,
                            HOST:ISIB,
                            PORT:45,
                            USER:Cohen ),
                          ENDLIST
                 OPERATION:DELIVER,
                 TYPE-OF-SERVICE:REGULAR,
                 TRACE:(PROPLIST:MPM:
                           (PROPLIST:
                              IA:12,1,0,52,0,45)
                            ENDLIST
                          DATE:1979-03-29-11:46-08:00,
                          ACTION:ORIGIN)),
                        ENDLIST
              ENDLIST
            DOC:<<document>>)
          ENDLIST

    [Page 46]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                        Examples & Scenarios

    Example 2:  Delivery and Acknowledgment

      The following are four views of the message of example 1 during the
      successive transmission from the origination MPM, through a relay MPM,
      to the destination MPM, and the return of the acknowledgment, through
      a relay MPM, to the originating MPM.

      +-----------------------------------------------------------------+
      |                          A         B                            |
      | sending --> originating --> relay --> destination --> receiving |
      |   user          MPM          MPM          MPM            user   |
      |                                                                 |
      |                          D         C                            |
      |             originating <-- relay <-- destination               |
      |                 MPM          MPM          MPM                   |
      +-----------------------------------------------------------------+

                               Transmission Path

                                   Figure 6.

    Postel                                                         [Page 47]

                                                                 August 1980
    Internet Message Protocol
    Examples & Scenarios

      A.  Between the originating MPM and the relay MPM.

        PROPLIST:
          NAME:"ID",
            PROPLIST:
              NAME:"MPM",
                PROPLIST:
                  NAME:"IA", NAME:"10,1,0,52,0,45"
                ENDLIST
              NAME:"TRANSACTION", INTEGER:37
            ENDLIST
          NAME:"CMD",
            PROPLIST:
              NAME:"MAILBOX",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", NAME:"10,3,0,52,0,45"
                    ENDLIST
                  NAME:"NET", NAME:"ARPA"
                  NAME:"HOST", NAME:"ISIB"
                  NAME:"PORT", NAME:"45"
                  NAME:"USER", NAME:"Cohen"
                ENDLIST
              NAME:"OPERATION", NAME:"DELIVER"
              NAME:"TYPE-OF-SERVICE", NAME:"REGULAR"
              NAME:"TRACE",
                LIST:
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,1,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:47.5-08:00"
                    NAME:"ACTION", NAME:"ORIGIN"
                  ENDLIST
                ENDLIST
            ENDLIST
          NAME:"DOC", <<document>>
        ENDLIST

    [Page 48]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                        Examples & Scenarios

      B.  Between the relay MPM and the destination MPM.

        PROPLIST:
          NAME:"ID",
            PROPLIST:
              NAME:"MPM",
                PROPLIST:
                  NAME:"IA", NAME:"10,1,0,52,0,45"
                ENDLIST
              NAME:"TRANSACTION", INTEGER:37
            ENDLIST
          NAME:"CMD",
            PROPLIST:
              NAME:"MAILBOX",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", NAME:"10,3,0,52,0,45"
                    ENDLIST
                  NAME:"NET", NAME:"ARPA"
                  NAME:"HOST", NAME:"ISIB"
                  NAME:"PORT", NAME:"45"
                  NAME:"USER", NAME:"Cohen"
                ENDLIST
              NAME:"OPERATION", NAME:"DELIVER"
              NAME:"TYPE-OF-SERVICE", NAME:"REGULAR"
              NAME:"TRACE",
                LIST:
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,1,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:47.5-08:00"
                    NAME:"ACTION", NAME:"ORIGIN"
                  ENDLIST
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,2,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:48-08:00"
                    NAME:"ACTION", NAME:"RELAY"
                  ENDLIST
                ENDLIST
            ENDLIST
          NAME:"DOC", <<document>>

    Postel                                                         [Page 49]

                                                                 August 1980
    Internet Message Protocol
    Examples & Scenarios

        ENDLIST

      C.  Between the destination MPM and the relay MPM.

        PROPLIST:
          NAME:"ID",
            PROPLIST:
              NAME:"MPM",
                PROPLIST:
                  NAME:"IA", NAME:"10,3,0,52,0,45"
                ENDLIST
              NAME:"TRANSACTION", INTEGER:1993
            ENDLIST
          NAME:"CMD",
            PROPLIST:
              NAME:"MAILBOX",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:"10,1,0,52,0,45"
                    ENDLIST
                  NAME:"NET", NAME:"ARPA"
                  NAME:"HOST", NAME:"ISIE"
                  NAME:"PORT", NAME:"45"
                  NAME:"USER", NAME:"*MPM*"
                ENDLIST
              NAME:"OPERATION", NAME:"ACKNOWLEDGE"
              NAME:"REFERENCE",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", NAME:"10,1,0,52,0,45"
                    ENDLIST
                  NAME:"TRANSACTION", INTEGER:37
                ENDLIST
              NAME:"ADDRESS",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:"10,3,0,52,0,45"
                    ENDLIST
                  NAME:"USER", NAME:"Cohen"
                ENDLIST
              NAME:"TYPE-OF-SERVICE", NAME:"REGULAR"
              NAME:"ERROR-CLASS", INDEX:0
              NAME:"ERROR-STRING", NAME:"Ok"
              NAME:"TRAIL",

    [Page 50]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                        Examples & Scenarios

                LIST:
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,1,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:47.5-08:00"
                    NAME:"ACTION", NAME:"ORIGIN"
                  ENDLIST
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,2,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:48-08:00"
                    NAME:"ACTION", NAME:"RELAY"
                  ENDLIST
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,3,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:51.567-08:00"
                    NAME:"ACTION", NAME:"DESTINATION"
                  ENDLIST
                ENDLIST
              NAME:"TRACE",
                LIST:
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,3,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:52-08:00"
                    NAME:"ACTION", NAME:"ORIGIN"
                  ENDLIST
                ENDLIST
            ENDLIST
        ENDLIST

    Postel                                                         [Page 51]

                                                                 August 1980
    Internet Message Protocol
    Examples & Scenarios

      D.  Between the relay MPM and the originating MPM.

        PROPLIST:
          NAME:"ID",
            PROPLIST:
              NAME:"MPM",
                PROPLIST:
                  NAME:"IA", NAME:"10,3,0,52,0,45"
                ENDLIST
              NAME:"TRANSACTION", INTEGER:1993
            ENDLIST
          NAME:"CMD",
            PROPLIST:
              NAME:"MAILBOX",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:"10,1,0,52,0,45"
                    ENDLIST
                  NAME:"NET", NAME:"ARPA"
                  NAME:"HOST", NAME:"ISIE"
                  NAME:"PORT", NAME:"45"
                  NAME:"USER", NAME:"*MPM*"
                ENDLIST
              NAME:"OPERATION", NAME:"ACKNOWLEDGE"
              NAME:"REFERENCE",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", NAME:"10,1,0,52,0,45"
                    ENDLIST
                  NAME:"TRANSACTION", INTEGER:37
                ENDLIST
              NAME:"ADDRESS",
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:"10,3,0,52,0,45"
                    ENDLIST
                  NAME:"USER", NAME:"Cohen"
                ENDLIST
              NAME:"TYPE-OF-SERVICE", NAME:"REGULAR"
              NAME:"ERROR-CLASS", INDEX:0
              NAME:"ERROR-STRING", NAME:"Ok"
              NAME:"TRAIL",
                LIST:
                  PROPLIST:

    [Page 52]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                        Examples & Scenarios

                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,1,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:47.5-08:00"
                    NAME:"ACTION", NAME:"ORIGIN"
                  ENDLIST
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,2,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:48-08:00"
                    NAME:"ACTION", NAME:"RELAY"
                  ENDLIST
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,3,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:51.567-08:00"
                    NAME:"ACTION", NAME:"DESTINATION"
                  ENDLIST
                ENDLIST
              NAME:"TRACE",
                LIST:
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,3,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:52-08:00"
                    NAME:"ACTION", NAME:"ORIGIN"
                  ENDLIST
                  PROPLIST:
                    NAME:"MPM",
                      PROPLIST:
                        NAME:"IA", NAME:"10,2,0,52,0,45"
                      ENDLIST
                    NAME:"DATE", NAME:"1979-03-29-11:52.345-08:00"
                    NAME:"ACTION", NAME:"RELAY"
                  ENDLIST
                ENDLIST
            ENDLIST
        ENDLIST

    Postel                                                         [Page 53]

                                                                 August 1980
    Internet Message Protocol

    [Page 54]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                           7.  SPECIFICATION SUMMARY

    7.1.  Message Fields

      All keywords used in this protocol are to be recognized independent of
      case.

      action: NAME (one of)
        "ORIGIN" | "RELAY" | "FORWARD" | "DESTINATION"

      address: PROPLIST (one of)

        NAME: "MPM", <mpm-identifier>
        NAME: "USER", <user>

          or

        NAME: "NET", <net>
        NAME: "HOST", <host>
        NAME: "PORT", <port>
        NAME: "USER", <user>

      answer: BOOLEAN

      city: NAME

      command: PROPLIST
        NAME: "MAILBOX", <mailbox>
        NAME: "OPERATION", <operation>
        <<arguments>>
        NAME: "ERROR-CLASS", <error-class>    (only in replies)
        NAME: "ERROR-STRING", <error-string>  (only in replies)
        NAME: "TRACE", <trace>

      country: NAME

      document: <<document>>

      error-class: INDEX

      error-string: NAME

      host: NAME

    Postel                                                         [Page 55]

                                                                 August 1980
    Internet Message Protocol

      handling-stamp: PROPLIST
        NAME: "MPM", <mpm-identifier>
        NAME: "DATE", <date>
        NAME: "ACTION", <action>

      identification: LIST
        NAME: "MPM", <mpm-identifier>
        NAME: "TRANSACTION", <transaction-number>

      internet-address: NAME

      mailbox:  PROPLIST (some of)
        NAME: "MPM", <mpm-identifier>
        NAME: "NET", <net>
        NAME: "HOST", <host>
        NAME: "PORT", <port>
        NAME: "USER", <user>
        NAME: "ORG", <organization>
        NAME: "CITY", <city>
        NAME: "STATE", <state>
        NAME: "COUNTRY", <country>
        NAME: "ZIP", <zip-code>
        NAME: "PHONE", <phone-number>
        <<other-items>>

      message: PROPLIST
        NAME: "ID", <identification>
        NAME: "CMD", <command>
        NAME: "DOC", <document> (only in deliver)

      mpm-identifier: PROPLIST (one of)

        NAME: "IA", <internet-address>

          or

        NAME: "X121", <x121-address>

      net: NAME

      operation: NAME (one of)
          "DELIVER" | "ACKNOWLEDGE
        | "PROBE"   | "RESPONSE
        | "CANCEL"  | "CANCELED"

      organization: NAME

      phone-number: NAME

    [Page 56]                                                         Postel

    August 1980
                                                   Internet Message Protocol

      port: NAME

      state: NAME

      trace: LIST
        <handling-stamp>
        ...

      trail: LIST
        <handling-stamp>
        ...

      transaction-number: INTEGER

      type-of-service: NAME (one or more of)
        "REGULAR" | "FORWARD" | "GENDEL" | "PRIORITY"

      user: NAME

      x121-address: NAME

      zip-code: NAME

    Postel                                                         [Page 57]

                                                                 August 1980
    Internet Message Protocol

    7.2.  Deliver Message

      PROPLIST:
        NAME:"ID",
          PROPLIST:
            NAME:"MPM",
              PROPLIST:
                NAME:"IA", NAME:<internet-address>
              ENDLIST
            NAME:"TRANSACTION", INTEGER:<transaction-number>
          ENDLIST
        NAME:"CMD",
          PROPLIST:
            NAME:"MAILBOX",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"NET", NAME:<net>
                NAME:"HOST", NAME:<host>
                NAME:"PORT", NAME:<port>
                NAME:"USER", NAME:<user>
                NAME:"ORG", NAME:<organization>
                NAME:"CITY", NAME:<city>
                NAME:"STATE", NAME:<state>
                NAME:"COUNTRY", NAME:<country>
                NAME:"ZIP", NAME:<zip-code>
                NAME:"PHONE", NAME:<phone-number>
                <<other-items>>
              ENDLIST
            NAME:"OPERATION", NAME:"DELIVER"
            NAME:"TYPE-OF-SERVICE", NAME:<type-of-service>
            NAME:"TRACE",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
          ENDLIST
        NAME:"DOC", <<document>>
      ENDLIST

    [Page 58]                                                         Postel

    August 1980
                                                   Internet Message Protocol

    7.3.  Acknowledge Message

      PROPLIST:
        NAME:"ID",
          PROPLIST:
            NAME:"MPM",
              PROPLIST:
                NAME:"IA", NAME:<internet-address>
              ENDLIST
            NAME:"TRANSACTION", INTEGER:<transaction-number>
          ENDLIST
        NAME:"CMD",
          PROPLIST:
            NAME:"MAILBOX",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"USER", NAME:"*MPM*"
                NAME:"NET", NAME:<net>
                NAME:"PORT", NAME:<port>
                NAME:"HOST", NAME:<host>
              ENDLIST
            NAME:"OPERATION", NAME:"ACKNOWLEDGE"
            NAME:"REFERENCE",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", NAME:<internet-address>
                  ENDLIST
                NAME:"TRANSACTION", INTEGER:<transaction-number>
              ENDLIST
            NAME:"ADDRESS",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"USER", NAME:<user>
              ENDLIST
            NAME:"TYPE-OF-SERVICE", NAME:<type-of-service>
            NAME:"ERROR-CLASS", INDEX:<error-class>
            NAME:"ERROR-STRING", NAME:<error-string>
            NAME:"TRAIL",
              LIST:
                PROPLIST:
                  NAME:"MPM",

    Postel                                                         [Page 59]

                                                                 August 1980
    Internet Message Protocol

                    PROPLIST:
                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
            NAME:"TRACE",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
          ENDLIST
      ENDLIST

    [Page 60]                                                         Postel

    August 1980
                                                   Internet Message Protocol

    7.4.  Probe Message

      PROPLIST:
        NAME:"ID",
          PROPLIST:
            NAME:"MPM",
              PROPLIST:
                NAME:"IA", NAME:<internet-address>
              ENDLIST
            NAME:"TRANSACTION", INTEGER:<transaction-number>
          ENDLIST
        NAME:"CMD",
          PROPLIST:
            NAME:"MAILBOX",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"NET", NAME:<net>
                NAME:"HOST", NAME:<host>
                NAME:"PORT", NAME:<port>
                NAME:"USER", NAME:<user>
                NAME:"ORG", NAME:<organization>
                NAME:"CITY", NAME:<city>
                NAME:"STATE", NAME:<state>
                NAME:"COUNTRY", NAME:<country>
                NAME:"ZIP", NAME:<zip-code>
                NAME:"PHONE", NAME:<phone-number>
                <<other-items>>
              ENDLIST
            NAME:"OPERATION", NAME:"PROBE"
            NAME:"TRACE",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
          ENDLIST
      ENDLIST

    Postel                                                         [Page 61]

                                                                 August 1980
    Internet Message Protocol

    7.5.  Response Message

      PROPLIST:
        NAME:"ID",
          PROPLIST:
            NAME:"MPM",
              PROPLIST:
                NAME:"IA", NAME:<internet-address>
              ENDLIST
            NAME:"TRANSACTION", INTEGER:<transaction-number>
          ENDLIST
        NAME:"CMD",
          PROPLIST:
            NAME:"MAILBOX",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"NET", NAME:<net>
                NAME:"HOST", NAME:<host>
                NAME:"PORT", NAME:<port>
                NAME:"USER", NAME:"*MPM*"
              ENDLIST
            NAME:"OPERATION", NAME:"RESPONSE"
            NAME:"REFERENCE",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", NAME:<internet-address>
                  ENDLIST
                NAME:"TRANSACTION", INTEGER:<transaction-number>
              ENDLIST
            NAME:"ADDRESS",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"USER", NAME:<user>
              ENDLIST
            NAME:"ERROR-CLASS", INDEX:<error-class>
            NAME:"ERROR-STRING", NAME:<error-string>
            NAME:"TRAIL",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:

    [Page 62]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
            NAME:"TRACE",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
          ENDLIST
      ENDLIST

    Postel                                                         [Page 63]

                                                                 August 1980
    Internet Message Protocol

    7.6.  Cancel Message

      PROPLIST:
        NAME:"ID",
          PROPLIST:
            NAME:"MPM",
              PROPLIST:
                NAME:"IA", NAME:<internet-address>
              ENDLIST
            NAME:"TRANSACTION", INTEGER:<transaction-number>
          ENDLIST
        NAME:"CMD",
          PROPLIST:
            NAME:"MAILBOX",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"NET", NAME:<net>
                NAME:"HOST", NAME:<host>
                NAME:"PORT", NAME:<port>
                NAME:"USER", NAME:<user>
                NAME:"ORG", NAME:<organization>
                NAME:"CITY", NAME:<city>
                NAME:"STATE", NAME:<state>
                NAME:"COUNTRY", NAME:<country>
                NAME:"ZIP", NAME:<zip-code>
                NAME:"PHONE", NAME:<phone-number>
                <<other-items>>
              ENDLIST
            NAME:"OPERATION", NAME:"CANCEL"
            NAME:"REFERENCE",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", NAME:<internet-address>
                  ENDLIST
                NAME:"TRANSACTION", INTEGER:<transaction-number>
              ENDLIST
            NAME:"TRACE",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>

    [Page 64]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
          ENDLIST
      ENDLIST

    Postel                                                         [Page 65]

                                                                 August 1980
    Internet Message Protocol

    7.7.  Canceled Message

      PROPLIST:
        NAME:"ID",
          PROPLIST:
            NAME:"MPM",
              PROPLIST:
                NAME:"IA", NAME:<internet-address>
              ENDLIST
            NAME:"TRANSACTION", INTEGER:<transaction-number>
          ENDLIST
        NAME:"CMD",
          PROPLIST:
            NAME:"MAILBOX",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"NET", NAME:<net>
                NAME:"HOST", NAME:<host>
                NAME:"PORT", NAME:<port>
                NAME:"USER", NAME:"*MPM*"
              ENDLIST
            NAME:"OPERATION", NAME:"CANCELED"
            NAME:"REFERENCE",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", NAME:<internet-address>
                  ENDLIST
                NAME:"TRANSACTION", INTEGER:<transaction-number>
              ENDLIST
            NAME:"ADDRESS",
              PROPLIST:
                NAME:"MPM",
                  PROPLIST:
                    NAME:"IA", INTEGER:<internet-address>
                  ENDLIST
                NAME:"USER", NAME:<user>
              ENDLIST
            NAME:"ERROR-CLASS", INDEX:<error-class>
            NAME:"ERROR-STRING", NAME:<error-string>
            NAME:"TRAIL",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:

    [Page 66]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
            NAME:"TRACE",
              LIST:
                PROPLIST:
                  NAME:"MPM",
                    PROPLIST:
                      NAME:"IA", INTEGER:<internet-address>
                    ENDLIST
                  NAME:"DATE", NAME:<date>
                  NAME:"ACTION", NAME:<action>
                ENDLIST
                ...
              ENDLIST
          ENDLIST
      ENDLIST

    Postel                                                         [Page 67]

                                                                 August 1980
    Internet Message Protocol

    7.8.  Data Element Summary

      CODE   NAME      STRUCTURE                                LENGTH
      ----   ----      ---------                                ------

       0     NOP       CODE(1)                                   1

       1     PAD       CODE(1),COUNT(3),DATA(C)                  C+4

       2     BOOLEAN   CODE(1),TRUE-FALSE(1)                     2

       3     INDEX     CODE(1),INDEX(2)                          3

       4     INTEGER   CODE(1),INTEGER(4)                        5

       5     EPI       CODE(1),COUNT(3),INTEGER(C)               C+4

       6     BITSTR    CODE(1),COUNT(3),BITS(C/8)                C/8+4

       7     NAME      CODE(1),COUNT(1),NAME(C)                  C+2

       8     TEXT      CODE(1),COUNT(3),TEXT(C)                  C+4

       9     LIST      CODE(1),COUNT(3),ITEMS(2),DATA(C-2)       C+4

      10     PROPLIST  CODE(1),COUNT(3),PAIRS(1),DATA(C-1)       C+4

      11     ENDLIST   CODE(1)                                   1

      12     S-TAG     CODE(1),INDEX(2)                          3

      13     S-REF     CODE(1),INDEX(2)                          3

      14     ENCRYPT   CODE(1),COUNT(3),ALG-ID(1),
                                       KEY-ID(2),DATA(C-3)       C+4

      The numbers in parentheses are the number of octets in the field.

    [Page 68]                                                         Postel

    August 1980
                                                   Internet Message Protocol

                                   REFERENCES

    [1]   Cerf, V., "The Catenet Model for Internetworking," Information
          Processing Techniques Office, Defense Advanced Research Projects
          Agency, IEN 48, July 1978.

    [2]   Postel, J.,  "DOD Standard Internet Protocol," USC/Information
          Sciences Institute, IEN 128, NTIS number AD A079730, January 1980.

    [3]   Postel, J.,  "DOD Standard Transmission Control Protocol,"
          USC/Information Sciences Institute, IEN 129, NTIS number AD
          A082609, January 1980.

    [4]   Postel, J., "Assigned Numbers," RFC 762, USC/Information Sciences
          Institute, January 1980.

    [5]   Feinler, E. and J. Postel, eds., "ARPANET Protocol Handbook,"
          NIC 7104, for the Defense Communications Agency by the Network
          Information Center of SRI International, Menlo Park, California,
          Revised January 1978.

    [6]   Neigus, N., "File Transfer Protocol for the ARPA Network,"
          RFC 542, NIC 17759, SRI International, August 1973.

    [7]   Bhushan, A., K. Progran, R. Tomlinson, and J. White,
          "Standardizing Network Mail Headers," RFC 561, NIC 18516,
          September 1973.

    [8]   Myer, T., and D. Henderson, "Message Transmission Protocol,"
          RFC 680, NIC 32116, 30 April 1975.

    [9]   Crocker, D., J. Vittal, K. Progran, and D. Henderson, "Standard
          for the Format of ARPA Network Text Messages," RFC 733, NIC 41952,
          21 November 1977.

    [10]  Barber, D., and J. Laws, "A Basic Mail Scheme for EIN," INWG 192,
          February 1979.

    [11]  Braaten, O., "Introduction to a Mail Protocol," Norwegian
          Computing Center, INWG 180, August 1978.

    [12]  Crocker, D., E. Szurkowski, and D. Farber, "An Internetwork Memo
          Distribution Capability - MMDF," Sixth Data Communications
          Symposium, ACM/IEEE, November 1979.

    Postel                                                         [Page 69]

                                                                 August 1980
    Internet Message Protocol
    References

    [13]  Haverty, J., D. Henderson, and D. Oestreicher, "Proposed
          Specification of an Inter-site Message Protocol," 8 July 1975.

    [14]  Thomas, R., "Providing Mail Services for NSW Users," BBN NSW
          Working Note 24, Bolt Beranek and Newman, October 1978.

    [15]  White, J., "A Proposed Mail Protocol," RFC 524, NIC 17140, SRI
          International, 13 June 1973.

    [16]  White, J., "Description of a Multi-Host Journal," NIC 23144, SRI
          International, 30 May 1974.

    [17]  White, J., "Journal Subscription Service," NIC 23143, SRI
          International, 28 May 1974.

    [18]  Levin, R., and M. Schroeder, "Transport of Electronic Messages
          Through a Network," Teleinformatics 79, Boutmy & Danthine (eds.)
          North Holland Publishing Co., 1979.

    [19]  Earnest, L., and J. McCarthy, "DIALNET: A Computer Communications
          Study," Computer Science Department, Stanford University, August
          1978.

    [20]  Crispin M., "DIALNET: A Telephone Network Data Communications
          Protocol," DECUS Proceedings, Fall 1979.

    [21]  Caulkins, D., "The Personal Computer Network (PCNET) Project: A
          Status Report," Dr. Dobbs Journal of Computer Calisthenics and
          Orthodontia,  v.5, n.6, June 1980.

    [22]  Postel, J., "NSW Transaction Protocol (NSWTP)," USC/Information
          Sciences Institute, IEN 38, May 1978.

    [23]  Haverty, J., "MSDTP -- Message Services Data Transmission
          Protocol," RFC 713, NIC 34739, April 1976.

    [24]  Haverty, J., "Thoughts on Interactions in Distributed Services,"
          RFC 722, NIC 36806, 16 September 1976.

    [25]  Postel, J., "A Structured Format for Transmission of Multi-Media
          Documents," RFC 767, USC/Information Sciences Institute,
          August 1980.

    [26]  ISO-2014, "Writing of calendar dates in all-numeric form,"
          Recommendation 2014, International Organization for
          Standardization, 1975.

    [Page 70]                                                         Postel

    August 1980
                                                   Internet Message Protocol
                                                                  References

    [27]  ISO-3307, "Information Interchange -- Representations of time of
          the day," Recommendation 3307, International Organization for
          Standardization, 1975.

    [28]  ISO-4031, "Information Interchange -- Representation of local time
          differentials," Recommendation 4031, International Organization
          for Standardization, 1978.

    [29]  CCITT-X.121, "International Numbering Plan for Public Data
          Networks," Recommendation X.121, CCITT, Geneva, 1978.

    [30]  Postel, J.,  "NSW Data Representation (NSWB8)," USC/Information
          Sciences Institute, IEN 39, May 1978.

    [31]  Cohen, D., "On Holy Wars and a Plea for Peace," IEN 137,
          USC/Information Sciences Institute, 1 April 1980.

    [32]  Hofstadter, D., "Godel, Escher, Bach: An Eternal Golden Braid,"
          Basic Books, New York, 1979..

    [33]  Harrenstien, K., "Field Addressing," ARPANET Message, SRI
          International, October 1977.

    [34]  Postel, J., "Out-of-Net Host Address for Mail," RFC 754,
          USC/Information Sciences Institute, April 1979.

    [35]  Shoch, J., "On Inter-Network Naming, Addressing, and Routing,"
          IEEE Computer Society, COMPCON, Fall 1978.

    [36]  National Bureau of Standards, "Data Encryption Standard," Federal
          Information Processing Standards Publication 46, January 1977.

    [37]  Diffie, W., and M. Hellman, "New Directions in Cryptology," IEEE
          Transactions on Information Theory, IT-22, 6, November 1976.

    [38]  Rivest, R., A. Shamir, and L. Adleman,  "A Method for Obtaining
          Digital Signatures and Public-Key Cryptosystems"  Communications
          of the ACM, Vol. 21, Number 2, February 1978.

    [39]  Merkle, R., and M. Hellman, "Hiding Information and Signatures in
          Trapdoor Knapsacks," IEEE Transactions of Information Theory,
          IT-24,5, September 1978.

    Postel                                                         [Page 71]