                                                                  March 1979

    IEN: 85
    RFC: 753

                           INTERNET MESSAGE PROTOCOL

                               Jonathan B. Postel

                                   March 1979

                         Information Sciences Institute
                       University of Southern California
                               4676 Admiralty Way
                       Marina del Rey, California  90291

                                 (213) 822-1511

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

    [Page 0]                                                          Postel

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                                                   Internet Message Protocol

                               TABLE OF CONTENTS

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

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

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

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

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

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

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

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

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

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

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

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

    Postel                                                          [Page i]

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    Internet Message Protocol

    [Page ii]                                                         Postel

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                                                   Internet Message Protocol

                                    PREFACE

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

                                                                  Jon Postel

    Postel                                                        [Page iii]

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    Internet Message Protocol

    [Page iv]                                                         Postel

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

                           INTERNET MESSAGE PROTOCOL

                                1.  INTRODUCTION

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

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

    1.1.  Motivation

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

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

    1.2.  Scope

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

    Postel                                                          [Page 1]

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    Internet Message Protocol
    Introduction

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

    1.3.  The Internetwork Environment

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

    1.4.  Operation

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

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

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

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

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

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                                                   Internet Message Protocol
                                                                Introduction

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

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

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

      When local message protocols are used, special conversion programs are
      required to transform local messages to internet format when they are
      going out, and to transform internet messages to local format when
      they come into the local environment.  Such transformations are
      potentially information lossy.  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 discard of 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 defered as long as possible,
      still the possibility remains, that in some cases, it is the only
      reasonable thing to do.

    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 TCP
      [20].  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).

    Postel                                                          [Page 3]

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    Internet Message Protocol
    Introduction

      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

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                                                   Internet Message Protocol

                           2.  FUNCTIONAL DESCRIPTION

    2.1.  Terminology

      The basic unit transferred between networks is called a message.  A
      message is made up of a transaction identifier (a number which
      uniquely identifies the message), a command list (which contains the
      necessary information for delivery), and the document list.  The
      document list consists of a header and a body, which contains the
      actual data of the message.

      For a personal letter the document body corresponds to the contents
      the a letter, the document header corresponds to the the address and
      return address on the envelope.

      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.

      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.

      Please see the Glossary section for a more complete list of
      terminology.

    2.2.  Assumptions

      The following assumptions are made about the internetwork environment:

      It is in general 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.

      We require each MPM to know completely only the addressing format of
      its own network.  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 number.

      Each MPM will have a unique internet address.  This feature will

    Postel                                                          [Page 5]

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    Internet Message Protocol
    Functional Description

      enable every MPM to place a unique "handling-stamp" on a message which
      passes through it en-route 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
              \                                          /
               \                                        /
                \                                      /
             --+----------------------------------------+-- 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 and 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 utilizing different
        communication services.

      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

    [Page 6]                                                          Postel

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                                                   Internet Message Protocol
                                                      Functional Description

        MPMs, though all communication services must meet the following
        service characteristics.

        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) [20].  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).  Data is delivered
             in the order sent with no gaps.

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

          o  A way to signal and be notified "out-of-band" (such as TCP's
             urgent) is available so that some messages can be labeled "more
             important" than others.

          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.  Complete breakdown on communication is reported
             to the user.

      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.  A MPM might exist in two (or more)
      interprocess communication environments, and such an MPM might act to
      relay messages between MPMs in the environments.

    Postel                                                          [Page 7]

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    Functional Description

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

                     Message Service with Internal Relaying

                                   Figure 2.

      The transfer of data between UIPs and MPMs is conceived of 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

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                                                   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 a MPM (or a UIP), a
      message may be represented in any convenient form.  As the following
      figure shows, when a message is ready for transmission, it moves from
      the processing routines to be encoded in the typed data elements and
      then to a data compression routine, and is finally transmitted.  On
      the receiving side, the message is first decompressed then decoded
      from the data element representation to the local representation for
      the processing routines.

    Postel                                                          [Page 9]

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    Functional Description

              +------------------------------------------------+
              !                                                !
              !  processing      DATA         DATA             !
              !  routines   ---> ENCODER ---> COMPRESSOR --->  !
              !                                                !
              +------------------------------------------------+
                                 Send MPM

              +------------------------------------------------+
              !                                                !
              !      DATA              DATA         processing !
              ! ---> DECOMPRESSOR ---> DECODER ---> routines   !
              !                                                !
              +------------------------------------------------+
                                Receive MPM

                                 Detailed View

                                   Figure 4.

    [Page 10]                                                         Postel

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                                                   Internet Message Protocol
                                                      Functional Description

    2.5.  Relation to Other Protocols

      The following diagram illustrates the place of the message protocol in
      the protocol hierarchy:

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

                             Protocol Relationships

                                   Figure 5.

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

    Postel                                                         [Page 11]

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    Internet Message Protocol

    [Page 12]                                                         Postel

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                                                   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 media it
    has to come in some order.  In this attempt, a very brief overview of
    the message structure is given, then a radical switch is made to
    defining the basic building blocks, and finally using the building
    blocks to reach the overall structure again.

    3.1.  Overview of Message Structure

      In general a message is composed of three parts:  the identification,
      the command, and the document.  Each part is in turn composed of
      message objects.

      The identification part is composed of a transaction number assigned
      by the originating MPM, and the internet host number of that MPM.

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

      The document part is composed of a header and a body.  The message
      delivery system does not depend on the contents of the document part,
      but this specification does make some recommendations for the document
      header.

      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.

    3.2.  Data Elements

      The data elements defined here are similar to the data structure and
      encoding used in NSW [18].

      Each of the diagrams which follow represent a sequence of octets.
      Field boundaries are denoted by the "!" character, octet boundaries by
      the "+" character. The diagrams are presented in left to right order.
      Each element begins with a one octet code.

    Postel                                                         [Page 13]

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    Specification

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

                          +------+
        0  No Operation   !  1   !
                          +------+

                          +------+------+------+------+------
        1  Padding        !  0   !     octet count    ! Data ...
                          +------+------+------+------+------

                          +------+------+
        2  Boolean        !  2   ! 1/0  !
                          +------+------+

                          +------+------+------+
        3  Index          !  3   !     Data    !
                          +------+------+------+

                          +------+------+------+------+------+
        4  Integer        !  4   !            Data           !
                          +------+------+------+------+------+

                          +------+------+------+------+------
        5  Bit String     !  5   !      bit count     ! Data ...
                          +------+------+------+------+------

                          +------+------+------+------+------
        6  Text String    !  6   !     octet count    !  Data ...
                          +------+------+------+------+------

                          +------+------+------+------+------+------+-----
        7  List           !  7   !     octet count    !  item count ! Data
                          +------+------+------+------+------+------+-----

                          +------+------+------+------+------
        8  Proplist       !  8   !     octet count    ! Data ...
                          +------+------+------+------+------

    [Page 14]                                                         Postel

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                                                   Internet Message Protocol
                                                               Specification

      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.  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 which has the value
      1 for True and 0 for False.

      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 (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 two octet count field instead of
      the number of octets.

      Element code 6 (TEXT) is used for the representation of text.  Seven
      bit ASCII characters are used, right justified in the octet.  The high
      order bit in the octet is zero.

      Element code 7 (LIST) can be used to create structures composed of
      other elements.  The item-count contains the number of elements which
      follow.  Any element may be used including List itself.  The octet
      count specifies the number of octets in the whole list.  A null or
      empty List, one with no elements, has an item-count of zero (0).

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    Specification

      Element code 8 (PROPLIST) is the Property-List element.  It has the
      following form:

        +------+------+------+------+------+
        !   8  !     octet          ! pair !
        !      !           count    ! count!
        +------+------+------+------+------+
                             +------+------+------+---------+---------+
                             ! name !    value    ! name    ! value   !
                 repeated    ! count!    count    !      ...!      ...!
                             +------+------+------+---------+---------+

      The Property-List structure consists of a set of unordered name/value
      pairs.  The pairs are a one octet name count and a two octet value
      count followed by the name and value strings.  The counts specify the
      length in octets of the name and value strings.  Each string has a
      length in octets which agrees with its respective count.  The count of
      octets until the next pair in the property list is  1 + 2 + name count
      + value count octets.  The entire Property-List is of course equal in
      length to the octet count of the element itself.  Immediately
      following the octet count for the entire element is a one octet pair
      count field which contains the total number of name/value pairs in the
      Proplist.

    3.3.  Message Objects

      In the composition of messages we use a set of objects such as
      address, or date.  These objects are encoded in the basic data
      elements.  The message objects are built of data elements.

      While data elements are typed, message objects are not.  This is
      because messages are structured to the extent that only one kind of
      message object may occur in any position of a message structure.

      The following is a list of some of the objects used in messages.  The
      object descriptions are grouped by the section of the message in which
      they normally occur.

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                                                   Internet Message Protocol
                                                               Specification

      Identification

        Internet Host Number (ihn)

          This identifies a host in the internetwork environment.  When used
          as a part of tid, it identifies the originating host of a message.
          The ihn 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.

          INTEGER

        Transaction Identifier (tid)

          This is the transaction identifier associated with a particular
          command.  It is a list of the transaction number and the internet
          host number of the originating host.

          LIST ( tn , ihn )

        Transaction Number (tn)

          This is a number which is uniquely associated with this
          transaction by  the originating host.  It identifies the
          transaction.  (A transaction is a message and acknowledgment, this
          is discussed in more detail in later sections.)  A tn must be
          unique for the time which the message (a request or reply)
          containing it could be active in the network.

          INDEX

      Command

        Address

          This is very similar to Mailbox in that it also is the "address"
          of a user.  However, Address is intended to contain the minimum
          information necessary for delivery, and no more.

          PROPLIST ( --- )

        Answer

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

          BOOLEAN

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    Specification

        Arguments

          This is the argument to many of the operations.  It consists of a
          List of different data types.  The List will have form and data
          relevant with the particular operation.

          LIST ( --- )

        Command-Type

          Gives the type of a command (e.g., request, reply, alarm).

          INDEX

        Error-List

          The error list contains information concerning an error which has
          occured.  It is a List comprised of the two objects error-class
          and error-string.

          LIST ( error class, error string )

        Error-Class

          A code for the class of the error.

          INDEX

        Error-String

          A text string explaining the error.

          TEXT

        How-Delivered

          A comment on the delivery of a messages, for instance a message
          could be delivered, forwarded, or turned over to general delivery.

          LIST ( TEXT )

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                                                   Internet Message Protocol
                                                               Specification

        Mailbox

          This is the "address" of a user of the internetwork mail system.
          Mailbox contains information such as net, host, location, and
          local user-id 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 could contain an Address (as opposed to Mailbox) which
          the user will use from then on.

            A mailbox is a PROPLIST.  A mailbox might contain the following
            name-value pairs:

              name    element  description
              ----    -------  -----------
              IA      INTEGER  internet address
              NET     TEXT     network name
              HOST    TEXT     host name
              USER    TEXT     user name
              CITY    TEXT     city
              COUNTRY TEXT     country
              STATE   TEXT     state
              ZIP     TEXT     zip code
              PHONE   TEXT     phone number

          PROPLIST ( --- )

        Operation

          This names the operation or procedure to be performed.

          TEXT

        Options

          REGULAR for normal delivery, FORWARD for message forwarding,
          GENDEL for general delivery, or other options which may be defined
          later.

          LIST ( TEXT, ... )

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        Reasons

          These could be mailbox does not exist, mailbox full, etc.

          LIST ( TEXT )

        Stamp

          Each MPM that handles the message must add a unique identifier
          (ihn, see above) to the list.  This will prevent messages from
          being sent back and forth through the internet mail system without
          eventually either being delivered or returned to the sender.

          LIST ( ihn, ihn, ... )

        Trail

          When a message is sent through the internetwork environment, it
          acquires a list of MPMs that have handled the message in "Stamp".
          This list is then carried as "Trail" upon reply or acknowledgment
          of that message. More simply, requests and replies always have a
          "Stamp" and each MPM adds its ihn to this "Stamp."  Replies, in
          addition, have a "Trail" which is the complete "Stamp" of the
          original message.

          LIST ( ihn, ihn, ... )

        Type

          The command type, e.g., request or reply.

          INDEX

      Document

        In this section, we define some objects useful in message document
        headers.  The ones we use are taken from the current ARPANET message
        syntax standard [6,8].

        CC

          When copies of a message are sent to others in addition to the
          addresses in the To object, those to whom the copies are sent will
          have their addresses recorded here.  CC will be a single TEXT
          element.

          TEXT

    [Page 20]                                                         Postel

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        Date

          The date and time are represented according to the International
          Standards Organization (ISO) recommendations [13,14,15].  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 4 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.

          TEXT

        Document-Body

          The document body will contain that portion of the message
          commonly thought of as the text portion.  It will be composed of a
          list of elements.  This will allow transmission of data other than
          pure text if such capabilities are needed.  We can, for instance,
          envision digital voice communication through the transmission of
          BITSTR element, or transmission of graphic data, etc.  Information
          regarding control of such features could be included in the header
          for cooperating sites, or in the body itself but such protocols
          would depend upon agreement among those sites involved.  It is
          expected of course that the majority of messages will contain body
          portions comprised of TEXT elements.

          LIST ( --- )

        Document-Header

          The document header contains the memo header presented to the
          user.  In principle this may be of any style or structure.  In
          this specification it is recommended that a PROPLIST be used and
          that the name-value pairs correspond to the header fields of
          RFC 733 [6].

          PROPLIST ( --- )

    Postel                                                         [Page 21]

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        From

          The From is meant to be the name of the author of a document.  It
          will be one TEXT element.

          TEXT

        Reply-To

          Sometimes it will be desired to direct the replies of a message to
          some address other than the From or the Sender.  In such a case
          the Reply-To object can be used.

          TEXT

        Sender

          The Sender will contain the address of the individual who sent the
          message. In some cases this is NOT the same as the author of the
          message. Under such a condition, the author should be specified in
          the From object.  The Sender is a single TEXT element.

          TEXT

        Subject

          The subject of the message.

          TEXT

        To

          To identifies the addressees of the message.  The To object is one
          TEXT element.

          TEXT

    [Page 22]                                                         Postel

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    3.4.  Command

      This section describes the commands which processes in the internet
      message system can use to communicate.  Several aspects of the command
      structure are based on the NSW Transaction Protocol [19].  The
      commands come in pairs, with each request having a corresponding
      reply.

       A command is a list:

        LIST ( mailbox, stamp, type, operation, arguments, error-list )

      The arguments are described generally here and more specifically, if
      necessary, in the description of each command.

        mailbox:  PROPLIST

          This is the "to" specification of the message.  Mailbox takes the
          form of 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 list
          without extra information.

        stamp:  LIST ( INTEGER, ...  )

          This is a list of the MPMs that have handled the message.  Each
          MPM must add its 32 bit Internet Host Number (ihn) to the LIST.

        type: INDEX

          type=1 a REQUEST operation.

          type=2 a REPLY operation.

          type=3 an ALARM operation. (A high priority message.)

          type=4 a RESPONSE to an alarm operation.

        operation: TEXT

          Operation is the name of the operation or procedure to be
          performed.  This string must be interpreted in an upper/lower case
          independent manner.

    Postel                                                         [Page 23]

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        arguments: LIST

          This is a list of arguments to the above operation.

        error-list:  LIST

          If message is type 1 or 3 (a request or an alarm):

            LIST ( )  (a zero length list)

          If message is a type 2 or 4 (a response or response to alarm)

            LIST ( error-class, error-string ) indicates what,if any, error
            occured

          error-class: INDEX

            =0: indicates success, no error
            =1: partial results returned.
              This error class is used when several steps are performed by
              one operation and some of them fail.
            =2: failure, resources unavailable.
            =3: failure, user error.
            =4: failure, MPM error. Recoverable.
            =5: failure, MPM error. Fatal.
            =6: User abort requested

          error-string: TEXT

            This is a human readable character string describing the error.

        Possible errors:

                  error-string                  error-class

          No errors                                  0
          Command not implemented                    2
          Syntax error, command unrecognized         3
          Syntax error, in arguments                 3
          Server error, try again later              4
          No service available                       5
          User requested abort                       6

    [Page 24]                                                         Postel

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      command:  DELIVER

        type:  1

        function:  Sends message to a mailbox

        reply:  The reply is ACKNOWLEDGE

        arguments:  LIST ( options )

          options:  one or more of the following

            "REGULAR"  regular delivery

            "FORWARD"  message forwarding

            "GENDEL"   general delivery

            other options which may be defined later

        argument structure:

          LIST ( LIST ( TEXT, ... ))

    Postel                                                         [Page 25]

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      command:  ACKNOWLEDGE

        type:  2

        function:  reply to DELIVER

        arguments: LIST ( tid, trail, answer, reasons, how-delivered )

          tid:  tid of the originating message

          trail:   the stamp from the deliver command

          answer:  yes if delivered successfully,
                   no if error in delivery.

          reasons:  if the answer is yes, the reason is "ok", if the answer
          is no the reason could be one of "no such user", "no such host",
          "no such network", "address ambiguous", or a similar response

          how-delivered:  one or more of the following:

            "FORWARD"  message was accepted for forwarding

            "GENDEL"   message was accepted for general delivery

            "ACCEPT"   message was accepted for normal delivery

            other types of delivery may be defined later

        argument structure:

          LIST ( LIST ( INDEX, INTEGER ),
                 LIST ( INTEGER, ...  ),
                 BOOLEAN,
                 LIST ( TEXT ),
                 LIST ( TEXT ))

    [Page 26]                                                         Postel

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      command:  PROBE

        type:  1

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

        reply:  the reply is RESPONSE

        arguments:  LIST ( --none-- )

        argument structure:

          LIST ( )

    Postel                                                         [Page 27]

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      command:  RESPONSE

        type:  2

        function:  reply to PROBE

        arguments:  LIST ( tid, trail, answer, address OR reasons )

          tid:  the tid which came from the originating PROBE

          trail:  the stamp which came from the originating PROBE

          answer:  Yes if mailbox found, or no for invalid mailbox

          if answer is yes the fourth argument is address
          if answer is no it is reasons

          address:  a specific address in the network

          reasons:  a reason why mailbox is invalid

            Possible reasons include:

              "Mailbox doesn't exist"

              "Mailbox full"

              "Mailbox has moved, try this new location", address

                address is a new address to try

        argument structure:

          if answer is yes

            LIST ( LIST ( INDEX, INTEGER ),
                   LIST ( INTEGER, ... ),
                   BOOLEAN,
                   PROPLIST )

          if answer is no

            LIST ( LIST ( INDEX, INTEGER ),
                   LIST ( INTEGER, ... ),
                   BOOLEAN,
                   LIST ( TEXT ))

    [Page 28]                                                         Postel

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        command:  CANCEL

          type:  3

          function:  abort request for specified transaction

          reply:  The reply is CANCELED

          arguments:  LIST ( tid )

            tid of transaction to be cancelled

          argument structure:

            LIST ( LIST ( INDEX, INTEGER ))

    Postel                                                         [Page 29]

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        command:  CANCELED

          type:  4

          function:  reply to CANCEL

          arguments:  LIST ( tid, trail, answer )

            tid:  tid of transaction to be cancelled

            trail:  the stamp of the CANCEL command

            answer:  yes if the command was canceled, no if not.

          argument structure:

            LIST ( LIST ( INDEX, INTEGER ),
                   LIST ( INTEGER, ... ),
                   BOOLEAN )

    [Page 30]                                                         Postel

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      To summarize again, a command consists of a LIST of the following
      objects:

        name        element
        ----        -------
        mailbox     PROPLIST
        stamp       LIST ( INTEGER, ... )
        type        INDEX
        operation   TEXT
        arguments   LIST ( --- )
        error       LIST ( INDEX, TEXT )

    3.5.  Document

      The actual document follows the command list.  It contains a header
      which usually contains such information as From, To, Date, CC, etc.;
      and the actual body of the message.  The message delivery system does
      not depend on the document.  The following section should be taken as
      a recommendation for common practice, not as a requirement.

      Document Header

        For the same reason that it is impossible to for see the many forms
        that intranet addresses will take, standardizing of document headers
        would also be a mistake. The approach we suggest is to lay the
        groundwork for a set of basic document header functions and provide
        for enough extensibility to allow nets to add whatever header
        features they desire.  Features added in this fashion, however, may
        not be understood by other networks.  It is suggested that subset
        defined here be implemented by all networks.

        This subset is taken from the current ARPANET standard for message
        headers in the text oriented computer message system [6,8].

        The document header will precede the document body portion of the
        message and will consist of a proplist data element.  The document
        header is meant to be used by individual networks to tailor the
        header to suit their individual needs.  As an example, consider the
        ARPA network.  Typically, the receiver's name is taken to be his
        network address.  It often prints in the document header in just
        that form: Frank@SITEX.  Such a salutation is unacceptable in some
        more formal modes of communication.  Some network might choose to
        place into header proplist the name-value pair ("SALUTATION:", "Mr.
        Frank Hacker").  Upon receipt of the message, the document handling
        program would then be able to scan the header proplist looking for
        such a pair and so be able to correctly address the recipient by
        name instead of by network address.  However, other networks or

    Postel                                                         [Page 31]

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        sites within the network may not understand such specific
        information.  Under such a condition it should be ignored.

        The minimum header is a PROPLIST of the following name-value pairs:

          Name     Value
          ----     -----
          DATE     TEXT
          FROM     TEXT

        A normal header is a PROPLIST containing the following name-value
        pairs:

          Name     Value
          ----     -----
          DATE     TEXT
          SENDER   TEXT
          FROM     TEXT
          TO       TEXT
          CC       TEXT
          SUBJECT  TEXT

      Document Body

        The Body of the message is just a sequence of data elements which
        contains the actual document.  Much of the time this will be a
        single TEXT element, but for some applications other data elements
        may be utilized.

        LIST ( --- )

    [Page 32]                                                         Postel

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    3.6.  Message Structure

      An internet message is composed of three parts.  The first is the tid
      which identifies the transaction; the second is the Command List; and
      the third part is the Document List, which is itself comprised of a
      Document-Header and a Document-Body.

      When shipped between two MPMs, a message will take the form of a LIST:

        Message is:

          LIST ( tid, Command-List, Document-List )

        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.

          Thus, a message-bag is:

            LIST ( Message1, Message2, ... )

      Message Sharing

        When messages are batched for delivery, 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
        parts of the message, much repeated data would be sent if a copy of
        the Mail for each recipient were to be shipped in the message-bag.

        To avoid this redundancy, messages are assembled in the message-bag
        so that actual data appears first and references to it appear later
        in the message-bag.  Since each message has a unique tid, the
        references will indicate the tid of the actual data.  In this sense,
        all references to copied data may be thought of as pointing earlier
        in the message-bag.  The data to be retrieved can be thought of as
        indexed by tid.  Note that the semantics require such references to
        point to data already seen.

        When a portion is Shared, that portion is determined by its position
        within a message, i.e., if the Command list was to be Shared, then
        its position within a Message would contain the tid of the message
        already seen whose Command list was identical to it.  The same is
        true of the Document Header and the Document Body.  Only a complete
        Command, Header, or Body may be Shared, never a partial one.

    Postel                                                         [Page 33]

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        If an encryption scheme is used, that portion of the message which
        is encrypted can not be shared.  This is due to the fact that
        encrypting keys will be specific between two individuals.

      Internal Message Organization

        The tid

          This is the transaction identifier.  It is assigned by the
          originating MPM.

        The Command List

          The command-list is a LIST which contains two elements, content
          and command.

          Content is one item of element type INDEX.  If content=0, the item
          is not shared and the next element of the LIST is the command.  If
          content=1 the item is shared.  In this case, the second element
          will contain the tid of the command to share from.  The tid must
          be of a prior message in the current message-bag.  Other values of
          content may be defined later for different data structures.

          Thus, command-list is:

            LIST ( content, tid )       if content=1

          Or,

            LIST ( content, command )    if content=0

          content is:

            INDEX     which is 0 if there is no sharing
                        and is 1 if sharing occurs

          tid is:

            the tid of the message to be shared from

          command is:

            LIST ( mailbox, stamp, type, operation, arguments, error-list )

        The document-list

          The document portion of an internet message is optional and when
          present is comprised of a LIST containing two elements:

    [Page 34]                                                         Postel

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            document-list is:

              LIST ( header-list, body-list )

          While either the header-list or the body-list may be shared, both
          elements must appear in the m.

        The document-header

          The header-list will be a List which will always contain two
          elements.  The first element will be content to indicate whether
          or not the header is to be shared.  The second element will either
          be the tid of the header to be copied (if content=1) or it will be
          the document-header (which is a PROPLIST) containing the actual
          header information (if content=0). The tid must point to a
          document-header already seen in the message-bag.

          The header-list is either:

            LIST ( content, tid )                if content=1

          Or,

            LIST ( content, document-header )     if content=0

          document-header is:

            PROPLIST which contains header information

        The document-body

          The body-list will be a LIST of two elements.  The first element
          will again be content, indicating whether or not the body is to be
          shared.  If it is shared, the second element will be tid
          indicating which body to copy.  This tid must be of a message
          already seen in the message-bag.  If content indicates no sharing,
          then the second item is a document-body.

          body-list is:

            LIST ( content, tid )           if content=1

          Or,

            LIST ( content, document-body )  if content=0

    Postel                                                         [Page 35]

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          document-body is:

            LIST ( items comprising the body ... )

      Message Fields

        message := ( tid, command-list, document-list )

        tid := ( tn, ihn )

        command-list := ( content, command )

        command := ( mailbox, stamp, type, operation,
                     arguments, error-list )

        document-list := ( header-list, body-list )

        header-list := ( content, document-header )

        body-list := ( content, document-body )

    3.7.  MPM Organization

      Introduction

        The heart of the internet message system is the MPM which is
        responsible for routing and delivering message between the networks.
        Each network must have at least one MPM.  These MPMs are connected
        together, and internet mail is always transferred along channels
        between them.  The system interfaces with the already existent local
        message system.

        Since the local network 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.

      The MPM

        Messages in the internet mail system are shipped in "bags," each bag
        containing one or more messages.  Each bag is addressed to a
        specific MPM and contains messages for the hosts on that MPM's
        network.

        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.

    [Page 36]                                                         Postel

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        Loosely, each MPM can be separated into five 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:

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

          3--Local Net Delivery

            Delivers local net and local host messages, may call on
            conversion program.

          4--Foreign Net Router

            Creation of new Message-Bags for forwarding to other MPMs,
            determines route.

          5--Foreign Net Shipper

            Activates foreign shipping channels and ships Message-Bag to
            foreign MPMs. Performs data compression while shipping bags.

        All of these components can be thought of as independent.  Of the
        five, the Acceptor, the Local-Net Delivery, and the Message-Bag
        Processor are fully self-contained and communicate with each other
        only through a queue, the Bag-Input Queue.  The function of the
        Acceptor is to await incoming Message-Bags and to insert them into
        the Bag-Input Queue.

        That queue is the input to the Message-Bag Processor component 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 a MPM decides to forward a message to another MPM, it must add
        its own identification (i.e., its ihn) to the stamp field of the
        command.  The stamp then becomes a record of the route the message

    Postel                                                         [Page 37]

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        has taken.  An MPM should examine the stamp field to see if the
        message is in a routing loop.  Some commands require the return of
        the stamp as a trail in the matching reply command.

        All of these queues have as elements complete Message-Bags (some of
        which may have been portions of the original Bag).

        The Local-Host and Local-Net queues serve as input to the Local-Net
        Delivery process.  This component is responsible for delivering
        messages to its local host and other hosts on its local net to which
        it is connected.  It must be capable of handling whatever error
        conditions the local net might return, including the ability to
        retransmit.  It may call on 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 one of the
        MPMs to which it is connected should receive the Bag.

        It then places the Bag along with the routing information into the
        Shippable Mail Queue.  The Foreign Net Shipper retrieves it from
        that queue and transmits it across a channel to the intended foreign
        MPM.

        The Foreign Net Router should be capable of receiving external input
        to its routing information table.  This may come from the Foreign
        Net Shipper 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 topological
        information to determine where to forward any incoming Message-Bag.
        Decisions concerning the return of undeliverable Bags are made by
        the Router.

        It should be stressed here that message delivery should be reliable.
        In the event that delivery is impossible, the message should be
        returned to the sender along with information regarding the reason
        for not delivering it.

      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.

    [Page 38]                                                         Postel

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    3.8.  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 weakly 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).

      Communication Interface

        It is assumed here that the MPM use an underlying communication
        system, and TCP [20] has been taken as the model.  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).

    Postel                                                         [Page 39]

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    [Page 40]                                                         Postel

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                                                   Internet Message Protocol

                            4.  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@ISIB>
        Subject: Meeting Thursday
        To: Dave Crocker <DCrocker@Rand-Unix>
        CC: Mamie

        Dave:

        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.

        1.  message

        2.  ( tid, command-list, document-list )

        3.  ( ( tn, ihn ),
            ( content, command ),
            ( header-list, body-list ) )

        4.  ( ( tn, ihn ),
              ( content,
                ( mailbox, stamp, type, operation,
                  arguments, error-list ) ),
              ( ( content, document-header ),
                ( content, document-body ) ) )

        5.  ( ( 37, 167772404 ),
            ( 0, (
                   ( IA: 167772359, NET: arpa, HOST: rand-unix,
                     USER: DCrocker ),
                   ( 167772404 ),
                   1
                   DELIVER
                   ( ( REGULAR ) ),
                   ( ) ) ),
            ( ( 0, (
                     Date: 1979-03-29-11:46-08:00
                     From: Jon Postel <Postel@ISIB>
                     Subject: Meeting Thursday

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    Examples & Scenarios

                     To: Dave Crocker <DCrocker@Rand-Unix>
                     CC: Mamie ) ),
              ( 0, ( Dave:

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

                     --jon. ) ) ) )

        6.  LIST( LIST( INDEX=37, INTEGER=167772404 ),
                  LIST( INDEX=0,
        command         LIST( PROPLIST( IA: 167772359,
                                        NET: arpa,
        mailbox                         HOST: rand-unix,
                                        USER: DCrocker ),
        stamp                 LIST( INTEGER=167772404 ),
        type                  INDEX=1
        operation             TEXT="DELIVER"
        arguments             LIST( LIST( TEXT="REGULAR" )),
        error-list            LIST( ) ) ),
                  LIST( LIST( INDEX=0,
        document-header       PROPLIST(
                                DATE: 1979-03-29-11:46-08:00
                                FROM: Jon Postel <Postel@ISIB>
                                SUBJECT: Meeting Thursday
                                TO: Dave Crocker <DCrocker@Rand-Unix>
                                CC: Mamie ) ),
                        LIST( INDEX=0,
        document-body         LIST( TEXT=
                                "Dave:

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

                                --jon." ) ) ) )

    [Page 42]                                                         Postel

    March 1979
                                                   Internet Message Protocol
                                                        Examples & Scenarios

    Example 2:  Delivery and Acknowledgment

      The following is 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.

      +-----------------------------------------------------------------+
      !                          1         2                            !
      ! sending --> originating --> relay --> destination --> receiving !
      !   user          MPM          MPM          MPM            user   !
      !                                                                 !
      !                          4         3                            !
      !             originating <-- relay <-- destination               !
      !                 MPM          MPM          MPM                   !
      +-----------------------------------------------------------------+

                               Transmission Path

                                   Figure 6.

    Postel                                                         [Page 43]

                                                                  March 1979
    Internet Message Protocol
    Examples & Scenarios

      1.  Between the originating MPM and the relay MPM.

            LIST( LIST( INDEX=37, INTEGER=167772404 ),
                  LIST( INDEX=0,
        command         LIST( PROPLIST( IA: 167772359,
                                        NET: arpa,
        mailbox                         HOST: rand-unix,
                                        USER: DCrocker ),
        stamp                 LIST( INTEGER=167772404 ),
        type                  INDEX=1
        operation             TEXT="DELIVER"
        arguments             LIST( LIST( TEXT="REGULAR" )),
        error-list            LIST( ) ) ),
                  LIST( LIST( INDEX=0,
        document-header       PROPLIST(
                                DATE: 1979-03-29-11:46-08:00
                                FROM: Jon Postel <Postel@ISIB>
                                SUBJECT: Meeting Thursday
                                TO: Dave Crocker <DCrocker@Rand-Unix>
                                CC: Mamie ) ),
                        LIST( INDEX=0,
        document-body         LIST( TEXT=
                                "Dave:

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

                                --jon." ) ) ) )

          The originating MPM sends the message of example 1 to a relay MPM.

    [Page 44]                                                         Postel

    March 1979
                                                   Internet Message Protocol
                                                        Examples & Scenarios

      2.  Between the relay MPM and the destination MPM.

            LIST( LIST( INDEX=37, INTEGER=167772404 ),
                  LIST( INDEX=0,
        command         LIST( PROPLIST( IA: 167772359,
                                        NET: arpa,
        mailbox                         HOST: rand-unix,
                                        USER: DCrocker ),
        stamp                 LIST( INTEGER=167772404,
                                    INTEGER=167772246 ),
        type                  INDEX=1
        operation             TEXT="DELIVER"
        arguments             LIST( LIST( TEXT="REGULAR" )),
        error-list            LIST( ) ) ),
                  LIST( LIST( INDEX=0,
        document-header       PROPLIST(
                                DATE: 1979-03-29-11:46-08:00
                                FROM: Jon Postel <Postel@ISIB>
                                SUBJECT: Meeting Thursday
                                TO: Dave Crocker <DCrocker@Rand-Unix>
                                CC: Mamie ) ),
                        LIST( INDEX=0,
        document-body         LIST( TEXT=
                                "Dave:

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

                                --jon." ) ) ) )

          The relay MPM adds its ihn to the stamp, but otherwise the message
          is unchanged.

    Postel                                                         [Page 45]

                                                                  March 1979
    Internet Message Protocol
    Examples & Scenarios

      3.  Between the destination MPM and the relay MPM.

            LIST( LIST( INDEX=1993, INTEGER=167772359 ),
                  LIST( INDEX=0,
        command         LIST( PROPLIST( IA: 167772404,
        mailbox                         USER: *MPM* ),
        stamp                 LIST( INTEGER=167772359 ),
        type                  INDEX=2
        operation             TEXT="ACKNOWLEDGE"
        arguments             LIST( LIST( INDEX=37,
         tid                              INTEGER=167772404 ),
                                    LIST( INTEGER=167772404,
         trail                            INTEGER=167772246,
                                          INTEGER=167772359 ),
         answer                     BOOLEAN=TRUE,
         reason                     LIST( TEXT="OK" ),
         how-delivered              LIST( TEXT="ACCEPT" ) ),
        error-list            LIST( INDEX=0,
                                    TEXT="No Errors") ),
        document  LIST( ) )

          The destination MPM delivers the message to the user's UIP, and
          composes an acknowledgment.  The acknowledgment is addressed to
          the originating MPM.  Note that the trail is the stamp of the
          incoming message plus the ihn of the destination MPM.

    [Page 46]                                                         Postel

    March 1979
                                                   Internet Message Protocol
                                                        Examples & Scenarios

      4.  Between the relay MPM and the originating MPM.

            LIST( LIST( INDEX=1993, INTEGER=167772359 ),
                  LIST( INDEX=0,
        command         LIST( PROPLIST( IA: 167772404,
        mailbox                         USER: *MPM* ),
        stamp                 LIST( INTEGER=167772359
                                    INTEGER=167772246),
        type                  INDEX=2
        operation             TEXT="ACKNOWLEDGE"
        arguments             LIST( LIST( INDEX=37,
         tid                              INTEGER=167772404 ),
                                    LIST( INTEGER=167772404,
         trail                            INTEGER=167772246,
                                          INTEGER=167772359 ),
         answer                     BOOLEAN=TRUE,
         reason                     LIST( TEXT="OK" ),
         how-delivered              LIST( TEXT="ACCEPT" ) ),
        error-list            LIST( INDEX=0,
                                    TEXT="No Errors") ),
        document  LIST( ) )

          The relay MPM adds its ihn to the stamp and forwards the
          acknowledgment.

    Postel                                                         [Page 47]

                                                                  March 1979
    Internet Message Protocol

    [Page 48]                                                         Postel

    March 1979
                                                   Internet Message Protocol

                                    GLOSSARY

    1822
              BBN Report 1822, "The Specification of the Interconnection of
              a Host and an IMP".  The specification of interface between a
              host and the ARPANET.

    Command List
              The part of a message used by the MPMs to determine the
              processing action to be taken.

    datagram
              A logical unit of data, in particular an internet datagram is
              the unit of data transfered between the internet module and a
              higher level module.

    Destination
              The destination address, an internet header datagram protocol
              field.

    Document List
              The part of the message created by or delivered to a user.

    header
              Control information at the beginning of a message, segment,
              datagram, packet or block of data.

    IMP
              The Interface Message Processor, the packet switch of the
              ARPANET.

    Internet Address
              A four octet (32 bit) source or destination address consisting
              of a Network field and a Local Address field.

    internet datagram
              The unit of data exchanged between a pair of internet modules
              (includes the internet header).

    Local Address
              The address of a host within a network.  The actual mapping of
              an internet local address on to the host addresses in a
              network is quite general, allowing for many to one mappings.

    Postel                                                         [Page 49]

                                                                  March 1979
    Internet Message Protocol
    Glossary

    message
              The unit of information transmitted between users of message
              systems.  As transmitted between MPMs a message consists of a
              Transaction Identifier, a Command List, and a Document List.

    module
              An implementation, usually in software, of a protocol or other
              procedure.

    MPM
              A Message Processing Module, the process which implements this
              internet message protocol.

    octet
              An eight bit byte.

    Rest
              The 3 octet (24 bit) local address portion of an Internet
              Address.

    RTP
              Real Time Protocol:  A host-to-host protocol for communication
              of time critical information.

    Source
              The source address, an internet header field.

    TCP
              Transmission Control Protocol:  A host-to-host protocol for
              reliable communication in internetwork environments.

    Transaction Identifier
              The unique identifier of a message.

    Type of Service
              An internet datagram protocol header field which indicates the
              type (or quality) of service for this internet packet.

    UIP
              A User Interface Program, a program which presents message
              data to a user and accepts message data from a user.  A
              program which interacts with the user in the composition and
              examination of messages.

    XNET
              A cross-net debugging protocol.

    [Page 50]                                                         Postel

    March 1979
                                                   Internet Message Protocol

                                   REFERENCES

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

    [2]   Bhushan, A., K. Progran, R. Tomlinson, and J. White,
          "Standardizing Network Mail Headers," RFC 561, NIC 18516, 5
          September 1973.

    [3]   Bolt Beranek and Newman, "Specification for the Interconnection of
          a Host and an IMP," BBN Technical Report 1822, May 1978 (Revised).

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

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

    [6]   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.

    [7]   Crocker, D., E. Szurkowski, and D. Farber, "Components of a
          Channel-independent Memo Transmission System," Department of
          Electrical Engineering, University of Delaware,, February 1979.

    [8]   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.

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

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

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

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

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

    Postel                                                         [Page 51]

                                                                  March 1979
    Internet Message Protocol
    References

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

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

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

    [17]  Postel, J.  "Internetwork Datagram Protocol, Version 4," USC
          Information Sciences Institute, IEN 80, February 1979.

    [18]  Postel, J.  "NSW Data Representation (NSWB8)," IEN 39, May 1978.

    [19]  Postel, J.  "NSW Transaction Protocol (NSWTP)," IEN 38, May 1978.

    [20]  Postel, J.  "Transmission Control Protocol, TCP, Version 4," USC
          Information Sciences Institute, IEN 81, February 1979.

    [21]  Postel, J., "Assigned Numbers," RFC 750, NIC 45500,
          26 September 1978.

    [22]  Postel, J., "Message System Transition Plan," JBP 64,
          USC-Information Sciences Institute, February 1979.

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

    [24]  Shoch, J., "A Note On Inter-Network Naming, Addressing, and
          Routing," Xerox Palo Alto Research Center, IEN 19, January 1978.

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

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

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

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

    [Page 52]                                                         Postel

    March 1979
                                                   Internet Message Protocol

                                   APPENDICES

    A.  Encryption

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

                          +------+------+------+------+-------
        9  Encrypt        !  9   !     octet count    ! Data ...
                          +------+------+------+------+--------

      Element code 9 (ENCRYPT) is Encrypt.  The format is the one octet type
      code, the three octet type count, and count octets of data.  Use of
      this element indicates that the data it contains is encrypted. The
      encryption scheme is yet to be decided but will probably be the Public
      Key Encryption technique [23] due to the capacity for coded
      signatures.

      To process this, the user is asked for the appropriate key the first
      time an encryption block is seen for a particular message.  The
      encrypted data is then decrypted.  The data thus revealed will be in
      the form of complete data type 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.

    Postel                                                         [Page 53]

                                                                  March 1979
    Internet Message Protocol
    Appendices

    B.  Data Compression

      When message-bags are shipped between MPMs the data should be
      compressed according to the following scheme:

        shipping-unit := compression-type message-bag

        compression-type := A one octet compression type indicator.

          compression-type value   description
          ----------------------   -----------
                     0             no compression used
                     1             basic compression

        basic compression

          This basic compression procedure is the same as that defined for
          use with the ARPANET FTP [8].  Three types of compression-units
          may be formed, sequence-units, replication-units, and
          filler-units.  The data is formed into a series of
          compression-units independent of the structure or object and
          element boundaries.

          sequence-unit

            A sequence-unit is a one octet flag and count followed by that
            many data octets.

              +-+-------+--------+--------+----
              !0!   n   !     n data octets ...
              +-+-------+--------+--------+----

            The flag and count octet has its high order bit zero and the
            remaining bits indicate the count (in the range 0 to 127) of
            following data octets.

          replication-unit

            A replication-unit is a one octet flag and count followed by one
            data octet, which is to be replicated count times.

              +--+------+--------+
              !10!   n  !   data !
              +--+------+--------+

            The flag and count octet has its high order two bits equal
            one-zero and the remaining six bits indicate the count (in the
            range 0 to 63) of number of time to replicate the data octet.

    [Page 54]                                                         Postel

    March 1979
                                                   Internet Message Protocol
                                                                  Appendices

          filler-unit

            A filler-unit is a one octet flag and count, indicating that a
            filler octet is to be inserted count times.

              +--+------+
              !11!   n  !
              +--+------+

            The flag and count octet has its high order two bits equal
            one-one and the remaining six bits indicate the count (in the
            range 0 to 63) of number of time to insert the filler octet.

            The filler octet is zero, the octet with all bits zero.

    Postel                                                         [Page 55]

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    Internet Message Protocol

    [Page 56]                                                         Postel