    Network Working Group                                         M. Banan
    Request for Comments: 2524                   Neda Communications, Inc.
    Category: Informational                                  February 1999

                                     Neda's
                 Efficient Mail Submission and Delivery (EMSD)
                       Protocol Specification Version 1.3

    Status of this Memo

       This memo provides information for the Internet community.  It does
       not specify an Internet standard of any kind.  Distribution of this
       memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (1999).  All Rights Reserved.

    IESG Note

       The protocol specified in this document may be satisfactory for
       limited use in private wireless IP networks.  However, it is
       unsuitable for general-purpose message transfer or for transfer of
       messages over the public Internet, because of limitations that
       include the following:

       - Lack of congestion control

          EMSD is layered on ESRO [RFC 2188], which does not provide
          congestion control.  This makes EMSD completely unsuitable for
          end-to-end use across the public Internet.  EMSD should be
          considered for use in a wireless network only if all EMSD email
          exchanged between the wireless network and the public Internet
          will transit an EMSD<->SMTP gateway between the two regions.

       - Inadequate security

          The document specifies only clear-text passwords for
          authentication.  EMSD should be used across a wireless network
          only if sufficiently strong encryption is in use to protect the
          clear-text password.

       - Lack of character set internationalization

          EMSD has no provision for representation of characters outside of
          the ASCII repertoire or for language tags.

    Banan                        Informational                      [Page 1]

    RFC 2524                          EMSD                     February 1999

       - Poorly defined gatewaying to and from Internet Mail

          Because Internet Mail and EMSD have somewhat different and
          conflicting service models and different data models, mapping
          between them may provide good service only in limited cases, and
          this may cause operational problems.

       The IESG therefore recommends that EMSD deployment be limited to
       narrow circumstances, i.e., only to communicate with devices that
       have inherent limitations on the length and format of a message (no
       more than a few hundred bytes of ASCII text), using either:

       a. wireless links with adequate link-layer encryption and gatewayed
          to the public Internet, or

       b. a private IP network that is either very over-provisioned or has
          some means of congestion control.

       In the near future, the IESG may charter a working group to define an
       Internet standards-track protocol for efficient transmission of
       electronic mail messages, which will be highly compatible with
       existing Internet mail protocols, and which wil be suitable for
       operation over the global Internet, including both wireless and wired
       links.

    ABSTRACT

       This document specifies the protocol and format encodings for
       Efficient Mail Submission and Delivery (EMSD). EMSD is a messaging
       protocol that is highly optimized for submission and delivery of
       short Internet mail messages.  EMSD is designed to be a companion to
       existing Internet mail protocols.

       This specification narrowly focuses on submission and delivery of
       short mail messages with a clear emphasis on efficiency.  EMSD is
       designed specifically with wireless network (e.g., CDPD, Wireless-IP,
       Mobile-IP) usage in mind.  EMSD is designed to be a natural
       enhancement to the mainstream of Internet mail protocols when
       efficiency in mail submission and mail delivery are important.  As
       such, EMSD is anticipated to become an initial basis for convergence
       of Internet Mail and IP-based Two-Way Paging.

       The reliability requirement for message submission and message
       delivery in EMSD are the same as existing email protocols.  EMSD
       protocol accomplishes reliable connectionless mail submission and
       delivery services on top of Efficient Short Remote Operations (ESRO)
       protocols as specified in RFC-2188 [1].

    Banan                        Informational                      [Page 2]

    RFC 2524                          EMSD                     February 1999

       Most existing Internet mail protocols are not efficient.  Most
       existing Internet mail protocols are designed with simplicity and
       continuity with SMTP traditions as two primary requirements.  EMSD is
       designed with efficiency as a primary requirement.

       The early use of EMSD in the wireless environment is manifested as
       IP-based Two-Way Paging services.  The efficiency of this protocol
       also presents significant benefits for large centrally operated
       Internet mail service providers.

    Table of Contents

       1  PRELIMINARIES                                                 4
          1.1 Internet Mail Submission and Delivery     .   .   .   .   4
          1.2 Relationship Of EMSD To Other Mail Protocols  .   .   .   5
          1.3 EMSD Requirements and Goals   .   .   .   .   .   .   .   7
          1.4 Anticipated Uses Of EMSD  .   .   .   .   .   .   .   .   8
          1.5 Definitions of Terms Used in this Specification   .   .   9
          1.6 Conventions Used In This Specification    .   .   .   .   9
          1.7 About This Specification  .   .   .   .   .   .   .   .  10
       2  EFFICIENT MAIL SUBMISSION AND DELIVERY OVERVIEW              10
       3  EFFICIENT MAIL SUBMISSION AND DELIVERY PROTOCOL              11
          3.1 Use Of Lower Layers   .   .   .   .   .   .   .   .   .  13
              3.1.1 Use of ESROS    .   .   .   .   .   .   .   .   .  13
              3.1.2 Use Of UDP  .   .   .   .   .   .   .   .   .   .  13
              3.1.3 Encoding Rules  .   .   .   .   .   .   .   .   .  13
              3.1.4 Presentation Context    .   .   .   .   .   .   .  14
          3.2 EMSD-UA Invoked Operations    .   .   .   .   .   .   .  14
              3.2.1 submit  .   .   .   .   .   .   .   .   .   .   .  14
              3.2.2 deliveryControl     .   .   .   .   .   .   .   .  17
              3.2.3 deliveryVerify  .   .   .   .   .   .   .   .   .  21
          3.3 EMSD-SA Invoked Operations    .   .   .   .   .   .   .  23
              3.3.1 deliver     .   .   .   .   .   .   .   .   .   .  23
              3.3.2 submissionControl   .   .   .   .   .   .   .   .  25
              3.3.3 submissionVerify    .   .   .   .   .   .   .   .  28
          3.4 EMSD Common Information Objects   .   .   .   .   .   .  30
              3.4.1 SecurityElements    .   .   .   .   .   .   .   .  30
              3.4.2 Message Segmentation and Reassembly     .   .   .  30
              3.4.3 Common Errors   .   .   .   .   .   .   .   .   .  33
              3.4.4 ContentType     .   .   .   .   .   .   .   .   .  35
              3.4.5 EMSDMessageId   .   .   .   .   .   .   .   .   .  35
              3.4.6 EMSDORAddress   .   .   .   .   .   .   .   .   .  36
              3.4.7 EMSDAddress     .   .   .   .   .   .   .   .   .  36
              3.4.8 DateTime    .   .   .   .   .   .   .   .   .   .  36
              3.4.9 AsciiPrintableString    .   .   .   .   .   .   .  37
              3.4.10 ProtocolVersionNumber  .   .   .   .   .   .   .  37
          3.5 Submission and Delivery Procedures    .   .   .   .   .  38
       4  DUPLICATE OPERATION DETECTION SUPPORT                        40

    Banan                        Informational                      [Page 3]

    RFC 2524                          EMSD                     February 1999

          4.1 Duplicate Operation Detection Support Overview    .   .  40
              4.1.1 Operation Value     .   .   .   .   .   .   .   .  40
              4.1.2 Operation Instance Identifier   .   .   .   .   .  41
       5  EMSD PROCEDURE FOR OPERATIONS                                42
          5.1 MTS Behavior  .   .   .   .   .   .   .   .   .   .   .  43
              5.1.1 MTS Performer   .   .   .   .   .   .   .   .   .  43
              5.1.2 Message-submission  .   .   .   .   .   .   .   .  44
              5.1.3 Delivery-control    .   .   .   .   .   .   .   .  46
              5.1.4 Delivery-verify     .   .   .   .   .   .   .   .  46
              5.1.5 MTS Invoker     .   .   .   .   .   .   .   .   .  46
          5.2 UA Behavior   .   .   .   .   .   .   .   .   .   .   .  49
              5.2.1 UA Performer    .   .   .   .   .   .   .   .   .  49
              5.2.2 UA Invoker  .   .   .   .   .   .   .   .   .   .  52
       6  EMSD FORMAT STANDARDS                                        54
          6.1 Format Standard Overview  .   .   .   .   .   .   .   .  54
          6.2 Interpersonal Messages    .   .   .   .   .   .   .   .  54
              6.2.1 Heading fields  .   .   .   .   .   .   .   .   .  55
              6.2.2 Body part types     .   .   .   .   .   .   .   .  61
       7  ACKNOWLEDGMENTS                                              62
       8  SECURITY CONSIDERATIONS                                      62
       9  AUTHOR'S ADDRESS                                             62
       A  EMSD-P ASN.1 MODULE                                          63
       B  EMSD-IPM ASN.1 MODULE                                        74
       C  RATIONALE FOR KEY DESIGN DECISIONS                           78
          C.1 Deviation From The SMTP Model     .   .   .   .   .   .  78
              C.1.1 Comparison of SMTP and EMSD Efficiency  .   .   .  78
          C.2 Use of ESRO Instead of TCP    .   .   .   .   .   .   .  79
          C.3 Use Of Remote Procedure Call (RPC) Model  .   .   .   .  79
          C.4 Use Of ASN.1  .   .   .   .   .   .   .   .   .   .   .  80
       D  FURTHER DEVELOPMENT                                          81
       E  REFERENCES                                                   82
       F  FULL COPYRIGHT STATEMENT                                     83

    1  PRELIMINARIES

       Mail in the Internet was not a well-planned enterprise, but instead
       arose in more of an "organic" way.

       This introductory section is not intended to be a reference model and
       concept vocabulary for mail in the Internet.  Instead, it only
       provides the necessary preliminaries for the concepts and terms that
       are essential to this specification.

    1.1  Internet Mail Submission and Delivery

       For the purposes of this specification, mail submission is the
       process of putting mail into the mail transfer system (MTS).

    Banan                        Informational                      [Page 4]

    RFC 2524                          EMSD                     February 1999

       For the purposes of this specification, mail delivery is the process
       of the MTS putting mail into a user's final mail-box.

       Throughout the Internet, presently most of mail submission and
       delivery is done through SMTP.

       SMTP was defined as a message *transfer* protocol, that is, a means
       to route (if needed) and deliver mail by putting finished (complete)
       messages in a mail-box.  Originally, users connected to servers from
       terminals, and all processing occurred on the server.  Now, a split-
       MUA (Mail User Agent) model is common, with MUA functionality
       occurring on both the user's own system and the server.

       In the split-MUA model, getting the messages to the user is
       accomplished through access to a mail-box on the server through such
       protocols as POP and IMAP. In the split-MUA model, user's access to
       its message is a "Message Pull" paradigm where the user is required
       to poll his mailbox.  Proper message delivery based on a "Message
       Push" paradigm is presently not supported.  The EMSD protocol
       addresses this shortcoming with an emphasis on efficiency.

       In the split-MUA model, message submission is often accomplished
       through SMTP. SMTP is widely used as a message *submission* protocol.
       Widespread use of SMTP for submission is a reality, regardless of
       whether this is good or bad.  EMSD protocol provides an alternative
       mechanism for message submission which emphasizes efficiency.

    1.2  Relationship Of EMSD To Other Mail Protocols

       Various Internet mail protocols facilitate accomplishment of various
       functions in mail processing.

    Banan                        Informational                      [Page 5]

    RFC 2524                          EMSD                     February 1999

       Figure 1, categorizes the capabilities of SMTP, IMAP, POP and EMSD
       based on the following functions:

       +------------------+------+-------+-----+------+
       |         Protocols| SMTP |  IMAP | POP | EMSD |
       |Functions         |      |       |     |      |
       |------------------|------|-------|-----|------|
       |Submission        | XX   |       |     | XXX  |
       |------------------|------|-------|-----|------|
       |Delivery          | XXX  |       |     | XXX  |
       |------------------|------|-------|-----|------|
       |Relay (Routing)   | XXX  |       |     |      |
       |------------------|------|-------|-----|------|
       |Retrieval         |      |  XXX  | XXX |  XX  |
       |------------------|------|-------|-----|------|
       |Mailbox Access    |      |  XXX  |  X  |      |
       |------------------|------|-------|-----|------|
       |Mailbox Synch.    |      |  XXX  |     |      |
       +------------------+------+-------+-----+------+

       Figure 1:  Messaging Protocols vs.  Supported Functions

         o Mail Submission

         o Mail Delivery

         o Mail Routing (Relay)

         o Mail Retrieval

         o Mail-box Access

         o Mail-box Synchronization

       In Figure 1, the number of "X"es in each box denotes the extent to
       which a particular function is supported by a particular protocol.

       Figure 1 clearly shows that combinations of these protocols can be
       used to complement each other in providing rich functionality to the
       user.  For example, a user interested in highly mobile messaging
       functionalities can use EMSD for "submission and delivery of time
       critical and important messages" and use IMAP for comprehensive
       access to his/her mail-box.

       For mail submission and delivery of short messages EMSD is up to 5
       times more efficient than SMTP both in terms of the number of packets
       transmitted and in terms of number of bytes transmitted.  Even with

    Banan                        Informational                      [Page 6]

    RFC 2524                          EMSD                     February 1999

       PIPELINING and other possible optimizations of SMTP, EMSD is up to 3
       times more efficient than SMTP both in terms of the number of packets
       transmitted and in terms of number of bytes transmitted.  Various
       efficiency studies comparing EMSD with SMTP, POP and IMAP are
       available.  See Section C.1.1 for more information about comparison
       of SMTP and EMSD's efficiency.

    1.3  EMSD Requirements and Goals

       The requirements and goals driving design of EMSD protocol are
       enumerated below.

        1. Provide for submission of short mail messages with the same level
           of functionality (or higher) that the existing Internet mail
           protocols provide.

        2. Provide for delivery of short mail messages with the same level
           of functionality (or higher) that the existing Internet mail
           protocols provide.

        3. Function as an extension of the existing mainstream Internet
           mail.

        4. Minimize the number of transmissions.

        5. Minimize the number of bytes transmitted.

        6. Be quick:  minimize latency of message submission and delivery.

        7. Provide the same level of reliability (or higher) that the
           existing email protocols provide.

        8. Accommodate varying sizes of messages:  the size of a message may
           determine how the system deals with the message, but the system
           must accommodate it.

        9. Be power efficient and respect mobile platform resources:
           including memory and CPU levels, as well as battery power
           longevity (i.e.  client-light and server-heavy).

        10. Highly extendible:  different users will demand different
            options, so the solution cannot require every feature to be a
            part of every message.  Likewise, usage will emerge that is not
            currently recognized as a requirement.  The solution must be
            extendible enough to handle new, emerging requirements.

    Banan                        Informational                      [Page 7]

    RFC 2524                          EMSD                     February 1999

        11. Secure:  provide the same level of security (or higher) that the
            existing email protocols provide.  Content confidentiality,
            originator/recipient authentication and message integrity must
            be available options to users.

        12. Easy to implement:  Re-use existing technology as much as
            possible.

    1.4  Anticipated Uses Of EMSD

       Any network and network operator which has significant bandwidth and
       capacity limitations can benefit from the use of EMSD. Any network
       user who must bear high costs for measured network usage can benefit
       from the use of EMSD.

       Initial uses of EMSD is anticipated to be primarily over IP-based
       wireless networks to provide two-way paging services.

       EMSD can also function as an adjunct to Mail Access Protocols for
       "Mail Notification Services".

       Considering:

          o that most wireless networks shall converge toward being IP-
            based;

          o that two-way paging is the main proven application in most
            wide-area wireless networks;

          o that two-way paging industry and the Internet Email industry can
            and should converge based on a set of open protocols that
            address the efficiency requirements adequately;

          o that existing Internet email protocols are not bandwidth
            efficient;

          o that existing Internet email protocols do not properly support
            the "push" model of delivery of urgent messages,

       the EMSD protocol is designed to facilitate the convergence of IP-
       based two-way paging and Internet email.

       Mail submission and delivery take place at the edges of the network.
       More than one mail submission and delivery protocols which address
       requirements specific to a particular user's environment are likely
       to be developed.  Such diversity on the edges of the network is

    Banan                        Informational                      [Page 8]

    RFC 2524                          EMSD                     February 1999

       desirable and with the right protocols, this diversity does not
       adversely impact the integrity of the mail transfer system.  EMSD is
       the initial basis for the mail submission and delivery protocol to be
       used when the user's environment demands efficiency.

    1.5  Definitions of Terms Used in this Specification

       The following informal definitions and acronyms are intended to help
       describe EMSD model described in this specification.

       Efficient Mail Submission and Delivery Protocol (EMSD-P): The
           protocol used to transfer messages between the EMSD - Server
           Agent (e.g., a Message Center) and the EMSD - User Agent (e.g., a
           Two-Way Pager), see Figure 2.
       Message Transfer Agent (MTA)

       Message Transfer Service (MTS)

       Message Routing Service (MRS): Collection of MTAs responsible for
           mail routing.
       Message User Agent (MUA)

       Efficient Mail Submission Server Agent (EMS-SA): An Application
           Process which conforms to this protocol specification and accepts
           mail from an EMS-UA and transfers it towards its recipients.

       Efficient Mail Delivery Server Agent (EMD-SA): An Application Process
           which conforms to this protocol specification and delivers mail
           to an EMD-UA.
       Efficient Mail Submission and Delivery Server Agent (EMSD-SA): An
           Application Process which incorporates both EMS-SA and EMD-SA
           capabilities.

       Efficient Mail Submission User Agent (EMS-UA): An Application Process
           which conforms to this protocol specification and submits mail to
           EMS-SA.

       Efficient Mail Delivery User Agent (EMD-UA): An Application Process
           which conforms to this protocol specification and accepts
           delivery of mail from EMD-SA.
       Efficient Mail Submission and Delivery User Agent (EMSD-UA): An
           Application Process which incorporates both EMS-UA and EMD-UA
           capabilities.

    1.6  Conventions Used In This Specification

       The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
       in this specification are to be interpreted as defined in [2].

    Banan                        Informational                      [Page 9]

    RFC 2524                          EMSD                     February 1999

       This specification uses the ES-OPERATION notation defined in
       Efficient Short Remote Operations (ESRO) protocols as specified in
       RFC-2188 [1].

       Operations and information objects are typically described using the
       ES-OPERATION and ASN.1 notations in the relevant sections of the
       specification.

       The complete machine verifiable ASN.1 modules are also compiled in
       one place in Appendix A and Appendix B.

    1.7  About This Specification

       This protocol specification constitutes a point-of-record.  It
       documents information exchanges and behaviors of existing
       implementations.  It is a basis for implementation of efficient mail
       submission and delivery user agents and servers.

       This specification has been developed entirely outside of IETF. It
       has had the benefit of review by many outside of IETF. Much has been
       learned from existing implementations of this protocol.  A number of
       deficiencies and areas of improvement have been identified and are
       documented in this specification.

       This protocol specification is being submitted on October 23, 1998
       for timely publication as an Informational RFC.

       Future development and enhancements to this protocol may take place
       inside of IETF.

    2  EFFICIENT MAIL SUBMISSION AND DELIVERY OVERVIEW

       This section offers a high level view of the Efficient Mail
       Submission and Delivery Protocol and Format Standards (EMSD-P&FS).

       The EMSD-P&FS are used to transfer messages between the EMSD - Server
       Agent (e.g., a Message Center) and the EMSD - User Agent (e.g., a
       Two-Way Pager), see Figure 2.

       This specification defines the protocols between an EMSD - User Agent
       (EMSD-UA) and an EMSD - Server Agent (EMSD-SA). The EMSD - P&FS
       consist of two independent components:

        1. EMSD Format Standard (EMSD-FS).

           EMSD-FS is a non-textual form of compact encoding of Internet
           mail (RFC-822) messages which facilitates efficient transfer of
           messages.  EMSD-FS is used in conjunction with the EMSD-P but is

    Banan                        Informational                     [Page 10]

    RFC 2524                          EMSD                     February 1999

           not a general replacement for RFC-822.  EMSD-FS defines a method
           of representation of short interpersonal messages.  It defines
           the "Content" encoding (Header + Body).  Although EMSD-FS
           contains end-to-end information its scope is purely point-to-
           point.  EMSD-FS relies on EMSD-P (see 2 below) for the transfer
           of the content to its recipients.

           This is described in the section entitled EMSD Format Standards.

        2. Efficient Mail Submission and Delivery Protocol (EMSD-P).

           EMSD-P is responsible for wrapping an EMSD-FS message (see 1
           above) in a point-to-point envelope and submitting or delivering
           it.  EMSD-P relies on the services of Efficient Short Remote
           Operation Services (ESROS) as specified in RFC-2188 [1] for
           transporting the point-to-point envelope.  Some of the services
           of EMSD-P include:  message originator authentication and
           optional message segmentation and reassembly.  The EMSD-P is
           expressed in terms of abstract services using the ESROS notation.
           This is described in the section entitled Efficient Mail
           Submission and Delivery Protocol.

       It is important to recognize that EMSD-P and EMSD-FS are not end-to-
       end, but focus on the point-to-point transfer of messages.  The two
       points being EMSD-SA and EMSD-UA. EMSD-P function as elements of the
       Internet mail environment, which provide end-to-end (EMSD-User to any
       other Messaging Originator or Recipient) services.

       Figure 2 illustrates how the EMSD-P&FS defines the communication
       between a specific EMSD-UA and a specific EMSD-SA. The Message
       Transfer System may include a number of EMSD-SAs.  Each EMSD-SA may
       have any number of EMSD-UAs with which it communicates.

       The Efficient Mail Submission and Delivery Services use the Efficient
       Short Remote Operation Services (ESROS). They also use the Duplicate
       Operation Detection Support Functions as described in the section
       entitled Duplicate Operation Detection Support Functions.  These
       functions guarantee that an operation is performed no more than once.

    3  EFFICIENT MAIL SUBMISSION AND DELIVERY PROTOCOL

       EM Submission is the process of transferring a message from EMSD-UA
       to EMSD-SA. EM Delivery is the process of transferring a message from
       EMSD-SA to EMSD-UA.

    Banan                        Informational                     [Page 11]

    RFC 2524                          EMSD                     February 1999

       The Message-submission service enables an EMSD-UA to submit a message
       to the EMSD-SA for transfer and delivery to one or more recipients.
       The Message-submission Service comprises of the submit operation --
       invoked by the EMSD-UA -- and possibly the submitVerify operation --
       invoked by the EMSD-SA.

       The Message-delivery service enables the EMSD-SA to deliver a message
       to an EMSD-UA. The Message-delivery Service comprises of the deliver
       operation -- invoked by the EMSD-SA -- and possibly the deliverVerify
       operation -- invoked by the EMSD-UA.

       EMSD-UA uses the following services:

            o Message-submission

       +---------------------------------------------+
       | MTS                                         |
       |                                             |
       |  +-------------------------+                |
       |  | MRS                     |                |
       |  |  +---+          +---+   |                |
       |  |  |   |          | M |   |         +---+  |
       |  |  |   |<-------->| T |<----------->|   |  |
       |  |  |   |          | A |   |         |   |  |               +---+
       |  |  |   |          +---+   |         | E |  |               | E |
       |  |  |   |                  |         | M |  |               | M |
       |  |  | M |                  |         | S |  |   EMSD-P&FS   | S |
       |  |  | T |<-------------------------->| D |<---------------->| D |
       |  |  | A |                  |         | - |  |               | - |
       |  |  |   |          +---+   |         | S |  |               | U |
       |  |  |   |          | M |   |         | A |  |               | A |
       |  |  |   |<-------->| T |<----------->|   |  |               +---+
       |  |  |   |          | A |   |         |   |  |
       |  |  +---+          +---+   |         +---+  |
       |  |                         |                |
       |  +-------------------------+                |
       |                                             |
       |                                             |
       +---------------------------------------------+

             Figure 2:  Efficient Mail Submission and Delivery Protocol

          o Delivery-control (the deliveryControl operation).

       EMSD-SA uses the following services:

          o Message-delivery

    Banan                        Informational                     [Page 12]

    RFC 2524                          EMSD                     February 1999

          o Submission-control (the submissionControl operation).

       This specification expresses information objects using ASN.1 [X.208].

       This specification expresses Remote Operations based on the model of
       ESROS as specified in Efficient Short Remote Operations (RFC-2188)
       [1].  The ES-OPERATION notation of (RFC-2188) is used throughout this
       specification to define specific operations.

       This specification uses the Duplicate Operation Detection Support
       functions as specified in Section 4.

    3.1  Use Of Lower Layers

    3.1.1  Use of ESROS

       ESRO protocol, as specified in (RFC-2188 [1]), provides reliable
       connectionless remote operation services on top of UDP [6] with
       minimum overhead.  ESRO protocol supports segmentation and
       reassembly, concatenation and separation.

       ESRO Services (2-Way and 3-Way handshake) shall be used by the EMSD-
       P.

       ESRO Service Access Point (SAP) selectors used by EMSD-P are
       enumerated in the protocol.

    3.1.2  Use Of UDP

       EMSD-P through ESRO MUST use UDP [6] port number 642 (esro-emsdp).

       Note that specification of Service Access Points (SAP) for EMSD-P
       include the UDP Port Number specification in addition to ESRO SAP
       selector specifications.  In other words, EMSD-P's use of ESRO SAPs
       does not preclude use of the same SAP selectors by other protocols
       which use a UDP port other than port 642.  Such usage of ESRO is a
       design characteristic of ESRO which results into bandwidth efficiency
       and is not a scalability limitation.

    3.1.3  Encoding Rules

       Use of Basic Encoding Rules (BER) [5] is mandatory for both EMSD
       Format Standards and EMSD Protocol.

       In order to minimize data transfer, the following restrictions shall
       be maintained in the formatting of EMSD PDUs:

          o Specifically, when ASN.1 Basic Encoding Rules are being used:

    Banan                        Informational                     [Page 13]

    RFC 2524                          EMSD                     February 1999

             A. Only the "Definite" form of Length encoding MUST be used,

             B. The "Short" form of Length encoding MUST be used whenever
                possible (i.e.  when the Length is less than 128), and

             C. OCTET STRING and BIT STRING values, and any other native
                ASN.1 types which may be encoded as either "Primitive" or
                "Constructed", MUST always be encoded as "Primitive" and
                MUST never be "Constructed".

    3.1.4  Presentation Context

       Parameter Encoding Type of "0" MUST be used in ESRO Protocol to
       identify Basic Encoding Rules for operation arguments.

    3.2  EMSD-UA Invoked Operations

       The following operations are invoked by EMSD-UA:

        a. submit

        b. deliveryControl

        c. deliveryVerify

       The submit operation uses the duplication detection functional unit
       while deliveryControl and deliveryVerify don't use the duplication
       detection.

       The complete definition of these operations follows.

    3.2.1  submit

       The submit ES-OPERATION enables an EMSD-UA to submit a message to the
       EMSD-SA for transfer and delivery to one or more recipients.

       submit ES-OPERATION

           ARGUMENT SubmitArgument
           RESULT SubmitResult
           ERRORS
           {
               submissionControlViolated,
               securityError,
               resourceError,
               protocolViolation,
               messageError
           } ::= 33;

    Banan                        Informational                     [Page 14]

    RFC 2524                          EMSD                     February 1999

       Duplicate operation detection is necessary for this operation.

       The successful completion of the ES-OPERATION signifies that the
       EMSD-SA has accepted responsibility for the message (but not that it
       has delivered it to its intended recipients).

       The disruption of the ES-OPERATION by an error signifies that the
       EMSD-SA cannot assume responsibility for the message.

       Arguments

       This operation's arguments are:

       SubmitArgument ::= SEQUENCE
       {
         -- Security features
         security                [0]    IMPLICIT SecurityElement OPTIONAL,

         -- Segmentation features for efficient transport
         segment-info                            SegmentInfo OPTIONAL,

         -- Content type of the message
         content-type                            ContentType,

         --
         -- THE CONTENT --
         --

         -- The submission content
         content                                 ANY DEFINED BY content-type
       };

       The fields are:

       Security

       See Section 3.4.1, "SecurityElements".

       Segment-info

       See Section 3.4.2, "Message Segmentation and Reassembly".

    Banan                        Informational                     [Page 15]

    RFC 2524                          EMSD                     February 1999

       Content-type

       This argument identifies the type of the content of the message.  It
       identifies the abstract syntax and the encoding rules used.

       Content

       This argument contains the information the message is intended to
       convey to the recipient(s).  It shall be generated by the originator
       of the message.

       Results

       This operation's results are:

       SubmitResult ::= SEQUENCE

           {
               -- Permanent identifier for this message.
               -- Also contains the message submission time.
               -- See comment regarding assignment of message identifiers,
               -- at the definition of EMSDLocalMessageId.

               message-id                              EMSDLocalMessageId

           };

       The fields are:

       Message-id

       This result contains an EMSD-SA-identifier that uniquely and
       unambiguously identifies the message-submission.  It shall be
       generated by the EMSD-SA.

       Errors

       See Section 3.4.3.

    Banan                        Informational                     [Page 16]

    RFC 2524                          EMSD                     February 1999

    3.2.2  deliveryControl

       The deliveryControl ES-OPERATION enables the EMSD-UA to temporarily
       limit the operations that the EMSD-SA may invoke, and the messages
       that the EMSD-SA may deliver to the EMSD-UA via the Message delivery
       ES-OPERATION.

       deliveryControl ES-OPERATION
           ARGUMENT DeliveryControlArgument
           RESULT DeliveryControlResult
           ERRORS
           {
               securityError,
               resourceError,
               protocolViolation
           } ::= 2;

       The duplicate operation detection is not required for this operation.

       The EMSD-SA shall hold until a later time, rather than abandon, ES-
       OPERATIONS and messages that are presently suspended.

       The successful completion of the ES-OPERATION signifies that the
       specified controls are now in force.

       The ES-OPERATION returns an indication of any ES-OPERATIONS that the
       EMSD-SA would invoke, or any message types that the EMSD-SA would
       deliver, were it not for the prevailing controls.

       Arguments

       This operation's arguments are:

       DeliveryControlArgument ::= SEQUENCE
       {
         -- Request an addition of or removal of a set of restrictions

         restrict                [0]     IMPLICIT Restrict DEFAULT update,

         -- Which operations are to be placed in the restriction set
         permissible-operations  [1]     IMPLICIT Operations OPTIONAL,

         -- What maximum content length should be allowed
         permissible-max-content-length

                                         [2]     IMPLICIT INTEGER
                                          (0..ub-content-length) OPTIONAL,

    Banan                        Informational                     [Page 17]

    RFC 2524                          EMSD                     February 1999

         -- What is the lowest priority message which may be delivered
         permissible-lowest-priority

                                         [3]     IMPLICIT ENUMERATED
                                                  {
                                                    non-urgent     (0),
                                                    normal         (1),
                                                    urgent         (2)
                                                  } OPTIONAL,

         -- Security features
         security                        [4]     IMPLICIT SecurityElement
                                                 OPTIONAL,

         -- User Feature selection
         user-features                   [5]     IMPLICIT OCTET STRING
                                                 OPTIONAL
       };

       Restrict

       This argument indicates whether the controls on ES-OPERATIONS are to
       be updated or removed.  It may be generated by the EMSD-UA.

       This argument may have one of the following values:

          o update:  The other arguments update the prevailing controls;

          o remove:  All temporary controls are to be removed

       In the absence of this argument, the default update shall be assumed.

       Permissible-operations

       This argument indicates the ES-OPERATIONS that the EMSD-SA may invoke
       on the EMSD-UA. It may be generated by the EMSD-UA.

       This argument may have the value allowed or prohibited for each of
       the following:

          o message-delivery:  The EMSD-SA may/may not invoke the deliver
            ES-OPERATIONS; and

          o Other ES-OPERATIONS are not subject to controls, and may be
            invoked at any time.

    Banan                        Informational                     [Page 18]

    RFC 2524                          EMSD                     February 1999

       In the absence of this argument, the ES-OPERATIONS that the EMSD-SA
       may invoke on the EMSD-UA are unchanged.

       Permissible-max-content-length

       This argument contains the content-length, in octets, of the
       longest-content message that the EMSD-SA shall deliver to the EMSD-UA
       via the deliver ES-OPERATIONS. It may be generated by the EMSD-UA.

       In the absence of this argument, the permissible-maximum-content-
       length of a message that the EMSD-SA may deliver to the EMSD-UA is
       unchanged.

       Permissible-lowest-priority

       This argument contains the priority of the lowest priority message
       that the EMSD-SA shall deliver to the EMSD-UA via the deliver ES-
       OPERATIONS. It may be generated by the EMSD-UA.

       This argument may have one of the following values of the priority
       argument of the submit ES-OPERATIONS: normal, non-urgent or urgent.

       In the absence of this argument, the priority of the lowest priority
       message that the EMSD-SA shall deliver to the EMSD-UA is unchanged.

       Security

       See Section 3.4.1, "SecurityElements".

       User-features

       This argument contains information that allows the EMSD-UA to convey
       to MTS the feature set that the user is capable of supporting.  This
       argument will be defined when the setConfiguration and
       getConfiguration operations are defined.

       Results

       DeliveryControlResult ::= SEQUENCE
       {
         -- Operation types queued at the EMSD-SA due to existing
         -- restrictions.
         waiting-operations      [0]     IMPLICIT Operations DEFAULT { },

    Banan                        Informational                     [Page 19]

    RFC 2524                          EMSD                     February 1999

         -- Types of messages queued at the EMSD-SA due to
         -- existing restrictions
         waiting-messages        [1]     IMPLICIT WaitingMessages
                                         DEFAULT { },

         -- Content Types of messages queued at the EMSD-SA
         waiting-content-types   SEQUENCE SIZE (0..ub-content-types) OF
                                                ContentType DEFAULT { }

       };

       Restrict ::= ENUMERATED
       {
           update                                      (1),
           remove                                      (2)
       };

       Operations ::= BIT STRING
       {
           submission                                  (0),
           delivery                                    (1)
       };

       WaitingMessages ::= BIT STRING
       {
           long-content                                (0),
           low-priority                                (1)

       };

       Waiting-operations

       This result indicates the ES-OPERATIONS being held by the EMSD-SA,
       and that the EMSD-SA would invoke on the EMSD-UA if it were not for
       the prevailing controls.  It may be generated by the EMSD-SA.

       This result may have the value holding or not-holding for each of the
       following:

          o message-delivery:  The EMSD-SA is/is not holding messages, and
            would invoke the deliver ES-OPERATIONS on the EMSD-UA if it were
            not for the prevailing controls.

       In the absence of this result, it may be assumed that the EMSD-SA is
       not holding any messages for delivery due to the prevailing controls.

    Banan                        Informational                     [Page 20]

    RFC 2524                          EMSD                     February 1999

       Waiting-messages

       This result indicates the kind of messages the EMSD-SA is holding for
       delivery to the EMSD-UA, and would deliver via the deliver ES-
       OPERATIONS, if it were not for the prevailing controls.  It may be
       generated by the EMSD-SA.

       This result may have one or more of the following values:

          o long-content:  The EMSD-SA has messages held for delivery to the
            EMSD-UA which exceed the permissible maximum-content-length
            control currently in force;

          o low-priority:  The EMSD-SA has messages held for delivery to the
            EMSD-UA of a lower priority than the permissible-lowest-priority
            control currently in force;

       In the absence of this result, it may be assumed that the EMSD-SA is
       not holding any messages for delivery to the EMSD-UA due to the
       permissible-maximum-content-length, permissible-lowest-priority or
       permissible-security context controls currently in force.

       Errors

       See Section 3.4.3.

    3.2.3  deliveryVerify

       The deliveryVerify ES-OPERATIONS enables the EMSD-UA to verify
       delivery of a message when it receives FAILURE.indication for deliver
       ES-OPERATIONS.

       deliveryVerify ES-OPERATION

           ARGUMENT DeliveryVerifyArgument
           RESULT DeliveryVerifyResult
           ERRORS
           {
               verifyError,
               resourceError,
               protocolViolation
           } ::= 5;

       The duplicate operation detection is not required for this operation.

       Arguments

    Banan                        Informational                     [Page 21]

    RFC 2524                          EMSD                     February 1999

       This operation's arguments are:

       DeliveryVerifyArgument ::= SEQUENCE

       {
         -- Identifier of this message. This is the same identifier that
         -- was provided to the originator in the Submission Result.
         -- See comment regarding assignment of message identifiers,
         -- at the definition of EMSDMessageId.
         message-id                                      EMSDMessageId
       };

       Message-id

       This argument contains an EMSD-SA-identifier that distinguishes the
       message from all other messages.  It shall be generated by the EMSD-
       SA, and shall have the same value as the message-submission-
       identifier supplied to the originator of the message when the message
       was submitted.

       Results

       DeliveryVerifyResult ::= SEQUENCE
       {
                status  DeliveryStatus
       };

        DeliveryStatus  ::= ENUMERATED
       {
               no-report-is-sent-out                   (1),
               delivery-report-is-sent-out             (2),
               non-delivery-report-is-sent-out         (3)
        };

       No-report-is-sent-out

       This result indicates that EMSD-SA has received the delivery verify
       and no report is sent out (either because it has not been requested
       or EMSD-SA has problems and can not send it out).

       Delivery-report-is-sent-out

       This result indicates that EMSD-SA has received the delivery verify

    Banan                        Informational                     [Page 22]

    RFC 2524                          EMSD                     February 1999

       and has sent the delivery report out.

       Non-Delivery-report-is-sent-out

       This result indicates that EMSD-SA has received the delivery verify
       but it has already sent out a non-Delivery report.  This should not
       happen in normal cases but a wrong user profile on EMSD-SA side can
       result in this outcome.

       Errors

       See Section 3.4.3.

    3.3  EMSD-SA Invoked Operations

       This section defines the operations invoked by the EMSD-SA:

          a. deliver;

          b. submissionControl;

          c. submissionVerify.

       The deliver operation uses 3-Way handshake service of ESROS. This
       operation always uses the duplication detection functional unit.

       The submissionControl and submissionVerify operations use 2-Way
       handshake service of ESROS without duplication detection.

    3.3.1  deliver

       The deliver ES-OPERATIONS enables the EMSD-SA to deliver a message to
       an EMSD-UA.

       deliver ES-OPERATION

           ARGUMENT DeliverArgument
           RESULT NULL
           ERRORS
           {
               deliveryControlViolated,
               securityError,
               resourceError,
               protocolViolation,
               messageError
           } ::= 35;

    Banan                        Informational                     [Page 23]

    RFC 2524                          EMSD                     February 1999

       The EMSD-UA MUST not refuse performing the deliver ES-OPERATION
       unless the delivery would violate the deliveryControl restrictions
       then in force.

       Arguments

       This operation's arguments are:

       DeliverArgument ::= SEQUENCE
       {
         -- Identifier of this message. This is the same identifier that
         -- was provided to the originator in the Submission Result.
         -- See comment regarding assignment of message identifiers,
         -- at the definition of EMSDMessageId.
         message-id                                      EMSDMessageId,

         -- Time the message was delivered to the recipient by EMSD-SA
         message-delivery-time                           DateTime,

         -- Time EMSD-SA originally took responsibility for processing
         -- of this message. This field shall be omitted if the message-id
         -- contains an EMSDLocalMessageId, because that field contains
         -- the submission time within it.
         message-submission-time [0]  IMPLICIT DateTime OPTIONAL,

         -- Security features
         security                [1]  IMPLICIT SecurityElement OPTIONAL,

         -- SegContentTypementation features for efficient transport
         segment-info                              SegmentInfo OPTIONAL,

         -- The type of the content
         content-type                                ContentType,

         --
         -- THE CONTENT --
         --

         -- The submitted (and now being delivered) content
         content                           ANY DEFINED BY content-type
       };

    Banan                        Informational                     [Page 24]

    RFC 2524                          EMSD                     February 1999

       message-id

       This argument contains an EMSD-SA-identifier that distinguishes the
       message from all other messages.  When within the EMSD, it MUST be
       generated by the EMSD-SA, and MUST have the same value as the
       message-submission-identifier supplied to the originator of the
       message when the message was submitted.

       Message-delivery-time

       This argument contains the Time at which delivery occurs and at which
       the EMSD-SA is relinquishing responsibility for the message.  It
       shall be generated by the EMSD-SA.

       Results

       This operation returns an empty result as indication of success.

       Errors

       See Section 3.4.3.

    3.3.2  submissionControl

       submissionControl ES-OPERATION
           ARGUMENT SubmissionControlArgument
           RESULT SubmissionControlResult
           ERRORS
           {
               securityError,
               resourceError,
               protocolViolation
           } ::= 4;

       The submissionControl ES-OPERATIONS enables the EMSD-SA to
       temporarily limit the operations that the EMSD-UA may invoke, and the
       messages that the EMSD-UA may submit to the EMSD-SA via the submit
       ES-OPERATIONS.

       The duplicate operation detection is not required for this operation.

       The EMSD-UA should hold until a later time, rather than abandon, ES-
       OPERATIONS and messages that are presently suspended.

    Banan                        Informational                     [Page 25]

    RFC 2524                          EMSD                     February 1999

       The successful completion of the ES-OPERATIONS signifies that the
       specified controls are now in force.  These controls supersede any
       previously in force, and remain in effect until the association is
       released or the EMSD-SA re-invokes the submissionControl ES-
       OPERATIONS.

       The ES-OPERATIONS returns an indication of any ES-OPERATIONS that the
       EMSD-UA would invoke were it not for the prevailing controls.

       Arguments

       This operation's arguments are:

       SubmissionControlArgument ::= SEQUENCE
       {
         -- Request an addition of or removal of a set of restrictions
         restrict               [0]     IMPLICIT Restrict DEFAULT update,

         -- Which operations are to be placed in the restriction set
         permissible-operations  [1]     IMPLICIT Operations OPTIONAL,

         -- What maximum content length should be allowed
         permissible-max-content-length
                                 [2]     IMPLICIT INTEGER
                                         (0..ub-content-length) OPTIONAL,

         -- Security features
         security                [3]     IMPLICIT SecurityElement
                                                         OPTIONAL
       };

       Restrict

       This argument indicates whether the controls on ES-OPERATIONS are to
       be updated or removed.  It may be generated by the EMSD-SA.

       This argument may have one of the following values:

          o update:  The other arguments update the prevailing controls;

          o remove:  All temporary controls are to be removed

       In the absence of this argument, the default update shall be assumed.

    Banan                        Informational                     [Page 26]

    RFC 2524                          EMSD                     February 1999

       Permissible-operations

       This argument indicates the ES-OPERATIONS that the EMSD-UA may invoke
       on the EMSD-SA. It may be generated by the EMSD-SA.

       This argument may have the value allowed or prohibited for each of
       the following:

          o submit:  The EMSD-UA may/may not invoke the submit ES-
            OPERATIONS; and

          o Other ES-OPERATIONS are not subject to controls, and may be
            invoked at any time.

       In the absence of this argument, the ES-OPERATIONS that the EMSD-UA
       may invoke on the EMSD-SA are unchanged.

       Permissible-max-content-length

       This argument contains the content-length, in octets, of the
       longest-content message that the EMSD-UA shall submit to the EMSD-SA
       via the submit ES-OPERATIONS. It may be generated by the EMSD-SA.

       In the absence of this argument, the permissible-maximum-content-
       length of a message that the EMSD-UA may submit to the EMSD-SA is
       unchanged.

       Security

       See Section 3.4.1, "SecurityElements".

       Results

       SubmissionControlResult ::= SEQUENCE
       {
         -- Operation types queued at the EMSD-SA due to existing
         -- restrictions.
         waiting-operations    [0]   IMPLICIT Operations DEFAULT { }

       };

    Banan                        Informational                     [Page 27]

    RFC 2524                          EMSD                     February 1999

       Waiting-operations

       This result indicates the ES-OPERATIONS being held by the EMSD-UA,
       and that the EMSD-UA would invoke if it were not for the prevailing
       controls.  It may be generated by the EMSD-UA.

       This result may have the value holding or not-holding for each of the
       following:

          o submit:  The EMSD-UA is/is not holding messages, and would
            invoke the submit ES-OPERATIONS if it were not for the
            prevailing controls.

       In the absence of this result, it may be assumed that the EMSD-UA is
       not holding any messages for submission due to the prevailing
       controls.

       Errors

       See Section 3.4.3.

    3.3.3  submissionVerify

       The submissionVerify ES-OPERATIONS enables the EMSD-SA to verify if
       the EMSD-UA has received the result of its submission.

       submissionVerify  ES-OPERATION

           ARGUMENT SubmissionVerifyArgument
           RESULT SubmissionVerifyResult
           ERRORS
           {
               submissionVerifyError,
               resourceError,
               protocolViolation
           } ::= 6;

       The duplicate operation detection is not required for this operation.

       Arguments

       This operation's arguments are:

       SubmissionVerifyArgument ::= SEQUENCE

    Banan                        Informational                     [Page 28]

    RFC 2524                          EMSD                     February 1999

         -- Identifier of this message. This is the same identifier that
         -- was provided to the originator in the Submission Result.
         -- See comment regarding assignment of message identifiers,
         -- at the definition of EMSDMessageId.
         {
            message-id                                  EMSDMessageId
         };

       Message-id

       This argument contains an EMSD-SA-identifier that distinguishes the
       message from all other messages.  It shall be generated by the EMSD-
       SA, and shall have the same value as the message-submission-
       identifier supplied to the originator of the message when the message
       was submitted.

       Results

       SubmissionVerifyResult ::= SEQUENCE
       {
               status  SubmissionStatus
       };

       SubmissionStatus::= ENUMERATED
       {
               send-message            (1),
               drop-message            (2)
       };

       Send-message

       This result indicates that EMSD-SA is supposed to send the message
       out.

       Drop-message

       This result indicates that EMSD-SA is supposed to drop the message.

       Errors

       See Section 3.4.3.

    Banan                        Informational                     [Page 29]

    RFC 2524                          EMSD                     February 1999

    3.4  EMSD Common Information Objects

    3.4.1  SecurityElements

       SecurityElement ::= SEQUENCE

       {
         credentials                          Credentials,
         contentIntegrityCheck                ContentIntegrityCheck OPTIONAL
       };

       Credentials ::= CHOICE
       {
         simple                          [0]     IMPLICIT SimpleCredentials

         -- Strong Credentials are for future study
         -- strong                       [1]     IMPLICIT StrongCredentials
         -- externalProcedure            [2]     EXTERNAL
       };

       SimpleCredentials ::= SEQUENCE
       {
         eMSDAddress                     EMSDAddress OPTIONAL,
         password                [0]     IMPLICIT OCTET STRING
                                 SIZE (0..ub-password-length)) OPTIONAL
       };

       -- StrongCredentials ::= NULL
       -- for now.
       -- ContentIntegrityCheck is a 16-bit checksum of content
       ContentIntegrityCheck ::= INTEGER (0..65535);

    3.4.2  Message Segmentation and Reassembly

       Small messages can benefit from the efficiencies of connectionless
       feature of ESROS (See Efficient Short Remote Operations, RFC-2188
       [1]).

       Very large messages are transferred using protocols (e.g., SMTP) that
       rely on Connection Oriented Transport Service (e.g., TCP).

       When a message is too large to fit in a single connectionless PDU but
       is not large enough to justify the overhead of connection
       establishment, it may be more efficient for the message to be
       segmented and reassembled while the connectionless service of ESROS
       is used.  If the underlying Remote Operation Service is capable of
       efficient segmentation/reassembly over connectionless (CL) services,

    Banan                        Informational                     [Page 30]

    RFC 2524                          EMSD                     February 1999

       then use of the segmenting/reassembly mechanism introduced in this
       section is not necessary.  This feature is accommodated in this layer
       by:

       SegmentInfo ::= CHOICE

       {
         first           [APPLICATION 2]         IMPLICIT FirstSegment,
         other           [APPLICATION 3]         IMPLICIT OtherSegment
       };

       FirstSegment ::= SEQUENCE
       {
         sequence-id                             INTEGER,
         number-of-segments                      INTEGER
         -- number-of-segments must not exceed ub-total-number-of-segments
       };

       OtherSegment ::= SEQUENCE
       {
         sequence-id                             INTEGER,
         segment-number                          INTEGER
       };

       Segmentation and reassembly only applies to Message-submission and
       Message-delivery.

       The sender of the message is responsible for segmenting the message
       content into segments that fit in CL PDUs.  The segmented content is
       sent in a sequence of message-segments each carrying a segment of the
       content.  sequence-Id is a unique identifier that is present in all
       message-segments.  In addition to sequence identifier, the first
       message-segment specifies the total number of segments (number-of-
       segments).  Other message-segments have a segment sequence number
       (segment-number).  The receiver is responsible for sequencing (based
       on segment-number) and reassembling the entire message.

       Segmenting over the Connectionless ESRO Service

       The sender of the message maps the original message into an ordered
       sequence of message-segments.  This sequence shall not be interrupted
       by other messages over the same ESROS association.

       All message-segments in the sequence shall be assigned a sequence
       identifier by sender.  The sequence identifier shall be incremented
       by one by the sender after transmission of a complete message
       sequence.

    Banan                        Informational                     [Page 31]

    RFC 2524                          EMSD                     February 1999

       The first message-segment specifies the total number of segments.
       All message-segments in the sequence except the first one shall be
       sequentially numbered, starting at 1 (first message-segment has
       implicit segment number of 0).

       Each message-segment is transmitted by issuing a Message-submission
       or Message-delivery ES-OPERATIONS. All segments of a segmented
       message are identified by the same sequence-id.  For a given message,
       the receiver should not impose any restriction on the order of
       arrival of message-segments.

       There is no requirement that any message-segment content be of
       maximum length allowed by ESROS for connectionless transmission;
       however, no more than ub-total-number-of-segments segments can be
       derived from a single message.

       The receiver reassembles a sequence of message-segments into a single
       message.  A message shall not be further processed unless all
       segments of the message are received.  Failure to receive the message
       shall be determined by the following events:

          o Expiration of Reassembly Timer (see Section 3.4.3).

          o Receipt of a message-segment with different sequence identifier.

       In the event of the above mentioned failures, the receiver shall
       discard a partially assembled sequence.

       In Reassembly process, all arguments other than content are ignored
       in all segments except the first one.  The content parts of all
       segments are concatenated to compose the original message content.

       When sender receives FAILURE.indication (as opposed to a
       resourceError) for a message-segment, the whole message shall be
       retransmitted.

       In the case of submission and delivery operations, the verify
       function is used as described below:

       Receiver ignores FAILURE.indications received for message-segments,
       and just collects the message-segments to complete the message.
       However, it keeps a failure status for a segmented message which says
       if any segment of the message has received FAILURE.indication.  When
       receiver succeeds to assemble the whole segmented message, then if
       the status of the message shows there has been a FAILURE.indication
       for any of the message-segments, it verifies the message through
       verify operation.  It's not enough to invoke verify operation just
       based on the last message-segment because the sender might send a

    Banan                        Informational                     [Page 32]

    RFC 2524                          EMSD                     February 1999

       segment without waiting for the result of the previous segment.  In
       such cases, there might be any combination of success and failure for
       message-segments on the sender side.

       Receiver uses the error code ResourceError (see Section 3.4.3) to ask
       for retransmission of a single segment and uses the error code
       MessageError (see Section 3.4.3) to ask for retransmission of all
       segments (the whole message).

       Reassembly Timer

       The Reassembly Timer is a local timer maintained by the receiver of
       message-segments that assists in performing the reassembly function.
       This timer determines how long a receiver waits for all segments of a
       message-segment sequence to be received.  The timer protects the
       receiver from the loss of a series of segments and possible sequence
       identifier wrap-around.

       The Reassembly Timer shall be started on receipt of a message-segment
       with different sequence identifier than that previously received.
       The timer shall be stopped on receipt of all segments composing the
       sequence.

       The value of Reassembly Timer is defined based on the network
       characteristics and the number of segments.  This requires that the
       transmission of all segments of a single message must be completed
       within this time limit.

    3.4.3  Common Errors

       protocolVersionNotRecognized  ERROR PARAMETER NULL ::= 1;

       submissionControlViolated  ERROR PARAMETER NULL ::= 2;

       messageIdentifierInvalid  ERROR PARAMETER NULL ::= 3;

       securityError ERROR PARAMETER security-problem SecurityProblem ::= 4;

       deliveryControlViolated   ERROR PARAMETER NULL ::= 5;

       resourceError  ERROR PARAMETER NULL ::= 6;

       protocolViolation  ERROR PARAMETER NULL ::= 7;

       messageError  ERROR PARAMETER NULL ::= 8;

       SecurityProblem ::= INTEGER (0..127);

    Banan                        Informational                     [Page 33]

    RFC 2524                          EMSD                     February 1999

       protocolVersionNotRecognized

       The major and minor protocol versions presented do not match those
       recognized as being valid.

       submissionControlViolated

       The Submission control violated error reports the violation by the
       MTS-user of a control on submission services imposed by the MTS via
       the Submission control service.  The Submission control violated
       abstract-error has no parameters.

       messageIdentifierInvalid

       The Message Identifier Invalid error reports that the Message
       Identifier presented to the MTS is not considered valid.

       securityError

       The Security error reports that the requested operation could not be
       provided by the MTS or MTS-user because it would violate the security
       policy in force.

       deliveryControlViolated

       The Delivery control violated error reports the violation by the MTS
       of a control on delivery operations imposed by the MTS-user via the
       Delivery-control operation.

       resourceError

       The messaging agent cannot currently support this operation.  In the
       case of segmentation and reassembly, resourceError is by the receiver
       used to request that the sender retransmit of a single segment.

       protocolViolation

       Indicates that one or more mandatory argument(s) were missing.

    Banan                        Informational                     [Page 34]

    RFC 2524                          EMSD                     February 1999

       messageError

       For a multi-segment message, this error indicates that the messaging
       agent has not received the message completely and that the message
       must be retransmitted.

       SecurityProblem

       To ensure the security-policy is not violated during delivery, the
       message-security-label is checked against the security-context.  If
       delivery is barred by the security-policy then, subject to the
       security policy, a report instruction for this is generated.

    3.4.4  ContentType

       ContentType ::=  INTEGER
       {
         -- Content type 0 is reserved and shall never be transmitted.
         reserved                                 (0),
         -- Content types between 1 and 31 (inclusive) are for
         -- internal-use only
         probe                                    (1), -- reserved
         delivery-report                          (2), -- reserved

         -- Content types between 32 and 63 (inclusive) are for
         -- message types  defined within this specifications.
         emsd-interpersonal-messaging-1995        (32),
         voice-messaging                          (33) -- reserved

         -- Content types beyond and including 64 are for
         -- bilaterally-agreed use between peers.
       } (0..127);

    3.4.5  EMSDMessageId

       If this message was originated as an RFC-822 message, then this
       EMSDMessageId shall be the "Message-Id:" field from that message.  If
       this message was originated within the EMSD domain, then this
       identifier shall be unique for the EMSD-SA generating this id.

       EMSDMessageId ::= CHOICE
       {
         EMSDLocalMessageId  [APPLICATION 4]
                             IMPLICIT EMSDLocalMessageId,

         rfc822MessageId     [APPLICATION 5]
                             IMPLICIT AsciiPrintableString

    Banan                        Informational                     [Page 35]

    RFC 2524                          EMSD                     February 1999

                             (SIZE (0..ub-message-id-length))
       };

       EMSDLocalMessageId ::= SEQUENCE
       {
         submissionTime            DateTime,
         messageNumber             INTEGER (0..ub-local-message-nu)
       };

    3.4.6 EMSDORAddress

       EMSDORAddress ::= CHOICE
       {
         -- This is the local-format address
         emsd-local-address-format            EMSDAddress,

         -- This is a globally-unique RFC-822 Address
         rfc822DomainAddress                 AsciiPrintableString
       };

       In the global sense Originators and Recipients are represented by
       EMSDORAddress.  The rfc822Domain may be used to address any
       recipient.

    3.4.7  EMSDAddress

       EMSDAddress ::= SEQUENCE
       {
         emsd-address        OCTET STRING (SIZE
                             (1..ub-emsd-address-length)),
         -- emsd-address is a decimal integer in BCD
            (Binary Encoded Decimal) format.
         -- If it had an odd number of digits, it is
         -- padded with 0 on the left.

         emsd-name          [0]  IMPLICIT OCTET STRING
                                 (SIZE (0..ub-emsd-name-length))
                                 OPTIONAL
       };

       Originator and Recipients in the scope of EMSD network are identified
       by a digit based addressing scheme.  EMSDAddress can only be used
       where the scope of addressing has clearly been limited to the EMSD
       network.

    3.4.8  DateTime

       DateTime ::= INTEGER;

    Banan                        Informational                     [Page 36]

    RFC 2524                          EMSD                     February 1999

       DateTime is a Julian date, expressed as the number of seconds since
       00:00:00 UTC, January 1, 1970.

    3.4.9  AsciiPrintableString

       Iso8859String ::=  GeneralString;

       AsciiPrintableString ::= [APPLICATION 0]
                                IMPLICIT Iso8859String (FROM

           (" "|"!"|"#"|"$"|"%"|"&"|"'"|"("|")"|"*"|"+"|","|"-"|"."|"/"|
            "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"|":"|";"|"<"|"="|">"|
            "?"|"@"|"A"|"B"|"C"|"D"|"E"|"F"|"G"|"H"|"I"|"J"|"K"|"L"|"M"|
            "N"|"O"|"P"|"Q"|"R"|"S"|"T"|"U"|"V"|"W"|"X"|"Y"|"Z"|"["|"]"|
            "^"|"_"|"`"|"a"|"b"|"c"|"d"|"e"|"f"|"g"|"h"|"i"|"j"|"k"|"l"|
            "m"|"n"|"o"|"p"|"q"|"r"|"s"|"t"|"u"|"v"|"w"|"x"|"y"|"z"|"{"|
            "|"|"}"|"~"|"\"|""""));

    3.4.10  ProtocolVersionNumber

       ProtocolVersionNumber ::= [APPLICATION 1]    SEQUENCE
       {
         version-major                   INTEGER,

      +------------------+-------+----+---------+----+---------+-----+-----+
      |Operation         |Invoker|Sap |Performer|Sap |Duplicate|OpId |ESROS|
      |                  |       |Sel |         |Sel |Detect   |     |Use  |
      |__________________|_______|____|_________|____|_________|_____|_____|
      |submit            |UA     |4   |MTS      |5   |Yes      |33   |3-Way|
      |__________________|_______|____|_________|____|_________|_____|_____|
      |deliver           |MTS    |2   |UA       |3   |Yes      |35   |3-Way|
      |__________________|_______|____|_________|____|_________|_____|_____|
      |deliveryControl   |UA     |8   |MTS      |9   |No       |2    |2-Way|
      |__________________|_______|____|_________|____|_________|_____|_____|
      |submissionControl |MTS    |6   |UA       |7   |No       |4    |2-Way|
      |__________________|_______|____|_________|____|_________|_____|_____|
      |submissionVerify  |MTS    |6   |UA       |7   |No       |6    |2-Way|
      |__________________|_______|____|_________|____|_________|_____|_____|
      |deliveryVerify    |UA     |8   |MTS      |9   |No       |5    |2-Way|
      |__________________|_______|____|_________|____|_________|_____|_____|
      |getConfiguration  |UA     |8   |MTS      |9   |No       |7    |2-Way|
      |__________________|_______|____|_________|____|_________|_____|_____|
      |setConfiguration  |MTS    |6   |UA       |7   |No       |8    |2-Way|
      +------------------+-------+----+---------+----+---------+-----+-----+

                       Table 1:  EMSD-P Operations Summary

    Banan                        Informational                     [Page 37]

    RFC 2524                          EMSD                     February 1999

         version-minor           [0]     IMPLICIT INTEGER DEFAULT 0
       }

    3.5  Submission and Delivery Procedures

       Table 1 provides a comprehensive summary of EMSD-P operations, the
       SAP selectors used and the operation IDs used.

       Submission

       The semantics of a submission operation is Exactly Once.  Exactly
       Once means that every operation is carried out exactly one time, no
       more and no less.  This semantic can not be fully implemented
       because, if after invoking the operation, an invoker has a Success
       (e.g.  result) indication and the performer has a FAILURE.indication,
       and the network goes down, the result of the operation will be Zero
       (and not Exactly Once).

       No more than one is controlled and guaranteed by the performer by
       using the Duplicate Operation Detection Support Functions (see the
       chapter entitled Duplicate Operation Detection Support).

       Not zero but one is realized by performer by using the
       SubmissionVerify operation.  When the performer receives
       FAILURE.indication, it's responsibility is to resolve the case by
       using SubmissionVerify resulting in Not zero but one.

       Submission procedure is as follows:

          o Submit operation with 3-Way handshake and Duplicate Operation
            Detection Support Function is invoked.

          o If performer at EMSD-SA receives FAILURE.indication, it invokes
            SubmissionVerify.

          o Message is sent out by EMSD-SA only if result operation is
            confirmed or the operation is verified (in the case of
            FAILURE.indication).

       The semantic of SubmissionVerify operation is At Least Once.  This
       type of semantics corresponds to the case that invoker keeps trying
       over and over until it gets a proper reply.  This operation can be
       performed more than once without any harm.

    Banan                        Informational                     [Page 38]

    RFC 2524                          EMSD                     February 1999

       Implications:

          o MTS sends out the message if and only if it's sure that UA knows
            about it.

       Delivery

       The semantics of Deliver operation is Exactly Once.  Exactly Once
       means that every operation is carried out exactly one time, no more
       and no less.  This semantic can not be fully implemented and if after
       invoking the operation, invoker has Success indication and performer
       has FAILURE.indication, and the network goes down, the result of the
       operation will be Zero (and not Exactly Once).

       No more than one is controlled and guaranteed by performer and by
       using the Duplicate Operation Detection Support Functions.

       Not zero but one is realized by performer by using the DeliveryVerify
       operation.  When performer receives FAILURE.indication, it's
       responsible to resolve the case by using DeliveryVerify resulting in
       Not zero but one.

       Delivery procedure is as follows:

          o Deliver operation with 3-Way handshake is invoked.

          o If performer at User Agent (device) receives FAILURE.indication,
            it invokes DeliveryVerify.

       The semantic of DeliveryVerify operation is At Least Once.  This type
       of semantics corresponds to the case that invoker keeps trying over
       and over until it gets a proper reply.  This operation can be
       performed more than once without any harm.

       Implications:

          o A non-delivery report is sent by MTS only if the message is not
            delivered.

          o The UA is responsible for notifying the MTS (through an explicit
            deliveryVerify) to make sure that a delivery report is sent out.

    Banan                        Informational                     [Page 39]

    RFC 2524                          EMSD                     February 1999

    4  DUPLICATE OPERATION DETECTION SUPPORT

    4.1  Duplicate Operation Detection Support Overview

       Some operations are idempotent in nature, i.e.  they can be performed
       more than once without any harm.  However, some other operations are
       non-idempotent in nature, i.e.  they should be performed only once.
       In the case of non-idempotent operations, performer should be able to
       detect duplicate operations and perform each non-idempotent operation
       only once.

       Examples of non-idempotent operations are Submission and Delivery of
       messages which shouldn't be performed more than once.  Examples of
       idempotent operations are Submission-control and Delivery-control
       which can be performed more than once with no harm.

       ESRO Services don't detect duplicate invocation of operations.  As a
       result, the Duplicate Operation Detection Support Functional Unit is
       used to detect duplication when the same operation instance is
       invoked more than once.  Invoker assigns an Operation Instance
       Identifier to an operation and this Operation Instance Identifier is
       used at the peer performer entity to detect the duplicate invocation
       of the same operation.

       Using this support, non-idempotent operations can be repeated over
       and over with no harm because the duplicate invocations are detected
       by this functional unit.  This support helps the performer not to
       perform an operation more than once.

       Support for duplication detection is realized through allocating
       Operation Instance Id (see Section 4.1.2, "Operation Instance
       Identifier") to an operation by invoker.  When an operation is
       invoked using duplication detection support, performer logs the
       Operation Instance Identifier and checks the next operations against
       duplication.

       Operation value identifies whether performer should detect duplicate
       operations (see Section 4.1.1, "Operation Value") and Operation
       Instance Id is assigned by invoker and sent as the first byte of
       operation's parameter.

    4.1.1  Operation Value

       Operation Values are divided into two groups.  Operation values from
       0 to 31 do not have Duplicate Operation Detection Support (0 to 31)
       and operation values from 32 to 63 have Duplicate Operation Detection
       Support.

    Banan                        Informational                     [Page 40]

    RFC 2524                          EMSD                     February 1999

       Duplicate Operation Detection Functional Unit checks for duplication
       only if Operation Value is in the range of 32 to 63.

       When invoker user uses an Operation Value in the range of 32 to 63
       which means operation with support for duplication detection, the
       user should specify an Operation Instance ID for the operation (see
       next section).

    4.1.2  Operation Instance Identifier

       To support duplication detection, an Operation Instance Identifier is
       assigned by invoker user and sent as the first byte of the
       operation's parameter.  This identifier is used on performer side to
       detect duplicate invocation of the same operation.  Characteristics
       of Operation Instance Identifier is as follows:

          o Operation Instance Identifier is one byte and can have values
            from 0 to 255.

          o Operation Instance Identifier is sent as the first byte of the
            operations parameter (without encoding).

          o The length of Operation Instance Identifier is 8-bit, but
            depending on the performer capabilities, it might keep 0 to 127
            Operation Instance Identifiers for duplication detection.  The
            performer profile defines the number of outstanding Operation
            Instance Identifiers that are checked against duplication.  When
            a performer profile indicates support for 0 outstanding
            Operation Instance Identifier, it means it does not have support
            for Duplicate Operation Detection.  In this case, there should
            be only one outstanding operation at any point of time.

          o Instance ID check is not part of ESROS, per se.  Use of
            Duplicate Detection is determined by EMSD-P. Operation Instance
            ID for operations 32-63 is the first byte of the argument.
            Duplicate Detection suuport strips that byte.

          o The Instance ID is not subject to Basic Encoding Rules (BER).

          o The invoker user assigns the Operation Instance Identifier to
            the operation at the time of requesting the invoke service.  The
            Operation Value should be in the range of operation values with
            duplication detection support, i.e.  32 to 63.

          o It's the responsibility of the user to choose Operation Instance
            Identifier in a way that uniqely and unambiguously identifies
            the operation.

    Banan                        Informational                     [Page 41]

    RFC 2524                          EMSD                     February 1999

          o From the invoker's perspective, assumption is that two
            operations with the same operation Instance Identifier are
            totally identical which means they produce exact same results.

          o Operation Instance Identifier uniqely specifies a non-idempotent
            operation and multiple invocations of such an operation will
            eventually result in the same outcome because the duplicate
            instances are identified and the operation is not performed more
            than once.

          o From the performer's perspective, assumption is that two
            operations with the same Operation Instance Identifier should be
            executed once and once only.

          o If requested, the degree of duplication checked by Duplicate
            Operation Detection Support Functional Unit on the performer's
            side (i.e.  the total number of outstanding Operation Instance
            Identifier kept) can be communicated with the invoker to
            synchronize the invocations.

          o User of Duplicate Operation Detection Support is responsible to
            behave based on the performer profile and its limitations in
            this regard.  This behavior is defined based on the desired
            semantic of the operation which is to be implemented.

          o On the performer side, when an Operation Instance Identifier is
            received, a previous Operation Instance Identifier whose
            distance to this latest one is greater than or equal to half of
            the wrap-around range of the Operation Instance Identifier
            number is expired, i.e.  for an 8-bit Operation Instance
            Identifier, the distance of 128 causes an old Operation Instance
            Identifier to expire.

          o It's the responsibility of the invoker user to use consecutive
            Operation Instance Identifier numbers, or when it skips some
            Operation Instance Identifiers, it should remember that if there
            is an smaller Operation Instance Identifier on performer side
            with the distance explained above, it will be expired.

    5  EMSD PROCEDURE FOR OPERATIONS

       The following sections shows the general procedures to be used in the
       implementation of the EMSD Message Transfer Server (MTS) and the EMSD
       User Agent (UA), with the option for 3-Way or 2-Way handshakes on
       operations which support them.  These procedures do not constitute
       complete behavior specifications for implementations.  The following
       sections contain information helpful to implementors.

    Banan                        Informational                     [Page 42]

    RFC 2524                          EMSD                     February 1999

       The MTS and the UA are event-driven.  Each waits for any of the
       possible event types, and, upon receiving an event, processes it.
       After processing the event, the next event is waited upon.

    5.1  MTS Behavior

       The MTS is event-driven.

       If it received an event from ESROS, then it could be any of the
       following types:

          o Message submit indication;

          o Message submit confirm and failure indication;

          o Result and Error indication for a deliver operation;

          o DeliveryVerify indication;

          o Result and Error indication for a submissionVerify operation;

          o Result and Error indication for a submissionControl operation;

          o DeliveryControl indication.

       For an ESROS event responsibility is passed to the MTS performer
       (Section 5.1.1).

       If the MTS received an event:

          o for message delivery, from the RFC-822 mailer;

          o requesting submission controls upon the UA, or;

          o indicating an elapsed timer (meaning that it's time to re-
            attempt a message delivery)

       then responsibility is passed to the MTS invoker (Section 5.1.5).

    5.1.1  MTS Performer

       The MTS performer is responsible for processing the following
       operations, received from ESROS:

          o Message-submission

          o Delivery-control

    Banan                        Informational                     [Page 43]

    RFC 2524                          EMSD                     February 1999

          o Delivery-verify

       The MTS performer should first make sure that it has received an
       INVOKE.indication.  Any other type of primitive shouldn't be
       occurring at this point, and should be ignored.

       If there's something wrong with the PDU or operation data, the MTS
       performer should send back an error to the proper invoker:

        1. Send an ESROS Error Request, then go wait for a response (either
           a confirmation or a failure indication).  The response is sent
           back on the same SAP type on which the event occurred.

        2. Keep track of the type of request that was issued.

       If there isn't anything wrong with the PDU or operation data, then
       the MTS performer has received a valid event from ESROS. This could
       be any of the defined Submission and Delivery Protocol operations.

    5.1.2  Message-submission

        1. The Message-submission operation first checks to see which SAP
           this Submit Request came in on.

        2. The request could have arrived as 2-Way SAP (see #3) or a 3-Way
           SAP (see #7).

        3. If the event arrived on the 2-Way SAP, consider this a protocol
           violation and ignore it.

        4. Wait for a response to the request.  The response could be either
           an ERROR.confirm (see #5) or a FAILURE.indication (see #6).

        5. The ERROR.request has been confirmed.  The UA knows that the
           submitted message wasn't sent.  Since there was an error, there
           is nothing more to do, so return.

         6. If the result to the ErrorRequest is a Failure.indication, it
           can be assumed that either the UA has received nothing (the
           ERROR.request PDU was lost), which means failure for the UA; or
           that the 3-Way acknowledgment was lost, which means that the UA
           has in fact received the ERROR.request PDU and knows about the
           delivery failure.  Either way, the message can be ignored.  There
           is nothing more to do, so return.

        7. If the event was received on the 3-Way SAP, then this is the
           correct SAP on which to receive a Submit Request.  Send back a
           Result Request and keep track of the primitive which was issued.

    Banan                        Informational                     [Page 44]

    RFC 2524                          EMSD                     February 1999

        8. Now wait for a response to our request.  The response will be
           either a Result.confirm (see #9) or a Failure.indication (see
           #13).

        9. The RESULT.request has been confirmed.

        10. Submit the message to the RFC-822 mailer.

        11. Attempt, a number of times, to send the submitted message via
            the RFC-822 mailer.  If the send was successful, then return.

        12. If, after the maximum number of retries, the message was not
            able to be sent, consider it a failure.  Since the UA assumption
            has been that submission was successful, but now it has not been
            sent, a brand new message, a Non-Delivery message, must be
            generated and delivered to the UA. When this is completed, then
            return.

        13. A FAILURE.indication has occurred due to the previously issued
            RESULT.request.

        14. A Submission Verification is issued to the UA to see if the
            RESULT.request was received.  There are three possible results
            from sending the submission verification to the UA: Fail (see
            #15), Send Message (see #16) or Drop Message (see #20).

        15. Fail -- The Submission-verify request didn't reach the UA, or
            the Submission Verify response didn't get back.  Ignore the
            message and return.

        16. The Submission Verify operation succeeded, meaning that the UA
            received the request, and responded with a message stating that
            it wants the message to be sent.

        17. Attempt, a number of times, to send the submitted message via
            the RFC-822 mailer.

        18. If the message was submitted to the RFC-822 mailer successfully,
            then return.  If, after the maximum number of retries, the
            message was not able to send the message, consider it a failure.

        19. The UA already assumes that the Message-submission was
            successful.  Now since the submitted message has not been sent,
            a brand new message, a Non-Delivery message, must be generated
            and delivered to the UA. After this is accomplished, then
            return.

    Banan                        Informational                     [Page 45]

    RFC 2524                          EMSD                     February 1999

        20. The UA responded with a message stating that the message should
            be dropped.  This may occur if the UA never received the result
            from the MTS, meaning that it never received the Message Id, and
            had to therefore inform the user that the message couldn't be
            submitted.  This may also occur if the UA doesn't have the
            record of the message being verified.  It can be because the
            message record has been aged and expired, or because the EMSD-UA
            has not been able to keep the record of the received message
            because of storage or memory limitations.  There is nothing to
            do, so return.

    5.1.3  Delivery-control

       This operation can be processed immediately.  After it is processed,
       the appropriate result is returned.

    5.1.4  Delivery-verify

       This operation occurs when the UA doesn't think that the MTS has
       received the RESULT.indication from a previously delivered message.
       The UA wants to make sure that the MTS knows it has been delivered.
       The MTS will determine what it knows of the specified message, and
       send back a result.  This can be processed immediately, as it doesn't
       need to deal with duplicate detection.

    5.1.5  MTS Invoker

       The MTS invoker is responsible for processing the following
       operations, received from ESROS:

          o Message-delivery

          o Submission-control

          o Submission-verify

       Submission-control

       Process the Submission Control request.

       Message-delivery

        1. Check the User Agent's profile to determine the SAP.

        2. Set the SAP to 3-Way.

    Banan                        Informational                     [Page 46]

    RFC 2524                          EMSD                     February 1999

        3. Issue the INVOKE.request on the appropriate SAP, with duplication
           detection enabled.  Since a local error is possible on issuing
           the INVOKE.request, a retry counter is needed.

        4. There are three possible events possible in result to the
           INVOKE.request:  an ERROR.indication (see #5), a
           RESULT.indication (see #9) or a FAILURE.indication (see #10).

        5. An ERROR.indication was received, which means that the UA can't
           accept the message right now.

        6. If the reason was one of a transient nature, wait for a while and
           then send the Deliver Request again.

        7. If the reason was one of a permanent nature, send back a non-
           delivery report to the originator.

        8. Since the error was one of a permanent nature, then the MTS must
           send back a non-delivery report, then log the unsuccessful
           delivery with error from UA and return.

        9. A RESULT.Indication was returned, which means that the Delivery
           was successful.  Send a delivery report to the originator if one
           was requested and log successful delivery and return.

           If the UA profile indicated that Complete mode was to be used,
           keep track of the fact that this message has been successfully
           delivered (as far as the MTS is concerned), so that if the UA
           sends us a Delivery Verify operation, we know that we consider
           the message to be delivered.

        10. A FAILURE.indication was returned, which means there was a
            problem getting the Deliver Request to the UA, or in getting the
            response back from the UA. In any case, a response was never
            received, so the request timed out.  Wait for a while, and then
            send the Deliver Request again.

            As long as a FAILURE.indication is returned and the number of
            retries has not been exceeded, keep trying to verify the
            delivery.

       Submission-verify

       The Submission-verify operation is always issued on the 2-Way SAP.
       The response is awaited.  If a response doesn't come, the request is
       queued and attempted again later.

    Banan                        Informational                     [Page 47]

    RFC 2524                          EMSD                     February 1999

        1. Issue the INVOKE.request on the 2-Way SAP, with duplication
           detection disabled.  Since a local error on issuing the invoke
           request is possible, a retry counter is needed.

        2. An INVOKE.Request has been issued and a response has been
           received.  The response will be either a a RESULT.indication (see
           #3) or a FAILURE.indication (see #4).  There are no defined
           errors to a Submission Verify operation, so an ERROR.indication
           should not be occurring here.

        3. A RESULT.indication was received.  Either ResponseSendMessage or
           ResponseDropMessage, as specified in the PDU, will be returned.

        4. A FAILURE.indication was received, which means that there was a
           problem getting the Submission Verify Request to the UA, or in
           getting the response back from the UA. In any case, the response
           was never received, so the request timed out.  Wait for a while,
           and then another attempt to send the Submission Verify request is
           needed.

       Non-Delivery Report

       Issue an INVOKE.request containing a Submit operation with a content
       type of Non-Delivery Report, to the UA. This operation is always
       issued on the 2-Way SAP. The response is awaited.  If a response
       doesn't come, the request is queued and attempted again later.

        1. Create a Submit operation.

        2. Issue the INVOKE.request on the 2-Way SAP, with duplication
           detection enabled.  Since a local error on issuing the invoke
           request is possible, a retry counter for is needed.

        3. A response to the INVOKE.Request has been received.  The response
           will be either a RESULT.indication (see #5), ERROR.indication
           (see #4), or a FAILURE indication (see #7).

        4. An ERROR.indication was received, which means that the UA doesn't
           know what to do with our non-delivery report.  That's the UAs
           problem, so just do nothing and return.

        5. A RESULT.indication was received, which means we delivered a
           successful non-delivery report.

        6. The result is logged.  Nothing more is needed, so return.

    Banan                        Informational                     [Page 48]

    RFC 2524                          EMSD                     February 1999

        7. A FAILURE.indication was received, which means there was a
           problem getting the Submit Request to the UA, or in getting the
           response back from the UA. In any case, the response was never,
           so the request timed out.  Wait for a while, and then send the
           Submission Verify request again.

    5.2  UA Behavior

       The User Agent is event-driven.

       If it received an event from ESROS, then it could be any of the
       following types:

          o Message deliver indication;

          o Message deliver confirm and failure indication;

          o Result and Error indication for a submit operation;

          o Submission verify indication;

          o Result and Error indication for a delivery verify operation;

          o Result and Error indication for a delivery control operation;

          o Submission control indication.

       For an ESROS event responsibility is passed to the UA performer
       (Section 5.2.1).

       IF the UA received an event indicating that there's a message from
       the user, for submission, then responsibility is passed to the UA
       invoker (Section 5.2.2).

    5.2.1  UA Performer

       The performer on the UA side is responsible for processing the
       following operations:

          o Message Delivery

          o Submission Verification

          o Submission Control

    Banan                        Informational                     [Page 49]

    RFC 2524                          EMSD                     February 1999

       Message-delivery

        1. A Message-delivery request is received.

        2. Check for the correctness of the PDU. If the PDU is bad the see
           #3.  If the PDU is good then see #8.

        3. Send an ESROS ERROR.request.  If the request arrived on a 3-Way
           SAP, use a 3-Way SAP for the result.  If the request arrived on a
           2-Way SAP, use a 2-Way SAP for the result.  Keep track of the
           type of request that was issued.

        4. Wait for the ESROS event.  The result could be an ERROR.confirm
           (see #5) or a FAILURE.indication (see #7).

        5. The ESROS event was an ERROR.confirm

        6. Log the message as the Non-Delivery was confirmed by the MTS and
           return.

        7. If the ESROS event was a FAILURE.indication, that means one of
           two things has occurred:

           A. The MTS has received nothing (the ERROR.request PDU was lost),
              which means that the MTS doesn't know that the message
              delivery has been rejected.  In this case, the MTS will
              eventually time out, and retransmit the message delivery
              request.

           B. The 3-Way acknowledgment was lost, which means that the MTS
              has in fact received the ERROR.request PDU and knows about the
              delivery failure.

           Either way, the message can now be ignored.

        8. Send an ESROS RESULT.request.  If the request arrived on a 3-Way
           SAP, use a 3-Way SAP for the result.  If the request arrived on a
           2-Way SAP, use a 2-Way SAP for the result.  Keep track of the
           type of request that was issued.

        9. Wait for the ESROS event.  The result could be an RESULT.confirm
           (see #10) or a FAILURE.indication (see #13).

        10. If the event is a RESULT.confirm, then the delivered message can
            now be given to the user.

        11. Deliver the message to the user.

    Banan                        Informational                     [Page 50]

    RFC 2524                          EMSD                     February 1999

        12. Log the message as Message Delivery Known to MTS.

        13. If the event is a FAILURE.indication, then, if the delivery was
            on a 3-Way SAP, a Delivery Verification request to the MTS can
            be issued to see if the MTS actually got the RSULT.request.  If
            the delivery was on a 2-Way SAP, then the message will delivered
            to the user and if the MTS has not received the RESULT.request,
            it will retransmit it later and the duplicate will be ignored.

        14. Deliver the message to the user.  Since a FAILRUE.indication was
            received in response to a RESULT.requst, it means that possible,
            the MTS didn't receive the RESULT.request.  The MTS could now
            time out, and send another copy of the same message.  Save the
            message for duplication detection.

        15. Log the fact that the message was delivered, but that the MTS
            might not be aware of it.

        16. If the UA supports Delivery Verification, and the Delivery
            Request was sent on the 3-Way SAP, then see #17.  If either of
            these conditions are not true, then return.

        17. Send a Delivery-verify request to see if the MTS got the
            RESULT.request.

            There are three possible results from sending the delivery
            verification to the MTS: Fail (see #18), ResponseNonDelivery
            (see #20) or ResponseDelivery (see #23).

        18. Fail -- Delivery Verify request didn't reach the MTS, or the
            Delivery Verify response didn't get back to the UA.

        19. Log this as delivering the message to the user, but the MTS
            having possibly sent a Non-Delivery report to the originator
            even though the UA did actually deliver the message to the user.
            Then return.

        20. ResponseNonDelivery -- Verify Response indicates that the MTS
            now knows (because of the Delivery Verify operation that the
            message has been delivered to the user, but had not received our
            RESULT.request nor a Delivery Verify operation in a timely
            manner, and had already sent out a Non-Delivery report to the
            originator.

        21. The MTS had not received, from the UA, in a timely manner, a
            RESULT.indication indicating that the message had been delivered
            to the user.  The MTS has already sent a Non-Delivery report to

    Banan                        Informational                     [Page 51]

    RFC 2524                          EMSD                     February 1999

            the originator.  The UA must let the user know about this.  Log
            the message as delivered to the user, but a Non-Delivery sent to
            the originator.

        22. Since the UA received a response to the Verify operation, it
            knows that the MTS knows about this message delivery, so the UA
            also knows that it won't be receiving a duplicate of it.  The UA
            can now remove this message's Message Id from the list of
            possible duplicates.

        23. ResponseDelivery -- Verify Response received from MTS.

        24. This means that the MTS knows (either because the MTS had
            received the RESULT.request that was sent by the UA or because
            the MTS has now received the UAs Delivery-verification message,
            informing that the UA received the message for delivery to the
            user.  The MTS is (or was) able to send a Delivery report to the
            originator if one was requested.  Log it as such.

        25. Since the UA received a response to the Verify operation, it
            knows that the MTS knows about this message delivery, so the UA
            also knows that it won't be receiving a duplicate of it.  The UA
            can now remove this message's Message Id from the list of
            possible duplicates and return.

       Submission-verify

       Process the Submission-verify request and return.

       Submission-control

       This operation can be processed immediately.  After it is processed,
       the appropriate result is returned.

    5.2.2  UA Invoker

       The invoker on the UA side is responsible for processing the
       following operations:

          o Message-submission

          o Delivery-control

          o Delivery-verify

    Banan                        Informational                     [Page 52]

    RFC 2524                          EMSD                     February 1999

       Message-submission

       General procedures for UA's Message-submission mirror that of MTS's
       Message-delivery.

       Delivery-control

        1. Issue the INVOKE.request on the 3-Way SAP, with duplication
           detection enabled.  Since the UA can get a local error on issuing
           the invoke request, a retry counter is needed.

           If we got a local failure in issuing the Invoke Request, wait a
           while and then try again (up to the limit of the maximum number
           of retries).

        2. The UA has issued an INVOKE.Request.  Wait for a response from
           ESROS. The response will be either a RESULT.indication (see #5),
           ERROR.indication (see #3), or FAILURE.indication (see #7).

        3. A ERROR.indicaiton was received, meaning that the MTS told says
           that it cannot accept the message.

        4. Log the MTS rejection and return

        5. A RESULT.indication was received, which means that the Submission
           was successful.

        6. Log successful submission and return.

        7. a FAILURE.indication was received, meaning that there was a
           problem getting the Submit Request to the MTS, or in getting the
           response back from the MTS. In any case, the UA never received
           the response, so the request timed out.  Wait for a while, and
           then send the Submit Request again.

        8. The UA has exceeded the maximum number of retries.  Let the user
           know, log the failure and return.

       Delivery-verify

       General procedures for UA's Delivery-verify mirror that of MTS's
       Submission-verify.

    Banan                        Informational                     [Page 53]

    RFC 2524                          EMSD                     February 1999

    6  EMSD FORMAT STANDARDS

    6.1  Format Standard Overview

       EMSD Format Standard (EMSD-FS) is a non-textual form of compact
       encoding of Internet mail (RFC-822) messages which facilitates
       efficient transfer of messages.  EMSD-FS is used in conjunction with
       the EMSD-P but is not a general replacement for RFC-822.  EMSD-FS
       defines a method of representation of short interpersonal message.
       It defines the "Content" encoding (Header + Body).  Although EMSD-FS
       contains end-to-end information its scope is purely point-to-point.

       The "Efficient InterPersonal Message Format Standard" is defined in
       this section.  This standard is primarily intended for communication
       among people.

       The EMSD Format Standard is designed to be fully consistent with
       RFC-822 [3].  In many ways EMSD-FS can be considered to be an
       efficiency oriented encoder and decoder.  Through use of EMSD-FS an
       RFC-822 message is converted to a more compact binary encoding.  This
       more compact message is then transfered between an EMSD-SA and EMSD-
       UA. The compact message (represented in EMSD-FS) may then be
       converted back to RFC-822 intact.

       For messages that are originated (submitted) with EMSD protocol,
       certain fields (e.g., addresses, message-id) can have special forms
       that are specialized and produce more compact EMSD-FS encoding.
       These special forms are legitimate values of RFC-822 messages.

       This specification expresses information objects using ASN.1 [X.208].
       Encoding of ASN.1 shall be based on Basic Encoding Rules (BER) [5].
       Future revisions of this specification will use Packed Encoding Rules
       (PER) [4].

       The convention of (O) "OPTIONAL", (D) "DEFAULT", (C) "CONDITIONAL"
       and (M) "MANDATORY" which express requirements for presence of
       information is used in this section.

    6.2  Interpersonal Messages

       An interpersonal message (IPM) consists of a heading and a body.

       IPM ::=   SEQUENCE

       {

         heading       Heading,

    Banan                        Informational                     [Page 54]

    RFC 2524                          EMSD                     February 1999

         body          Body OPTIONAL

       };

    6.2.1  Heading fields

       The fields that may appear in the Heading of an IPM are defined and
       described below.

       Heading ::= SEQUENCE
       {
         -- Address of the sending agent (person, program, machine) of
         -- this message. This field is mandatory if the sender
         -- is different than the originator.
         sender                      [0]     EMSDORAddress OPTIONAL,

         -- Address of the originator of the message
         -- (not necessarily the sender)
         originator                          EMSDORAddress,

         -- List of recipients and flags associated with each.
         recipient-data                      SEQUENCE SIZE (1..ub-recipients)
                                             OF PerRecipientFields,

         -- Flags applying to this entire message
         per-message-flags           [1]     IMPLICIT BIT STRING

          {
          -- Priority values
          -- At most one of "non-urgent" and "urgent" may be specified
          -- concurrently.  If neither is specified, then a Priority
          -- level of "normal" is assumed.
          priority-non-urgent             (0),
          priority-urgent                 (1),

          -- Importance values

          -- At most one of "low" and "high" may be specified
          -- concurrently.  If neither is specified, then an
          -- Importance level of "normal" is  assumed.
          importance-low                  (2),
          importance-high                 (3),

          -- Indication of whether this message has been
             automatically forwarded
          auto-forwarded                  (4)
          } OPTIONAL,

    Banan                        Informational                     [Page 55]

    RFC 2524                          EMSD                     February 1999

         -- User-specified recipient who is to receive replies
            to this message.
         reply-to                    [2]     IMPLICIT SEQUENCE SIZE
                                             (1..ub-reply-to)
                                            OF EMSDORAddress OPTIONAL,

         -- Identifier of a previous message, for which this message
         -- is a reply
         replied-to-IPM                       EMSDMessageId OPTIONAL,

         -- Subject of the message.
         subject                     [3]     IMPLICIT AsciiPrintableString
                                             (SIZE (0..ub-subject-field))
                                                       OPTIONAL,

         -- RFC-822 header fields not explicitly provided for in
         -- this Heading. For messages incoming from the external
         -- world (i.e. in RFC-822 format), the Message-Id: field
         -- need not go here, as it is placed in the
         -- Envelope's EMSDMessageId (message-id) field.
         extensions        [4]  IMPLICIT  SEQUENCE
                                (SIZE (0..ub-header-extensions))
                                OF  IPMSExtension OPTIONAL,

         -- MIME Version (if other than 1.0)
         mime-version            [5]     IMPLICIT AsciiPrintableString
                                         (SIZE (0..ub-mime-version-length))
                                                      OPTIONAL,

         -- Top-level MIME Content Type
         mime-content-type       [6]     IMPLICIT AsciiPrintableString
                                         (SIZE (0..
                                          ub-mime-content-type-length))
                                                   OPTIONAL,

         -- MIME Content Id
         mime-content-id         [7]     IMPLICIT AsciiPrintableString
                                         (SIZE (0..
                                          ub-mime-content-id-length))
                                                   OPTIONAL,

         -- MIME Content Description
         mime-content-description [8]    IMPLICIT AsciiPrintableString
                                         (SIZE (0..ub-mime-content-
                                         description-length))
                                                   OPTIONAL,
         -- Top-level MIME Content Type
         mime-content-transfer-encoding

    Banan                        Informational                     [Page 56]

    RFC 2524                          EMSD                     February 1999

                                  [9]     IMPLICIT AsciiPrintableString
                                          (SIZE (0..ub-mime-content-
                                          transfer-encoding))
                                                   OPTIONAL
       };

       Some fields have components and thus are composite, rather than
       indivisible.  A field component is called a sub-field.

       Sender

       This field is mandatory if the sender is different from the
       originator.

       Originator

       The Originator heading field (O) identifies the IPM's originator.

       Recipient-data

       PerRecipientFields ::= SEQUENCE
       {
         recipient-address                            EMSDORAddress,
         per-recipient-flags                          BIT STRING

         {
         -- Recipient Types.
         -- At most one of "copy" and "blind-copy" may be
         -- specified concurrently for a single recipient.  If
         -- neither is specified, than the recipient
         -- is assumed to be a "primary" recipient.
         recipient-type-copy                             (0),
         recipient-type-blind-copy                       (1),

         -- Notification Request Types.
         -- Only one of "rn" and "nrn" may be specified
         -- concurrently, \x110011 for a single recipient.
         -- "rn" implies "nrn" in addition.
         notification-request-rn                         (2),
         notification-request-nrn                        (3),

         notification-request-ipm-return                 (4),

         -- Report Request Types

    Banan                        Informational                     [Page 57]

    RFC 2524                          EMSD                     February 1999

         -- At most one of these should be set for a
         -- particular recipient. "delivery" implies "non-delivery"
         -- in addition.
         report-request-non-delivery                     (5),
         report-request-delivery                         (6),

         -- Originator-to-Recipient request for a reply.
         reply-requested                                 (7)
         } DEFAULT { report-request-non-delivery }

       };

       recipient-address

       The Primary Recipients heading field identifies the zero or more
       users who are the "primary recipients" of the IPM. The primary
       recipients might be those users who are expected to act upon the IPM.

       per-recipient-flags

       The Copy Recipients heading field identifies the zero or more users
       who are the "copy recipients" of the IPM. The copy recipients might
       be those users to whom the IPM is conveyed for information.

       recipient-type-copy

       This field is set if the recipient is on the Carbon Copy (CC) list.

       recipient-type-blind-copy

       This field is set if the recipient is on the Blind Carbon Copy (BCC)
       list.

       The Blind Copy Recipients heading field (C) identifies zero or more
       users who are the intended blind copy recipients of the IPM.

       The phrase "copy recipients" above has the same meaning as in "Copy
       Recipients" from Section 6.2.1 .  A blind copy recipient is one whose
       role as such is disclosed to neither primary nor copy recipients.

    Banan                        Informational                     [Page 58]

    RFC 2524                          EMSD                     February 1999

       In the instance of an IPM intended for a blind copy recipient, this
       conditional field shall be present and identify that user.  Whether
       it shall also identify the other blind copy recipients is a local
       matter.  In the instance of the IPM intended for a primary or copy
       recipient, the field shall be absent.

       notification-request-rn

       A receipt notification (rn) reports its originator's receipt, or his
       expected and arranged future receipt, of an IPM.

       notification-request-nrn

       A non-receipt notification (nrn) reports its originator's failure to
       receive, to accept, or his delay in receiving, an IPM.

       notification-request-ipm-return

       When this field is set, the contents of the message are returned
       along with the notification.

       report-request-non-delivery

       The report request enables the MTS to acknowledge to the MTS-user one
       or more outcomes of a previous invocation of the message-submission
       or probe-submission abstract-operations.

       A report is returned only in case of non-delivery.

       report-request-delivery

       For the message-submission, report-delivery indicates the delivery or
       non-delivery of the submitted message to one or more recipients.  For
       the probe-submission, the report-delivery indicates whether or not a
       message could be delivered if the message were to be submitted.

       reply-requested

       When set this field indicates that the originator requests that a
       recipient send a message in reply to the message which carries the
       request.

    Banan                        Informational                     [Page 59]

    RFC 2524                          EMSD                     February 1999

       per-message-Flags

       Priority

       The Priority field (default is normal) identifies the priority that
       the authorizing users attach to the IPM. It may assume any one of the
       following values:  urgent, normal, or non-urgent.

       At most one of either "non-urgent" or "urgent" may be specified
       concurrently.  If neither is specified, then a Priority level of
       "normal" is assumed.

       Importance

       The Importance heading field (default normal) identifies the
       importance that the authorizing users attach to the IPM. It may
       assume any one of the following values:  low, normal, or high.

       At most one of either "low" or "high" may be specified concurrently.
       If neither is specified, then a Importance level of "normal" is
       assumed.

       The values above are not defined by this specification; they are
       given meaning by users.

       auto-forwarded

       The Auto-forwarded heading field (default is false) indicates whether
       the IPM is the result of auto-forwarding.  It is a Boolean value.

       reply-to

       User-specified recipient or recipients who are to receive replies to
       this message.

       replied-to IPM

       The Replied-to IPM heading field (C) identifies the IPM to which the
       present IPM is a reply.  It comprises an IPM identifier.

       This conditional field shall be present if, and only if, the IPM is a
       reply.

    Banan                        Informational                     [Page 60]

    RFC 2524                          EMSD                     February 1999

       Note - In the context of forwarding, care should be taken to
       distinguish between the forwarding IPM and the forwarded IPM. This
       field should identify whichever of these two IPMs to which the reply
       responds.

       subject

       The Subject heading field (O) identifies the subject of the IPM. It
       corresponds to the "Subject:" field of RFC-822.

       extensions

       The Extensions heading field [D no extensions (i.e.  members)]
       conveys information accommodated by no other heading field.  It
       comprises a Set of zero or more IPMS extensions, each conveying one
       such information item.

       IPMSExtension ::= SEQUENCE
       {
           x-header-label                      AsciiPrintableString,
           x-header-value                      AsciiPrintableString
       };

    6.2.2  Body part types

       The types of body parts that may appear in the Body of an IPM are
       structured using the MIME specification.

       Body ::= SEQUENCE
       {
         compression-method          [0]     IMPLICIT CompressionMethod
                                                      OPTIONAL,
         -- If compression method is not specified,
         -- "no-compression" is implied.

         message-body                        OCTET STRING
         -- See MIME for structure of the Body.
         -- If a compression method is specified, the entire text containing
         -- the Content-Type: element followed by the RFC-822 body are
         -- compressed using the specified method, and placed herein.
       };

       CompressionMethod ::= INTEGER
       {
         -- Compression Methods numbered 0 to 63 are reserved for
         -- assignment within this and associated specifications.

    Banan                        Informational                     [Page 61]

    RFC 2524                          EMSD                     February 1999

         no-compression                  (0),
         lempel-ziv                      (1)

         -- Compression Methods numbered between 64 and 127 may be
         --  used on a bilaterally-agreed basis between peers.
       } (0..127)

    7  ACKNOWLEDGMENTS

       In the context of Limited Size Messaging (LSM) over CDPD and pACT
       over Narrowband PCS, AT&T Wireless Services (AWS), funded work which
       was relevant to the development of the EMSD protocols.

    8  SECURITY CONSIDERATIONS

       This protocol supports simple authentication of the originator's
       address by the EMSD-SA and simple authentication of EMSD-SA by EMSD-
       UA.

       Mainstream Internet mail security mechanisms can be used in
       conjunction with the EMSD protocol.

    9  AUTHOR'S ADDRESS

       Mohsen Banan
       Neda Communications, Inc.
       17005 SE 31st Place
       Bellevue, WA 98008

       EMail: mohsen@neda.com

    Banan                        Informational                     [Page 62]

    RFC 2524                          EMSD                     February 1999

    A  EMSD-P ASN.1 MODULE

       This section compiles in one place the complete ASN.1 Module for EM
       Submission and Delivery Protocol.

       EMSD-SubmissionAndDeliveryProtocol DEFINITIONS ::=

       BEGIN

       EXPORTS EMSDORAddress, AsciiPrintableString, ContentType,
       DateTime, EMSDMessageId, EMSDORAddress, ProtocolVersionNumber;

       -- Upper bounds

       ub-recipients  INTEGER ::= 256;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-reply-to INTEGER ::= 256;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-subject-field INTEGER ::= 128;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-password-length INTEGER ::= 16;
       ub-content-length INTEGER ::= 65535;
       -- also defined in EMSD-Probe
       ub-content-types INTEGER ::= 128;
       ub-message-id-length INTEGER ::= 127;
       ub-total-number-of-segments INTEGER ::= 32;
       ub-header-extensions INTEGER ::= 64;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-emsd-name-length INTEGER ::= 64;
       ub-emsd-address-length INTEGER ::= 20;
       ub-rfc822-name-length INTEGER ::= 127;
       ub-mime-version-length INTEGER ::= 8;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-mime-content-type-length INTEGER ::= 127;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-mime-content-id-length INTEGER ::= 127;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-mime-content-description-length INTEGER ::= 127;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-mime-content-transfer-encoding INTEGER ::= 127;
       -- also defined in EMSD-InterpersonalMessaging1995
       ub-local-message-nu INTEGER ::= 4096;

       ----------------------
       -- SUBMIT Operation --
       ----------------------

       submit ES-OPERATION

    Banan                        Informational                     [Page 63]

    RFC 2524                          EMSD                     February 1999

           ARGUMENT SubmitArgument
           RESULT SubmitResult
           ERRORS
           {
               submissionControlViolated,
               securityError,
               resourceError,
               protocolViolation,
               messageError
           } ::= 33;

       SubmitArgument ::= SEQUENCE
       {
         -- Security features
         security           [0]    IMPLICIT SecurityElement
                                   OPTIONAL,

         -- Segmentation features for efficient transport
         segment-info                  SegmentInfo OPTIONAL,

         -- Content type of the message
         content-type                            ContentType,

         --
         -- THE CONTENT --
         --

         -- The submission content
         content                       ANY DEFINED BY content-type

       };

       SubmitResult ::= SEQUENCE

       {

         -- Permanent identifier for this message.
         -- Also contains the message submission time.
         -- See comment regarding assignment of message
         -- identifiers, at the definition of EMSDLocalMessageId.
         message-id                        EMSDLocalMessageId
           };

       --------------------------------
       -- Delivery Control Operation --
       --------------------------------

       deliveryControl ES-OPERATION

    Banan                        Informational                     [Page 64]

    RFC 2524                          EMSD                     February 1999

           ARGUMENT DeliveryControlArgument
           RESULT DeliveryControlResult
           ERRORS
           {
               securityError,
               resourceError,
               protocolViolation
           } ::= 2;

       DeliveryControlArgument ::= SEQUENCE
       {
         -- Request an addition of or removal of a set of restrictions
         restrict             [0]     IMPLICIT Restrict DEFAULT update,

         -- Which operations are to be placed in the restriction set
         permissible-operations  [1]     IMPLICIT Operations OPTIONAL,

         -- What maximum content length should be allowed
         permissible-max-content-length
                                 [2]     IMPLICIT INTEGER
                                         (0..ub-content-length) OPTIONAL,

         -- What is the lowest priority message which may be delivered
         permissible-lowest-priority
                                 [3]     IMPLICIT ENUMERATED
                                         {
                                            non-urgent     (0),
                                            normal         (1),
                                            urgent         (2)
                                         } OPTIONAL,

         -- Security features
         security                  [4]     IMPLICIT SecurityElement
                                                         OPTIONAL,

         -- User Feature selection
         user-features             [5]     IMPLICIT OCTET STRING OPTIONAL
       };

       DeliveryControlResult ::= SEQUENCE
       {
         -- Operation types queued at the EMSD-SA due to existing
         -- restrictions.
         waiting-operations    [0]   IMPLICIT Operations DEFAULT { },

         -- Types of messages queued at the EMSD-SA due to
         -- existing restrictions

    Banan                        Informational                     [Page 65]

    RFC 2524                          EMSD                     February 1999

         waiting-messages      [1]   IMPLICIT WaitingMessages DEFAULT { },

         -- Content Types of messages queued at the EMSD-SA
         waiting-content-types   SEQUENCE SIZE (0..ub-content-types) OF
                                               ContentType DEFAULT { }
       };

       Restrict ::= ENUMERATED
       {
           update                                      (1),
           remove                                      (2)
       };

       Operations ::= BIT STRING
       {
           submission                                  (0),
           delivery                                    (1)
       };

       WaitingMessages ::= BIT STRING
       {
           long-content                                (0),
           low-priority                                (1)
       };

       -- Delivery Verify Operation

       deliveryVerify ES-OPERATION

           ARGUMENT DeliveryVerifyArgument
           RESULT DeliveryVerifyResult
           ERRORS
           {
               verifyError,
               resourceError,
               protocolViolation
           } ::= 5;

       DeliveryVerifyArgument ::= SEQUENCE
       {
         -- Identifier of this message. This is the same identifier that
         -- was provided to the originator in the Submission Result.
         -- See comment regarding assignment of message identifiers,
         -- at the definition of EMSDMessageId.
         message-id                                      EMSDMessageId
       };

    Banan                        Informational                     [Page 66]

    RFC 2524                          EMSD                     February 1999

       DeliveryVerifyResult ::= SEQUENCE
       {
                                status  DeliveryStatus
       };

        DeliveryStatus  ::= ENUMERATED
       {
               no-report-is-sent-out                   (1),
               delivery-report-is-sent-out             (2),
               non-delivery-report-is-sent-out         (3)
       };

       -----------------------
       -- DELIVER Operation --
       -----------------------

       deliver ES-OPERATION
           ARGUMENT DeliverArgument
           RESULT NULL
           ERRORS
           {
               deliveryControlViolated,
               securityError,
               resourceError,
               protocolViolation,
               messageError
           } ::= 35;

       DeliverArgument ::= SEQUENCE
       {
         -- Identifier of this message. This is the same identifier that
         -- was provided to the originator in the Submission Result.
         -- See comment regarding assignment of message identifiers,
         -- at the definition of EMSDMessageId.
         message-id                                      EMSDMessageId,

         -- Time the message was delivered to the recipient by EMSD-SA
         message-delivery-time                           DateTime,

         -- Time EMSD-SA originally took responsibility for processing
         -- of this message. This field shall be omitted if the message-id
         -- contains an EMSDLocalMessageId, because that field contains
         -- the submission time within it.
         message-submission-time [0]     IMPLICIT   DateTime OPTIONAL,

         -- Security features
         security                [1]     IMPLICIT   SecurityElement OPTIONAL,

    Banan                        Informational                     [Page 67]

    RFC 2524                          EMSD                     February 1999

         -- SegContentTypementation features for efficient transport
         segment-info                               SegmentInfo OPTIONAL,

         -- The type of the content
         content-type                               ContentType,

         --
         -- THE CONTENT --
         --

         -- The submitted (and now being delivered) content
         content                       ANY DEFINED BY content-type

       };

       -- Submission Control Operation

       submissionControl ES-OPERATION
           ARGUMENT SubmissionControlArgument
           RESULT SubmissionControlResult
           ERRORS
           {
               securityError,
               resourceError,
               protocolViolation
           } ::= 4;

       SubmissionControlArgument ::= SEQUENCE
       {
         -- Request an addition of or removal of a set of restrictions
         restrict               [0]     IMPLICIT Restrict DEFAULT update,

         -- Which operations are to be placed in the restriction set
         permissible-operations  [1]     IMPLICIT Operations OPTIONAL,

         -- What maximum content length should be allowed
         permissible-max-content-length
                                 [2]     IMPLICIT INTEGER
                                         (0..ub-content-length) OPTIONAL,

         -- Security features
         security                [3]     IMPLICIT SecurityElement
                                                         OPTIONAL
       };

       SubmissionControlResult ::= SEQUENCE
       {
         -- Operation types queued at the EMSD-SA due to existing

    Banan                        Informational                     [Page 68]

    RFC 2524                          EMSD                     February 1999

         -- restrictions.
         waiting-operations    [0]   IMPLICIT Operations DEFAULT { }

       };

       ----------------------------------
       -- Submission Verify Operation --
       ----------------------------------

       submissionVerify  ES-OPERATION

           ARGUMENT SubmissionVerifyArgument
           RESULT SubmissionVerifyResult
           ERRORS
           {
               submissionVerifyError,
               resourceError,
               protocolViolation
           } ::= 6;

       SubmissionVerifyArgument ::= SEQUENCE
         -- Identifier of this message. This is the same identifier that
         -- was provided to the originator in the Submission Result.
         -- See comment regarding assignment of message identifiers,
         -- at the definition of EMSDMessageId.
         {
            message-id                       EMSDMessageId
         };

       SubmissionVerifyResult ::= SEQUENCE
           {
               status  SubmissionStatus
           };

       SubmissionStatus::= ENUMERATED
       {
               send-message            (1),
               drop-message            (2)
       };

       -- GetConfiguration Operation
       -- To be fully defined later. This will possibly include,
       -- but not be limited to:
       --      get-local-time-zone
       --      get-protocol-version
       --      etc.

       getConfiguration ES-OPERATION

    Banan                        Informational                     [Page 69]

    RFC 2524                          EMSD                     February 1999

               ARGUMENT NULL
               RESULT NULL
               ERRORS
               {
                   resourceError,
                   protocolViolation
               } ::= 7;

       -- SetConfiguration Operation
       -- To be fully defined later.

       setConfiguration ES-OPERATION

               ARGUMENT NULL
               RESULT NULL
               ERRORS
               {
                   resourceError,
                   protocolViolation
               } ::= 8;

       -- Security --

       SecurityElement ::= SEQUENCE

       {
         credentials                   Credentials,
         contentIntegrityCheck         ContentIntegrityCheck OPTIONAL
       };

       Credentials ::= CHOICE
       {
         simple                          [0]   IMPLICIT SimpleCredentials
         -- Strong Credentials are for future study
         -- strong                       [1]   IMPLICIT StrongCredentials
         -- externalProcedure            [2]   EXTERNAL
       };

       SimpleCredentials ::= SEQUENCE

       {
         eMSDAddress                         EMSDAddress OPTIONAL,
         password                    [0]     IMPLICIT OCTET STRING
                                     (SIZE (0..ub-password-length)) OPTIONAL
       };

       -- StrongCredentials ::= NULL
       -- for now.

    Banan                        Informational                     [Page 70]

    RFC 2524                          EMSD                     February 1999

       -- ContentIntegrityCheck is a 16-bit checksum of content
       ContentIntegrityCheck ::= INTEGER (0..65535);

       SegmentInfo ::= CHOICE

       {
         first           [APPLICATION 2]         IMPLICIT FirstSegment,
         other           [APPLICATION 3]         IMPLICIT OtherSegment
       };

       FirstSegment ::= SEQUENCE

       {
         sequence-id                             INTEGER,
         number-of-segments                      INTEGER
         -- number-of-segments must not exceed ub-total-number-of-segments

       };

       OtherSegment ::= SEQUENCE

       {
         sequence-id                             INTEGER,
         segment-number                          INTEGER
       };

       -----------
       -- Errors --
       ------------

       protocolVersionNotRecognized  ERROR PARAMETER NULL ::= 1;

       submissionControlViolated  ERROR PARAMETER NULL ::= 2;

       messageIdentifierInvalid  ERROR PARAMETER NULL ::= 3;

       securityError ERROR PARAMETER security-problem SecurityProblem ::= 4;

       deliveryControlViolated   ERROR PARAMETER NULL ::= 5;

       resourceError  ERROR PARAMETER NULL ::= 6;

       protocolViolation  ERROR PARAMETER NULL ::= 7;

       messageError  ERROR PARAMETER NULL ::= 8;

       SecurityProblem ::= INTEGER (0..127);

    Banan                        Informational                     [Page 71]

    RFC 2524                          EMSD                     February 1999

       --
       -- EXPORTED Definitions (for use by associated specifications) --
       --

       ContentType ::=  INTEGER
       {
         -- Content type 0 is reserved and shall never be transmitted.
         reserved                                 (0),
         -- Content types between 1 and 31 (inclusive) are for
         -- internal-use only
         probe                                    (1), -- reserved
         delivery-report                          (2), -- reserved

         -- Content types between 32 and 63 (inclusive) are for
         -- message types  defined within this specifications.
         emsd-interpersonal-messaging-1995        (32),
         voice-messaging                          (33) -- reserved

         -- Content types beyond and including 64 are for
         -- bilaterally-agreed use between peers.
       } (0..127);

       -- If this message was originated as an RFC-822 message, then this
       -- EMSDMessageId shall be the "Message-Id:" field from that message.
       -- If this message was originated within the EMSD domain,
       -- then this identifier shall be unique for the Message Center
       -- generating this id.

       EMSDMessageId ::= CHOICE
       {
         emsdLocalMessageId     [APPLICATION 4]  IMPLICIT
                                EMSDLocalMessageId,
         rfc822MessageId        [APPLICATION 5]  IMPLICIT
                                AsciiPrintableString
                                (SIZE (0..ub-message-id-length))

       };

       EMSDLocalMessageId ::= SEQUENCE
       {
         submissionTime                  DateTime,
         messageNumber                   INTEGER (0..ub-local-message-nu)
       };

       -- An Originator/Recipient Address in EMSD Environment

       EMSDORAddress ::= CHOICE
       {

    Banan                        Informational                     [Page 72]

    RFC 2524                          EMSD                     February 1999

         -- This is the local-format address
         emsd-local-address-format            EMSDAddress,

         -- This is a globally-unique RFC-822 Address
         rfc822DomainAddress                 AsciiPrintableString
       };

       EMSDAddress ::= SEQUENCE
       {
         emsd-address         OCTET STRING
                                        (SIZE (1..ub-emsd-address-length)),

         -- emsd-address is a decimal integer in BCD (Binary Encoded Decimal)
         -- format.
         -- If it had an odd number of digits, it is padded with 0 on
         -- the left.

         emsd-name                [0]     IMPLICIT OCTET STRING
                                          (SIZE (0..ub-emsd-name-length))
                                          OPTIONAL
       };

       DateTime ::= INTEGER;

       Iso8859String ::=  GeneralString;

       AsciiPrintableString ::= [ APPLICATION 0 ]
                                IMPLICIT Iso8859String (FROM

           (" "|"!"|"#"|"$"|"%"|"&"|"'"|"("|")"|"*"|"+"|","|"-"|"."|"/"|
            "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"|":"|";"|"<"|"="|">"|
            "?"|"@"|"A"|"B"|"C"|"D"|"E"|"F"|"G"|"H"|"I"|"J"|"K"|"L"|"M"|
            "N"|"O"|"P"|"Q"|"R"|"S"|"T"|"U"|"V"|"W"|"X"|"Y"|"Z"|"["|"]"|
            "^"|"_"|"`"|"a"|"b"|"c"|"d"|"e"|"f"|"g"|"h"|"i"|"j"|"k"|"l"|
            "m"|"n"|"o"|"p"|"q"|"r"|"s"|"t"|"u"|"v"|"w"|"x"|"y"|"z"|"{"|
            "|"|"}"|"~"|"\"|""""));

       ProtocolVersionNumber ::= [APPLICATION 1]    SEQUENCE
       {
         version-major                   INTEGER,
         version-minor           [0]     IMPLICIT INTEGER DEFAULT 0
       }
       END  -- end of EMSD-SubmissionAndDeliveryProtocol

    Banan                        Informational                     [Page 73]

    RFC 2524                          EMSD                     February 1999

    B  EMSD-IPM ASN.1 MODULE

       This section compiles in one place the complete ASN.1 Module for
       EMSD-IPM.

       EMSD-InterpersonalMessaging1995 DEFINITIONS ::=

       BEGIN

       IMPORTS EMSDORAddress, EMSDMessageId, AsciiPrintableString
         FROM EMSD-SubmissionAndDeliveryProtocol;

       ub-recipients  INTEGER ::= 256;
       ub-reply-to INTEGER ::= 256;
       ub-subject-field INTEGER ::= 128;
       ub-header-extensions INTEGER ::= 64;
       ub-emsd-name-length INTEGER ::= 64;
       ub-mime-version-length INTEGER ::= 8;
       ub-mime-content-type-length INTEGER ::= 127;
       ub-mime-content-id-length INTEGER ::= 127;
       ub-mime-content-description-length INTEGER ::= 127;
       ub-mime-content-transfer-encoding INTEGER ::= 127;

       IPM ::=   SEQUENCE

       {
         heading                              Heading,
         body                                 Body OPTIONAL
       };

       Heading ::= SEQUENCE
       {
         -- Address of the sending agent (person, program, machine) of
         -- this message. This field is mandatory if the sender
         -- is different than the originator.
         sender                      [0]     EMSDORAddress OPTIONAL,

         -- Address of the originator of the message
         -- (not necessarily the sender)
         originator                          EMSDORAddress,

         -- List of recipients and flags associated with each.
         recipient-data                      SEQUENCE SIZE (1..ub-recipients)
                                             OF PerRecipientFields,

         -- Flags applying to this entire message
         per-message-flags           [1]     IMPLICIT BIT STRING

    Banan                        Informational                     [Page 74]

    RFC 2524                          EMSD                     February 1999

         {
            -- Priority values
            -- At most one of "non-urgent" and "urgent" may be specified
            -- concurrently.  If neither is specified, then a Priority
            -- level of "normal" is assumed.
            priority-non-urgent             (0),
            priority-urgent                 (1),

            -- Importance values
            -- At most one of "low" and "high" may be specified
            --  concurrently.  If neither is specified, then an
            -- Importance level of "normal" is  assumed.
            importance-low                  (2),
            importance-high                 (3),

            -- Indication of whether this message has been automatically
            -- forwarded
            auto-forwarded                  (4)
          }  OPTIONAL,

         -- User-specified recipient who is to receive replies to this
         -- message.
         reply-to                    [2]     IMPLICIT SEQUENCE SIZE
                                             (1..ub-reply-to)
                                             OF EMSDORAddress OPTIONAL,

         -- Identifier of a previous message, for which this message
         -- is a reply
         replied-to-IPM                       EMSDMessageId OPTIONAL,

         -- Subject of the message.
         subject                     [3]     IMPLICIT AsciiPrintableString
                                             (SIZE (0..ub-subject-field))
                                                       OPTIONAL,

         -- RFC-822 header fields not explicitly provided for in
         -- this Heading. For messages incoming from the external
         -- world (i.e. in RFC-822 format), the Message-Id: field
         -- need not go here, as it is placed in the
         -- Envelope's EMSDMessageId (message-id) field.
         extensions                [4]   IMPLICIT  SEQUENCE
                                   (SIZE (0..ub-header-extensions))
                                         OF  IPMSExtension OPTIONAL,

         -- MIME Version (if other than 1.0)
         mime-version            [5]     IMPLICIT AsciiPrintableString
                                         (SIZE
                                         (0..ub-mime-version-length))

    Banan                        Informational                     [Page 75]

    RFC 2524                          EMSD                     February 1999

                                                 OPTIONAL,

         -- Top-level MIME Content Type
         mime-content-type       [6]     IMPLICIT AsciiPrintableString
                                         (SIZE (0..
                                          ub-mime-content-type-length))
                                                    OPTIONAL,

         -- MIME Content Id
         mime-content-id         [7]     IMPLICIT AsciiPrintableString
                                         (SIZE (0..
                                          ub-mime-content-id-length))
                                                   OPTIONAL,

         -- MIME Content Description
         mime-content-description [8]    IMPLICIT AsciiPrintableString
                                         (SIZE (0..
                                      ub-mime-content-description-length))
                                                   OPTIONAL,

         -- Top-level MIME Content Type
         mime-content-transfer-encoding
                                  [9]     IMPLICIT AsciiPrintableString
                            (SIZE (0..ub-mime-content-transfer-encoding))
                                                      OPTIONAL
       };

       PerRecipientFields ::= SEQUENCE
       {
         recipient-address                            EMSDORAddress,
         per-recipient-flags                          BIT STRING

          {
             -- Recipient Types.
             -- At most one of "copy" and "blind-copy" may be
             -- specified concurrently for a single recipient.  If
             -- neither is specified, than the recipient
             -- is assumed to be a "primary" recipient.
             recipient-type-copy                             (0),
             recipient-type-blind-copy                       (1),

             -- Notification Request Types.
             -- Only one of "rn" and "nrn" may be specified
             -- concurrently, \x110011 for a single recipient.
             -- "rn" implies "nrn" in addition.
             notification-request-rn                         (2),
             notification-request-nrn                        (3),
             notification-request-ipm-return                 (4),

    Banan                        Informational                     [Page 76]

    RFC 2524                          EMSD                     February 1999

             -- Report Request Types
             -- At most one of these should be set for a
             -- particular recipient. "delivery" implies "non-delivery"
             -- in addition.
             report-request-non-delivery                     (5),
             report-request-delivery                         (6),

             -- Originator-to-Recipient request for a reply.
             reply-requested                                 (7)
          }  DEFAULT { report-request-non-delivery }

       };

       IPMSExtension ::= SEQUENCE
       {
         x-header-label                      AsciiPrintableString,
         x-header-value                      AsciiPrintableString
       };

       Body ::= SEQUENCE
       {
         compression-method          [0]     IMPLICIT CompressionMethod
                                                        OPTIONAL,
         -- If compression method is not specified,
         -- "no-compression" is implied.

         message-body                        OCTET STRING
         -- See MIME for structure of the Body.
         -- If a compression method is specified, the entire text containing
         -- the Content-Type: element followed by the RFC-822 body are
         -- compressed using the specified method, and placed herein.
       };

       CompressionMethod ::= INTEGER
       {
         -- Compression Methods numbered 0 to 63 are reserved for
         -- assignment within this and associated specifications.
         no-compression                  (0),
         lempel-ziv                      (1)

         -- Compression Methods numbered between 64 and 127 may be
         --  used on a bilaterally-agreed basis between peers.
       } (0..127)

       END  -- end of EMSD-InterpersonalMessaging1995

    Banan                        Informational                     [Page 77]

    RFC 2524                          EMSD                     February 1999

    C  RATIONALE FOR KEY DESIGN DECISIONS

       This section summarizes the rationale behind key design decisions
       that were made while developing the EMSD Protocols.

    C.1  Deviation From The SMTP Model

       SMTP is the main mail transport mechanism throughout the Internet.
       SMTP is widely deployed and well understood by many engineers who
       specialize in Internet email.  Because of these reasons, works based
       on SMTP or derived from it have a higher likelyhood of being widely
       deployed throughout the Internet.

       However, SMTP is highly inefficient for transfer of short messages.
       SMTP's inefficiency applies to both the number of transmissions and
       also to the number of bytes transmitted.

       Even when fully optimized with PIPELINING, SMTP is still quite
       inefficient.

       Submission of a short message with SMTP involves 15 transmissions.
       Submission of a short message with SMTP and PIPELINING involves 9
       transmissions.  Submission of a short message with EMSD (EMSD-P and
       ESRO) involves 3 transmissions (in typical cases).

       The key requirement driving the design of EMSD is efficiency.  It was
       determined that the at least 3 fold gains in efficiency justifies the
       deviation from the SMTP model.

    C.1.1  Comparison of SMTP and EMSD Efficiency

       The table below illustrates the number of N-PDUs exchanged for
       transfer of a short Internet email when using SMTP, SMTP and
       PIPELINING, QMTP and EMSD. The names used for identifying the PDUs
       are informal names.

               SMTP      SMTP + pipelining   QMTP, QMQP,   EMSD
               -------   -----------------   ------------  -----------
       client: SYN       SYN                 SYN           Submit.Req
       server: SYN ok    SYN ok              SYN           Submit.Resp
       client: HELO      EHLO                message       ack
       server: ok        PIPELINING          accept close
       client: MAIL      MAIL RCPT DATA      close
       server: ok        ok
       client: RCPT      message QUIT
       server: ok        accept ok close
       client: DATA      close
       server: ok

    Banan                        Informational                     [Page 78]

    RFC 2524                          EMSD                     February 1999

       client: message
       server: accept
       client: QUIT
       server: ok close
       client: close

    C.2  Use of ESRO Instead of TCP

       In order to provide the same level of reliability that the existing
       email protocols provide for short messages, it is clear that a
       reliable underlying service is needed.  UDP [6], by itself, is
       clearly not adequate.

       Use of TCP however, involves three phases:

          1. Connection Establishment

          2. Data Transfer

          3. Disconnect

       Reliable transfer of a short message using TCP at a minimum involves
       5 transmissions as it is the case with QMTP.

       The key requirement driving the design of EMSD is Efficiency.  It was
       determined that elimination of the extra 2 transmissions that are an
       inherent characteristic of TCP, justifies deviation from it.

       ESRO protocol, as specified in (RFC-2188 [1]), provides reliable
       connectionless remote operation services on top of UDP [6] with
       minimum overhead.  ESRO protocol supports segmentation and
       reassembly, concatenation and separation.

       Reliable transfer of a short message using ESRO involves 3
       transmissions as it is the case with EMSD-P.

    C.3  Use Of Remote Procedure Call (RPC) Model

       Many Internet protocols are "text-based".  Few Internet protocols are
       RPC based.  Protocols designed around the "text-based" approach have
       a better track record of acceptance throughout the Internet.

       Considering that message submission and delivery in EMSD involve no
       more than two data exchanges, the text-based model becomes the same
       as an operation.  Furthermore, the RPC model is the natural way of
       using ESRO.

    Banan                        Informational                     [Page 79]

    RFC 2524                          EMSD                     February 1999

    C.4  Use Of ASN.1

       In order to minimize the number of bytes transferred, efficient
       encoding mechanisms are needed.

       Amongst today's encoding mechanisms, ASN.1 has the unique feature of
       separating the abstract syntax from the encoding rules.  By selecting
       ASN.1 as the notation used for expressing EMSD's information objects,
       EMSD has the flexibility of using the most efficient encoding rules
       such as Packed Encoding Rules (PER) when they are available.

       Efficient encoding can always be better performed when the syntax of
       the information is known.  In general, encoding and compression
       techniques which use the knowledge of the syntax of the information
       produce better results than those compression techniques that work on
       arbitrary text.

    Banan                        Informational                     [Page 80]

    RFC 2524                          EMSD                     February 1999

    D  FURTHER DEVELOPMENT

       Beyond this documentation of existing implementations, further
       development of EMSD protocol is anticipated.

       The following deficiencies and areas of improvement are identified.

          o Mapping of RFC-822 to EMSD-FS needs to be more explicit.

          o Mapping of EMSD-FS to RFC-822 needs to be more explicit.

          o Text of duplicate detection section needs more structure.

          o SubmissionControl operation needs more informative description.

          o Based on implementor's feedback the "EMSD PROCEDURE FOR
            OPERATIONS" section needs to be adjusted or re-done.

          o The EMSD protocol can be extended to also support transfer of
            raw RFC-822 text-based messages in addition to EMSD-FS. This
            would be a trade-off in favor of "ease of implementation"
            against "efficiency of bytes transfered".

          o Provide mechanisms to support fully automated initial
            provisioning of mail-boxes.

       Future development of the EMSD Protocol is anticipated to take place
       at http://www.emsd.org/.  Those interested in further development and
       maintenance of this protocol are invited to join the various mailing
       lists hosted at http://www.emsd.org/.

    Banan                        Informational                     [Page 81]

    RFC 2524                          EMSD                     February 1999

    E. References

       [1] Banan, M., Cheng, J. and M. Taylor, "At&t/neda's efficient short
           remote operations (ESRO) protocol specification version 1.2.",
           RFC 2188, September 1997.

       [2] Bradner, S., "Key words for use in RFCs to indicate requirement
           levels", BCP 14, RFC 2119, March 1997.

       [3] Crocker, D., "Standard for the format of ARPA internet text
           messages", STD 11, RFC 822, August 1982.

       [4] Information Processing --- Open Systems Interconnection ---
           Specification of Packed Encoding Rules for Abstract Syntax
           Notation One (ASN.1). International Organization for
           Standardization and International Electrotechnical Committee.
           International Standard 8825-2.

       [5] Information Processing --- Open Systems Interconnection ---
           Specification of Basic Encoding Rules for Abstract Syntax
           Notation One (ASN.1). International Organization for
           Standardization and International Electrotechnical Committee,
           1987. International Standard 8825.

       [6] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
           1980.

    Banan                        Informational                     [Page 82]

    RFC 2524                          EMSD                     February 1999

    F. Full Copyright Statement

       Copyright (C) The Internet Society (1999).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Banan                        Informational                     [Page 83]