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RFC 2156

Proposed STD
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MIXER (Mime Internet X.400 Enhanced Relay): Mapping between X.400 and RFC 822/MIME

Part 1 of 5, p. 1 to 27
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Obsoletes:    0987    1026    1138    1148    1327    1495
Updates:    0822


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Network Working Group                                         S. Kille
Request for Comments: 2156                                  Isode Ltd.
Obsoletes: 987, 1026, 1138, 1148, 1327, 1495              January 1998
Updates: 822
Category: Standards Track


              MIXER (Mime Internet X.400 Enhanced Relay):
                 Mapping between X.400 and RFC 822/MIME

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Table of Contents

   1          - Overview ......................................    3
   1.1        - X.400 .........................................    3
   1.2        - RFC 822 and MIME ..............................    3
   1.3        - The need for conversion .......................    4
   1.4        - General approach ..............................    4
   1.5        - Gatewaying Model ..............................    5
   1.6        - Support of X.400 (1984) .......................    8
   1.7        - X.400 (1992) ..................................    8
   1.8        - MIME ..........................................    8
   1.9        - Body Parts ....................................    8
   1.10       - Local and Global Scenarios ....................    9
   1.11       - Compatibility with previous versions ..........   10
   1.12       - Aspects not covered ...........................   10
   1.13       - Subsetting ....................................   11
   1.14       - Specification Language ........................   11
   1.15       - Related Specifications ........................   11
   1.16       - Document Structure ............................   12
   1.17       - Acknowledgements ..............................   12
   2          - Service Elements ..............................   13
   2.1        - The Notion of Service Across a Gateway ........   13
   2.2        - RFC 822 .......................................   15
   2.3        - X.400 .........................................   18
   3          - Basic Mappings ................................   27
   3.1        - Notation ......................................   27

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   3.2        - ASCII and IA5 .................................   29
   3.3        - Standard Types ................................   29
   3.4        - Encoding ASCII in Printable String ............   33
   3.5        - RFC 1522 ......................................   34
   4          - Addressing and Message IDs ....................   35
   4.1        - A textual representation of MTS.ORAddress .....   36
   4.2        - Global Address Mapping ........................   43
   4.3        - EBNF.822-address <-> MTS.ORAddress ............   46
   4.4        - Repeated Mappings .............................   59
   4.5        - Directory Names ...............................   62
   4.6        - MTS Mappings ..................................   62
   4.7        - IPMS Mappings .................................   67
   5          - Detailed Mappings .............................   71
   5.1        - RFC 822 -> X.400: Detailed Mappings ...........   71
   5.2        - Return of Contents ............................   86
   5.3        - X.400 -> RFC 822: Detailed Mappings ...........   86
   Appendix A - Mappings Specific to SMTP .....................  114
   1          - Probes ........................................  114
   2          - Long Lines ....................................  114
   3          - SMTP Extensions ...............................  114
   3.1        - SMTP Extension mapping to X.400 ...............  114
   3.2        - X.400 Mapping to SMTP Extensions ..............  115
   Appendix B - Mapping with X.400(1984) ......................  116
   Appendix C - RFC 822 Extensions for X.400 access ...........  118
   Appendix D - Object Identifier Assignment ..................  119
   Appendix E - BNF Summary ...................................  120
   Appendix F - Text format for MCGAM distribution ............  127
   1          - Text Formats ..................................  127
   2          - Mechanisms to register and to distribute
                MCGAMs ........................................  127
   3          - Syntax Definitions ............................  128
   4          - Table Lookups .................................  129
   5          - Domain -> OR Address MCGAM format .............  129
   6          - OR Address -> Domain MCGAM format .............  129
   7          - Domain -> OR Address of Preferred Gateway
                table .........................................  130
   8          - OR Addresss -> domain of Preferred Gateway
                table .........................................  130
   Appendix G - Conformance ...................................  131
   Appendix H - Change History: RFC 987, 1026, 1138, 1148
                ...............................................  133
   1          - Introduction ..................................  133
   2          - Service Elements ..............................  133
   3          - Basic Mappings ................................  133
   4          - Addressing ....................................  134
   5          - Detailed Mappings .............................  134
   6          - Appendices ....................................  134
   Appendix I - Change History: RFC 1148 to RFC 1327 ..........  135

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   1          - General .......................................  135
   2          - Basic Mappings ................................  135
   3          - Addressing ....................................  135
   4          - Detailed Mappings .............................  135
   5          - Appendices ....................................  136
   Appendix J - Change History: RFC 1327 to this Document
                ...............................................  137
   1          - General .......................................  137
   2          - Service Elements ..............................  137
   3          - Basic Mappings ................................  137
   4          - Addressing ....................................  137
   5          - Detailed Mappings .............................  138
   6          - Appendices ....................................  138
   Appendix L - ASN.1 Summary .................................  139
   Security Considerations ....................................  141
   Author's Address ...........................................  141
   References .................................................  141
   Full Copyright Statement ...................................  144

Chapter 1 -- Overview

1.1.  X.400

   This document relates primarily to the ITU-T 1988 and 1992 X.400
   Series Recommendations / ISO IEC 10021 International Standard.  This
   ISO/ITU-T standard is referred to in this document as "X.400", which
   is a convenient shorthand.  Any reference to the 1984 Recommendations
   will be explicit.  Any mappings relating to elements which are in the
   1992 version and not in the 1988 version will be noted explicitly.
   X.400 defines an Interpersonal Messaging System (IPMS), making use of
   a store and forward Message Transfer System.  This document relates
   to the IPMS, and not to wider application of X.400, such as EDI as
   defined in X.435.

1.2.  RFC 822 and MIME

   RFC 822 evolved as a messaging standard on the DARPA (the US Defense
   Advanced Research Projects Agency) Internet.  RFC 822 specifies an
   end to end message format, consisting of a header and an unstructured
   text body.  MIME (Multipurpose Internet Mail Extensions) specifies a
   structured message body format for use with RFC 822.  The term "RFC
   822" is used in this document to refer to the combination of MIME and
   RFC 822. RFC 822 and MIME are used in conjunction with a number of
   different message transfer protocol environments.  The core of the
   MIXER specification is designed to work with any supporting message
   transfer protocol.

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   One transfer protocol, SMTP, is of particular importance and is
   covered in MIXER.  On the Internet and other TCP/IP networks, RFC 822
   is used in conjunction with RFC 821, also known as Simple Mail

   Transfer Protocol (SMTP) [30], in a manner conformant with the host
   requirements specification [10].  Use of MIXER with SMTP is defined
   in Appendix A.

1.3.  The need for conversion

   There is a large community using RFC 822 based protocols for mail
   services, who will wish to communicate with users of the IPMS
   provided by X.400 systems.  This will also be a requirement in cases
   where communities intend to make a transition between the different
   technologies, as conversion will be needed to ensure a smooth service
   transition.  It is expected that there will be more than one gateway,
   and this specification will enable them to behave in a consistent
   manner.  Note that the term gateway is used to describe a component
   performing the mapping between RFC 822 and X.400.  This is standard
   usage amongst mail implementors, but differs from that used by
   transport and network service implementors.

   Consistency between gateways is desirable to provide:

   1.   Consistent service to users.

   2.   The best service in cases where a message passes through
        multiple gateways.

1.4.  General approach

   There are a number of basic principles underlying the details of the
   specification.  These principles are goals, and are not achieved in
   all aspects of the specification.

   1.   The specification should be pragmatic.  There should not be
        a requirement for complex mappings for "Academic" reasons.
        Complex mappings should not be required to support trivial
        additional functionality.

   2.   Subject to 1), functionality across a gateway should be as
        high as possible.

   3.   It is always a bad idea to lose information as a result of
        any transformation.  Hence, it is a bad idea for a gateway
        to discard information in the objects it processes.  This
        includes requested services which cannot be fully mapped.

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   4.   Mail gateways  operate at a level above the layer on which
        they perform mappings.  This implies that the gateway shall
        not only be cognisant of the semantics of objects at the
        gateway level, but also be cognisant of higher level
        semantics.  If meaningful transformation of the objects that
        the gateway operates on is to occur, then the gateway needs
        to understand more than the objects themselves.

   5.   Subject to 1), the mapping should be reversible.  That is, a
        double transformation should bring you back to where you
        started.

1.5.  Gatewaying Model

1.5.1.  X.400

   X.400 defines the IPMS Abstract Service in X.420 , [11] which
   comprises of three basic services:

   1.   Origination

   2.   Reception

   3.   Management

   Management is a local interaction between the user and the IPMS, and
   is therefore not relevant to gatewaying.  The first two services
   consist of operations to originate and receive the following two
   objects:

   1.   IPM (Interpersonal Message). This has two components: a
        heading, and a body.  The body is structured as a sequence
        of body parts, which may be basic components (e.g., IA5
        text, or G3 fax), or forwarded Interpersonal Messages.  The
        heading consists of fields containing end to end user
        information, such as subject, primary recipients (To:), and
        importance.

   2.   IPN (Inter Personal Notification).  A notification  about
        receipt of a given IPM at the UA level.

   The Origination service also allows for origination of a probe, which
   is an object to test whether a given IPM could be correctly received.

   The Reception service also allows for receipt of Delivery Reports
   (DR), which indicate delivery success or failure.

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   These IPMS Services utilise the Message Transfer System (MTS)
   Abstract Service [12].  The MTS Abstract Service provides the
   following three basic services:

   1.   Submission (used by IPMS Origination)

   2.   Delivery (used by IPMS Reception)

   3.   Administration (used by IPMS Management)

   Administration is a local issue, and so does not affect this
   standard.  Submission and delivery relate primarily to the MTS
   Message (comprising Envelope and Content), which carries an IPM or
   IPN (or other uninterpreted contents).  The Envelope includes a
   message identifier, an originator, and a list of recipients.
   Submission also includes the probe service, which supports the MTS
   Probe. Delivery also includes Reports, which indicate whether a given
   MTS Message has been delivered or not (or for a probe if delivery
   would have happened).

   The MTS is provided by MTAs which interact using the MTA (Message
   Transfer Agent) Service, which defines the interaction between MTAs,
   along with the procedures for distributed operation.  This service
   provides for transfer of MTS Messages, Probes, and Reports.

1.5.2.  RFC 822

   RFC 822 is based on the assumption that there is an underlying
   service, which is here called the 822-MTS service.  The 822-MTS
   service provides three basic functions:

   1.   Identification of a list of recipients.

   2.   Identification of an error return address.

   3.   Transfer of an RFC 822 message.

   It is possible to achieve 2) within the RFC 822 header.

   This specification will be used most commonly with SMTP as the 822-
   MTS service.  The core MIXER specification is written so that it does
   not rely on non-basic 822-MTS services.  Use of non-basic SMTP
   services is described in Appendix A.  The core of this document is
   written using SMTP terminology for 822-MTS services.

   An RFC 822 message consists of a header, and content which is
   uninterpreted ASCII text.  The header is divided into fields, which
   are the protocol elements.  Most of these fields are analogous to IPM

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   heading fields, although some are analogous to MTS Service Elements
   or MTA Service Elements.

   RFC 822 supports delivery status notifications by use of the NOTARY
   mechanisms [28].

1.5.3.  The Gateway

   Given this functional description of the two services, the functional
   nature of a gateway can now be considered.  It would be elegant to
   consider the SMTP (822-MTS) service mapping onto the MTS Service
   Elements and RFC 822 mapping onto an IPM, but there is a not a clear
   match between these services.  Another elegant approach would be to
   treat this document as the definition of an X.400 Access Unit (AU).
   In this case, the abstraction level is too high, and some necessary
   mapping function is lost.  It is necessary to consider that the IPM
   format definition, the IPMS Service Elements, the MTS Service
   Elements, and MTA Service Elements on one side are mapped into RFC
   822 + SMTP on the other in a slightly tangled manner.  The details of
   the tangle will be made clear in Chapter 5.  Access to the MTA
   Service Elements is minimised.

   The following basic mappings are thus defined.  When going from RFC
   822 to X.400, an RFC 822 message and the associated SMTP information
   is always mapped into an IPM (MTA, MTS, and IPMS Services) and a
   Delivery Status Notification is mapped onto a Report.  Going from
   X.400 to RFC 822, an RFC 822 message and the associated SMTP
   information may be derived from:

   1.   An IPN (MTA, MTS, and IPMS services)

   2.   An IPM (MTA, MTS, and IPMS services)

   A Report (MTA, and MTS Services) is mapped onto a delivery status
   notification.

   Probes (MTA Service) shall be processed by the gateway, as discussed
   in Chapter 5.  MTS Messages containing Content Types other than those
   defined by the IPMS are not mapped by the gateway, and shall be
   rejected at the gateway if no other gatewaying procedure is defined.

   This specification is concerned with X.400 IPMS.  Future
   specifications may defined mappings for other X.400 content types.

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1.5.4.  Repeated Mappings

   The primary goal of this specification is to support single mappings,
   so that X.400 and RFC 822 users can communicate with maximum
   functionality.

   The mappings specified here are designed to work where a message
   traverses multiple times between X.400 and RFC 822. This is often
   essential, particularly in the case of distribution lists.  However,
   in general, this will lead to a level of service which is the lowest
   common denominator (approximately the services offered by RFC 822).

   Some RFC 822 networks may wish to use X.400 as an interconnection
   mechanism (typically for policy reasons), and this is fully
   supported.

   Where an X.400 message transfers to RFC 822 and then back to X.400,
   there is no expectation of X.400 services which do not have an
   equivalent service in standard RFC 822 being preserved - although
   this may be possible in some cases.

1.6.  Support of X.400 (1984)

   The MIXER definition is based on the initial specification of RFC 987
   and in its addendum RFC 1026, which defined a mapping between
   X.400(1984) and RFC 822.  The core MIXER mapping is defined using the
   full 1988 version of X.400, and not to a 1984 compatible subset. New
   features of X.400(1988) can be used to provide a much cleaner mapping
   than that defined in RFC 987.  To interwork with 1984 systems,
   Appendix B shall be followed.

   If a message is being transferred to an X.400(1984) system by way of
   X.400(1988) MTA it will give a slightly better service to follow the
   rules of Appendix B, than to downgrade without this knowledge.
   Downgrading specifications which supplement those specified in X.400
   (X.419) are given in RFC 1328 [22] and RFC 1496 (HARPOON) [5].

1.7.  X.400 (1992)

   X.400 (1992) features are not used by the core of this mapping, and
   so there is not an equivalent downgrade problem.

1.8.  MIME

   MIME format messages are generated by this mapping.  As MIME messages
   are fully RFC 822 compliant, this will not cause problems with
   systems which are not MIME capable.

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1.9.  Body Parts

   MIME and X.400 IPMS can both carry arbitrary body parts. MIME defines
   a mechanism for adding new body parts, and new body parts are
   registered with the IANA. X.400 defines a mechanism adding new body
   parts, usually referred to as Body Part 15.  Extensions are defined
   by Object Identifiers, so there is no requirement for a central body
   part registration authority.  The Electronic Messaging Association
   (EMA) maintains a list of some commonly used body parts.  The EMA has
   specified a mechanism to use the File Transfer Body Part (FTBP) as a
   more generic means to support message attachments.  This approach is
   gaining widespread commercial support.

   The mapping between X.400 and MIME body parts is defined in the
   companion MIXER specification, referenced here as RFC 2157 [8].  This
   document is an update of RFC 1494 [6].

   Editor's Note:
      References to 2157 will be resolved as these two
      documents are expected to progress in parallel.

   These two specifications together form the complete MIXER Mapping.

1.10.  Local and Global Scenarios

   There are two basic scenarios for X.400/MIME interworking:

   Global Scenario

      There are two global mail networks (Internet/MIME and X.400),
      interconnected by multiple gateways.   Objects may be transferred
      over multiple gateways, and so it is important that gateways
      behave in a coherent fashion.  MIXER is critical to support this
      scenario.

   Local Scenario

      A gateway is used to connect a closed community to a global mail
      network (this could be enforced by connectivity or gateway
      authorisation policy).  This is a common commercial scenario.
      MIXER is useful to support this scenario, as it allows an industry
      standard provision of service, but this could be supported by
      something which was MIXER-like.

   A solution for the global scenario will work for the local scenario.
   However, there are aspects of MIXER which have significant
   implementation or deployment effort (the global mapping is the major
   one, but there are other details too) which and are needed to support

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   the global scenario, but are not needed in the local scenario.

   Note that the local scenario may be the driving force for most
   deployments, and support of the global scenario may be an important
   secondary goal.

   There is also a transition effect.  Gateways which are initially
   deployed in a strict local scenario situation start to find
   themselves in a global scenario.  A common case is ADMD provided
   gateways, which are targeted strictly at the local scenario.  In
   practice they soon start to operate in the global scenario, because
   of distribution lists and messages exchanged with X.400 users that
   are not customers of the ADMD.  At this point, users are hurt by the
   restrictions of a local scenario gateway.

   Note that conformance to MIXER applies to an instantiation of a
   gateway, not just an implementation (although clearly it is critical
   that the implementation is capable of being operated in a conformant
   manner).

   MIXER's conformance target is the global scenario, and the
   specification of MIXER defines operation in this way.

1.11.  Compatibility with previous versions

   The changes between this and older versions of the document are given
   in Appendices H, I and J.  These are RFCs 987, 1026, 1138, 1148 and
   1327.  This document is a revision of RFC 1327 [21].  As far as
   possible, changes have been made in a compatible fashion.

1.12.  Aspects not covered

   There have been a number of cases where previous versions of this
   document were used in a manner which was not intended.  This section
   is to make clear some limitations of scope.  In particular, this
   specification does not specify:

   -    Extensions of RFC 822 to provide access to all X.400
        services

   -    X.400 user interface definition

   These are really coupled.  To map the X.400 services, this
   specification defines a number of extensions to RFC 822.  As a side
   effect, these give the 822 user access to SOME X.400 services.
   However, the aim on the RFC 822 side is to preserve current service,
   and it is intentional that access is not given to all X.400 services.
   Thus, it will be a poor choice for X.400 implementors to use MIXER as

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   an interface - there are too many aspects of X.400 which cannot be
   accessed through it.  If a text interface is desired, a specification
   targeted at X.400, without RFC 822 restrictions, would be more
   appropriate.  Some optional and limited extensions in this area have
   proved useful, and are defined in Appendix C.

1.13.  Subsetting

   This proposal specifies a mapping which is appropriate to preserve
   services in existing RFC 822 communities.  Implementations and
   specifications which subset this specification are non-conformant and
   strongly discouraged.

1.14.  Specification Language

   ISO and Internet standards have clear definitions as to the style of
   language used.  This specification maps between ISO/ITU-T protocol
   and Internet protocols.  This document uses ISO terminology for the
   following reasons:

   1.   This was done in previous versions.

   2.   ISO language may be mechanically converted to Internet
        language, but not vice versa.

   The key elements of the ISO rules are:

   1.   All mandatory features shall clearly be indicated by
        imperative statements or the word "shall" or "shall not".

   2.   Optional features shall be indicated by the word "may".

   3.   The word "should" and the phrase "may not" shall not be
        used.

   In some cases the specification issues guidance on use of optional
   features, by use of the the phrase word "recommended" or "not
   recommended".

   To interpet this document according to Internet rules, replace every
   occurrence of "shall" with "must".

1.15.  Related Specifications

   Mappings between Mail-11 and X.400 and Mail-11 and RFC 822 are
   described in RFC 2162, using mappings related to those defined here
   [2].

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1.16.  Document Structure

   This document has five chapters:

   1.   Overview - this chapter.

   2.   Service Elements - This describes the (end user) services
        mapped by a gateway.

   3.   Basic mappings - This describes some basic notation used in
        Chapters 3-5, the mappings between character sets, and some
        fundamental protocol elements.

   4.   Addressing - This considers the mapping between X.400 OR
        names and RFC 822 addresses, which is a fundamental gateway
        component.

   5.   Detailed Mappings - This describes the details of all other
        mappings.

   There are also ten appendices.

   WARNING:

      THE REMAINDER OF THIS SPECIFICATION IS TECHNICALLY DETAILED.  IT
      WILL NOT MAKE SENSE, EXCEPT IN THE CONTEXT OF RFC 822 AND X.400
      (1988).  DO NOT ATTEMPT TO READ THIS DOCUMENT UNLESS YOU ARE
      FAMILIAR WITH THESE SPECIFICATIONS.

1.17.  Acknowledgements

   The work in this specification was substantially based on RFC 987 and
   RFC 1148, which had input from many people, who are credited in the
   respective documents.

   A number of comments from people on RFC 1148 lead to RFC 1327.  In
   particular, there were comments and suggestions from: Maurice Abraham
   (HP); Harald Alvestrand (Sintef); Peter Cowen (X-Tel); Jim Craigie
   (JNT); Ella Gardner (MITRE); Christian Huitema (Inria); Erik Huizer
   (SURFnet); Neil Jones (DEC); Ignacio Martinez (IRIS); Julian Onions
   (X-Tel); Simon Poole (SWITCH); Clive Roberts (Data General); Pete
   Vanderbilt (SUN); Alan Young (Concurrent).

   RFC 1327 has been widely adopted, and a review team was formed.  This
   comprised of: Urs Eppenberger (SWITCH)(Chair); Claudio Allocchio
   (INFN); Harald Alvestrand (UNINETT); Dave Crocker (Brandenburg); Ned
   Freed (Innosoft); Erik Huizer (SURFnet); Steve Kille (Isode); Peter
   Sylvester (GC Tech).

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   Harald Alvestrand also supplied the tables mapping DSN status codes
   with X.400 codes.  Ned Freed defined parts of the File Transfer Body
   Part mapping.

   Comment and input has also been received from: Bengt Ackzell (Generic
   Systems); Samir Albadine (Transpac); Mark Boyes (DEC); Larry Campbell
   (Boston Software Works); Jacqui Caren (Cray); Allan Cargille (MCI);
   Kevin Carrosso (Innosoft); Charlie Combs (OIW); Jim Craigie (Net-
   Tel); Eamon Doyle (Isocor); Efifion Edem (SITA); Jyrki Heikkinen
   (ICL); Edward Hibbert (DCL); Jeroun Houttin (Terena); Kevin Jordan
   (CDS); Paul Kingsnorth (DEC); Carl-Uno Manros (Manros Consulting);
   Suzan Mendes (Telis); Robert Miles (Softswitch); Roger Mizumorri
   (Enterprise Solutions Ltd); Keith Moore (University of Tennessee);
   Ruth Moulton (Net-Tel) Michel Musy (Bull); Kenji Nonaka (NTT): The
   OIW MHSIG; Tom Oliphant (SWITCH); Julian Onions (NEXOR); Jacob Palme
   (KTH); Olivier Paridaens (ULB); Mary la Roche (Citicorp); John
   Setsaas (Maxware); Russell Sharpe (DCL); Patrick Soulier (CCETT);
   Eftimios Tsigros (Universite Libre de Bruxelles); Sean Turner (IECA);
   Mark Wahl (Isode); David Wilson (Isode); Bill Wohler (Worldtalk);
   Alan Young (Isode); Alain Zahm (Telis).

Chapter 2 - Service Elements

   This chapter considers the services offered across a gateway built
   according to this specification.  It gives a view of the
   functionality provided by such a gateway for communication with users
   in the opposite domain.  This chapter considers service mappings in
   the context of SINGLE transfers only, and not repeated mappings
   through multiple gateways.

2.1.  The Notion of Service Across a Gateway

   RFC 822 and X.400 provide a number of services to the end user.  This
   chapter describes the extent to which each service can be supported
   across an X.400 <-> RFC 822 gateway.  The cases considered are single
   transfers across such a gateway, although the problems of multiple
   crossings are noted where appropriate.

2.1.1.  Origination of Messages

   When a user originates a message, a number of services are available.
   Some of these imply actions (e.g., delivery to a recipient), and some
   are insertion of known data (e.g., specification of a subject field).
   This chapter describes, for each offered service, to what extent it
   is supported for a recipient accessed through a gateway.  There are
   three levels of support:

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   Supported
      The corresponding protocol elements map well, and so the service
      can be fully provided.

   Not Supported
      The service cannot be provided, as there is a complete mismatch.

   Partial Support
      The service can be partially fulfilled.

   In the first two cases, the service is simply marked as "Supported"
   or "Not Supported".  Some explanation may be given if there are
   additional implications, or the (non) support is not intuitive.  For
   partial support, the level of partial support is summarised.  Where
   partial support is good, this will be described by a phrase such as
   "Supported by use of.....".  A common case of this is where the
   service is mapped onto a non-standard service on the other side of
   the gateway, and this would have lead to support if it had been a
   standard service.  In many cases, this is equivalent to support.  For
   partial support, an indication of the mechanism is given, in order to
   give a feel for the level of support provided.  Note that this is not
   a replacement for Chapter 5, where the mapping is fully specified.

      If a service is described as supported, this implies:

   -    Semantic correspondence.

   -    No (significant) loss of information.

   -    Any actions required by the service element.

   An example of a service gaining full support: If an RFC 822
   originator specifies a Subject: field, this is considered to be
   supported, as an X.400 recipient will get a subject indication.

   In many cases, the required action will simply be to make the
   information available to the end user.  In other cases, actions may
   imply generating a delivery report.

   All RFC 822 services are supported or partially supported for
   origination.  The implications of non-supported X.400 services is
   described under X.400.

2.1.2.  Reception of Messages

   For reception, the list of service elements required to support this
   mapping is specified.  This is really an indication of what a
   recipient might expect to see in a message which has been remotely

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   originated.

2.2.  RFC 822

   RFC 822 does not explicitly define service elements, as distinct from
   protocol elements.  However, all of the RFC 822 header fields, with
   the exception of trace, can be regarded as corresponding to implicit
   RFC 822 service elements.

2.2.1.  Origination in RFC 822

   A mechanism of mapping, used in several cases, is to map the RFC 822
   header into a heading extension in the IPM (InterPersonal Message).
   This can be regarded as partial support, as it makes the information
   available to any X.400 implementations which are interested in these
   services. Communities which require significant RFC 822 interworking
   are recommended to require that their X.400 User Agents are able to
   display these heading extensions.  Support for the various service
   elements (headers) is now listed.

   Date:
        Supported.

   From:
        Supported.  For messages where there is also a sender field,
        the mapping is to "Authorising Users Indication", which has
        subtly different semantics to the general RFC 822 usage of
        From:.

   Sender: Supported.

   Reply-To: Supported.

   To:  Supported.

   Cc:  Supported.

   Bcc: Supported.

   Message-Id: Supported.

   In-Reply-To:
      Supported, for a single reference.  Where multiple references are
      given, partial support is given by mapping to "Cross Referencing
      Indication".  This gives similar semantics.

   References: Supported.

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   Keywords: Supported by use of a heading extension.

   Subject: Supported.

   Comments: Supported by use of a heading extension.

   Encrypted: Supported by use of a heading extension.

   Content-Language: Supported.

   Resent-*

      Supported by use of a heading extension.  Note that addresses in
      these fields are mapped onto text, and so are not accessible to
      the X.400 user as addresses.  In principle, fuller support would
      be possible by mapping onto a forwarded IP Message, but this is
      not suggested.

   Other Fields

      In particular X-* fields, and "illegal" fields in common usage
      (e.g., "Fruit-of-the-day:") are supported by use of heading
      extensions.

   MIME introduces a number of headings.  Support is defined in RFC
   2157.

2.2.2.  Reception by RFC 822

   This considers reception by an RFC 822 User Agent of a message
   originated in an X.400 system and transferred across a gateway.  The
   following standard services (headers) may be present in such a
   message:

   Date:

   From:

   Sender:

   Reply-To:

   To:

   Cc:

   Bcc:

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   Message-Id:

   In-Reply-To:

   References:

   Subject:

   Content-Type: (See RFC 2157)

   Content-Transfer-Encoding: (See RFC 2157)

   MIME-Version: (See RFC 2157)

   The following services (headers) may be present in the header of a
   message. These are defined in more detail in Chapter 5 (5.3.4, 5.3.6,
   5.3.7):

   Autoforwarded:

   Autosubmitted:

   X400-Content-Identifier:

   Content-Language:

   Conversion:

   Conversion-With-Loss:

   Delivery-Date:

   Discarded-X400-IPMS-Extensions:

   Discarded-X400-MTS-Extensions:

   DL-Expansion-History:

   Deferred-Delivery:

   Expires:

   Importance:

   Incomplete-Copy:

   Latest-Delivery-Time:

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   Message-Type:

   Original-Encoded-Information-Types:

   Originator-Return-Address:

   Priority:

   Reply-By:

   Sensitivity:

   Supersedes:

   X400-Content-Type:

   X400-MTS-Identifier:

   X400-Originator:

   X400-Received:

   X400-Recipients:

2.3.  X.400

2.3.1.  Origination in X.400

   When mapping services from X.400 to RFC 822 which are not supported
   by RFC 822, new RFC 822 headers are defined, and registered by
   publication in this standard. It is intended that co-operating RFC
   822 systems may also use them.  Where these new fields are used, and
   no system action is implied, the service can be regarded as being
   partially supported.  Chapter 5 describes how to map X.400 services
   onto these new headers.  Other elements are provided, in part, by the
   gateway as they cannot be provided by RFC 822.

   Some service elements are marked N/A (not applicable).  There are
   five cases, which are marked with different comments:

   N/A (local)
      These elements are only applicable to User Agent / Message
      Transfer Agent interaction and so they cannot apply to RFC 822
      recipients.

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   N/A (PDAU)
      These service elements are only applicable where the recipient is
      reached by use of a Physical Delivery Access Unit (PDAU), and so
      do not need to be mapped by the gateway.

   N/A (reception)
      These services  are only applicable for reception.

   N/A (prior)
      If requested, this service shall be performed prior to the
      gateway.

   N/A (MS)
      These services are only applicable to Message Store (i.e., a local
      service).

   Finally, some service elements are not supported.  In particular, the
   new security services are not mapped onto RFC 822.  Unless otherwise
   indicated, the behaviour of service elements marked as not supported
   will depend on the criticality marking supplied by the user.  If the
   element is marked as critical for transfer or delivery, a non-
   delivery notification will be generated.  Otherwise, the service
   request will be ignored.

2.3.1.1.  Basic Interpersonal Messaging Service

   These are the mandatory IPM services as listed in Section 19.8 of
   X.400 / ISO/IEC 10021-1, listed here in the order given. Section 19.8
   has cross references to short definitions of each service.

   Access management
      N/A (local).

   Content Type Indication
      Supported by a new RFC 822 header (X400-Content-Type:).

   Converted Indication
      Supported by a new RFC 822 header (X400-Received:).

   Delivery Time Stamp Indication
      N/A (reception).

   IP Message Identification
      Supported.

   Message Identification
      Supported, by use of a new RFC 822 header (X400-MTS-Identifier).
      This new header is required, as X.400 has two message-ids whereas

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      RFC 822 has only one (see IP Message Identification

   Non-delivery Notification
      Not supported in all cases.  Supported where the recipient system
      supports NOTARY DSNs.  In general all RFC 822 systems will return
      error reports by use of IP messages.  In other service elements,
      this pragmatic result can be treated as effective support of this
      service element.

   Original Encoded Information Types Indication
      Supported as a new RFC 822 header (Original-Encoded-Information-
      Types:).

   Submission Time Stamp Indication
      Supported.

   Typed Body
      Support is defined in RFC 2157.

   User Capabilities Registration
      N/A (local).

2.3.1.2.  IPM Service Optional User Facilities

   This section describes support for the optional (user selectable) IPM
   services as listed in Section 19.9 of X.400 / ISO/IEC 10021- 1,
   listed here in the order given.  Section 19.9 has cross references to
   short definitions of each service.

   Additional Physical Rendition
      N/A (PDAU).

   Alternate Recipient Allowed
      Not supported.  There is no RFC 822 service equivalent to
      prohibition of alternate recipient assignment (e.g., an RFC 822
      system may freely send an undeliverable message to a local
      postmaster).  A MIXER gateway has two conformant options.  The
      first is not to gateway a message requesting prohibition of
      alternate recipient, as this control cannot be guaranteed.  This
      option supports the service, but may cause unacceptable level of
      message rejections. The second is to gateway the message on the
      basis that there is no alternate recipient service in RFC 822. RFC
      1327 allowed only the second option.   If the first option is
      shown to be operationally effective, it may be the only option in
      future versions of MIXER.

   Authorising User's Indication
      Supported.

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   Auto-forwarded Indication
      Supported as new RFC 822 header (Auto-Forwarded:).

   Basic Physical Rendition
      N/A (PDAU).

   Blind Copy Recipient Indication
      Supported.

   Body Part Encryption Indication
      Supported by use of a new RFC 822 header (Original-Encoded-
      Information-Types:), although in most cases it will not be
      possible to map the body part in question.

   Content Confidentiality
      Not supported.

   Content Integrity
      Not supported.

   Conversion Prohibition
      Supported. Operation defined in RFC 2157.

   Conversion Prohibition in Case of Loss of Information
      Supported.  Operation defined in RFC 2157.

   Counter Collection
      N/A (PDAU).

   Counter Collection with Advice
      N/A (PDAU).

   Cross Referencing Indication
      Supported.

   Deferred Delivery
      N/A (prior).  This service shall always be provided by the MTS
      prior to the gateway.  A new RFC 822 header (Deferred-Delivery:)
      is provided to transfer information on this service to the
      recipient.

   Deferred Delivery Cancellation
      N/A (local).

   Delivery Notification
      Supported.  This is performed at the gateway, but may be performed
      at the end system if the end system supports NOTARY.  Thus, a
      notification is sent by the gateway to the originator.

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   Delivery via Bureaufax Service
      N/A (PDAU).

   Designation of Recipient by Directory Name
      N/A (local).

   Disclosure of Other Recipients
      Supported by use of a new RFC 822 header (X400-Recipients:).  This
      is descriptive information for the RFC 822 recipient, and is not
      reverse mappable.

   DL Expansion History Indication
      Supported by use of a new RFC 822 header (DL-Expansion-History:).

   DL Expansion Prohibited
      Distribution List means MTS supported distribution list, in the
      manner of X.400.  This service does not exist in the RFC 822
      world, although RFC 822 supports distribution list functionality.
      There is no SMTP leve control to prohibit distribution list
      expansion.   A MIXER gateway has two conformant options.  The
      first is not to gateway a message requesting DL expansion
      prohibition, as this control cannot be guaranteed.  This option
      supports the service, but may cause unacceptable level of message
      rejections. The second is to gateway the message on the basis that
      there is no distribution list service in RFC 822. RFC 1327 allowed
      only the second option.   If the first option is shown to be
      operationally effective, it may be the only option in future
      versions of MIXER.

   Express Mail Service
      N/A (PDAU).

   Expiry Date Indication
      Supported as new RFC 822 header (Expires:).  In general, no
      automatic action can be expected.

   Explicit Conversion
      N/A (prior).

   Forwarded IP Message Indication
      Supported.

   Grade of Delivery Selection
      Not Supported.  There is no equivalent service in RFC 822.

   Importance Indication
      Supported as new RFC 822 header (Importance:).

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   Incomplete Copy Indication
      Supported as new RFC 822 header (Incomplete-Copy:).

   Language Indication
      Supported as new RFC 822 header (Content-Language:).

   Latest Delivery Designation
      Not supported.  A new RFC 822 header (Latest-Delivery-Time:) is
      provided, which may be used by the recipient for general
      information, but will not be acted on by the SMTP infrastrucuture.

   Message Flow Confidentiality
      Not supported.

   Message Origin Authentication
      N/A (reception).

   Message Security Labelling
      Not supported.

   Message Sequence Integrity
      Not supported.

   Multi-Destination Delivery Supported.

   Multi-part Body
      Supported.

   Non Receipt Notification Request
      Not supported.

   Non Repudiation of Delivery
      Not supported.

   Non Repudiation of Origin
      N/A (reception).

   Non Repudiation of Submission
      N/A (local).

   Obsoleting Indication
      Supported as new RFC 822 header (Supersedes:).

   Ordinary Mail
      N/A (PDAU).

   Originator Indication
      Supported.

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   Originator Requested Alternate Recipient
      Not supported, but is placed as comment next to address (X400-
      Recipients:).

   Physical Delivery Notification by MHS
      N/A (PDAU).

   Physical Delivery Notification by PDS
      N/A (PDAU).

   Physical Forwarding Allowed
      Supported by use of a comment in a new RFC 822 header (X400-
      Recipients:), associated with the recipient in question.

   Physical Forwarding Prohibited
      Supported by use of a comment in a new RFC 822 header (X400-
      Recipients:), associated with the recipient in question.

   Prevention of Non-delivery notification
      Supported where SMTP and NOTARY are available.  In other cases
      formally supported, as delivery notifications cannot be generated
      by RFC 822.  In practice, errors will be returned as IP Messages,
      and so this service may appear not to be supported (see Non-
      delivery Notification).

   Primary and Copy Recipients Indication
      Supported

   Probe
      Supported at the gateway (i.e., the gateway services the probe).

   Probe Origin Authentication
      N/A (reception).

   Proof of Delivery
      Not supported.

   Proof of Submission
      N/A (local).

   Receipt Notification Request Indication
      Not supported.

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   Redirection Disallowed by Originator
      Redirection means MTS supported redirection, in the manner of
      X.400.  This service does not exist in the RFC 822 world.  RFC 822
      redirection (e.g., aliasing) is regarded as an informal
      redirection mechanism, beyond the scope of this control.  Messages
      will be sent to RFC 822, irrespective of whether this service is
      requested. In practice, control of this service is not supported.

   Registered Mail
      N/A (PDAU).

   Registered Mail to Addressee in Person
      N/A (PDAU).

   Reply Request Indication
      Supported as comment next to address.

   Replying IP Message Indication
      Supported.

   Report Origin Authentication
      N/A (reception).

   Request for Forwarding Address
      N/A (PDAU).

   Requested Delivery Method
      N/A (local).   The service request is dealt with at submission
      time.  Any such request is made available through the gateway by
      use of a comment associated with the recipient in question.

   Return of Content
      Supported where SMTP and NOTARY are used. In principle for other
      situations, this is N/A, as non-delivery notifications are not
      supported.  In practice, most RFC 822 systems will return part or
      all of the content along with the IP Message indicating an error
      (see Non-delivery Notification).

   Sensitivity Indication
      Supported as new RFC 822 header (Sensitivity:).

   Special Delivery
      N/A (PDAU).

   Stored Message Deletion
      N/A (MS).

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   Stored Message Fetching
      N/A (MS).

   Stored Message Listing
      N/A (MS).

   Stored Message Summary
      N/A (MS).

   Subject Indication
      Supported.

   Undeliverable Mail with Return of Physical Message
      N/A (PDAU).

   Use of Distribution List
      In principle this applies only to X.400 supported distribution
      lists (see DL Expansion Prohibited).  Theoretically, this service
      is N/A (prior).  In practice, because of informal RFC 822 lists,
      this service can be regarded as supported.

   Auto-Submitted Indication
      Supported

2.3.2.  Reception by X.400

2.3.2.1.  Standard Mandatory Services

   The following standard IPM mandatory user facilities are required for
   reception of RFC 822 originated mail by an X.400 UA.

   Content Type Indication

   Delivery Time Stamp Indication

   IP Message Identification

   Message Identification

   Non-delivery Notification

   Original Encoded Information Types Indication

   Submission Time Stamp Indication

   Typed Body

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2.3.2.2.  Standard Optional Services

   The following standard IPM optional user facilities are required for
   reception of RFC 822 originated mail by an X.400 UA.

   Authorising User's Indication

   Blind Copy Recipient Indication

   Cross Referencing Indication

   Originator Indication

   Primary and Copy Recipients Indication

   Replying IP Message Indication

   Subject Indication

2.3.2.3.  New Services

   A new X.400 service "RFC 822 Header Field" is defined using the
   extension facilities.  This allows for any RFC 822 header field to be
   represented.  It may be present in RFC 822 originated messages which
   are received by an X.400 UA.



(page 27 continued on part 2)

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