[Docs] [txt|pdf] [draft-kille-mixer...] [Diff1] [Diff2]

PROPOSED STANDARD

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



Kille                       Standards Track                     [Page 1]

RFC 2156                         MIXER                      January 1998


   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



Kille                       Standards Track                     [Page 2]

RFC 2156                         MIXER                      January 1998


   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.





Kille                       Standards Track                     [Page 3]

RFC 2156                         MIXER                      January 1998


   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.




Kille                       Standards Track                     [Page 4]

RFC 2156                         MIXER                      January 1998


   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.





Kille                       Standards Track                     [Page 5]

RFC 2156                         MIXER                      January 1998


   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



Kille                       Standards Track                     [Page 6]

RFC 2156                         MIXER                      January 1998


   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.








Kille                       Standards Track                     [Page 7]

RFC 2156                         MIXER                      January 1998


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.




Kille                       Standards Track                     [Page 8]

RFC 2156                         MIXER                      January 1998


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



Kille                       Standards Track                     [Page 9]

RFC 2156                         MIXER                      January 1998


   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



Kille                       Standards Track                    [Page 10]

RFC 2156                         MIXER                      January 1998


   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].




Kille                       Standards Track                    [Page 11]

RFC 2156                         MIXER                      January 1998


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).



Kille                       Standards Track                    [Page 12]

RFC 2156                         MIXER                      January 1998


   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:





Kille                       Standards Track                    [Page 13]

RFC 2156                         MIXER                      January 1998


   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



Kille                       Standards Track                    [Page 14]

RFC 2156                         MIXER                      January 1998


   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.




Kille                       Standards Track                    [Page 15]

RFC 2156                         MIXER                      January 1998


   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:




Kille                       Standards Track                    [Page 16]

RFC 2156                         MIXER                      January 1998


   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:




Kille                       Standards Track                    [Page 17]

RFC 2156                         MIXER                      January 1998


   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.







Kille                       Standards Track                    [Page 18]

RFC 2156                         MIXER                      January 1998


   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



Kille                       Standards Track                    [Page 19]

RFC 2156                         MIXER                      January 1998


      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.



Kille                       Standards Track                    [Page 20]

RFC 2156                         MIXER                      January 1998


   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.



Kille                       Standards Track                    [Page 21]

RFC 2156                         MIXER                      January 1998


   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:).




Kille                       Standards Track                    [Page 22]

RFC 2156                         MIXER                      January 1998


   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.



Kille                       Standards Track                    [Page 23]

RFC 2156                         MIXER                      January 1998


   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.









Kille                       Standards Track                    [Page 24]

RFC 2156                         MIXER                      January 1998


   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).





Kille                       Standards Track                    [Page 25]

RFC 2156                         MIXER                      January 1998


   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





Kille                       Standards Track                    [Page 26]

RFC 2156                         MIXER                      January 1998


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.

Chapter 3 Basic Mappings

3.1.  Notation

   The X.400 protocols are encoded in a structured manner according to
   ASN.1, whereas RFC 822 is text encoded.  To define a detailed
   mapping, it is necessary to refer to detailed protocol elements in
   each format.  A notation to achieve this is described in this
   section.

3.1.1.  RFC 822

   Structured text is defined according to the Extended Backus Naur Form
   (EBNF) defined in Section 2 of RFC 822 [16].  In the EBNF definitions
   used in this specification, the syntax rules given in Appendix D of
   RFC 822 are assumed.  When these EBNF tokens are referred to outside
   an EBNF definition, they are identified by the string "822." appended
   to the beginning of the string (e.g., 822.addr-spec).  Additional
   syntax rules, to be used throughout this specification, are defined
   in this chapter.

   The EBNF is used in two ways.



Kille                       Standards Track                    [Page 27]

RFC 2156                         MIXER                      January 1998


   1.   To describe components of RFC 822 messages (or of SMTP
        components).  When these new EBNF tokens are referred to
        outside an EBNF definition, they are identified by the
        string "EBNF." appended to the beginning of the string
        (e.g., EBNF.importance).

   2.   To describe the structure of IA5 or ASCII information not in
        an RFC 822 message.

   For all new EBNF, tokens will either be self delimiting, or be
   delimited by self delimiting tokens.  Comments and LWSP are not used
   as delimiters, except for the following cases, where LWSP may be
   inserted according to RFC 822 rules.

   -    Around the ":" in all headers

   -    EBNF.labelled-integer

   -    EBNF.object-identifier

   -    EBNF.encoded-info

   RFC 822 folding rules are applied to all headers.  Comments are never
   used in these new headers.

   This notation is used in a modified form to refer to NOTARY EBNF
   [28].  For this EBNF, the keyword EBNF it replaces with DSN, for
   example DSN.final-recipient-field fields.

3.1.2.  ASN.1

   An element is referred to with the following syntax, defined in EBNF:

      element         = service "." definition *( "." definition )
      service         = "IPMS" / "MTS" / "MTA"
      definition      = identifier / context
      identifier      = ALPHA *< ALPHA or DIGIT or "-" >
      context         = "[" 1*DIGIT "]"

   The EBNF.service keys are shorthand for the following service
   specifications:

   IPMS IPMSInformationObjects defined in Annex E of X.420 / ISO 10021-
   7.

   MTS MTSAbstractService defined in Section 9 of X.411 / ISO 10021-4.

   TA MTAAbstractService defined in Section 13 of X.411 / ISO 10021-4.



Kille                       Standards Track                    [Page 28]

RFC 2156                         MIXER                      January 1998


   FTBP File Transfer Body Part, as defined in [27].

   The first EBNF.identifier identifies a type or value key in the
   context of the defined service specification.  Subsequent
   EBNF.identifiers identify a value label or type in the context of the
   first identifier (SET or SEQUENCE).  EBNF.context indicates a context
   tag, and is used where there is no label or type to uniquely identify
   a component.  The special EBNF.identifier keyword "value" is used to
   denote an element of a sequence.  For example, IPMS.Heading.subject
   defines the subject element of the IPMS heading.  The same syntax is
   also used to refer to element values.  For example,
   MTS.EncodedInformationTypes.[0].g3Fax refers to a value of
   MTS.EncodedInformationTypes.[0] .

3.2.  ASCII and IA5

   A gateway will interpret all IA5 as ASCII.  Thus, mapping between
   these forms is conceptual.

3.3.  Standard Types

   There is a need to convert between ASCII text and some of the types
   defined in ASN.1 [14].  For each case, an EBNF syntax definition is
   given, for use in all of this specification, which leads to a mapping
   between ASN.1, and an EBNF construct.  All EBNF syntax definitions of
   ASN.1 types are in lower case, whereas ASN.1 types are referred to
   with the first letter in upper case.  Except as noted, all mappings
   are symmetrical.

3.3.1.  Boolean

   Boolean is encoded as:

      boolean = "TRUE" / "FALSE"

3.3.2.  NumericString

   NumericString is encoded as:

      numericstring = *(DIGIT / " ")











Kille                       Standards Track                    [Page 29]

RFC 2156                         MIXER                      January 1998


3.3.3.  PrintableString

   PrintableString is a restricted IA5String defined as:

      printablestring  = *( ps-char )
      ps-restricted-char      = 1DIGIT /  1ALPHA / " " / "'" / "+"
                         / "," / "-" / "." / "/" / ":" / "=" / "?"
      ps-delim         = "(" / ")"
      ps-char          = ps-delim / ps-restricted-char

   This can be used to represent real printable strings in EBNF.

3.3.4.  T.61String

   In cases where T.61 strings are only used for conveying human
   interpreted information, the aim of a mapping is to render the
   characters appropriately in the remote character set, rather than to
   maximise reversibility.  For these cases, there are two options, both
   of which are conformant to this specification:

   1.   The mappings to IA5 defined in ITU-T Recommendation X.408
        (1988) may be used [13].  These will then be encoded in
        ASCII.   This is the approach mandated in RFC 1327.

   2.   This mapping may be used if the characters are not contained
        within ASCII repertoire, but are all in an IANA-registered
        character set.  Use the encoding defined in RFC 1522 [9] to
        generate appropriate encoded-words.  If this mapping is
        used, the character set ISO-8859-1 shall be used if all of
        the characters needed are available in this repertoire.  In
        other cases, the character set TELETEX shall be used.  The
        details of this character set is defined in the Appendix C
        of RFC 2157.

   There is also a need to represent Teletex Strings in ASCII, for some
   aspects of OR Address.  For these, the following encoding is used:

      teletex-string   = *( ps-char / t61-encoded )
      t61-encoded      = "{" 1* t61-encoded-char "}"
      t61-encoded-char = 3DIGIT

   Characters in EBNF.ps-char are mapped simply.  Other octets,
   including control characters, are mapped using a quoting mechanism
   similar to the printable string mechanism.  Each octet is represented
   as 3 decimal digits.  For example, the Yen character (hex A5) is
   represented as {165}.  As the three character string, a, yen
   character, b, would be represented as either "a{165}b".




Kille                       Standards Track                    [Page 30]

RFC 2156                         MIXER                      January 1998


   The use of escape sequences follows that set down for ASN1.  in ISO
   8825-1, with the additional specifiction that the default G1 page is
   ISO Latin 1.  The page settings may be changed by escape sequences.
   Changes of the settings hold within a pair of curly brackets ({}),
   and the settings revert to the default after the right bracket (})
   (i.e., they do not carry forward to subsequent T.61 encoding).

   There are a number of places where a string may have a Teletex and/or
   Printable String representation.  The following EBNF is used to
   represent this.

      teletex-and-or-ps = [ printablestring ] [ "*" teletex-string ]

   The natural mapping is restricted to EBNF.ps-char, in order to make
   the full BNF easier to parse.  An example is:

         "yen*{165}"

3.3.5.  UTCTime

   Both UTCTime and the RFC 822 822.date-time syntax contain: Year,
   Month, Day of Month, hour, minute, second (optional), and Timezone
   (technically a time differential in UTCTime).  822.date-time also
   contains an optional day of the week, but this is redundant.  With
   the exception of Year, a symmetrical mapping can be made between
   these constructs.

   Note:
      In practice, a gateway will need to parse various illegal variants
      on 822.date-time.  In cases where 822.date-time cannot be parsed,
      it is recommended that the derived UTCTime is set to the value at
      the time of translation.  Such errors may be noted in an RFC 822
      comment, to aid detection and correction.

   When mapping to X.400, the UTCTime format which specifies the
   timezone offset shall be used.

   When mapping to RFC 822, the 822.date-time format shall include a
   numeric timezone offset (e.g., -0500).

   When mapping time values, the timezone shall be preserved as
   specified.  The date shall not be normalised to any other timezone.









Kille                       Standards Track                    [Page 31]

RFC 2156                         MIXER                      January 1998


   RFC 822, as modified by RFC 1123, requires use of a four digit year.
   Note that the original RFC 822 uses a two digit date, which is no
   longer legal.  UTCTime uses a two digit date.  To map a year from RFC
   822 to X.400, simply use the last two digits.  To map a year from
   X.400 to RFC 822, assume that the two digit year refers to a year in
   the 10 year epoch 1980-2079.

3.3.6.  Integer

   A basic ASN.1 Integer will be mapped onto EBNF.numericstring.  In
   many cases ASN.1 will enumerate Integer values or use ENUMERATED.  An
   EBNF encoding labelled-integer is provided. When mapping from EBNF to
   ASN.1, only the integer value is mapped, and the associated text is
   discarded.  When mapping from ASN.1 to EBNF, a text label may be
   added.  It is recommended that this is done wherever possible and
   that clear text labels are chosen.

   A second encoding labelled-integer-2 is provided. This is used in
   DSNs, where the parsing rules will treat the text as a comment. This
   definition was not present in RFC 1327.

      labelled-integer ::= [ key-string ] "(" numericstring ")"

      labelled-integer-2 ::= [ numericstring ] "(" key-string ")"

      key-string      = *key-char
      key-char        = <a-z, A-Z, 0-9, and "-">


3.3.7.  Object Identifier

   Object identifiers are represented in a form similar to that given in
   ASN.1.  The order is the same as for ASN.1 (big-endian).  The numbers
   are mandatory, and used when mapping from the ASCII to ASN.1.  The
   key-strings are optional.  It is recommended that as many strings as
   possible are generated when mapping from ASN.1 to ASCII, to
   facilitate user recognition.

      object-identifier  ::= oid-comp object-identifier
                      | oid-comp

      oid-comp ::= [ key-string ] "(" numericstring ")"









Kille                       Standards Track                    [Page 32]

RFC 2156                         MIXER                      January 1998


   An example representation of an object identifier is:

      joint-iso-ccitt(2) mhs (6) ipms (1) ep (11) ia5-text (0)

      or

      (2) (6) (1)(11)(0)

   Because of the use of brackets and the conflict with the RFC 822
   comment convention, MIXER is defines so that the EBNFobject-
   identifier definition is not used in structured fields.

3.4.  Encoding ASCII in Printable String

   Some information in RFC 822 is represented in ASCII, and needs to be
   mapped into X.400 elements encoded as printable string.  For this
   reason, a mechanism to represent ASCII encoded as PrintableString is
   needed.

   A structured subset of EBNF.printablestring is now defined.  This
   shall be used to encode ASCII in the PrintableString character set.

      ps-encoded       = *( ps-restricted-char / ps-encoded-char )
      ps-encoded-char  = "(a)"               ; (@)
                       / "(p)"               ; (%)
                       / "(b)"               ; (!)
                       / "(q)"               ; (")
                       / "(u)"               ; (_)
                       / "(l)"               ; "("
                       / "(r)"               ; ")"
                       / "(" 3DIGIT ")"

   The 822.3DIGIT in EBNF.ps-encoded-char shall have range 0-127, and is
   interpreted in decimal as the corresponding ASCII character.  Special
   encodings are given for: at sign (@), percent (%), exclamation
   mark/bang (!), double quote ("), underscore (_), left bracket ((),
   and right bracket ()).  These characters, with the exception of round
   brackets, are not included in PrintableString, but are common in RFC
   822 addresses.  The abbreviations will ease specification of RFC 822
   addresses from an X.400 system.  These special encodings shall be
   interpreted in a case insensitive manner, but always generated in
   lower case.

   A reversible mapping between PrintableString and ASCII can now be
   defined.  The reversibility means that some values of printable
   string (containing round braces) cannot be generated from ASCII.
   Therefore, this mapping shall only be used in cases where the
   printable strings have been derived from ASCII (and will therefore



Kille                       Standards Track                    [Page 33]

RFC 2156                         MIXER                      January 1998


   have a restricted domain).  For example, in this specification, it is
   only applied to a Domain Defined Attribute which will have been
   generated by use of this specification and a value such as "(" would
   not be possible.

   To encode ASCII as PrintableString, the EBNF.ps-encoded syntax is
   used, with all EBNF.ps-restricted-char mapped directly.  All other
   822.CHAR are encoded as EBNF.ps-encoded-char.

   To encode PrintableString as ASCII, parse PrintableString as
   EBNF.ps-encoded, and then reverse the previous mapping.  If the
   PrintableString cannot be parsed, then the mapping is being applied
   in to an inappropriate value, and an error shall be given to the
   procedure doing the mapping. In some cases, it may be preferable to
   pass the printable string through unaltered.

   Some examples are now given.  Note the arrows which indicate
   asymmetrical mappings:

         PrintableString           ASCII

         'a demo.'         <->   'a demo.'
         foo(a)bar         <->   foo@bar
         (q)(u)(p)(q)      <->   "_%"
         (a)               <->   @
         (A)               ->    @
         (l)a(r)           <->   (a)
         (126)             <->   ~
         (                 ->    (
         (l)               <->   (

3.5.  RFC 1522

   RFC 1522 defines a mechanism for encoding other character set
   information into elements of RFC 822 Headers.  A gateway may ignore
   this encoding and treat the elements as ASCII.

   A preferred approach is for the gateway to interpret the RFC 1522
   encoding. This will not always be straightforward, because:

   1.   RFC 1522 permits an openly extensible character set choice,
        which may be broader than T.61.

   2.   It is not always possible to map all characters into the
        equivalent X.400 field.

   RFC 1522 is only applied to fields which are "for information only".
   A gateway which interprets header elements according to RFC 1522 may



Kille                       Standards Track                    [Page 34]

RFC 2156                         MIXER                      January 1998


   apply reasonable heuristics to minimise information loss.

Chapter 4 - Addressing and Message IDs

   Addressing is the most complex aspect of X.400 <-> RFC 822 gateway
   and is therefore  given a separate chapter.  This chapter also
   discusses message identifiers, as they are closely linked to
   addresses.  This chapter, as a side effect, also defines a textual
   representation of an X.400 OR Address.   This specification has much
   similarity to the X.400(92) representation of addresses.   This was
   because early versions of this specification were a major input to
   this work.  This specification retains compatibility with earlier
   versions.  The X.400 specification of address representation can be
   parsed but is not generated.

   Initially we consider an address in the (human) mail user sense of
   "what is typed at the mailsystem to reference a mail user".  A basic
   RFC 822 address is defined by the EBNF EBNF.822-address:

         822-address     = [ route ] addr-spec

   These definitions are taken from RFC 822.  In SMTP (or another 822-
   MTS protocol), the originator and each recipient are considered to be
   defined by such a construct.  In an RFC 822 header, the EBNF.822-
   address is encapsulated in the 822-address syntax rule, and there may
   also be associated comments.  None of this extra information has any
   semantics, other than to the end user.

   The basic X.400 OR Address, used by the MTS for routing, is defined
   by MTS.ORAddress.  In IPMS, the MTS.ORAddress is encapsulated within
   IPMS.ORDescriptor.

   The RFC 822 822.address is mapped with IPMS.ORDescriptor, and that
   RFC 822 EBNF.822-address is mapped with MTS.ORAddress.

   Section 4.1 defines a textual representation of an OR Address, which
   is used throughout the rest of this specification.  This text
   representation is designed to represent an X.400 address in the LHS
   (left hand side) or local part of an RFC 822 address, and so this
   representation gives a mechanism to represent X.400 addresses within
   RFC 822 addresses.

   Section 4.2 describes global equivalence mapping between parts of the
   X.400 and RFC 822 name spaces, and defines the concept of a MIXER
   Conformant Global Address Mapping (MCGAM).  Gateways conforming to
   this specification shall support MCGAMs.





Kille                       Standards Track                    [Page 35]

RFC 2156                         MIXER                      January 1998


   Section 4.3 is the core part of this chapter, and defines the mapping
   mechanism.

4.1.  A textual representation of MTS.ORAddress

   MTS.ORAddress is structured as an ordered set of attributes
   (type/value pairs).  It is clearly necessary to be able to encode
   this in ASCII for gatewaying purposes.  All components shall be
   encoded, in order to guarantee return of error messages, and to
   optimise third party replies.

4.1.1.  Basic OR Address Representation

   An OR Address has a number of structured and unstructured attributes.
   For each unstructured attribute, a key and an encoding is specified.
   For structured attributes, the X.400 attribute is mapped onto one or
   more attribute value pairs.  For domain defined attributes, each
   element of the sequence will be mapped onto a triple (key and two
   values), with each value having the same encoding.  The attributes
   are as follows, with 1984 attributes given in the first part of the
   attribute key table.  For each attribute, a reference is given,
   consisting of the relevant sections in X.402 / ISO 10021-2, and the
   extension identifier for 88 only attributes.  The attribute key table
   follows:

Attribute (Component)               Key         Enc    Ref     Id

84/88 Attributes

MTS.CountryName                      C                P     18.3.3
MTS.AdministrationDomainName         ADMD             P     18.3.1
MTS.PrivateDomainName                PRMD             P     18.3.21
MTS.NetworkAddress                   X121             N     18.3.7
MTS.TerminalIdentifier               T-ID             P     18.3.23
MTS.OrganizationName                 O                P/T   18.3.9
MTS.OrganizationalUnitNames.value    OU               P/T   18.3.10
MTS.NumericUserIdentifier            UA-ID            N     18.3.8
MTS.PersonalName                     PN               P/T   18.3.12
MTS.PersonalName.surname             S                P/T   18.3.12
MTS.PersonalName.given-name          G                P/T   18.3.12
MTS.PersonalName.initials            I                P/T   18.3.12
MTS.PersonalName
   .generation-qualifier             GQ               P/T   18.3.12
MTS.DomainDefineAttribute.value      DD               P/T   18.1







Kille                       Standards Track                    [Page 36]

RFC 2156                         MIXER                      January 1998


88 Attributes

MTS.CommonName                       CN               P/T   18.3.2    1
MTS.TeletexCommonName                CN               P/T   18.3.2    2
MTS.TeletexOrganizationName          O                P/T   18.3.9    3
MTS.TeletexPersonalName              PN               P/T   18.3.12   4
MTS.TeletexPersonalName.surname      S                P/T   18.3.12   4
MTS.TeletexPersonalName.given-name   G                P/T   18.3.12   4
MTS.TeletexPersonalName.initials     I                P/T   18.3.12   4
MTS.TeletexPersonalName
   .generation-qualifier             GQ               P/T   18.3.12   4
MTS.TeletexOrganizationalUnitNames
   .value                            OU               P/T   18.3.10   5
MTS.TeletexDomainDefinedAttribute
   .value                            DD               P/T   18.1      6
MTS.PDSName                          PD-SERVICE       P     18.3.11   7
MTS.PhysicalDeliveryCountryName      PD-C             P     18.3.13   8
MTS.PostalCode                       PD-CODE          P     18.3.19   9
MTS.PhysicalDeliveryOfficeName       PD-OFFICE        P/T   18.3.14   10
MTS.PhysicalDeliveryOfficeNumber     PD-OFFICE-NUM    P/T   18.3.15   11
MTS.ExtensionORAddressComponents     PD-EXT-ADDRESS   P/T   18.3.4    12
MTS.PhysicalDeliveryPersonName       PD-PN            P/T   18.3.17   13
MTS.PhysicalDeliveryOrganizationName PD-O             P/T   18.3.16   14
MTS.ExtensionPhysicalDelivery
   AddressComponents                 PD-EXT-DELIVERY  P/T   18.3.5    15
MTS.UnformattedPostalAddress         PD-ADDRESS       UPA   18.3.25   16
MTS.StreetAddress                    PD-STREET        P/T   18.3.22   17
MTS.PostOfficeBoxAddress             PD-BOX           P/T   18.3.18   18
MTS.PosteRestanteAddress             PD-RESTANTE      P/T   18.3.20   19
MTS.UniquePostalName                 PD-UNIQUE        P/T   18.3.26   20
MTS.LocalPostalAttributes            PD-LOCAL         P/T   18.3.6    21
MTS.ExtendedNetworkAddress
   .e163-4-address.number            NET-NUM          N     18.3.7    22
MTS.ExtendedNetworkAddress
   .e163-4-address.sub-address       NET-SUB          N     18.3.7    22
MTS.ExtendedNetworkAddress
   .psap-address                     NET-PSAP         X     18.3.7    22
MTS.TerminalType                     T-TY             I     18.3.24   23













Kille                       Standards Track                    [Page 37]

RFC 2156                         MIXER                      January 1998


   The following keys identify different EBNF encodings, which are
   associated with the ASCII representation of MTS.ORAddress.


         Key         Encoding

         P     printablestring
         N     numericstring
         T     teletex-string
         P/T   teletex-and-or-ps
         UPA   upa-string
         I     labelled-integer
         X     presentation-address

   The EBNF for presentation-address is taken from the specification RFC
   1278 "A String Encoding of Presentation Address" [23].

   In most cases, the EBNF encoding maps directly to the ASN.1 encoding
   of the attribute.  There are a few exceptions. In cases where an
   attribute can be encoded as either a PrintableString or NumericString
   (Country, ADMD, PRMD), either form is mapped into the EBNF.  When
   generating ASN.1, the NumericString encoding shall be used if the
   string contains digits and only digits.

   There are a number of cases where the P/T (teletex-and-or-ps)
   representation is used.  Where the key maps to a single attribute,
   this choice is reflected in the encoding of the attribute (attributes
   10-21). For example:

          /CN=yen*{165}/

   For most of the 1984 attributes and common name, there is a
   printablestring and a teletex variant.   This pair of attributes is
   mapped onto the single component here.  This will give a clean
   mapping for the common cases where only one form of the name is used.
   If there is  teletex attribute or teletex component only, and it
   contains only characters in the printable string character set, it
   shall be represented in the EBNF as if it had been encoded as
   printable string.   A single printable string representation shall
   also be done when both forms are present and they have the same
   printable string representation.

   The Unformatted Postal Address has a slightly more complex mapping
   onto a variant of   (teletex-and-or-ps), defined as:

        upa-string = [ printable-upa ] [ "*" teletex-string ]
        printable-upa = printablestring *( "|" printablestring )




Kille                       Standards Track                    [Page 38]

RFC 2156                         MIXER                      January 1998


   The optional teletex part is straightforward.  There is an (optional)
   sequence of printable strings which are mapped in order.  For
   example:

      /PD-ADDRESS=The Dome|The Square|Richmond|England/

   X.400 (1992) has introduced a string representation of OR Addresses
   (see F.401, Annex B).  This has specified a number of string keywords
   for attributes.  As earlier versions of this specification  were an
   input to this work, many of the keywords are the same.  To increase
   compatibility, the following alternative values shall be recognised
   when mapping from RFC 822 to X.400.  These shall not be generated
   when mapping from X.400 to RFC 822.  The following keyword
   alternative table and the subsequent paragraph lists alternative
   keywords.


                        Keyword         Alternative

                    ADMD              A
                    PRMD              P
                    GQ                Q
                    X121              X.121
                    UA-ID             N-ID
                    PD-OFFICE-NUM     PD-OFFICE NUMBER
                    PD-OFFICE-NUM     PD-OFN
                    PD-EXT-ADDRESS    PD-EA
                    PD-EXT-DELIVERY   PD-ED
                    PD-OFFICE         PD-OF
                    PD-STREET         PD-S
                    PD-UNIQUE         PD-U
                    PD-LOCAL          PD-L
                    PD-RESTANTE       PD-R
                    PD-BOX            PD-B
                    PD-CODE           PD-PC
                    PD-SERVICE        PD-SN
                    DD                DDA
                    NET-NUM           E.164
                    NET-PSAP          PSAP
                    PD-ADDRESS        PD-A

   When mapping from RFC 822 to X.400, the keywords defined in this
   paragraph shall be recognized.  The ordered keywords: OU1, OU2,
   OU3, and OU4, shall be recognised.  If these are present, no
   keyword OU shall be present.  These will be treated as ordered
   values of OU.  PD-A1, PD-A2, PD-A3, PD-A4, PD-A5, PD-A6 shall be
   treated as ordered lines.  If present, these will be assembled
   with separating line feeds to form a single physical address.  In



Kille                       Standards Track                    [Page 39]

RFC 2156                         MIXER                      January 1998


   this case PD-ADDRESS (or PD-A) shall not be present.   Similarly,
   there are ordered keywords for domain defined attributes: DD1,
   DD2, DD3, DD4,

   If ISDN is present, it may be interpreted as an E.163/164
   address, using local heuristics to parse the string.  X.400
   defines the key, but does not give an interpretation of the
   value.

   For T-TY (Terminal Type), the X.400 recommended values are
   preferred, but other values are allowed.  These values are: tlx
   (3); ttx (4); g3fax (5); g4fax (6); ia5 (7); and vtx (8).

4.1.2.  Encoding of Personal Name

   Handling of Personal Name and Teletex Personal Name  is a common
   requirement.   Therefore MIXER defines an alternative to the
   EBNF.standard-type syntax, which utilises the "human" conventions for
   encoding these components.  A syntax is defined, which is designed to
   provide a clean encoding for the common cases of OR Address
   specification where:

   1.   There is no generational qualifier

   2.   Initials, if present, contain only letters

   3.   Given Name, if present, does not contain full stop ("."),
        and is at least two characters long.

   4.   Surname does not contain full stop in the first two
        characters.

   5    If Surname is the only component, it does not contain full
        stop.

   The following EBNF is defined:

         encoded-pn      = [ given "." ] *( initial "." ) surname

         given           = 2*<ps-char not including ".">

         initial         = ALPHA

         surname         = printablestring







Kille                       Standards Track                    [Page 40]

RFC 2156                         MIXER                      January 1998


   This is used to map from any string containing only printable string
   characters to an OR address personal name.  To map from a string to
   OR Address components, parse the string according to the EBNF.  The
   given name and surname are assigned directly.  All EBNF.initial
   tokens are concatenated without intervening full stops to generate
   the initials component.

   For an OR address which follows the above restrictions, a string is
   derived in the natural manner.  In this case, the mapping will be
   reversible.

   For example:

         GivenName       = "Marshall"
         Surname         = "Rose"

         Maps with  "Marshall.Rose"

         Initials        = "MT"
         Surname         = "Rose"

         Maps with  "M.T.Rose"

         GivenName       = "Marshall"
         Initials        = "MT"
         Surname         = "Rose"

         Maps with  "Marshall.M.T.Rose"

   Note that X.400 suggests that Initials is used to encode all initials
   except the surname (X.402 section 18.3.12).  Therefore, the defined
   encoding is "natural" when either GivenName or Initials, but not
   both, are present.  The case where both are present can be encoded.

4.1.3.  Standard Encoding of MTS.ORAddress

   Given this structure, we can specify an EBNF representation of an OR
   Address. The output format of addresses is defined by EBNF.std-or-
   address.  The more flexible input format is defined by EBNF.std-or-
   address-input. The input EBNF has been added subsequent to RFC 1327,
   to reflect the formal incorporation of a number of heuristics.  The
   address element separator on input may be "/", ";", or a mixture of
   these.  The output format is used in all examples.

         std-or-address  = 1*( "/" attribute "=" value ) "/"
         attribute       = standard-type
                         / "RFC-822"
                         / dd-key "." std-printablestring



Kille                       Standards Track                    [Page 41]

RFC 2156                         MIXER                      January 1998


         std-or-address-input =  [ sep pair ] sep  pair *( sep pair )
                                sep  [ pair sep ]

         sep             = "/" / ";"
         pair            = input-attribute "=" value
         input-attribute = attribute
                         / dd-key ":" std-printablestring

         standard-type   = key-string

         dd-key          = key-string

         value           = std-printablestring

         std-printablestring
                         = *( std-char / std-pair )

         std-char        = <"{", "}", "*", and any ps-char
                                         except "/" and "=" >
         std-pair        = "$" ps-char

   For address generation, the standard-type is any key defined in the
   key table in Section 4.1, except PN, and DD.  For address parsing,
   other key values from Section 4.1 are also valid.  The EBNF leads to
   a set of attribute/value pairs. The value is interpreted according to
   the EBNF encoding defined in the table.

   If the standard-type is PN, the value is interpreted according to
   EBNF.encoded-pn, and the components of MTS.PersonalName and/or
   MTS.TeletexPersonalName derived accordingly.

   If dd-key is the recognised Domain Defined string (DD) or one of the
   alternatives defined in Section 4.1, then the type and value are
   interpreted according to the syntax implied from the encoding, and
   aligned to either the teletex or printable string form.  Key and
   value shall have the same encoding.

   If value is "RFC-822", then the (printable string) Domain Defined
   Type of "RFC-822" is assumed.  This is an optimised encoding of the
   domain defined type defined by this specification.

   The matching of all keywords shall be done in a case-independent
   manner.

   EBNF.std-or-address uses the characters "/" and "=" as delimiters.
   Domain Defined Attributes and any value may contain these characters.
   A quoting mechanism, using the non-printable string "$" is used to
   allow these characters to be represented.



Kille                       Standards Track                    [Page 42]

RFC 2156                         MIXER                      January 1998


   If an address of this syntax is parsed, and a country value is
   present, but no ADMD, the string shall be interpreted as if an ADMD
   value of single space had been specified.

4.2.  Global Address Mapping

   From a user perspective, the ideal mapping  would be entirely
   symmetrical and global, to enable addresses to be referred to
   transparently in the remote system, with the choice of gateway being
   left to the Message Transfer Service.  There are two fundamental
   reasons why this is not possible:

   1.   The syntaxes are sufficiently different to make this
        impossible.

   2    There is insufficient administrative co-operation between
        the X.400 and RFC 822 name registration authorities for this
        to work.

   Another way to view this situation is to see that there is not a full
   global equivalence between X.400 and RFC 822 addressing.  To meet
   user needs to the extent possible, this specification provides for
   equivalence where there is sufficient co-operation.  To be useful,
   this equivalence shall be recognised and interpreted in the same way
   by all gateways.  Therefore, an asymmetrical mapping is defined,
   which can be symmetrical where there is appropriate administrative
   co-operation.  Section 4.3 describes the asymetrical aspects.   This
   section describes a mechanism to enable the administrative co-
   ordination for symmetrical mappings.

   In order to achieve a symmetrical mapping there is a need to define
   an administrative equivalence between parts of the OR Address and
   Domain namespaces.  Previous version of this specification did this
   by definition of a global set of mappings.  MIXER defines the concept
   of a MIXER Conformant Global Address Mapping (MCGAM).  This acronym
   is defined so that it is very clear what is being referenced.

   The X.400 and Internet Mail address spaces are hierarchical.  It is
   possible to define an equivalence between two points in the
   hierarchies, such that addresses below that point can be derived in
   an algorithmic manner.  An MCGAM is a mapping from a point in one
   hierarchy to a point in the other hierarchy.  An "MGGAM pair" is a
   pair of symmetrical mappings between two points.  To define an MCGAM,
   the following shall apply:

   1.   The authority defining the MCGAM shall have responsibility
        for BOTH of the namespaces between which the MCGAM is
        defined.



Kille                       Standards Track                    [Page 43]

RFC 2156                         MIXER                      January 1998


   2.   The authority defining the MCGAM is responsible to ensure
        that addresses allocated below the two equivalence points
        conform to the rules set out below.

   3.   The authority defining the MCGAM is responsible to ensure
        that addresses which are generated according to the MCGAM
        are routed correctly.

   In general, MCGAMs will be independent.   In some cases, a set of
   MCGAMs may be related (e.g., where one MCGAM defines a mapping for an
   organization and a second MCGAM defines an excpetion for a subtree
   within the organization).   In this case, the related set of MCGAMs
   shall be treated as a single MCGAM for distribution purposes.

   The existence of an MCGAM does not imply routability and access for
   all users.

   The authority defining an MCGAM may simply use this mapping locally.
   This will often be the case in a "local scenario" gateway.   Because
   of third party addressing, a MIXER gateway will work best with the
   maximum number of MCGAMs.   Therefore, three mechanisms are defined
   to enable publication and exchange of MCGAMs:

   1.   Distribution of text tables.  This is described in Appendix
        F of this specification.

   2.   Distribution by Domain Name Service.   This is described in
        RFC 2163 [3].

   3.   Distribution by X.500 Directory Service.   This is defined
        in RFC 2164 [26].

   The following sections define how the MCGAM namespace equivalence is
   modelled.  The Internet Domain Namespace defines a simple hierarchy.
   For the purposes of this mapping, only parts of the namespace where
   domains conform to the EBNF domain-syntax are allowed.

         domain-syntax   = alphanum [ *alphanumhyphen alphanum ]
         alphanum        = <ALPHA or DIGIT>
         alphanumhyphen  = <ALPHA or DIGIT or HYPHEN>

   Although RFC 822 allows for a more general syntax, this restricted
   syntax is used in MIXER as it is the one chosen by the various domain
   service administrations.  In practice, it reflects all RFC 822 usage.







Kille                       Standards Track                    [Page 44]

RFC 2156                         MIXER                      January 1998


   The following OR Address attributes are considered as a hierarchy,
   and may be specified by the domain.  They are (in order of the
   hierarchy defined by MIXER):

         Country, ADMD, PRMD, Organization, Organizational Units

   There may be up to four ordered Organizational Units.   This
   hierarchy reflects most usage of X.400, although X.400 may be used in
   other ways. In particular, it covers the Mnemonic OR Address using a
   1984 compatible encoding.  This is seen as the dominant form of OR
   Address. MCGAMs may only be used when this hierarchy applies.

   An equivalence mapping is defined between two nodes in the respective
   hierarchies. For example:

         => "AC.UK" might be mapped with
         PRMD="UK.AC", ADMD="GOLD 400", C="GB"

   The mapping identifies that the management of these points in the
   respective hierarchies is the same (or co-operate very closely).  The
   equivalence means that the namespaces below this equivalence point
   map 1:1, except where the mapping is overridden by further
   equivalence mappings lower down the hierarchy.   This equivalence may
   be achieved in three ways:

   1.   All of the nodes below this point are RFC 822, and the MIXER
        mapping defines the X.400 addresses for these nodes.

   2.   All of the nodes below this point are X.400, and the MIXER
        mapping defines the  RFC 822 addresses for these nodes.

   3.   There are X.400 and RFC 822 nodes below this point, and
        addressing is managed in a manner which  ensures the
        equivalence.   The rules to achieve this are  defined by
        MIXER.

   Each of these ways gives a framework for MCGAM definition.

   When an MCGAM is defined, a systematic mapping for the inferior nodes
   in the two hierarchies follows.   This is a 1:1 mapping between the
   nodes in the subtrees.  For example, given the MCGAM pair defined
   above:

         the domain "R-D.Salford.AC.UK" algorithmically maps with
         OU="R-D", O="Salford", PRMD="UK.AC", ADMD="GOLD 400", C="GB"






Kille                       Standards Track                    [Page 45]

RFC 2156                         MIXER                      January 1998


   Note that when an equivalence is defined, that this can be re-defined
   for lower points in the hierarchy.  However, it is not possible to
   declare contained subtrees to be un-mappable.

   The equivalence mapping also provides a mechanism to deal with
   missing elements in the X.400 hierarchy (most commonly the PRMD,
   which is the only element that may be ommitted when conforming to
   recent versions of X.400).  A domain may be associated with an
   omitted attribute in conjunction with several present ones.  When
   performing the algorithmic insertion of components lower in the
   hierarchy, the omitted value shall be skipped.  For example:

         If there is an MCGAM pair between domain HNE.EGM" and "O=HNE",
         "ADMD=ECQ", "C=TC", and omitted PRMD

   then

         "ZI.HNE.EGM" is algorithmically mapped with "OU=I", "O=HNE",
         "ADMD=ECQ", "C=TC"

   Attributes may have null values, and  this is treated separately from
   omitted attributes (while it is not ideal to make this distinction,
   it is useful in practice).

4.2.1.  Directory and Nameserver Mappings

   When a set of MCGAMs are supported by X.500 or DNS, there is the
   possibility that results will be indeterminate due to timeout.
   Lookup shall be repeated until a value is determined, in order to
   maintain  consistent gateway operation.

   Where the mapping relates to an envelope address, the gateway shall
   non-deliver messages according to the associated MTA's normal timeout
   policy.  Where the mapping relates to addresses in the message
   header, there shall be a timeout in the range of 1-4 hours or shorter
   if this is required to maintain quality of service constraints.   If
   a mapping cannot be done in this time, address encapsulation shall be
   used.

4.3.  EBNF.822-address <-> MTS.ORAddress

   This section defines the basic address mapping.

4.3.1.  X.400 encoded in RFC 822

   This section defines how X.400 addresses are represented in RFC 822
   addresses.




Kille                       Standards Track                    [Page 46]

RFC 2156                         MIXER                      January 1998


   The std-or-address syntax is  used to encode OR Address information
   in the 822.local-part of EBNF.822-address.  Where there is an
   applicable equivalence mapping, further  OR Address information is
   associated with the 822.domain component.  This cannot be used in the
   general case, due to character set problems, and to the variants of
   X.400 OR Addresses which use different attribute types.  The only way
   to encode the full PrintableString character set in a domain is by
   use of the 822.domain-ref syntax (i.e. 822.atom).  This is likely to
   cause problems on many systems.  The effective character set of
   domains is in practice reduced from the RFC 822 set, by restrictions
   imposed by domain conventions and policy [10], and by the EBNF
   definition in SMTP.

   A generic 822.address consists of a 822.local-part and a sequence of
   822.domains (e.g., <@domain1,@domain2:user@domain3>).  All except the
   822.domain associated with the 822.local-part (domain3 in this case)
   are considered to specify routing within the RFC 822 world, and will
   not be interpreted by the gateway (although they may have identified
   the gateway from within the RFC 822 world).

   The  822.domain associated with the 822.local-part identifies the
   gateway from within the RFC 822 world.  This final 822.domain may be
   used to determine some number of OR Address attributes, where this
   does not conflict with the first role.  RFC 822 routing to gateways
   will usually be set up to facilitate the 822.domain being used for
   both purposes.

   In the case that there is no applicable equivalence mapping, all of
   the X.400 address is encoded in the 822.local-part and the 822.domain
   identifies the gateway to which the message is being sent.  This
   technique may be used by the RFC 822 user for any X.400 address where
   the equivalence mapping is not known.

   In the case that there is an applicable MCGAM, the maximum number of
   attributes are encoded in the 822.domain.  The remaining attributes
   are encoded on the LHS, using the EBNF.std-or-address syntax.  For
   example:

         /I=J/S=Linnimouth/GQ=5/@Marketing.Widget.COM

   encodes the MTS.ORAddress consisting of:


         MTS.CountryName                       = "TC"
         MTS.AdministrationDomainName          = "BTT"
         MTS.OrganizationName                  = "Widget"
         MTS.OrganizationalUnitNames.value     = "Marketing"
         MTS.PersonalName.surname              = "Linnimouth"



Kille                       Standards Track                    [Page 47]

RFC 2156                         MIXER                      January 1998


         MTS.PersonalName.initials             = "J"
         MTS.PersonalName.generation-qualifier = "5"

   on the basis of an MCGAM pair between:

         Domain: Widget.COM
         OR Address: O="Widget", ADMD="BTT", C="TC"

   Given the OR address, the domain Widget.COM is determined from the
   equivalence mapping and the next component is determined
   algorithmically to give Marketing.Widget.COM.  The remaining
   attributes are encoded on the LHS in 822.local-part.

   There is a further mechanism to simplify the encoding of common
   cases, where the only attributes to be encoded on the LHS are (non-
   Teletex) Personal Name attributes which comply with the restrictions
   of 4.1.2.  To achieve this, the 822.local-part shall be encoded as
   EBNF.encoded-pn.  In the previous example, if the GenerationQualifier
   was not present in the OR Address, it would map with the RFC 822
   address:  J.Linnimouth@Marketing.Widget.COM.

   From the standpoint of the RFC 822 Message Transfer System, the
   domain specification is used to route the message in the standard
   manner.  The standard domain mechanisms are used to select
   appropriate gateways for the corresponding OR Address space.  It is
   the responsibility of the management that defines the equivalence
   mapping to define routing in the manner which will enable the message
   to be delivered.

4.3.2.  RFC 822 encoded in X.400

   The previous section showed a mapping from X.400 to RFC 822.  In the
   case where  the mapping was symmetrical and based on the equivalence
   mapping, this has also shown how RFC 822 is encoded in the X.400.
   This equivalence cannot be used for all RFC 822 addresses.

   The general case is mapped by use of domain defined attributes.  A
   (Printable String) Domain defined type "RFC-822" is defined. The
   associated attribute value is an ASCII string encoded according to
   Section 3.3.3 of this specification. The interpretation of the ASCII
   string follows RFC 822, and RFC 1123 [10,16].  Domains shall always
   be fully qualified.









Kille                       Standards Track                    [Page 48]

RFC 2156                         MIXER                      January 1998


   Other OR Address attributes will be used to identify a context in
   which the OR Address will be interpreted.  This might be a Management
   Domain, or some part of a Management Domain which identifies a
   gateway MTA.  For example:

         C               = "GB"
         ADMD            = "GOLD 400"
         PRMD            = "UK.AC"
         O               = "UCL"
         OU              = "CS"
         "RFC-822"      =  "Jimmy(a)WIDGET-LABS.CO.UK"

   OR

         C               = "TC"
         ADMD            = "Wizz.mail"
         PRMD            = "42"
         "rfc-822"       = "postel(a)venera.isi.edu"

   Note in each case the PrintableString encoding of "@" as "(a)".  In
   the second example, the "RFC-822" domain defined attribute is
   interpreted everywhere within the (Private) Management Domain.  In
   the first example, further attributes are needed within the
   Management Domain to identify a gateway.  Thus, this scheme can be
   used with varying levels of Management Domain co-operation.

   There is a limit of 128 characters in the length of value of a domain
   defined attribute, and an OR Address can have a maxmimum of four
   domain defined attributes.  Where the printable string generated from
   the RFC 822 address exceeds 128 characters, additional domain defined
   attributes are used to enable up to 512 characters to be encoded.
   These attributes shall be filled completely before the next one is
   started.   The (Printable String) DDA keywords are:  RFC822C1;
   RFC822C2; RFC822C3.  Longer addresses cannot be encoded.

   MIXER defines a representation of RFC 822 addresses in printable
   string domain defined attributes.  Teletex domain defined attributes
   with a key of RFC-822, RFC822C1; RFC822C2; RFC822C3 shall not be
   generated.  This is for backwards compatibility reasons.












Kille                       Standards Track                    [Page 49]

RFC 2156                         MIXER                      January 1998


   Reception of these attributes in the manner defined below is
   mandatory.  This is to allow the possibility for future versions of
   MIXER to allow generation of teletex domain defined attributes.
   Where the values of all of these teletex domain defined attributes
   are printable string characters, they shall be interpreted in the
   same way as the printable string domain defined attributes.   If this
   is not the case, the printable string encoding translation shall be
   omitted.  If both teletex and printable string attributes are
   present, this is valid if and only if they represent exactly the same
   RFC 822 address.

4.3.3.  Component Ordering

   In most cases, ordering of OR Address components is not significant
   for the mappings specified.  However, Organizational Units (printable
   string and teletex forms) and Domain Defined Attributes are specified
   as SEQUENCE in MTS.ORAddress, and so their order may be significant.
   This specification needs to take account of this:

   1.   To allow consistent mapping into the domain hierarchy

   2.   To ensure preservation of order over multiple mappings.

   There are three places where an order is specified:

   1.   The text encoding (std-or-address) of MTS.ORAddress as used
        in the local-part of an RFC 822 address.  An order is needed
        for those components which may have multiple values
        (Organizational Unit, and Domain Defined Attributes). When
        generating an 822.std-or-address, components of a given type
        shall be in hierarchical order with the most significant
        component on the RHS (right hand side or domain part).  If
        there is an Organization Attribute, it shall be to the right
        of any Organizational Unit attributes.  These requirements
        are for the following reasons:

   -         Alignment to the hierarchy of other components in RFC
             822 addresses (thus, Organizational Units will appear
             in the same order, whether encoded on the RHS or LHS).

   -         Backwards compatibility with RFC 987/1026.

   -         To ensure that gateways generate consistent addresses.
             This is both to help end users, and to generate
             identical message ids.






Kille                       Standards Track                    [Page 50]

RFC 2156                         MIXER                      January 1998


   Further, it is recommended that all other attributes are generated
   according to this ordering, so that all attributes so encoded follow
   a consistent hierarchy.  When generating 822.msg-id, this order shall
   be followed.

   2.   For the Organizational Units (OU) in MTS.ORAddress, the
        first OU in the SEQUENCE is the most significant, as specified
        in X.400.

        3.   For the Domain Defined Attributes in MTS.ORAddress, the
        First Domain Defined Attribute in the SEQUENCE is the most
        significant.

   Note that although this ordering is mandatory for this mapping, MIXER
   does not give additional implications on the ordering significance
   within X.400.

4.3.4.  RFC 822 -> X.400 Basic Address Mapping

   There are two basic cases:

   1.   X.400 addresses encoded in RFC 822.  This will also include
        RFC 822 addresses which are given reversible encodings.

   2.   "Genuine" RFC 822 addresses.

   The mapping shall proceed as follows, by first assuming case 1).

   STAGE I.

   1.   If the 822-address is not of the form:

         local-part "@" domain

       take the domain which will be routed on and apply step 2 of stage
       1 to derive (a possibly null) set of attributes. Then go to stage
       II.

       The gateway may  reduce a source route address to this form by
       removal of all but the last domain.  In terms of the design
       intentions of RFC 822, this would be an incorrect action. (Note
       that an address of the form local%part@domain is not a source
       route).  However, in most cases, it will provide a better service








Kille                       Standards Track                    [Page 51]

RFC 2156                         MIXER                      January 1998


       to the end user, and is in line with the Internet Host
       Requirements.  This is a reflection on the common inappropriate
       use of source routing in RFC 822 based systems, despite the
       discussion in the Host Requirements [10].  Either approach, or
       the intermediate approach of stripping only domain references
       which reference the local gateway are conformant to this
       specification.

   2.   If the 822.local-part uses the 822.quoted-string encoding,
        remove this quoting.  If the resulting unquoted
        822.local-part has leading space, trailing space, or two
        adjacent spaces go to stage II.

   3.   If the unquoted 822.local-part contains any characters not
        in PrintableString, "{", "}", "*", and "$", go to stage II.

   4.   Parse the (unquoted) 822.local-part according to the EBNF
        EBNF.std-or-address-input.  Checking of upper bounds shall
        not be done at this point.  If this parse fails, parse the
        local-part according to the EBNF EBNF.encoded-pn.  If this
        parse fails, go to stage II.  The result is a set of
        type/value pairs.

   5.   Associate the EBNF.attribute-value syntax (determined from
        the identified type) with each value, and check that it
        conforms.  If not, go to stage II.

   6.   If the set of attributes forms a valid X.400 address,
        according to X.402, then go to step 9.  All forms of X.400
        address are allowed at this stage.  Steps 7-8 default
        attributes for certain types of OR Address.

   7.   If the set of attributes cannot form a mnemonic form of
        X.400 address after addition of attributes which may be
        derived from the EBNF.domain (C, ADMD, PRMD, O, OU), go to
        stage II.

   8.   Attempt to parse EBNF.domain as:

         *( domain-syntax "." ) known-domain

        Where EBNF.known-domain is the longest possible match in the set
        of MCGAMs being used by the gateway (described in Section 4.2).
        EBNF.domain-syntax is the restricted domain syntax defined in
        Section 4.2, to which all of the domain components shall conform
        for the parse to be successful.  If this fails, go to stage II.





Kille                       Standards Track                    [Page 52]

RFC 2156                         MIXER                      January 1998


        For each component, systematically allocate the attribute
        implied by each EBNF.domain-syntax component in the order: C,
        ADMD, PRMD, O, OU.  Note that if the MCGAM used identifies an
        "omitted attribute", then this attribute shall be omitted in the
        systematic allocation.  If this new component exceed an upper
        bound (ADMD: 16; PRMD: 16; O: 64; OU:  32) or it would lead to
        more than four OUs, then go to stage II with the attributes
        derived.

        The attributes derived in this step (referred to as RHS
        attributes) are merged with the ones derived from the LHS (step
        6).  In some cases, not all of the RHF attributes are used.  LHS
        attributes are all used.  C will not be in the LHS attributes.
        If ADMD is in the LHS attributes,  only C is taken from the RHS
        attributes. If PRMD is in the LHS attributes, C and ADMD are
        taken from the RHS attributes.  If O is on the LHS, C, ADMD and
        PRMD (if present) are taken from the RHS attributes.  In other
        cases all RHS attributes are taken.

   9.   Ensure that the set of attributes conforms both to the
        MTS.ORAddress specification and to the restrictions on this
        set given in X.400, and that no upper bounds are exceeded
        for any attribute.  If not go to stage II.

   10.  Build the OR Address from this information.

   STAGE II.

   This will only be reached if the RFC 822 EBNF.822-address is not a
   valid X.400 encoding.  This implies that the address  refers to a
   recipient on an RFC 822 system or that the encoding of the address is
   invalid.  Such addresses shall be encoded in an X.400 OR Address
   using a domain defined attribute.

   1.   Convert the EBNF.822-address to PrintableString, as
        specified in Chapter 3.

   2.   Generate the "RFC-822" domain defined attribute  from this
        string.

   3.   Build the rest of the OR Address in the manner described
        below.









Kille                       Standards Track                    [Page 53]

RFC 2156                         MIXER                      January 1998


   It is not always possible to encode the domain defined attribute
   due to length restrictions.  If the limit is exceeded by a
   mapping at the MTS level, then the gateway shall reject the
   message in question.  If this occurs at the IPMS level, then the
   action will depend on the policy being taken for IPMS encoding,
   which is discussed in Section 5.1.3.

   Use Stage I, step 8, to generate a set of attributes to build the
   remainder of the address.  The administrative equivalence of the
   mappings will ensure correct routing through X.400 to a gateway
   back to RFC 822.

   If Stage I, step 8 does not generate a set of attributes or
   the address generated is unroutable, the remained of the OR
   address is generated as follows.  The remainder of the OR address
   effectively identifies a source route to a gateway from the X.400
   side.  There are three cases, which are handled differently:

   SMTP Return Address
      This shall be set up so that errors are returned through the
      same gateway.  Therefore, the OR Address of the local
      gateway shall be used.

   IPMS Addresses
      These are optimised for replying.  In general, the message
      may end up anywhere within the X.400 world, and so this
      optimisation identifies a gateway appropriate for  the RFC
      822 address being converted.  The 822.domain to which the
      address would be routed is used to select an appropriate
      gateway.

      In this case, it may be useful to use a non-local gateway,
      which will optimise the reply address.   This information
      may be looked up in gateway tables in a manner equivalent to
      the MCGAM lookup.  Because of the similarity of lookup, the
      three MCGAM lookup mechanisms (table, X.500, DNS) are also
      available to look up this information.   This information is
      local, and a gateway may insert any appropriate  (gateway)
      OR Address.  The longest possible match on the 822.domain
      defines which gateway to use.  This mechanism is used for
      any part of the X.400 namespace for which it is desirable to
      identify a preferred X.400 gateway in order to optimise
      routing.

      If no mapping is found for the 822.domain, a default value
      (typically that of the local gateway) is used.  It is never
      appropriate to ignore the locally used MCGAMs.




Kille                       Standards Track                    [Page 54]

RFC 2156                         MIXER                      January 1998


   SMTP Recipient
      As the RFC 822 and X.400 worlds are in principle fully
      connected, there is no technical reason for this situation
      to occur. In practice, this is not the case.  In some cases,
      routing may be configured to use X.400 to connect an RFC 822
      island to the Internet.  The information that this part of
      the domain space is to be routed by X.400 rather than
      remaining within the RFC 822 world shall be configured
      privately into the gateway in question. X.400 routing shall
      not make use of the presence of the RFC-822 DDA to perform
      X.400 routing.  The OR address shall then be generated in
      the same manner as for an IPMS address, using the locally
      available MCGAMs.  It is to support this case that the
      definition of the global domain to gateway mapping is
      important, as the use of this mapping will lead to a remote
      X.400 address, which can be routed by X.400 routing
      procedures.  The information in this mapping shall not be
      used as a basis for deciding to convert a message from RFC
      822 to X.400.

   Three examples are given, neither of which has applicable MCGAMs.

   Example 1: (Address not in "localpart" "@" "domainpart")

   @relay.co.uk:userb@host2

            maps to

   c=gb; a= ; p=uk.ac; o=mr; dd.rfc-822=(a)relay.co.uk:userb(a)host2;

   Example 2: (Address with non printablestring characters)

   Tom_Harris@cs.widget.com

            maps to

   c=us; a=MCI; P=relay; dd.rfc-822=Tom(u)Harris(a)cs.widget.com;


   Example 3: (Address with an entry for alter.net into the OR Address
   of Preferred Gateway table, pointing to c=gb; A=BTglobal; P=relay)

   postmaster@UK.alter.net

      maps to

   c=gb; a=BTglobal; P=relay; dd.rfc-822=postmaster(a)UK.alter.net;




Kille                       Standards Track                    [Page 55]

RFC 2156                         MIXER                      January 1998


4.3.4.1.  Heuristic for mapping RFC 822 to X.400

   The following heuristic, which  relates to ordering of address
   components, may be used when mapping from RFC 822 to X.400.  The
   ordering of attributes may be inverted or mixed, and so the following
   heuristics may be applied:

       If there is an Organization attribute to the left of any Org Unit
       attribute, assume that the hierarchy is inverted.  This is to
       facilitate the situation where a user has input the attributes in
       reverse hierarchical order.  To do this the gateway shall first
       map according to the order defined in 4.3.3.    If this mapping
       generates an address which X.400 address verification shows to be
       invalid, this heuristic may be applied as an alternative to
       immediate rejection of the address.

4.3.5.  X.400 -> RFC 822 Basic Address Mapping

   There are two basic cases:

   1.   RFC 822 addresses encoded in X.400.

   2.   "Genuine" X.400 addresses.  This may include symmetrically
        encoded RFC 822 addresses.

   When an MTS Recipient OR Address is interpreted, gatewaying will be
   selected if there is a single "RFC-822" domain defined attribute
   present.  In this case, use mapping A and in other cases, use mapping
   B.

   RFC 1327 specified that this shall only be done when the gateway
   identfied is local or otherwise known, and identified the approach
   specified here as a pragmatic option.  Experience has shown that this
   is effective in practice, despite theoretical problems.

   If a gateway wishes to make a mapping in a manner similar to RFC
   1327, but does not wish for this global interpretation (e.g., to
   support an RFC 822 local system, which does not use global
   addressing), then it may choose a private domain defined attribute,
   different to "RFC-822".  An RFC 1327 gateway might be configurable to
   operate in this manner.

   Mapping A

   1.   Map the domain defined attribute value to ASCII, as defined
        in Chapter 3, and drop all other attributes.





Kille                       Standards Track                    [Page 56]

RFC 2156                         MIXER                      January 1998


   Mapping B

   This is used for X.400 addresses which do not use the explicit RFC
   822 encoding.

   1.   For all string encoded attributes, remove any leading or
        trailing spaces, and replace adjacent spaces with a single
        space.

        The only attribute which is permitted to have zero length is
        the ADMD.  This shall be mapped onto a single space.

        These transformations are for lookup only.   If an
        EBNF.std-or-address mapping is used as in 4), then the
        original values shall be used.

   2.   The numeric country codes may be mapped to the two letter
        values (as defined in ISO 3166).  Global mappings are
        usually only defined in terms of the ISO 3166 codes.

   3.   Noting the hierarchy specified in 4.3.1 and including
        omitted attributes, determine the maximum set of attributes
        which have an associated domain specification in the local
        set of MCGAMs.  If no match is found, allocate the domain as
        described below, and go to step 5. The default domain to be
        used is the specification of the local gateway.   A gateway
        may use other domains according to private mapping tables or
        heuristics.   For example, it may choose a domain which it
        knows to provide a free gateway service to the mapped
        address.

        In cases where the address refers to an X.400 UA, it is
        important that the generated domain will correctly route to
        a gateway.  In general, this is achieved by carefully co-
        ordinating RFC 822 routing with the definition of the
        MCGAMs, as there is no easy way for the gateway to make this
        check.  One rule that shall be used is that domains with
        only one component will not route to a gateway.  If the
        generated domain does not route correctly, the address is
        treated as if no match is found.

        The gateway may also make use of a mapping equivalent to the
        MCGAM mapping to determine the domain to use.  This mapping
        is done from the OR Address hierarchy.   This is not a
        global mapping, but is a routing style mapping from the OR
        Address space, to enable a best choice domain to be
        inserted.   This mapping is supported by the three MCGAM
        lookup mechanisms.



Kille                       Standards Track                    [Page 57]

RFC 2156                         MIXER                      January 1998


   4.   The mapping identified  in 3) gives a domain, and an OR
        address prefix.  Follow the hierarchy: C, ADMD, PRMD, O, OU.
        For each successive component below the OR address prefix, which
        conforms to the syntax EBNF.domain-syntax (as defined in 4.3.1),
        allocate the next subdomain.  At least one attribute of the
        X.400 address shall not be mapped onto subdomain, as 822.local-
        part cannot be null.  If there are omitted attributes in the OR
        address prefix, these will have correctly and uniquely mapped to
        a domain component.   Where there is an attribute omitted below
        the prefix, all attributes remaining in the OR address shall be
        encoded on the LHS.  This is to ensure a reversible mapping. For
        example, if there is an address /S=XX/O=YY/ADMD=A/C=NN/ and a
        mapping for /ADMD=A/C=NN/ is used, then /S=XX/O=YY/ is encoded
        on the LHS.

   5.   If the address contains any attribute not used in mnemonic
        form, then all of the attributes in the address shall be encoded
        on the LHS in EBNF.std-or-address syntax, as described below.

        For addresses of mnemonic form, if the remaining components are
        personal-name components, conforming to the restrictions of
        4.2.1, then EBNF.encoded-pn is derived to form 822.local-part.
        In other cases the remaining components are simply encoded as
        822.local-part using the EBNF.std-or-address syntax.  If
        necessary, the 822.quoted-string encoding is used.  The
        following are examples of legal quoting: "a b".c@x; "a b.c"@x.
        Either form may be generated.  Generation of the latter style is
        strongly recommended.

   Four examples are given.

   Example 1: (Address with missing X.400 elements and no specific
   mapping rule for "o=sales; a=Master400; C=it", where a mapping exists
   for a=master400; C=it;)

   S=Support; O=sales;  A=Master400; C=it;

       maps to

   /S=Support/o=sales/@Master400.it











Kille                       Standards Track                    [Page 58]

RFC 2156                         MIXER                      January 1998


   Example 2: (Address with illegal characters in RFC822 generated
   domain if default hierarchical translation (specific mapping rule is
   existing for c=fr; a=atlas; p=autoroutes) is used)

   S=renseignements; O=Region Parisienne; P=autoroutes; A=atlas; C=fr;

       maps to

   "/S=renseignements/o=Region Parisienne/"@autoroutes.fr


   Example 3:  (Address containing elements not mappable into RFC822
   local part)

   S=Rossi; DD.cap=20100; DD.ph1=Via Larga 11; DDA.city=Milano;
   A=PtPostel; C=it;

       maps to

   "/DD.cap=20100/DD.ph1=Via Larga
   11/DD.city=Milano/S=Rossi/"@ptpostel.it


   Example 4:   (Address with an entry for A=ATT; C=us; into the domain
   of Preferred Gateway table, pointing to attmail.com)

   G=Andy; S=Wharol; O=MMNY; A=ATT; C=us;

      maps to

   /G=Andy/S=Wharol/O=MMNY@attmail.com

4.4.  Repeated Mappings

   There are two types of repeated mapping:

   1.   A recursive mapping, where the repeat is within one gateway

   2    A source route, where the repetition occurs across multiple
        gateways











Kille                       Standards Track                    [Page 59]

RFC 2156                         MIXER                      January 1998


4.4.1.  Recursive Mappings

   It is possible to supply an address which is recursive at a single
   gateway.  For example:

              C          = "XX"
              ADMD       = "YY"
              O          = "ZZ"
              "RFC-822"  = "Smith(a)ZZ.YY.XX"

   This is mapped first to an RFC 822 address, and then back to the
   X.400 address:

              C          = "XX"
              ADMD       = "YY"
              O          = "ZZ"
              Surname    = "Smith"

   In some situations this type of recursion may be frequent.  It is
   important where this occurs, that no unnecessary protocol conversion
   occurs. This will minimise loss of service.

4.4.2.  Source Routes

   The mappings defined are symmetrical and reversible across a single
   gateway.  The symmetry is particularly useful in cases of (mail
   exploder type) distribution list expansion.  For example, an X.400
   user sends to a list on an RFC 822 system which he belongs to.  The
   received message will have the originator and any 3rd party X.400 OR
   Addresses in correct format (rather than doubly encoded).  In cases
   (X.400 or RFC 822) where there is common agreement on gateway
   identification, then this will apply to multiple gateways.

   When a message traverses multiple gateways, the mapping will always
   be reversible, in that a reply can be generated which will correctly
   reverse the path.  In many cases, the mapping will also be
   symmetrical, which will appear clean to the end user.  For example,
   if countries "AB" and "XY" have RFC 822 networks, but are
   interconnected by X.400, the following may happen:  The originator
   specifies:

          Joe.Soap@Widget.PTT.XY









Kille                       Standards Track                    [Page 60]

RFC 2156                         MIXER                      January 1998


   This is routed to a gateway, which generates:

              C               = "XY"
              ADMD            = "PTT"
              PRMD            = "Griddle MHS Providers"
              Organization    = "Widget Corporation"
              Surname         = "Soap"
              Given Name      = "Joe"

   This is then routed to another gateway where the mapping is reversed
   to give:

          Joe.Soap@Widget.PTT.XY

   Here, use of the gateway is transparent.

   Mappings will only be symmetrical where mapping equivalences are
   defined. In other cases, the reversibility is more important, due to
   the (far too frequent) cases where RFC 822 and X.400 services are
   partitioned.

   The syntax may be used to source route.  THIS IS STRONGLY
   DISCOURAGED.  For example:

      X.400 -> RFC 822  -> X.400

      C             = "UK"
      ADMD          = "Gold 400"
      PRMD          = "UK.AC"
      "RFC-822"     = "/PN=Duval/DD.Title=Manager/(a)Inria.ATLAS.FR"

   This will be sent to an arbitrary UK Academic Community gateway by
   X.400.  Then it will be sent by JNT Mail to another gateway
   determined by the domain Inria.ATLAS.FR (FR.ATLAS.Inria).  This will
   then derive the X.400 OR Address:

      C             = "FR"
      ADMD          = "ATLAS"
      PRMD          = "Inria"
      PN.S          = "Duval"
      "Title"       = "Manager"










Kille                       Standards Track                    [Page 61]

RFC 2156                         MIXER                      January 1998


   Similarly:

   RFC 822 -> X.400 -> RFC 822

   "/RFC-822=jj(a)seismo.css.gov/PRMD=AC/ADMD=BT/C=GB/"@monet.berkeley.edu

   This will be sent to monet.berkeley.edu by RFC 822, then to the
   AC PRMD by X.400, and then to jj@seismo.css.gov by RFC 822.

4.5.  Directory Names

   Directory Names are an optional part of OR Name, along with OR
   Address.  The RFC 822 addresses are mapped onto the OR Address
   component. As there is no functional mapping for the Directory Name
   on the RFC 822 side, a textual mapping is used.  There is no
   requirement for reversibility in terms of the goals of this
   specification.  There may be some loss of functionality in terms of
   third party recipients where only a directory name is given, but this
   seems preferable to the significant extra complexity of adding a full
   mapping for Directory Names.

   The Directory Name shall be represented within an RFC 822 comment
   using the comaptible formats of RFC 1484 or RFC 1485.  It is
   recommended that the directory string format of RFC 1485 is used
   [24].  The User Friendly Name form of RFC 1484 may be used [25].

4.6.  MTS Mappings

   The basic mappings at the MTS level are:

      1) SMTP originator ->
                    MTS.PerMessageSubmissionFields.originator-name
         MTS.OtherMessageDeliveryFields.originator-name ->
                    SMTP originator

      2) SMTP recipient ->
                    MTS.PerRecipientMessageSubmissionFields
         MTS.OtherMessageDeliveryFields.this-recipient-name ->
                    SMTP recipient

   SMTP recipients and return addresses are encoded as EBNF.822-address.

   The MTS Originator is always encoded as MTS.OriginatorName, which
   maps onto MTS.ORAddressAndOptionalDirectoryName, which in turn maps
   onto MTS.ORName.






Kille                       Standards Track                    [Page 62]

RFC 2156                         MIXER                      January 1998


4.6.1.  RFC 822 -> X.400 MTS Mappings

   From the SMTP Originator, use the basic ORAddress mapping, to
   generate MTS.PerMessageSubmissionFields.originator-name (MTS.ORName),
   without a DirectoryName.

   For recipients, the following settings are made for each component of
   MTS.PerRecipientMessageSubmissionFields.

   recipient-name
      This is derived from the SMTP recipient by the basic ORAddress
      mapping.

   originator-report-request
      This may either be set to "delivery-report", or set according to
      SMTP extensions as set out in Appendix A.

   explicit-conversion
      This optional component is omitted, as this service is not needed

   extensions
      The default value (no extensions) is used

4.6.2.  X.400 -> RFC 822 MTS Mappings

   The basic functionality is to generate the SMTP originator and
   recipients.  There is information present on the X.400 side, which
   cannot be mapped into analogous SMTP services.  For this reason, new
   RFC 822 fields are added for the MTS Originator and Recipients.  The
   information discarded at the SMTP level will be present in these
   fields. In some cases a (positive) delivery report will be generated.

4.6.2.1.  SMTP Mappings

   Use the basic ORAddress mapping, to generate the SMTP originator
   (return address) from MTS.OtherMessageDeliveryFields.originator-name
   (MTS.ORName).  If MTS.ORName.directory-name is present, it is
   discarded.  (Note that it will be presented to the user, as described
   in 4.6.2.2).

   The mapping  uses the MTA level information, and maps each value of
   MTA.PerRecipientMessageTransferFields.recipient-name, where the
   responsibility bit is set, onto an SMTP recipient.

      Note:The SMTP recipient is conceptually generated from
      MTS.OtherMessageDeliveryFields.this-recipient-name.  This is done
      by taking MTS.OtherMessageDeliveryFields.this-recipient-name, and
      generating an SMTP recipient according to the basic ORAddress



Kille                       Standards Track                    [Page 63]

RFC 2156                         MIXER                      January 1998


      mapping, discarding MTS.ORName.directory-name if present.
      However, if this model was followed exactly, there would be no
      possibility to have multiple SMTP recipients on a single message.
      This is unacceptable, and so layering is violated.

4.6.2.2.  Generation of RFC 822 Headers

   Not all per-recipient information can be passed at the SMTP level.
   For this reason, two new RFC 822 headers are created, in order to
   carry this information to the RFC 822 recipient.  These fields are
   "X400-Originator:"  and "X400-Recipients:".

   The "X400-Originator:" field is set to the same value as the SMTP
   originator.  In addition, if
   MTS.OtherMessageDeliveryFields.originator-name (MTS.ORName) contains
   MTS.ORName.directory-name then this Directory Name shall be
   represented in an 822.comment.

   Recipient names, taken from each value of
   MTS.OtherMessageDeliveryFields.this-recipient-name and
   MTS.OtherMessageDeliveryFields.other-recipient-names are made
   available to the RFC 822 user by use of the "X400-Recipients:" field.
   By taking the recipients at the MTS level, disclosure of recipients
   will be dealt with correctly.  However, this conflicts with a desire
   to optimise mail transfer.  There is no problem when disclosure of
   recipients is allowed. Similarly, there is no problem if there is
   only one RFC 822 recipient, as the "X400-Recipients" field is only
   given one address.

   There is a problem if there are multiple RFC 822 recipients, and
   disclosure of recipients is prohibited.  In this case, discard the
   per-recipient information.

   If any MTS.ORName.directory-name is present, it shall be represented
   in an 822.comment.

   If MTS.OtherMessageDeliveryFields.orignally-intended-recipient-name
   is present, then there has been redirection,  or there has been
   distribution list expansion.  Distribution list expansion is a per-
   message option, and the information associated with this is
   represented by the "DL-Expansion-History:" field described in Section
   5.3.6.  Other information is represented in an 822.comment associated
   with MTS.OtherMessageDeliveryFields.this-recipient-name, The message
   may be delivered to different RFC 822 recipients, and so several
   addresses in the "X400-Recipients:" field may have such comments.
   The non-commented recipient is the RFC 822 recipient. The EBNF of the
   comment is defined by redirect-comment.




Kille                       Standards Track                    [Page 64]

RFC 2156                         MIXER                      January 1998


         redirect-comment  = redirect-first *( redirect-subsequent )

         redirect-first = "Originally To:"  mailbox  "Redirected on"
            date-time "To:"  redirection-reason

         redirect-subsequent = mailbox  "Redirected Again on"
            date-time "To:"  redirection-reason

         redirection-history-item = "intended recipient" mailbox
            "redirected to"  redirection-reason
            "on" date-time

         redirection-reason =
            "Recipient Assigned Alternate Recipient"
            / "Originator Requested Alternate Recipient"
            / "Recipient MD Assigned Alternate Recipient"
            / "Directory Look Up"
            / "Alias"

   It is derived from
   MTA.PerRecipientMessageTransferFields.extension.redirection-history.
   The values are taken from the X.400(92) Implementor's guide (Version
   13, July 1995).   The first three values are in X.400(88).   The
   fourth value is in X.400(92), but has the name "recipient-directory-
   substitution-alternate-recipient". An example of this with two
   redirects is:

   X400-Recipients: postmaster@widget.com (Originally To:
             sales-manager@sales.widget.com
         Redirected on Thu, 30 May 91 14:39:40 +0100
             To: Originator Requested Alternate Recipient
             postmaster@sales.widget.com
         Redirected Again on Thu, 30 May 91 14:41:20 +0100
             To: Recipient MD Assigned Alternate Recipient)

   In addition the following per-recipient services from
   MTS.OtherMessageDeliveryFields.extensions are represented in comments
   if they are used.  None of these services can be provided on RFC 822
   networks, and so in general these will be informative strings
   associated with other MTS recipients. In some cases, string values
   are defined.  For the remainder, the string value shall be chosen by
   the implementor.   If the parameter has a default value, then no
   comment shall be inserted when the parameter has that default value.

   requested-delivery-method

   physical-forwarding-prohibited
        "(Physical Forwarding Prohibited)".



Kille                       Standards Track                    [Page 65]

RFC 2156                         MIXER                      January 1998


   physical-forwarding-address-request
        "(Physical Forwarding Address Requested)".

   physical-delivery-modes

   registered-mail-type

   recipient-number-for-advice

   physical-rendition-attributes

   physical-delivery-report-request
       "(Physical Delivery Report Requested)".

   proof-of-delivery-request
       "(Proof of Delivery Requested)".

4.6.2.3.  Delivery Report Generation

   If SMTP is used, the behaviour is specified in Appendix A.  In other
   cases, if MTA.PerRecipientMessageTransferFields.per-recipient-
   indicators requires a positive delivery notification, this shall be
   generated by the gateway.  Supplementary Information shall be set to
   indicate that the report is gateway generated.  This information
   shall include the name of the gateway generating the report.

4.6.3.  Message IDs (MTS)

   A mapping from 822.msg-id to MTS.MTSIdentifier is defined.  The
   reverse mapping is not needed, as MTS.MTSIdentifier is always mapped
   onto new RFC 822 fields.  The value of MTS.MTSIdentifier.local-part
   will facilitate correlation of gateway errors.

   To map from 822.msg-id, apply the standard mapping to 822.msg-id, in
   order to generate an MTS.ORAddress.  The Country, ADMD, and PRMD
   components of this are used to generate MTS.MTSIdentifier.global-
   domain-identifier.  MTS.MTSIdentifier.local-identifier is set to the
   822.msg-id, including the braces "<" and ">".   If this string is
   longer than MTS.ub-local-id-length (32), then it is truncated to this
   length.

   The reverse mapping is not used in this specification.  It would be
   applicable where MTS.MTSIdentifier.local-identifier is of syntax
   822.msg-id, and it algorithmically identifies MTS.MTSIdentifier.







Kille                       Standards Track                    [Page 66]

RFC 2156                         MIXER                      January 1998


4.7.  IPMS Mappings

   All RFC 822 addresses are assumed to use the 822.mailbox syntax.
   This includes all 822.comments associated with the lexical tokens of
   the 822.mailbox.  In the IPMS OR Names are encoded as MTS.ORName.
   This is used within the  IPMS.ORDescriptor, IPMS.RecipientSpecifier,
   and IPMS.IPMIdentifier.  An asymmetrical mapping is defined between
   these components.

4.7.1.  RFC 822 -> X.400

   To derive IPMS.ORDescriptor from an RFC 822 address.

   1.   Take the address, and extract an EBNF.822-address.  Any
        source routing shall be removed.  This can be derived trivially
        from either the 822.addr-spec or 822.route-addr syntax.  This is
        mapped to MTS.ORName as described above, and used as
        IMPS.ORDescriptor.formal-name.

   2.   A string shall be built consisting of (if present):

   -         The 822.phrase component if the 822.address is an
             822.phrase 822.route-addr construct.

   -         Any 822.comments, in order, retaining the parentheses.

         This string is then encoded into T.61 using a human oriented
         mapping (as described in Section 3.5).  If the string is not
         null, it is assigned to IPMS.ORDescriptor.free-form-name.

3.   IPMS.ORDescriptor.telephone-number is omitted.

   If IPMS.ORDescriptor is being used in IPMS.RecipientSpecifier,
   IPMS.RecipientSpecifier.reply-request and
   IPMS.RecipientSpecifier.notification-requests are set to default
   values (false and none).

   If the 822.group construct is present, any included 822.mailbox is
   encoded as above to generate a separate IPMS.ORDescriptor.  The
   822.group is  mapped to T.61 (as described in Section 3.5), and a
   IPMS.ORDescriptor with only an free-form-name component built from
   it.

4.7.2.  X.400 -> RFC 822

   Mapping from IPMS.ORDescriptor to RFC 822 address.  In the basic
   case, where IPMS.ORDescriptor.formal-name is present, proceed as
   follows.



Kille                       Standards Track                    [Page 67]

RFC 2156                         MIXER                      January 1998


   1.   Encode IPMS.ORDescriptor.formal-name (MTS.ORName) as
        EBNF.822-address.

   2a.  If IPMS.ORDescriptor.free-form-name is present, convert it
        to ASCII or T.61 (Section 3.5), and use this as the 822.phrase
        component of 822.mailbox using the 822.phrase 822.route-addr
        construct.

   2b.  If IPMS.ORDescriptor.free-form-name is absent.  If
        EBNF.822-address is parsed as 822.addr-spec use this as the
        encoding of 822.mailbox.  If EBNF.822-address is parsed as
        822.route 822.addr-spec, then an 822.phrase taken from
        822.local-part is added.

   3    If IPMS.ORDescriptor.telephone-number is present, this is
        placed in an 822.comment, with the string "Tel ".  The normal
        international form of number is used.  For example:

         (Tel +44-181-333-7777)

   4.   If IPMS.ORDescriptor.formal-name.directory-name is present,
        then a text representation is placed in a trailing 822.comment.

   5.   If IPMS.RecipientSpecifier.report-request has any non-
        default values, then an 822.comment "(Receipt Notification
        Requested)", and/or "(Non Receipt Notification Requested)",
        and/or "(IPM Return Requested)" may be appended to the address.
        "(Receipt Notification Requested)" may be used to infer "(Non
        Receipt Notification Requested)".  The effort of correlating P1
        and P2 information is too great to justify the gateway sending
        Receipt Notifications.

        In RFC 1327, inclusion of these comments was mandatory.
        Experience has shown that the clutter and confusion caused to
        RFC 822 users does not justify the information conveyed.
        Implementors are recommended to not include these comments.
        Unless an application is found where retention of these comments
        is desirable, they will be dropped from the next version.

   6.   If IPMS.RecipientSpecifier.reply-request is True, an
        822.comment "(Reply requested)"  is appended to the address.

   If IPMS.ORDescriptor.formal-name is absent, IPMS.ORDescriptor.free-
   form-name is converted to ASCII (see section 3.5), and used as
   822.phrase within the RFC 822 822.group syntax.  For example:

         Free Form Name ":" ";"




Kille                       Standards Track                    [Page 68]

RFC 2156                         MIXER                      January 1998


   Steps 3-6 are then followed.

4.7.3.  IP Message IDs

   There is a need to map both ways between 822.msg-id and
   IPMS.IPMIdentifier.  This allows for X.400 Receipt Notifications,
   Replies, and Cross References to reference an RFC 822 Message ID,
   which is preferable to a gateway generated ID.  A reversible and
   symmetrical mapping is defined.  This provides fully reversible
   mappings when messages pass multiple times across the X.400/RFC 822
   boundary.

   An important issue with messages identifiers is mapping to the exact
   form, as many systems use these ids as uninterpreted keys.  The use
   of table driven mappings is not always symmetrical, particularly in
   the light of alternative domain names, and alternative management
   domains.  For this reason, a purely algorithmic mapping is used.  A
   mapping which is simpler than that for addresses can be used for two
   reasons:

   -    There is no major requirement to make message IDs "natural"

   -    There is no issue about being able to reply to message IDs.
        (For addresses, creating a return path which works is more
        important than being symmetrical).

   The mapping works by defining a way in which message IDs generated on
   one side of the gateway can be represented on the other side in a
   systematic manner.  The mapping is defined so that the possibility of
   clashes is low enough to be treated as impossible.

4.7.3.1.  822.msg-id represented in X.400

   IPMS.IPMIdentifier.user is omitted.  The IPMS.IPMIdentifier.user-
   relative-identifier is set to a printable string encoding of the
   822.msg-id with the angle braces ("<" and ">") removed.  The upper
   bound on this component is 64.  The options for handling this are
   discussed in Section 5.1.3.

4.7.3.2.  IPMS.IPMIdentifier represented in RFC 822

   The 822.domain of 822.msg-id is set to the value "MHS". The
   822.local-part of 822.msg-id is constructed by building a string of
   syntax EBNF.id-loc from IPMS.IPMIdentifier.

          id-loc ::= [ printablestring ] "*"  [ std-or-address ]





Kille                       Standards Track                    [Page 69]

RFC 2156                         MIXER                      January 1998


   EBNF.printablestring is the IPMS.IPMIdentifier.user-relative-
   identifier, and EBNF.std-or-address being an encoding of the
   IPMS.IPMIdentifier.user derived according to this specification.
   822.local-part is derived from EBNF.id-loc, if necessary using the
   822.quoted-string encoding.  For example:

         <"147*/S=Dietrich/O=Siemens/ADMD=DBP/C=DE/"@MHS>

4.7.3.3.  822.msg-id -> IPMS.IPMIdentifier

   If the 822.local-part can be parsed as:

         [ printablestring ] "*"  [ std-or-address ]

   and the 822.domain is "MHS", then this ID was X.400 generated.  If
   EBNF.printablestring is present, the value is assigned to
   IPMS.IPMIdentifier.user-relative-identifier.  If EBNF.std-or-address
   is present, the OR Address components derived from it are used to set
   IPMS.IPMIdentifier.user.

   Otherwise, this is an RFC 822 generated ID.  In this case, set
   IPMS.IPMIdentifier.user-relative-identifier to a printable string
   encoding of the 822.msg-id without the angle braces and omit
   IPMS.IPMID.user.

4.7.3.4.  IPMS.IPMIdentifier -> 822.msg-id

   If IPMS.IPMIdentifier.user is absent, and IPMS.IPMIdentifier.user-
   relative-identifier mapped to ASCII and angle braces added parses as
   822.msg-id, then this is an RFC 822 generated ID.

   Otherwise, the ID is X.400 generated.  Use the
   IPMS.IPMIdentifier.user to generate an EBNF.std-or-address form
   string.  Build the 822.local-part of the 822.msg-id with the syntax:

         [ printablestring ] "*"  [ std-or-address ]

   The printablestring is taken from IPMS.IPMIdentifier.user-relative-
   identifier.  Use 822.quoted-string if necessary.  The 822.msg-id is
   generated with this 822.local-part, and "MHS" as the 822.domain.

4.7.3.5.  Phrase form

   In "In-Reply-To:" and "References:", the encoding 822.phrase may be
   used as an alternative to 822.msg-id.  To map from 822.phrase to
   IPMS.IPMIdentifier, assign IPMS.IPMIdentifier.user-relative-
   identifier to the phrase.  When mapping from IPMS.IPMIdentifier for
   "In-Reply-To:" and "References:", if IPMS.IPMIdentifier.user is



Kille                       Standards Track                    [Page 70]

RFC 2156                         MIXER                      January 1998


   absent and IPMS.IPMIdentifier.user-relative-identifier does not parse
   as 822.msg-id, generate an 822.phrase rather than adding the domain
   MHS.

4.7.3.6.  RFC 987 backwards compatibility

   The mapping defined here is different to that used in RFC 987, as the
   RFC 987 mapping lead to changed message IDs in many cases.  Fixing
   the problems is preferable to retaining backwards compatibility.  An
   implementation of this standard may recognise message IDs generated
   by RFC 987.  This is not recommended.

   RFC 987 generated encodings may be recognised as follows.  When
   mapping from X.400 to RFC 822, if the IPMS.IPMIdentifier.user-
   relative-identifier is "RFC-822" the id is RFC 987 generated. When
   mapping from RFC 822 to X.400, if the 822.domain is not "MHS", and
   the 822.local-part can be parsed as

         [ printablestring ] "*"  [ std-or-address ]

   then it is RFC 987 generated.  In each of these cases, it is
   recommended to follow the RFC 987 rules.

Chapter 5 - Detailed Mappings

   This chapter specifies  detailed mappings for the functions outlined
   in Chapters 1 and 2.  It makes extensive use of the notations and
   mappings defined in Chapters 3 and 4.

5.1.  RFC 822 -> X.400: Detailed Mappings

   The mapping of RFC 822/MIME messages to X.400 InterPersonal Messages
   is described in Sections 5.1.1 to 5.1.7.   Mapping of NOTARY format
   delivery status notifications, which are all messages of type
   multipart/report and subtype delivery-status-notifications to X.400
   delivery reports is covered in Section 5.1.8.

5.1.1.  Basic Approach

   A single IP Message is generated from an RFC 822 message.  The RFC
   822 headers are used to generate the IPMS.Heading.

   Some RFC 822 fields cannot be mapped onto a standard IPM Heading
   field, and so an extended field is defined in Section 5.1.2.  This is
   then used for fields which cannot be mapped onto existing services.






Kille                       Standards Track                    [Page 71]

RFC 2156                         MIXER                      January 1998


   The message is submitted to the MTS, and the services required can be
   defined by specifying MTS.MessageSubmissionEnvelope.  A few
   parameters of the MTA Abstract service are also specified, which are
   not in principle available to the MTS User.  Use of these services
   allows RFC 822 MTA level parameters to be carried in the analogous
   X.400 service elements.  The advantages of this mapping far outweigh
   the layering violation.

5.1.2.  X.400 Extension Field

   An IPMS Extension is defined:

   rfc-822-field HEADING-EXTENSION
              VALUE RFC822FieldList
              ::= id-rfc-822-field-list


   RFC822FieldList ::= SEQUENCE OF RFC822Field

   RFC822Field ::= IA5String

   The Object Identifier id-rfc-822-field-list is defined in Appendix D.

   To encode any RFC 822 Header using this extension, an RFC822Field
   element is built using the 822.field omitting the trailing CRLF
   (e.g., "Fruit-Of-The-Day: Kiwi Fruit"). All fields shall be unfolded.
   There shall be no space before the ":".  The reverse mapping builds
   the RFC 822 field in a straightforward manner.  This RFC822Field is
   appended to the RFC822FieldList, which is added to the IPM Heading as
   an extension field.

5.1.3.  Generating the IPM

   The IPM (IPMS Service Request) is generated according to the rules of
   this section. The IPMS.IPM.body is generated from the RFC 822 message
   body in the manner described in Section 5.1.5.

   If no specific 1988 features are used, the IPM generated is encoded
   as content type 2.  Otherwise, it is encoded as content type 22.  The
   latter will always be the case if extension heading fields are
   generated.










Kille                       Standards Track                    [Page 72]

RFC 2156                         MIXER                      January 1998


   When generating the IPM, the issue of upper bounds are handled as
   follows. Truncate fields to the upper bounds specified in X.400.
   This will prevent problems with UAs which enforce upper bounds, but
   will sometimes discard useful information.  This approach will cause
   more problems for some fields than others (e.g., truncating an OR
   Address component that would be used to route a reply would be a more
   severe problem than truncating a Free Form Name).  If the Free Form
   name is truncated, it shall be done so that it does not break RFC 822
   comments and RFC 1522 encoding.

   Note:This approach removes a choice of options given in RFC 1327,
        based on operational experience.

   The rest of this section concerns IPMS.IPM.heading (IPMS.Heading).
   The only mandatory component of IPMS.Heading is the
   IPMS.Heading.this-IPM (IPMS.IPMIdentifier).  A default is generated
   by the gateway.  With the exception of "Received:", the values of
   multiple fields are merged (e.g., If there are two "To:" fields, then
   the mailboxes of both are merged to generate a single list which is
   used in the IPMS.Heading.primary-recipients.  Information shall be
   generated from the standard RFC 822 Headers as follows:

   Date:
        Ignore (Handled at MTS level)

   Received:
        Ignore (Handled at MTA level)

   Message-Id:
        Mapped to IPMS.Heading.this-IPM.  For these, and all other
        fields containing 822.msg-id the mappings of Chapter 4 are used
        for each 822.msg-id.

   From:
        If Sender: is present, this is mapped to
        IPMS.Heading.authorizing-users.  If not, it is mapped to
        IPMS.Heading.originator.  For this, and other components
        containing addresses, the mappings of Chapter 4 are used for
        each address.

   Sender:
        Mapped to IPMS.Heading.originator.  Because X.400 does not have
        the same From/Sender distinction as RFC 822, this mapping is not
        always natural and may lead to unexpected results in some cases.

   Reply-To:
        Mapped to IPMS.Heading.reply-recipients.




Kille                       Standards Track                    [Page 73]

RFC 2156                         MIXER                      January 1998


   To:  Mapped to IPMS.Heading.primary-recipients

   Cc:  Mapped to IPMS.Heading.copy-recipients.

   Bcc: Mapped to IPMS.Heading.blind-copy-recipients if there is at
        least one BCC: recipient.  If there are no recipients in this
        field, it shall either be mapped to a zero length sequence or
        mapped to a single recipient that has a free from name "BCC" and
        no other addressing information.  This alternate treatment is
        allowed because some X.400 systems cannot handle a zero lenght
        sequence of addresses.

   In-Reply-To:
        If there is one value, it is mapped to IPMS.Heading.replied-to-
        IPM, using the 822.phrase or 822.msg-id mapping as appropriate.
        If there are multiple values, this cannot be done as the X.400
        heading is single valued. In this case no IPMS.Heading.replied-
        to-IPM is generated and the values are mapped to
        IPMS.Heading.related-IPMs, along with any values from a
        "References:" field.

   References:
        Mapped to IPMS.Heading.related-IPMs.

   Keywords:
        Mapped onto a heading extension.

   Subject:
        Mapped to IPMS.Heading.subject.  The field-body uses the human
        oriented mapping referenced in Section 3.3.4.

   Comments:
        Mapped onto a heading extension.

        This is a change from 1327, which specified to generate an
        IPMS.BodyPart of type IPMS.IA5TextBodyPart with
        IPMS.IA5TextBodyPart.parameters.repertoire set to the default
        (ia5), containing the value of the fields, preceded by the
        string "Comments: " and that this body part shall precede the
        other one. Experience has shown that this complexity is not
        justified.  This text is retained to facilitate backwards
        compatibility.

   Encrypted:
        Mapped onto a heading extension.

   Resent-*
        Mapped onto a heading extension.



Kille                       Standards Track                    [Page 74]

RFC 2156                         MIXER                      January 1998


        Note that it would be possible to use a ForwardedIPMessage for
        these fields, but the semantics are (arguably) slightly
        different, and it is probably not worth the effort.

   Content-Language:
        This field is defined in RFC 1766 [7].  Map the first two
        characters of each value given onto the IPM Languages extension.
        If any comments or values longer than two characters occur, a
        header extension shall also be generated.

   Other Fields
        In particular X-* fields, and "illegal" fields in common usage
        (e.g., "Fruit-of-the-day:") are mapped onto a heading extension,
        unless covered by another section or appendix of this
        specification.  The same treatment is applied to RFC 822 fields
        where the content of the field does not conform to RFC 822
        (e.g., a Date: field with unparseable syntax).

   The mapping of the following headings is defined in RFC 2157.

   MIME-Version: 5
   Content-Transfer-Encoding:
   Content-Type
   Content-ID
   Content-Description

5.1.4.  Generating the IPM Body

   Generation of the IPM Body is defined in RFC 2157.

5.1.5.  Mappings to the MTS Abstract Service

   The MTS.MessageSubmissionEnvelope comprises
   MTS.PerMessageSubmissionFields, and
   MTS.PerRecipientMessageSubmissionFields.  The mandatory parameters
   are defaulted as follows.

   MTS.PerMessageSubmissionFields.originator-name
      This is always generated from SMTP, as defined in Chapter 4.

   MTS.PerMessageSubmissionFields.content-type
      Set to the value implied by the encoding of the IPM (2 or 22).

   MTS.PerRecipientMessageSubmissionFields.recipient-name
      These will always be supplied from SMTP, as defined in Chapter 4.






Kille                       Standards Track                    [Page 75]

RFC 2156                         MIXER                      January 1998


   Optional components are omitted, and default components defaulted.
   This means that disclosure of recipients is prohibited and conversion
   is allowed.  There are two exceptions to the defaulting. For
   MTS.PerMessageSubmissionFields.per-message-indicators, the following
   settings are made:

   -    Alternate recipient is allowed, as it seems desirable to
        maximise the opportunity for (reliable) delivery.

   If SMTP is used, Appendix A shall be followed in setting these
   parameters.

   The trace is set to indicate conversion (described below) and the
   encoded information types in the trace is derived from the message
   generated by the gateway, and shall reflect all body parts (including
   those in enclosed messages).  In addition it shall include the
   Encoded Information Type "eit-mixer", which is defined in Appendix D.
   The presence of the EIT will indicate to the X.400 recipient that a
   MIXER conversion has occurred.
   MTS.PerMessageSubmissionFields.original-encoded-information-types
   will include all of the values used in the trace, unless specified
   otherwise in RFC 2157.

   This type of conversion will prevent the normal loop detection from
   working in certain circumstances, and introduces the possiblity of
   gateway loops.  MIXER gateways shall therefore count the number of
   MIXER conversions made.  If this count exceeds five in one direction,
   the message shall be treated as if a loop has been detected.

   The MTS.PerMessageSubmissionFields.content-correlator is encoded as
   IA5String, and contains the Subject:, Message-ID:, Date:,  and To:
   fields (if present) in this order.  This includes the strings
   "Subject:", "Date:", "To:", "Message-ID:", and appropriate folding to
   make the field appear readable.  This shall be truncated to MTS.ub-
   content-correlator-length (512) characters.  In addition, if there is
   a "Subject:" field, the MTS.PerMessageSubmissionFields.content-
   identifier, is set to a printable string representation of the
   contents of it.   If the length of this string is greater than
   MTS.ub-content-id-length (16), it shall be truncated to 13 characters
   and the string "..." appended. Both are used, due to the much larger
   upper bound of the content correlator, and that the content id is
   available in X.400(1984).









Kille                       Standards Track                    [Page 76]

RFC 2156                         MIXER                      January 1998


5.1.6.  Mappings to the MTA Abstract Service

   There is a need to map directly onto some aspects of the MTA Abstract
   service, for the following reasons:

   -    So the  MTS Message Identifier can be generated from the RFC
        822 Message-ID:.

   -    So that the submission date can be generated from the
        822.Date.

   -    To prevent loss of trace information

   -    To prevent RFC 822/X.400 looping caused by distribution
        lists or redirects

   The following mappings are defined.

   Message-Id:
      If this is present and no Resent: fields are present, the
      MTA.PerMessageTransferFields.message-identifier may be generated
      from it, using the mappings described in Chapter 4.

      This mapping arguably generates messages which do not conform to
      US GOSIP (1984 version only), which states:

      6.7.e MPDU Identifier Validation

      (1) Validation of the GlobalDomainIdentifier component of the MPDU
      Identifier is performed on reception of a message (i.e. the result
      of a TRANSFER.Indication).

      (2) The country name should be known to the validating domain, and
      depending on the country name, validation of the

      ADMD name may also be possible.

      (3) Additional validation of the GlobalDomainIdentifier is
      performed against the corresponding first entry in the
      TraceInformation. If inconsistencies are found during the
      comparison, a non-delivery notice with the above defined reason
      and diagnostic code is generated.

      (4) A request will be generated to the CCITT for a more meaningful
      diagnostic code (such as "InconsistentMPUTIdentifier").






Kille                       Standards Track                    [Page 77]

RFC 2156                         MIXER                      January 1998


   This applies to ADMDs only, and is specified in the 1984 version and
   not the 1988 version. Conformance depends on the interpretation of
   "inconsistency".   The specification makes the most sensible choice,
   and so is not being changed in the update from RFC 1327.

   Date: (and Resent-Date:)
      If one or more Resent-Date: fields is present, the most recent
      Resent-Date: field shall be used instead of the Date: field in the
      following description.

      The Date: field is used to set the first component of
      MTA.PerMessageTransferFields.trace-information
      (MTA.TraceInformationElement).  The SMTP originator is mapped into
      an MTS.ORAddress, and used to derive
      MTA.TraceInformationElement.global-domain-identifier.  The
      optional components of MTA.TraceInformationElement.domain-
      supplied-information are omitted, and the mandatory components are
      set as follows:

      MTA.DomainSuppliedInformation.arrival-time
         This is set to the date derived from Date:

      MTA.DomainSuppliedInformation.routing-action
         Set to relayed.

      The first element of MTA.PerMessageTransferFields.internal-trace-
      information is generated in an analogous manner, although this can
      be dropped later in certain circumstances (see the procedures for
      "Received:").  The MTA.InternalTraceInformationElement.mta-name is
      derived from the 822.domain in the 822 MTS Originator address.

   Received:
      All RFC 822 trace is used to derive
      MTA.PerMessageTransferFields.trace-information and
      MTA.PerMessageTransferFields.internal-trace-information.
      Processing of Received: lines  follows processing of Date:, and is
      done from the bottom to the top of the RFC 822 header (i.e., in
      chronological order).  When other trace elements (in particular
      X400-Received:)  are processed the relative ordering (top to
      bottom of the header) shall be retained correctly.

      The initial element of MTA.PerMessageTransferFields.trace-
      information shall be generated from Date: as described above,
      unless the message has previously been in X.400, when it will be
      derived from the X.400 trace information.






Kille                       Standards Track                    [Page 78]

RFC 2156                         MIXER                      January 1998


      For each  Received: field, the following processing shall be done.
      If the "by"  part of the received is present and there is an
      available MCGAM which can map this domain, use it to derive an
      MTS.GlobalDomainIdentifier.  Otherwise MTS.GlobalDomainIdentifier
      is set from local information.  If this is different from the one
      in the last element of MTA.PerMessageTransferFields.trace-
      information (MTA.TraceInformationElement.global-domain-identifier)
      create a new MTA.TraceInformationElement, and optionally remove
      MTA.PerMessageTransferFields.internal-trace-information.
      Requirements on trace stripping are discussed below.

      Then add a new element (MTA.InternalTraceInformationElement) to
      MTA.PerMessageTransferFields.internal-trace-information, creating
      this if needed.  This shall be done, even if nter-MD trace is
      created.  The MTA.InternalTraceInformationElement.global-domain-
      identifier is set to the value derived.  The
      MTA.InternalTraceInformationElement.mta-supplied-information
      (MTA.MTASuppliedInformation) is set as follows:

         MTA.MTASuppliedInformation.arrival-time
            Derived from the date of the Received: line

         MTA.MTASuppliedInformation.routing-action
            Set to relayed

      The MTA.InternalTraceInformationElement.mta-name is taken from the
      "by" component of the "Received:" field, truncated to MTS.ub-mta-
      name-length (32).  For example:

         Received: from computer-science.nottingham.ac.uk by
            vs6.Cs.Ucl.AC.UK via Janet with NIFTP  id aa03794;
            28 Mar 89 16:38 GMT

   Generates the string

         vs6.Cs.Ucl.AC.UK

   The gateway shall add in a single element of trace information,
   reflecting the gateway's local information and the time of
   conversion.  The MTA.InternalTraceInformationElement.mta-supplied-
   information (MTA.MTASuppliedInformation) is set as follows:

   MTA.DomainSuppliedInformation.arrival-time
      Set to the time of conversion

   MTA.DomainSuppliedInformation.routing-action
      Set to relayed




Kille                       Standards Track                    [Page 79]

RFC 2156                         MIXER                      January 1998


   MTA.AdditionalAcctions.converted-encoded-information-types Set to
   correct set of EITs for the message that is generated by the gateway.
   This trace element will thus reflect gateway operation as a
   conversion.

   This trace generation will often lead to generation of substantial
   amounts of trace information, which does not reflect X.400 transfers.
   Stripping of some of this trace may be necessary in some operational
   environments.   This stripping shall be considered a function of the
   associated X.400 MTA, and not of the MIXER gateway.

5.1.7.  Mapping New Fields

   This specification defines a number of new fields for Reports,
   Notifications and IP Messages. A gateway conforming to this
   specification shall  map all of these fields to X.400, except as
   defined below.

   The mapping of two  extended fields is particularly important, in
   order to prevent looping.  "DL-Expansion-History:" is mapped to
   MTA.PerMessageTransferFields.extensions.dl-expansion-history X400-
   Received: shall be mapped to MTA.PerMessageTransferFields.trace-
   information and MTA.PerMessageTransferFields.internal-trace-
   information.  In cases where X400-Received: is present, the usual
   mapping of Date: to generate the first element of trace shall not be
   done.   This is because the message has come from X.400, and so the
   first element of trace can be taken from the first X400-Received:.

   The following fields shall not be mapped, and shall be

   -    Discarded-X400-MTS-Extensions:

   -    Message-Type:

   -    Discarded-X400-IPMS-Extensions:

   -    X400-Content-Type:

   -    X400-Originator:

   -    X400-Recipients:

   -    X400-MTS-Identifier:  Mapping this field would be useful in
        some circumstances, but very dangerous in others (e.g.,
        following an internet list expansion).  Therefore it is not
        mapped.





Kille                       Standards Track                    [Page 80]

RFC 2156                         MIXER                      January 1998


5.1.8.  Mapping Delivery Status Notifications to X.400

5.1.8.1.  Basic Model

   Internet Mail delivery status notifications (DSN) are mapped to X.400
   delivery reports.   With message mapping, information without a
   mapping is carried by an IPM Extension.   This cannot be done for
   delivery reports.   Two mechanisms are used for information where
   there is not a direct mapping.

   The first mechanism is to define extensions, which allow all of the
   DSN information to be carried in the delivery report.  This is not
   completely satisfactory for two reasons:

   1.   User defined extensions are supported by the ISO version of
        the standard, but not the CCITT one.  Therefore,
        implementation support for these extensions will not be
        universal.

   2.   X.400 User Agent implementations will not in general
        recognise these extensions.   Therefore, although the
        information will be present, it will often not be available
        to the user.    This may be very problematic, as this
        information may be critical to diagnosing the reason for a
        failure.

   Therefore a second mechanism is defined.  This shall always be used
   when the DSN contains non-delivery information, and may be used in
   other cases.  This mechanism is to map the whole DSN (as if it were
   an ordinary multipart) into the return of content.  This will make
   the DSN information available as a text body part in the outer
   message, with the real returned content as an enclosed message.  This
   mechanism will ensure that information is not lost at the gateway.

5.1.8.2.  DSN Extensions

   Two X.400 MTS extensions are defined as follows:

   dsn-header-list EXTENSION
      RFC822FieldList
      ::= id-dsn-header-list

   dsn-field-list EXTENSION
      RFC822FieldList
      ::= id-dsn-field-list






Kille                       Standards Track                    [Page 81]

RFC 2156                         MIXER                      January 1998


   The Object Identifiers id-dsn-header-list and id-dsn-field-list are
   defined in Appendix D.  Theses extensions are used in the same way as
   the IPM extension rfc-822-field, described in Section 5.1.2.   These
   extensions may only be used with ISO-10021, and not X.400 (which does
   not allow user extensions at the MTS level).

5.1.8.3.  DSN to Delivery Report Mapping

   Some DSNs are mapped to Delivery Reports and some to IPMs, according
   to the value of the action field.   The mapping to an IPM is exactly
   as for a normal IPM mapping.   The choice of IPM and Delivery report
   is made for each reported recipient.   If this choice is different
   for different reported recipients both a Delivery Report and an IPM
   shall be generated.

   Reports are not be submitted in the X.400 model, and so the report
   submission is considered in terms of the MTA Abstract Service.  An
   MTA.Report is constructed. The MTA.ReportTransferEnvelope.report-
   identifier is generated from the Message-Id of the DSN (if present)
   and otherwise generated as the MTA would generate one for a submitted
   message.

   The DSN has an RFC 822 header.  Trace is mapped in the same manner as
   for a message to MTA.ReportTransferEnvelope.trace-information.  All
   other headers are used to create a dsn-header-list extension, which
   is added to MTA.PerReportTransferFields.extensions.  The DSN will
   have a single SMTP recipient.   This is mapped to the
   MTA.ReportTransferEnvelope.report-destination-name.

   The DSN is then treated as a normal MIME message, and an X.400 IPM is
   generated.   This IPM is used as
   MTA.PerReportTransferFields.returned-content, and its type is used to
   set MTA.PerReportTransferFields.content-type.  The DSN body part is
   mapped as if it was IA5 text/plain.

   The mandatory MTA.PerReportTransferFields.subject-identifier shall be
   generated from the DSN.per-message-field original-envelope-id, if
   this starts with the string "X400-MTS-Identifier: ", and derived from
   the rest of the field, which is encoded as EBNF.mts-msg-id.  In other
   cases, this field shall be generated by the MIXER Gateway.

   All other mappings are made from the DSN body part. A dsn-field-list
   extension is created and added to
   MTA.ReportTransferFields.extensions.  This is referred to as the per
   report extension list.  The DSN.per-message-fields are mapped as
   follows:





Kille                       Standards Track                    [Page 82]

RFC 2156                         MIXER                      January 1998


   original-envelope-id-field
   reporting-mta-field
   dsn-gateway-field
   received-from-mta-field
   arrival-date-field
   extension-field
   other

      All of these fields are added to the per report extension list.
      Currently there are no other mappings defined.

   Each reported recipient is considered in turn, and a
   MTA.PerRecipientReportTransferFields created for each.  The
   parameters of this are defaulted as follows:

   originally-specified-recipient-number
      In general, these are not available, and so are assigned
      incrementally.

   last-trace-information
      The arrival-time is generated from DSN.arrival-date if present,
      and if not from the Date: of the DSN.  This is a strucutred field,
      and the Report element contains the key information on the
      recipient.  For a DeliveryReport, the type-ofMTS-user is defaulted
      to public and the message-deliery-time is set to the same as the
      arrival-time.  For a NonDeliveryReport, the code mappings are
      define in Section 5.1.8.4.

   A dsn-field-list extension is created  and added to
   MTA.PerRecipientTransferFields.extensions.  This is referred to as
   the per recipient extension list.  The DSN.per-recipient-fields are
   mapped as follows

   original-recipient-field
      Mapped to MTA.PerRecipientReportTransferFields.originally-
      intended-recipient-name.

   final-recipient-field
      Mapped to MTA.PerRecipientReportTransferFields.actual-recipient-
      name.

   action-field
      If this is set to "failed", a non-delivery report is generated.
      If this is set to "delivered" a delivery report is generated.
      Bit one or two of MTA.PerRecipientTransferFields.per-recipient-
      indicators is set accordingly.  This also controls the encoding of
      MTA.PerRecipientTransferFields.last-trace-information, and the
      selection of the report type.



Kille                       Standards Track                    [Page 83]

RFC 2156                         MIXER                      January 1998


      For other values of the action-field ("delayed", "relayed",
      "expanded"), an IPM is generated.   This enables the status
      information to be communicated to the X.400 user, without the
      confusion of multiple delivery reports.

   status-field
      This is added to the per report extension list.  For non-delivery,
      it is also used to generate the reason and diagnostic codes
      contained within MTA.PerRecipientReportTransferFields.last-trace.
      The mappings are defined below.

   remote-mta-field

   diagnostic-code-field

   last-attempt-date-field

   will-retry-until-field

   extension-field

   other
      All of these fields are added to the per recipient extension list.

5.1.8.4.  Status Value Mappings

   Status values are mapped to X.400 reason and diagnostic codes as
   follows.

   If a status value is found that is not in this table, the gateway may
   use the same mapping as for "X.n.0" (1/None or 0/None), or it may map
   to another, configurable code.  Implementors are requested to forward
   new codes to the mixer list for inclusion in future versions of this
   standard.  So for instance. "5.2.37", currently undefined, would map
   onto the same as "5.2.0", namely 1/None.
















Kille                       Standards Track                    [Page 84]

RFC 2156                         MIXER                      January 1998


DSN code  Meaning                               X400 code Meaning

X.0.0     Other status                          1/None

X.1.0     Other Address Status                  1/None
X.1.1     Bad mailbox address                   1/0     Unrecognized
X.1.2     Bad system address                    1/0     Unrecognized
X.1.3     Bad mailbox address syntax            1/0     Unrecognized
X.1.4     Mailbox address ambiguous             1/1
X.1.5     Only used for positive reports, not applicable
X.1.6     Destination mailbox has moved         1/43  New addr unknown
X.1.7     Bad sender's mailbox address syntax   1/11  Invalid arguments
X.1.8     Bad sender's system address           1/11  Invalid arguments

X.2.0     Other or undefined mailbox status     1/None
X.2.1     Mailbox disabled, not accepting       1/4   Recipient unavail
X.2.2     Mailbox full                          1/4
X.2.3     Message length exceeds admin limit.   1/7     Content too long
X.2.4     Mailing list expansion problem        1/30  DL expansion fail

X.3.0     Other or undefined system status      0/None
X.3.1     System full                           1/2     MTS congestion
X.3.2     System not accepting network messages 1/2     MTS congestion
X.3.3     System not capable of selected feat   1/18    Unsupp crit func
X.3.4     Message too big for system            1/7
X.3.5     System incorrectly configured      1/None

X.4.0     Other or undefined network or routing 0/None
X.4.1     No answer from host                   0/None
X.4.2     Bad connection                        0/None
X.4.3     Routing server failure                6/None  Dir op unsucc.
X.4.4.    Unable to route                       0/None
X.4.5     Network congestion                    1/2     MTS congest.
X.4.6     Routing loop detected                 1/3
X.4.7     Delivery time expired                 1/5

X.5.0     Other or undefined protocol status    1/None

X.5.1     Invalid command                       1/14    Protocol viol.
X.5.2     Syntax error                          1/14
X.5.3     Too many recipients                   1/16
X.5.4     Invalid command arguments             1/14
X.5.5     Wrong protocol version                1/18    Unsupp.crit.func








Kille                       Standards Track                    [Page 85]

RFC 2156                         MIXER                      January 1998


X.6.0     Other or undefined media error        2/None  Conv. not perf
X.6.1     Media not supported                   1/6     EIT unsupp.
X.6.2     Conversion required and prohibited    1/9
X.6.3     Conversion required but not supported 2/8
X.6.4     Conversion with loss performed        POSITIVE only
X.6.5     Conversion failed                  2/47   Unable to downgrade

X.7.0     Other or undefined security status    1/46
X.7.1     Delivery not authorized, message ref  1/29  No DL submit perm
X.7.2     Mailing list expansion prohibited     1/28
X.7.3     Security conversion req but not poss  1/46  Secure mess. error
X.7.4     Security features not supported       1/46
X.7.5     Cryptographic failure                 1/46
X.7.6     Cryptographic algorithm not supported 1/46
X.7.7     Message integrity failure             1/46

5.1.8.5.  DSNs that originated in X.400

   The mapping of X.400 delivery reports to DSNs will in general provide
   sufficient information to make a useful reverse mapping.  Messages
   will often be mapped multiple times, commonly due to forwarding
   messages and to distribution lists.   Multiple mappings for delivery
   reports will be a good deal less common.  For this reason, the
   reverse mapping of the X.400 DSN extensions defined in MIXER is
   optional.

5.2.  Return of Contents

   RFC 1327 offered two approaches for return of content, as this
   service is optional in X.400 and expected in RFC 822.   MIXER simply
   requires that a gateway requests the return of content service from
   X.400.

5.3.  X.400 -> RFC 822: Detailed Mappings

5.3.1.  Basic Approach

   A single RFC 822 message is generated from the incoming IP Message,
   Report, or IP Notification.   All IPMS.BodyParts are mapped onto a
   single RFC 822 body.  Other services are mapped onto RFC 822 header
   fields.  Where there is no appropriate existing field, new fields are
   defined for IPMS, MTS and MTA services.

   The gateway mechanisms will correspond to MTS Delivery.  As with
   submission, there are aspects where the MTA (transfer) services are
   also used. In particular, there is an optimisation to allow for
   multiple SMTP recipients.




Kille                       Standards Track                    [Page 86]

RFC 2156                         MIXER                      January 1998


5.3.2.  RFC 822 Settings

   An RFC 822 Message has a number of mandatory fields in the RFC 822
   Header.  Some SMTP services mandate specification of an SMTP
   Originator.  Even in cases where this is optional, it is usually
   desirable to specify a value.  The following defaults are defined,
   which shall be used if the mappings specified do not derive a value:

   SMTP Originator
      If this is not generated by the mapping (e.g., for a Delivery
      Report), a value pointing at a gateway administrator shall be
      assigned.

   Date:
      A value will always be generated

   From:
      If this is not generated by the mapping, it is assigned equal to
      the SMTP Originator.  If this is gateway generated, an appropriate
      822.phrase shall be added.

   At least one recipient field
      If no recipient fields are generated, a field "To: list:;", shall
      be added.

   This will ensure minimal RFC 822 compliance.  When generating RFC 822
   headers, folding may be used.  It is recommended to do this,
   following the guidelines of RFC 822.

5.3.3.  Basic Mappings

5.3.3.1.  Encoded Information Types

   This mapping from MTS.EncodedInformationTypes is needed in several
   disconnected places.  EBNF is defined as follows:

      encoded-info    = 1#encoded-type

      encoded-type    = built-in-eit / object-identifier












Kille                       Standards Track                    [Page 87]

RFC 2156                         MIXER                      January 1998


      built-in-eit    = "Undefined"         ; undefined (0)
                      / "Telex"             ; tLX (1)
                      / "IA5-Text"          ; iA5Text (2)
                      / "G3-Fax"            ; g3Fax (3)
                      / "TIF0"              ; tIF0 (4)
                      / "Teletex"           ; tTX (5)
                      / "Videotex"          ; videotex (6)
                      / "Voice"             ; voice (7)
                      / "SFD"               ; sFD (8)
                      / "TIF1"              ; tIF1 (9)

   MTS.EncodedInformationTypes is mapped onto EBNF.encoded-info.
   MTS.EncodedInformationTypes.non-basic-parameters is ignored.  Built
   in types are mapped onto fixed strings (compatible with X.400(1984)
   and RFC 987), and other types are mapped onto EBNF.object-identifier.

5.3.3.2.  Global Domain Identifier

   The following simple EBNF is used to represent
   MTS.GlobalDomainIdentifier:

      global-id = std-or-address

   This is encoded using the std-or-address syntax, for the attributes
   within the Global Domain Identifier.

5.3.4.  Mappings from the IP Message

   Consider that an IPM has to be mapped to RFC 822.  The IPMS.IPM
   comprises an IPMS.IPM.heading and IPMS.IPM.body.   The heading is
   considered first.  Some EBNF for new fields is defined:

ipms-field = "Supersedes" ":" 1*msg-id
             / "Expires" ":" date-time
             / "Reply-By" ":" date-time
             / "Importance" ":" importance
             / "Sensitivity" ":" sensitivity
             / "Autoforwarded" ":" boolean
             / "Incomplete-Copy" ":"
             / "Content-Language" ":" 1#language
             / "Message-Type" ":" message-type
             / "Discarded-X400-IPMS-Extensions" ":" 1#object-identifier
             / "Autosubmitted" ":" autosubmitted


importance      = "low" / "normal" / "high"





Kille                       Standards Track                    [Page 88]

RFC 2156                         MIXER                      January 1998


sensitivity     = "Personal" / "Private" /
                  "Company-Confidential"

language        = 2*ALPHA [ "(" language-description ")" ]
     language-description = printable-string


message-type    = "Delivery Report"
                / "InterPersonal Notification"
                / "Multiple Part"

autosubmitted   = "not-auto-submitted"
                / "auto-generated"
                / "auto-replied"
                / "auto-forwarded"

   The mappings and actions for the IPMS.Heading are now specified for
   each element.  Addresses and Message Identifiers are mapped according
   to Chapter 4.  Other mappings are explained, or are straightforward
   (algorithmic).  If a field with addresses contains zero elements, it
   shall be discarded, except for IPMS.Heading.blind-copy-recipients,
   which can be mapped onto BCC: (the only RFC 822 field which allows
   zero recipients).

   IPMS.Heading.this-IPM
      Mapped to "Message-ID:".

   IPMS.Heading.originator
      If IPMS.Heading.authorizing-users is present this is mapped to
      Sender:, if not to "From:".

   IPMS.Heading.authorizing-users
      Mapped to "From:".

   IPMS.Heading.primary-recipients
      Mapped to "To:".

   IPMS.Heading.copy-recipients
      Mapped to "Cc:".

   IPMS.Heading.blind-copy-recipients
      Mapped to "Bcc:".

   IPMS.Heading.replied-to-ipm
      Mapped to "In-Reply-To:".






Kille                       Standards Track                    [Page 89]

RFC 2156                         MIXER                      January 1998


   IPMS.Heading.obsoleted-IPMs
      Mapped to the extended RFC 822 field "Supersedes:".   The replaces
      the RFC 1327 field "Obsoletes:".   Reverse mapping of the RFC 1327
      field may be supported.

   IPMS.Heading.related-IPMs
      Mapped to "References:".

   IPMS.Heading.subject
      Mapped to "Subject:".  The contents are converted to ASCII or T.61
      (as defined in Section 3.5).  CRLF will not be present in a valid
      X.400 field.  Any CRLF present are not mapped, but are used as
      points at which the subject field shall be folded, unless an RFC
      1522 encoding is used.

   IPMS.Heading.expiry-time
      Mapped to the extended RFC 822 field "Expires:".  The replaces the
      RFC 1327 field "Expiry-Date:".   Reverse mapping of the RFC 1327
      field may be supported.

   IPMS.Heading.reply-time
      Mapped to the extended RFC 822 field "Reply-By:".

   IPMS.Heading.reply-recipients
      Mapped to "Reply-To:".

   IPMS.Heading.importance
      Mapped to the extended RFC 822 field "Importance:".

   IPMS.Heading.sensitivity
      Mapped to the extended RFC 822 field "Sensitivity:".

   IPMS.Heading.autoforwarded
      Mapped to the extended RFC 822 field "Autoforwarded:".

   The standard extensions (Annex H of X.420 / ISO 10021-7) are mapped
   as follows:

   incomplete-copy
      Mapped to the extended RFC 822 field "Incomplete-Copy:".

   language
      Mapped to the  RFC 822 field "Content-Language:", defined in RFC
      1766 [7].  This mapping may be made without loss of information.

   auto-submitted
      Map to the extended RFC 822 field "Autosubmitted:".




Kille                       Standards Track                    [Page 90]

RFC 2156                         MIXER                      January 1998


   If the RFC 822 extended header is found, this shall be mapped onto an
   RFC 822 header, as described in Section 5.1.2.

   If a non-standard extension is found, it shall be discarded, unless
   the gateway understands the extension and can perform an appropriate
   mapping onto an RFC 822 header field.  If extensions are discarded,
   the list is indicated in the extended RFC 822 field "Discarded-X400-
   IPMS-Extensions:".

5.3.4.1.  Mapping the IPMS Body

   The mapping of the IPMS Body is defined in RFC 2157.

5.3.4.2.  Example Message

   An example message, illustrating a number of aspects is given below.

Received: from mhs-relay.ac.uk by bells.cs.ucl.ac.uk via JANET with
          NIFTP id <7906-0@bells.cs.ucl.ac.uk>;
          Thu, 30 May 1991 18:24:55 +0100
X400-Received: by mta "mhs-relay.ac.uk" in /PRMD=uk.ac/ADMD= /C=gb/;
               Relayed; Thu, 30 May 1991 18:23:26 +0100
X400-Received: by /PRMD=HMG/ADMD=GOLD 400/C=GB/; Relayed;
               Thu, 30 May 1991 18:20:27 +0100
Message-Type: Multiple Part
Date: Thu, 30 May 1991 18:20:27 +0100
X400-Originator: Stephen.Harrison@gosip-uk.hmg.gold-400.gb
X400-MTS-Identifier:
     [/PRMD=HMG/ADMD=GOLD 400/C=GB/;PC1000-910530172027-57D8]
Original-Encoded-Information-Types: ia5
X400-Content-Type: P2-1984 (2)
X400-Content-Identifier: Email Problems
From: Stephen.Harrison@gosip-uk.hmg.gold-400.gb (Tel +44 71 217 3487)
Message-ID: <PC1000-910530172027-57D8*@MHS>
To: Jim Craigie <NTIN36@gec-b.rutherford.ac.uk>,
    Tony Bates <tony@ean-relay.ac.uk>,
    Steve Kille <S.Kille@cs.ucl.ac.uk>
Subject: Email Problems
Sender: Stephen.Harrison@gosip-uk.hmg.gold-400.gb
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=boundary-1

--boundary-1
Content-Type: text/plain; charset=US-ASCII

Hope you gentlemen.......





Kille                       Standards Track                    [Page 91]

RFC 2156                         MIXER                      January 1998


Regards,

Stephen Harrison
UK GOSIP Project

--boundary-1
Content-Type: message/rfc822

From: Urs Eppenberger <Eppenberger@verw.switch.ch>
Message-ID:
<562*/S=Eppenberger/OU=verw/O=switch/PRMD=SWITCH/ADMD=ARCOM/C=CH/@MHS>
To: "Stephen.Harrison" <Stephen.Harrison@gosip-uk.hmg.gold-400.gb>
Cc: kimura@bsdarc.bsd.fc.nec.co.jp
Subject: Response to Email link
Content-Type: multipart/mixed; boundary=boundary-2


--boundary-2

Dear Mr Harrison......


--boundary-2--

--boundary-1--

5.3.5.  Mappings from an IP Notification

   Because of the service setting, IP Notifications will not usually
   need to be mapped by a MIXER gateway.  A message is generated, with
   the following fields:

   From:
      Set to the IPMS.IPN.ipn-originator.

   To:  Set to the recipient from MTS.MessageSubmissionEnvelope.
      If there have been redirects, the original address shall be used.

   Subject:
      Set to the string  "X.400 Inter-Personal Notification" for a
      receipt notification and to "X.400 Inter-Personal Notification
      (failure)" for a non-receipt notification.

   Message-Type:
      Set to "InterPersonal Notification"

   References:
      Set to IPMS.IPN.subject-ipm



Kille                       Standards Track                    [Page 92]

RFC 2156                         MIXER                      January 1998


   Discarded-X400-IPMS-Extensions:
      Used for any discarded IPN extensions.

   The following EBNF is defined for the body of the Message.  This
   format is defined to ensure that all information from an
   interpersonal notification is available to the end user in a uniform
   manner.

         ipn-body-format = ipn-description <CRLF>
                         [ ipn-extra-information <CRLF> ]
                         [ ipn-content-return ]

         ipn-description = ipn-receipt / ipn-non-receipt

         ipn-receipt = "Your message to:" preferred-recipient <CRLF>
                  "was received at" receipt-time <CRLF> <CRLF>
                  "This notification was generated"
                   acknowledgement-mode <CRLF>
                  "The following extra information was given:" <CRLF>
                   ipn-suppl <CRLF>

         ipn-non-receipt = "Your message to:"
                      preferred-recipient <CRLF>
                      ipn-reason

         ipn-reason = ipn-discarded / ipn-auto-forwarded

         ipn-discarded = "was discarded for the following reason:"
                          discard-reason <CRLF>

         ipn-auto-forwarded = "was automatically forwarded." <CRLF>
                              [ "The following comment was made:"
                              auto-comment ]


         ipn-extra-information =
                "The following information types were converted:"
                 encoded-info

         ipn-content-return = "The Original Message is not available"
                              / "The Original Message follows:"

         preferred-recipient = mailbox
         receipt-time        = date-time
         auto-comment        = printablestring
         ipn-suppl           = printablestring





Kille                       Standards Track                    [Page 93]

RFC 2156                         MIXER                      January 1998


         discard-reason     = "Expired" / "Obsoleted" /
                     "User Subscription Terminated" / "IPM Deleted"

         acknowledgement-mode = "Manually" / "Automatically"

   The mappings for elements of the common fields of IPMS.IPN
   (IPMS.CommonFields) onto this structure and the message header are:

   subject-ipm
      Mapped to "References:"

   ipn-originator
      Mapped  to "From:".

   ipn-preferred-recipient
      Mapped to EBNF.preferred-recipient

   conversion-eits
      Mapped to EBNF.encoded-info in EBNF.ipn-extra-information

   The mappings for elements of IPMS.IPN.non-receipt-fields
   (IPMS.NonReceiptFields) are:

   non-receipt-reason
      Used to select between EBNF.ipn-discarded and EBNF.ipn-auto-
      forwarded

   discard-reason
      Mapped to EBNF.discard-reason

   auto-forward-comment
      Mapped to EBNF.auto-comment

   returned-ipm
      This applies only to non-receipt notifications.  EBNF.ipn-
      content-return shall always be omitted for receipt notifications,
      and always be present in non-receipt notifications.  If present,
      the second option of EBNF.ipn-content-return is chosen, and the
      message is included.  In this case, the message is formatted as
      multipart/mixed, and the returned message included as
      message/rfc822 after the text body part. Otherwise the first
      option is chosen.

   The mappings for elements of IPMS.IPN.receipt-fields
   (IPMS.ReceiptFields) are:

   receipt-time
      Mapped to EBNF.receipt-time



Kille                       Standards Track                    [Page 94]

RFC 2156                         MIXER                      January 1998


   acknowledgement-mode
      Mapped to EBNF.acknowledgement-mode

   suppl-receipt-info
      Mapped to EBNF.ipn-suppl

   An example notification is:

         From: Steve Kille <steve@cs.ucl.ac.uk>
         To: Julian Onions <jpo@computer-science.nottingham.ac.uk>
         Subject: X.400 Inter-personal Notification
         Message-Type: InterPersonal Notification
         References: <1229.614418325@UK.AC.NOTT.CS>
         Date: Wed, 21 Jun 89 08:45:25 +0100

         Your message to: Steve Kille <steve@cs.ucl.ac.uk>
         was automatically forwarded.
         The following comment was made:
            Sent on to a random destination

         The following information types were converted: g3fax

5.3.6.  Mappings from the MTS Abstract Service

   This section describes the MTS mappings for User Messages (IPM and
   IPN).  This mapping is defined by specifying the mapping of
   MTS.MessageDeliveryEnvelope.  The following extensions to RFC 822 are
   defined to support this mapping:

         mts-field = "X400-MTS-Identifier" ":" mts-msg-id
                   / "X400-Originator" ":" mailbox
                   / "X400-Recipients" ":" 1#mailbox
                   / "Original-Encoded-Information-Types" ":"
                      encoded-info
                   / "X400-Content-Type" ":" mts-content-type
                   / "X400-Content-Identifier" ":" printablestring
                   / "Priority" ":" priority
                   / "Originator-Return-Address" ":" 1#mailbox
                   / "DL-Expansion-History" ":" mailbox ";" date-time
                      ";"
                   / "Conversion" ":" prohibition
                   / "Conversion-With-Loss" ":" prohibition
                   / "Delivery-Date" ":" date-time
                   / "Discarded-X400-MTS-Extensions" ":"
                      1#( object-identifier / labelled-integer )


         prohibition     = "Prohibited" / "Allowed"



Kille                       Standards Track                    [Page 95]

RFC 2156                         MIXER                      January 1998


         mts-msg-id       = "[" global-id ";" *text "]"

         mts-content-type = "P2" /  labelled-integer
                          / object-identifier

         priority        = "normal" / "non-urgent" / "urgent"


   The mappings for each element of MTS.MessageDeliveryEnvelope can now
   be considered.  Where the specified action does not result in an
   extended element being mapped, the criticality associated with this
   element shall be considered.  If the element is marked as critical
   for transfer or for delivery, the message shall be non delivered by
   the gateway because a critical extension cannot be correctly handled.

   MTS.MessageDeliveryEnvelope.message-delivery-identifier
      Mapped to the extended RFC 822 field "X400-MTS-Identifier:".

   MTS.MessageDeliveryEnvelope.message-delivery-time
      Discarded, as this time will be represented in an appropriate
      trace element.

   The mappings for elements of MTS.MessageDeliveryEnvelope.other-fields
   (MTS.OtherMessageDeliveryFields) are:

   content-type
      Mapped to the extended RFC 822 field "X400-Content-Type:".  The
      string "P2" is retained for backwards compatibility with RFC 987.
      This shall not be generated, and either the EBNF.labelled-integer
      or EBNF.object-identifier encoding used.

   originator-name
      Mapped to the SMTP originator, and to the extended RFC 822 field
      "X400-Originator:".  This is described in Section 4.6.2.

   original-encoded-information-types
      Mapped to the extended RFC 822 field "Original-Encoded-
      Information-Types:".

   priority
      Mapped to the extended RFC 822 field "Priority:".

   delivery-flags
      If the conversion-prohibited bit is set, add an extended RFC 822
      field "Conversion:".

   this-recipient-name and other-recipient-names
      The handling of these elements is described in Section 4.6.2.



Kille                       Standards Track                    [Page 96]

RFC 2156                         MIXER                      January 1998


   originally-intended-recipient-name
      The handling of this element is described in Section 4.6.2.

   converted-encoded-information-types
      Discarded.  This information will be mapped in the trace.

   message-submission-time
      Mapped to Date:.

   content-identifier
      Mapped to the extended RFC 822 field "X400-Content-Identifier:".
      In RFC 1327, this was "Content-Identifier:".  This has been
      changed to avoid confusion with MIME defined fields.   Gateways
      which reverse map, may support the old field.

   If any extensions (MTS.MessageDeliveryEnvelope.other-
   fields.extensions) are present, and they are marked as critical for
   transfer or delivery, then the message shall be rejected.  The
   extensions (MTS.MessageDeliveryEnvelope.other-fields.extensions) are
   mapped as follows.

   conversion-with-loss-prohibited
      If set to MTS.ConversionWithLossProhibited.conversion-with-loss-
      prohibited, then add the extended RFC 822 field "Conversion-With-
      Loss:".

   requested-delivery-method
      Mapped to a comment, as described in Section 4.6.2.2.

   originator-return-address
      Mapped to the extended RFC 822 field "Originator-Return-Address:".

   physical-forwarding-address-request
   physical-delivery-modes
   registered-mail-type
   recipient-number-for-advice
   physical-rendition-attributes
   physical-delivery-report-request
   physical-forwarding-prohibited


      These elements are only appropriate for physical delivery.
      They are represented as comments in the "X400-Recipients:"
      field, as described in Section 4.6.2.2.

   originator-certificate
   message-token
   content-confidentiality-algorithm-identifier



Kille                       Standards Track                    [Page 97]

RFC 2156                         MIXER                      January 1998


   content-integrity-check
   message-origin-authentication-check
   message-security-label
   proof-of-delivery-request

      These elements imply use of security services not available in the
      RFC 822 environment.  If they are marked as critical for transfer
      or delivery, then the message shall be rejected.  Otherwise they
      are discarded.

   redirection-history
      This is described in Section 4.6.2.

   dl-expansion-history
      Each element is mapped to an extended RFC 822 field "DL-
      Expansion-History:".  These fileds shall be ordered in the message
      header, so that the most recent expansion comes first (same order
      as trace).

      If any MTS (or MTA) Extensions not specified in X.400 are present,
      and they are marked as critical for transfer or delivery, then the
      message shall be rejected.  If they are not so marked, they can
      safely be discarded.  The list of discarded fields shall be
      indicated in the extended header "Discarded-X400-MTS-Extensions:".

5.3.7.  Mappings from the MTA Abstract Service

   There are some mappings at the MTA Abstract Service level which are
   done for IPM and IPN.  These can be derived from
   MTA.MessageTransferEnvelope.  The reasons for the mappings at this
   level, and the violation of layering are:

   -    Allowing for multiple recipients to share a single RFC 822
        message

   -    Making the X.400 trace information available on the RFC 822
        side

   -    Making any information on deferred delivery available

   The SMTP recipients are calculated from the full list of X.400
   recipients.  This is all of the members of
   MTA.MessageTransferEnvelope.per-recipient-fields being passed through
   the gateway, where the responsibility bit is set.  In some cases, a
   different RFC 822 message would be calculated for each recipient, due
   to differing service requests for each recipient.  As discussed in
   4.6.2.2, this specification allows either for multiple messages to be
   generated, or for the per-recipient information to be discarded.



Kille                       Standards Track                    [Page 98]

RFC 2156                         MIXER                      January 1998


   The following EBNF is defined for extended RFC 822 headers:

   mta-field       = "X400-Received" ":" x400-trace
                   / "Deferred-Delivery" ":" date-time
                   / "Latest-Delivery-Time" ":" date-time

   x400-trace       = "by" md-and-mta ";"
                    [ "deferred until" date-time ";" ]
                    [ "converted" "(" encoded-info ")" ";" ]
                    [ "attempted" md-or-mta ";"  ]
                       action-list
                    ";" arrival-time

   md-and-mta       = [ "mta" mta "in" ]  global-id
   mta              = word
   arrival-time     = date-time

   md-or-mta        = "MD" global-id
                    / "MTA" mta

   Action-list      = 1#action
   action           = "Redirected"
                    / "Expanded"
                    / "Relayed"
                    / "Rerouted"

   Note the EBNF.mta is encoded as 822.word.  If the character set does
   not allow encoding as 822.atom, the 822.quoted-string encoding is
   used.

   If MTA.PerMessageTransferFields.deferred-delivery-time is present, it
   is used to generate a Deferred-Delivery: field.  X.400 does not make
   this information available at the MTS level on delivery, because it
   requires that this service is provided by the first MTA. In the event
   that the first MTA does not provide this service, the function may
   optionally be implemented by the gateway: that is, the gateway may
   hold the message until the time specified in the protocol element.
   Thus, the value of this element will usually be in the past.  For
   this reason, the extended RFC 822 field is primarily for information.

   If MTA.PerMessageTransferFields.extensions.dl-expansion-prohibited is
   present and set to dl-expansion-probited, the gateway may reject that
   message on the basis that it is unable to control distribution list
   expansion beyond the gateway.  The service relating to this is
   described in Section 2.3.1.2.  This approach was not specified in RFC
   1327.  If it is found to be useful, it may be made mandatory in
   future versions of MIXER.




Kille                       Standards Track                    [Page 99]

RFC 2156                         MIXER                      January 1998


   If MTA.PerMessageTransferFields.extensions.recipient-reassignment-
   prohibited is present and set to recipeint-reassignment-probited, the
   gateway may reject that message on the basis that it is unable to
   control distribution list expansion beyond the gateway.  The service
   relating to this is described in Section 2.3.1.2.  This approach was
   not specified in RFC 1327.  If it is found to be useful, it may be
   made mandatory in future versions of MIXER.

   Merge MTA.PerMessageTransferFields.trace-information, and
   MTA.PerMessageTransferFields.internal-trace-information to produce a
   single ordered trace list.  If Internal trace from other management
   domains has not been stripped, this may require complex interleaving.
   Where an element of internal trace and external trace are identical,
   except for the MTA in the internal trace, only the internal trace
   element shall be presented. Use this to generate a sequence of
   "X400-Received:" fields. The only difference between external trace
   and internal trace will be the extra MTA information in internal
   trace elements.

   When generating an RFC 822 message all trace fields (X400-Received
   and Received) shall be at the beginning of the header, before any
   other fields.  Trace shall be in chronological order, with the most
   recent element at the front of the message.  This ordering is
   determined from the order of the fields, not from timestamps in the
   trace, as there is no guarantee of clock synchronisation.  A simple
   example trace (external) is:

   X400-Received: by /PRMD=UK.AC/ADMD=Gold 400/C=GB/ ; Relayed ;
           Tue, 20 Jun 89 19:25:11 +0100

   A more complex example (internal):

   X400-Received: by mta "UK.AC.UCL.CS" in
          /PRMD=UK.AC/ADMD=Gold 400/C=GB/ ;
          deferred until  Tue, 20 Jun 89 14:24:22 +0100 ;
           converted (undefined, g3fax) ; attempted MD /ADMD=Foo/C=GB/ ;
           Relayed, Expanded, Redirected ; Tue, 20 Jun 89 19:25:11 +0100

   The gateway itself shall add a single line of trace information,
   indicating MIXER conversion by use of a comment.  For example:

   Received: from isode.com by isode.com
          (MIXER Conversion following RFC 1327);
          Thu, 2 Jan 1997 14:46:03 +0000

   If SMTP is being used, Appendix A shall also be followed, which
   includes optional mappings to extension parameters.




Kille                       Standards Track                   [Page 100]

RFC 2156                         MIXER                      January 1998


5.3.8.  Mappings from Report Delivery

   that only reports destined for the MTS user will be mapped.  Some
   additional services are also taken from the MTA service.  X.400
   Delivery Reports are Mapped onto Delivery Status Notifications, as
   defined by NOTARY [28].

5.3.8.1.  MTS Mappings

   A Delivery Report service will be represented as
   MTS.ReportDeliveryEnvelope, which comprises of per-report-fields
   (MTS.PerReportDeliveryFields) and per-recipient-fields.

   The enclosing message is a MIME message of content type
   multipart/report, with report-type=delivery-status, which is
   generated with the following fields:

   From:
        An administrator at the gateway system.

   To:  A mapping of the
        MTA.ReportTransferEnvelope.report-destination-name.  This is
        also the SMTP recipient.

   Message-Type:
        Set to "Delivery Report".  This is strictly redundant, but
        retained for backwards compatibility with RFC 1327.

   Subject:
        The EBNF for the subject line is:

       subject-line  = "Delivery-Report" "(" status ")"
                       [ "for" destination ]

       status        = "success" / "failure" / "success and failures"

       destination   = mailbox / "MTA" word

   The subject is intended to give a clear indication as to the nature
   of the message, and summarise its contents. EBNF.status is set
   according to whether the recipients reported on are all successes,
   all failures, or a mixture.  It is common for a report to reference a
   single recipient, in which case a subject line giving using all of
   the options of EBNF.status can be used.  This gives useful
   information to the recipient.  Where information varies between
   reported recpients, the options cannot be used.  The EBNF.destination
   is used to indicate the addresses in the reports.  If the report is
   for a single address, EBNF.mailbox is used to give the RFC 822



Kille                       Standards Track                   [Page 101]

RFC 2156                         MIXER                      January 1998


   representation of the address.  If all of the reported recpients
   reference the same MTA this is included in EBNF.word.   The MTA is
   determined from the delivery report's trace.

   The format of the body of the message follows the NOTARY delivery
   status notification format, and is defined to ensure that all
   information is conveyed to the RFC 822 user in a consistent manner.
   The format is structured as if it was a message coming from the
   gateway, with three body parts. The first body part is ASCII text
   structured as follows:

   1.   A few lines giving keywords to indicate the original
        message.

   2.   A human summary of the status of each recipient being
        reported on.

   The second (mandatory)  body part is the NOTARY delivery status
   notification, which contains detailed information extracted from the
   report.  This information may be critical to diagnosing an obscure
   problem.

   The third (optional) body part contains the returned message (return
   of content).  This structure is useful to the RFC 822 recipient, as
   it enables the original message to be extracted.  For negative
   reports it shall be included if the original message is available.
   For positive reports headers from the message shall be included if
   the original message is available.

   The first body part containing the user oriented description is of
   type text/plain.  The format of this body part is defined below as
   EBNF.dr-user-info.

         dr-user-info = dr-summary <CRLF>
                         dr-recipients <CRLF>
                         dr-content-return

         dr-content-return = "The Original Message is not available"
              / "The Original Message follows:"

         dr-summary = "This report relates to your message:" <CRLF>
                         content-correlator <CRLF> <CRLF>
                      "of" date-time <CRLF> <CRLF>

         dr-recipients = *(dr-recipient <CRLF> <CRLF>)

         dr-recipient = dr-recip-success / dr-recip-failure




Kille                       Standards Track                   [Page 102]

RFC 2156                         MIXER                      January 1998


         dr-recip-success =
                         "Your message was successfully delivered to:"
                          mailbox "at" date-time


         dr-recip-failure = "Your message was not delivered to:"
                                 mailbox <CRLF>
                         "for the following reason:" *word report-point
         = [ "mta" mta-name "in" ] global-id content-correlator = *word
         mta-name = word

   EBNF.dr-summary
      The EBNF.content-correlator is taken from the content correlator
      (or content identifier if there is no content correlator) and the
      EBNF.date-time from the trace, as described in Section 5.3.8.3.
      LWSP may be added to improve the layout of the body part.

   EBNF.dr-recipients
      There is an element for each recipient in the delivery report.  In
      each case, EBNF.mailbox is taken from the RFC 822 form of the
      originally specified recipient, which is taken from the originally
      specified recipient element if present or from the actual
      recipient.  When reporting success, the message delivery time is
      used to derive EBNF.date-time.  When reporting failure, the
      information includes a human readable interpretation of the X.400
      diagnostic and reason codes, and the supplementary information.

   EBNF.dr-content-return
      This is set according to whether or not the content is being
      returned.

   The EBNF of this body part is designed for english-speaking users.
   The language of the strings in the EBNF may be altered.


















Kille                       Standards Track                   [Page 103]

RFC 2156                         MIXER                      January 1998


   The EBNF used in the delivery status notification is:

      dr-per-message-fields =
         / "X400-Conversion-Date" ":" date-time
         / "X400-Subject-Submision-Identifier" ":"
                                   mts-msg-id
         / "X400-Content-Identifier" ":" printablestring
         / "X400-Content-Type" ":" mts-content-type
         / "X400-Original-Encoded-Information-Types" ":"
                              encoded-info
         / "X400-Originator-and-DL-Expansion-History" ":"
                              mailbox ";" date-time ";"
         / "X400-Reporting-DL-Name" ":" mailbox
         / "X400-Content-Correlator" ":" content-correlator
         / "X400-Recipient-Info" ":" recipient-info
         / "X400-Subject-Intermediate-Trace-Information" ":"
                              x400-trace
         / dr-extensions

      dr-per-recipient-fields =
         / "X400-Redirect-Recipient" ":" "x400" ";" std-or
         / "X400-Mapped-Redirect-Recipient" ":" "rfc822" ";" mailbox
         / "X400-Converted-EITs" ":" encoded-info ";"
         / "X400-Delivery-Time" ":" date-time
         / "X400-Type-of-MTS-User" ":" labelled-integer
         / "X400-Last-Trace" ":" [ encoded-info ] date-time
         / "X400-Supplementary-Info" ":"
               <"> printablestring <"> ";"
         / "X400-Redirection-History" ":" redirect-history-item
         / "X400-Physical-Forwarding-Address" ":" mailbox
         / "X400-Originally-Specified-Recipient-Number" ":"
               integer
         / dr-extensions

      dr-extensions = "X400-Discarded-DR-Extensions" ":"
                        1# (object-identifier / labelled-integer)

      dr-diagnostic = "Reason" labelled-integer-2
                        [ ";" "Diagnostic" labelled-integer-2 ]

   A body part of type delivery status, as defined by NOTARY, is
   generated.  MIXER extends this delivery status notification (DSN)
   specification, by defining additional per message fields in EBNF.dr-
   per-message-fields and additional per recipient fields in EBNF.dr-
   per-recipient-fields.   These are used as extensions to DSN.per-
   message-fields and DSN.per-recipient-fields.  MIXER also defines a
   new NOTARY address type "x400", with encoding of EBNF.std-or.   A
   directory name may be inluded as an RFC 822 comment.



Kille                       Standards Track                   [Page 104]

RFC 2156                         MIXER                      January 1998


   The following DSN.per-message-fields are always generated:

   DSN.reporting-mta-field
      The DSN.mta-name-type is set to "x400", and this string is
      reserved by MIXER.  The DSN.mta-name has its syntax specified by
      EBNF.report-point, with the information derived from the first
      element of the DR's trace.

   DSN.arrival-date-field
      This is derived from the date of the
      MTA.PerRecipientReportTransferFields.last-trace-info.arrival-time
      of the first recipient in the report.

   The following two EBNF.per-message-fields are generated by the MIXER
   gateway:

   DSN.dsn-gateway-field
      The type is set to "dns" and the  domain  set to the local domain
      of the gateway.

   X400-Conversion-Date:
      The EBNF.date-time is set to the time of the MIXER conversion.

   The elements of MTS.ReportDeliveryEnvelope.per-report-fields are
   mapped as follows onto the DSN per message fields as follows:

   subject-submission-identifier
      Mapped to DSN.original-envelope-id-field.  The encoding of this
      MTS Identifier follows the format EBNF.mts-msg-id.

   content-identifier
      Mapped to X400-Content-Identifier:

   content-type
      Mapped to X400-Content-Type:

   original-encoded-information-types
      Mapped to X400-Encoded-Info:

   The extensions from MTS.ReportDeliveryEnvelope.per-report-
   fields.extensions are mapped as follows:

   originator-and-DL-expansion-history
      Each element is mapped to an "X400-Originator-and-DL-Expansion-
      History:"  They shall be ordered so that the most recent expansion
      comes first in the header (same order as trace).





Kille                       Standards Track                   [Page 105]

RFC 2156                         MIXER                      January 1998


   reporting-DL-name
      Mapped to X400-Reporting-DL-Name:

   content-correlator
      If the content correlator starts with the string "SMTP/NOTARY
      ENVID: ", then the remainder of the content correlator is mapped
      to the DSN original-envelope-id field.  If this is not the case,
      the content correlator is mapped to X400-Content-Correlator:,
      provided that the encoding is IA5String (this will always be the
      case).

   message-security-label
   reporting-MTA-certificate
   report-origin-authentication-check

      These security parameters will not be present unless there is an
      error in a remote MTA.  If they are present, they shall be
      discarded in preference to discarding the whole report.  They
      shall be listed in the X400-Discarded-DR-Extensions: field.

   If there are any other DR extensions, they shall also be discarded
   and listed in the X400-Discarded-DR-Extensions: field.

   For each element of MTS.ReportDeliveryEnvelope.per-recipient-fields,
   a set of DSN.per-recipient-fields is generated.  The fields are
   filled in as follows:

   actual-recipient-name
      If originally-intended-recipient-name is not present, generate a
      DSN.original-recipient-field fields, with DSN.address-type of
      "rfc822", and with an RFC 822 mailbox generated from the address
      encoded as specified by NOTARY.  Also generate a DSN.final-
      recipient-field field, which holds the X.400 representation of the
      same address.  If the directory name is present, it shall be added
      as a trailing comment in the X.400 form.

      If originally-intended-recipient-name is present, generate an
      "X400-Mapped-Redirect-Recipient:" field, with DSN.address-type of
      "rfc822", and with an RFC 822 mailbox generated from the address
      encoded as specified by NOTARY.  Also generate an "X400-Redirect-
      Recipient:" field, which holds the X.400 representation of the
      same address.  If the directory name is present, it shall be added
      as a trailing comment in the X.400 form.








Kille                       Standards Track                   [Page 106]

RFC 2156                         MIXER                      January 1998


   report
      If it is MTS.Report.delivery, then set DSN.action-field to
      "delivered", and set "X400-Delivery-Time:" and "X400-Type-of-MTS-
      User:" from the information in the report.  DSN.status field is
      set to "2.0.0".

      If it is MTS.Report.non-delivery, then set DSN.action-field to
      "failed".   DSN.diagnostic-code-field is encoded according to the
      syntax EBNF.dr-diagnostic, with the labelled integers set from the
      reason and diagnostic codes.  DSN.status-field is derived from the
      reason and diagnostic codes, as described below.

   converted-encoded-information-types
      Set X400-Converted-EITs:

   originally-intended-recipient
      Generate a DSN.final-recipient-field field, with DSN.address-type
      of "rfc822", and with an RFC 822 mailbox generated from the
      address encoded as specified by NOTARY.  Also generate a
      DSN.original-recipient-field field, which holds the X.400
      representation of the same address.  If the directory name is
      present, it shall be added as a trailing comment in the X.400
      form.

   supplementary-info
      Set X400-Supplementary-Info:

   redirection-history
      Generate an "X400-Redirection-History:" field for each redirect
      history element.  The fields are ordered with the earliest
      redirect first.

   physical-forwarding-address
      Set X400-Physical-Forwarding-Address as a mailbox, with directory
      name in comment if present.

   recipient-certificate
      Discard

   proof-of-delivery
      Discard

   Any unknown extensions shall be discarded, irrespective of
   criticality.  All discarded extensions shall be included in a "X400-
   Discarded-DR-Extensions:" field.






Kille                       Standards Track                   [Page 107]

RFC 2156                         MIXER                      January 1998


   The number from the MTA.PerRecipientReportTransferFields.originally-
   specified-recipient-number shall be mapped to "X400-Originally-
   Specified-Recipient-Number:", in order to facilitate reverse mapping
   of delivery reports.

   The original message shall be included in the delivery status
   notification if it is available. The original message will usually be
   available at the gateway, as discussed in Section 5.2.  If the
   original message is available, but is not a legal message format, a
   dump of the ASN.1 may be included, encoded as application/octet-
   string.  This is recommended, but not required.

   Where the original message is included, it shall be encoded according
   to the MIME specifications as content type message/rfc822.

5.3.8.2.  Status Code Mappings

   This section defines the mappings from X.400 diagnostic and status
   codes to the NOTARY Status field.

C/D     X400 meaning                            DSN code        Means

0/Any   Transfer failure (may be temporary)     4.4.0 Other net/route
1/Any   Unable to transfer                      5.0.0 Other, unknown
2/Any   Conversion not performed                5.6.3 Conv not supported
3/Any   Physical rendition not performed        5.6.0 Other media error
4/Any   Physical delivery not performed         5.1.0 Other address
                                                      status
5/Any   Restricted delivery                     5.7.1
6/Any   Directory operation unsuccessful        5.4.3 Routing server
                                                      failure
7/Any   Deferred delivery not performed         5.3.3 Not capable

1/0     Unrecognized OR name                    5.1.1
1/1     Ambiguous OR name                       5.1.4
1/2     MTS congestion                          4.3.1
1/3     Loop detected                           5.4.6
1/4     Recipient unavailable                   4.2.1
1/5     Delivery time expired                   4.4.7
1/6     Encoded information types unsupported   5.6.1 Media unsupp.
1/7     Content too long                        5.2.3
2/8     Conversion impractical                  5.6.3
2/9     Conversion prohibited                   5.6.3
1/10    Implicit conversion not subscribed      5.6.3
1/11    Invalid arguments                       5.5.2
1/12    Content syntax error                    5.5.2
1/13    Size constraint violation               5.5.2
1/14    Protocol violation                      5.5.0



Kille                       Standards Track                   [Page 108]

RFC 2156                         MIXER                      January 1998


1/15    Content type not supported              5.6.1 Media unsupp.
1/16    Too many recipients                     5.5.3
1/17    No bilateral agreement                  5.4.4
1/18    Unsupported critical function           5.3.3 System not capable
2/19    Conversion with loss prohibited         5.6.2
2/20    Line too long                           5.6.0
2/21    Page split                              5.6.0
2/22    Pictorial symbol loss                   5.6.2
2/23    Punctuation symbol loss                 5.6.2
2/24    Alphabetic character loss               5.6.2
2/25    Multiple information loss               5.6.2
1/26    Recipient reassignment prohibited       5.4.0 Undefined net/route
1/27    Redirection loop detected               5.4.6
1/28    DL expansion prohibited                 5.7.2
1/29    No DL submit permission                 5.7.1 Delivery not
                                                      authorized
1/30    DL expansion failure                    4.2.4
4/31    Physical rendition attrs not supported  5.6.0 Undefined media
                                                      error
4/32-45 Various physical mail stuff             5.1.0 Other address
                                                      status
1/43    New address unknown                     5.1.6 Destination mbox
                                                      moved
1/46    Secure messaging error                  5.7.0 Other security
                                                      status
2/47    Unable to downgrade                     5.3.3 System not capable
0/48    Unable to complete transfer             5.3.4 Message too big
0/49    Transfer attempts limit reached         4.4.7 Delivery time
                                                      expired

5.3.8.3.  MTA Mappings

   The single SMTP recipient is constructed from
   MTA.ReportTransferEnvelope.report-destination-name, using the
   mappings of Chapter 4.  Unlike with a user message, this information
   is not available at the MTS level.

   The following additional mappings are made, which results in fields
   in the outer header of the DSN.

   MTA.ReportTransferEnvelope.report-destination-name
      This is used to generate the To: field.

   MTA.ReportTransferEnvelope.identifier
      Mapped to the extended RFC 822 field "X400-MTS-Identifier:".  It
      may also be used to derive a "Message-Id:" field.





Kille                       Standards Track                   [Page 109]

RFC 2156                         MIXER                      January 1998


   MTA.ReportTransferEnvelope.trace-information
      and
   MTA.ReportTransferEnvelope.internal-trace-information

      Mapped onto the extended RFC 822 field "X400-Received:", as
      described in Section 5.3.7.  Date: is generated from the first
      element of trace.

   The following additional mappings are made, which result in per
   message fields in the DSN body part:

   MTA.PerRecipientReportTransferFields.last-trace-information
      Mapped to X400-Last-Trace:".

   MTA.PerReportTransferFields.subject-intermediate-trace-
      information Mapped to "X400-Subject-Intermediate-Trace-
      Information:". These fields are ordered so that the most recent
      trace element comes first.

5.3.8.4.  Example Delivery Reports

   This section contains sample delivery reports.   These are the same
   examples used in RFC 1327, and so they also illustrate the changes
   between RFC 1327 and this document.  Example Delivery Report 1:

   Received: from cs.ucl.ac.uk by bells.cs.ucl.ac.uk
      via Delivery Reports Channel id <27699-0@bells.cs.ucl.ac.uk>;
      Thu, 7 Feb 1991 15:48:39 +0000 From: UCL-CS MTA
   <postmaster@cs.ucl.ac.uk> To: S.Kille@cs.ucl.ac.uk Subject: Delivery
   Report (failure) for H.Hildegard@bbn.com Message-Type: Delivery
   Report Date: Thu, 7 Feb 1991 15:48:39 +0000 Message-ID:
   <"bells.cs.u.694:07.01.91.15.48.34"@cs.ucl.ac.uk> X400-Content-
   Identifier: Greetings.  MIME-Version: 1.0 Content-Type:
   multipart/report; report-type=delivery-status;
       boundary=boundary-1

   --boundary-1

   This report relates to your message:
           Greetings.

           of Thu, 7 Feb 1991 15:48:20 +0000

   Your message was not delivered to
           H.Hildegard@bbn.com for the following reason:
           Bad Address
           MTA 'bbn.com' gives error message  (USER) Unknown user name
   in



Kille                       Standards Track                   [Page 110]

RFC 2156                         MIXER                      January 1998


           "H.Hildegard@bbn.com"

   The Original Message follows:


   --boundary-1 content-type: message/delivery-status

   Reporting-MTA: x400;  bells.cs.ucl.ac.uk in /PRMD=uk.ac/ADMD=gold
   400/C=gb/ Arrival-Date: Thu, 7 Feb 1991 15:48:34 +0000 DSN-Gateway:
   dns;  bells.cs.ucl.ac.uk X400-Conversion-Date: Thu, 7 Feb 1991
   15:48:40 +0000 Original-Envelope-Id:
            [/PRMD=uk.ac/ADMD=gold
   400/C=gb/;<1803.665941698@UK.AC.UCL.CS>] X400-Content-Identifier:
   Greetings.  X400-Subject-Intermediate-Trace-Information:
   /PRMD=uk.ac/ADMD=gold 400/C=gb/;
            arrival Thu, 7 Feb 1991 15:48:20 +0000 action Relayed X400-
   Subject-Intermediate-Trace-Information:  /PRMD=uk.ac/ADMD=gold
   400/C=gb/;
            arrival Thu, 7 Feb 1991 15:48:18 +0000 action Relayed



   Original-Recipient: rfc822; H.Hildegard@bbn.com Final-Recipient:
   x400;
     /RFC-822=H.Hildegard(a)bbn.com/OU=cs/O=ucl/PRMD=uk.ac/ADMD=gold
   400/C=gb/; Action: failure Status: 5.1.1 Diagnostic-Code: x400;
   Reason 1 (Unable-To-Transfer);
        Diagnostic 0 (Unrecognised-ORName) X400-Last-Trace: (ia5) Thu, 7
   Feb 1991 15:48:18 +0000; X400-Originally-Specified-Recipient-Number:
   1 X400-Supplementary-Info: "MTA 'bbn.com' gives error message  (USER)
       Unknown user name in "H.Hildegard@bbn.com"";


   --boundary-1 Content-Type: message/rfc822

   Received: from glenlivet.cs.ucl.ac.uk by bells.cs.ucl.ac.uk
     with SMTP inbound id <27689-0@bells.cs.ucl.ac.uk>;
     Thu, 7 Feb 1991 15:48:21 +0000 To: H.Hildegard@bbn.com Subject:
   Greetings.  Phone: +44-71-380-7294 Date: Thu, 07 Feb 91 15:48:18
   +0000 Message-ID: <1803.665941698@UK.AC.UCL.CS> From: Steve Kille
   <S.Kille@cs.ucl.ac.uk>


   Steve

   --boundary-1--





Kille                       Standards Track                   [Page 111]

RFC 2156                         MIXER                      January 1998


   Example Delivery Report 2:

   Received: from cs.ucl.ac.uk by bells.cs.ucl.ac.uk
     via Delivery Reports Channel id <27718-0@bells.cs.ucl.ac.uk>;
     Thu, 7 Feb 1991 15:49:11 +0000
   X400-Received: by mta "bells.cs.ucl.ac.uk" in
     /PRMD=uk.ac/ADMD=gold 400/C=gb/;
     Relayed; Thu, 7 Feb 1991 15:49:08 +0000
   X400-Received: by /PRMD=DGC/ADMD=GOLD 400/C=GB/; Relayed;
     Thu, 7 Feb 1991 15:48:40 +0000
   From: UCL-CS MTA <postmaster@cs.ucl.ac.uk>
   To: S.Kille@cs.ucl.ac.uk
   Subject: Delivery Report (failure) for
            j.nosuchuser@dle.cambridge.DGC.gold-400.gb
   Message-Type: Delivery Report
   Date: Thu, 7 Feb 1991 15:46:11 +0000
   Message-ID: <"DLE/910207154840Z/000"@cs.ucl.ac.uk>
   X400-Content-Identifier: A useful mess...
   MIME-Version: 1.0
   Content-Type: multipart/report; report-type=delivery-status;
       boundary=boundary-1

   --boundary-1

   This report relates to your message:
           A useful mess...

           of Thu, 7 Feb 1991 15:43:20 +0000


   Your message was not delivered to
           j.nosuchuser@dle.cambridge.DGC.gold-400.gb
           for the following reason:
           Bad Address
           DG 21187: (CEO POA) Unknown addressee.

   The Original Message is not available


   --boundary-1
   content-type: message/delivery-status


   Reporting-MTA: x400; /PRMD=DGC/ADMD=GOLD 400/C=GB/
   Arrival-Date: Thu, 7 Feb 1991 15:48:40 +0000
   DSN-Gateway: dns;  bells.cs.ucl.ac.uk
   X400-Conversion-Date: Thu, 7 Feb 1991 15:49:12 +0000
   Original-Envelope-Id:



Kille                       Standards Track                   [Page 112]

RFC 2156                         MIXER                      January 1998


     [/PRMD=uk.ac/ADMD=gold 400/C=gb/;<1796.665941626@UK.AC.UCL.CS>]
   X400-Content-Identifier: A useful mess...


   Original-Recipient: rfc822; j.nosuchuser@dle.cambridge.DGC.gold-400.gb
   Final-Recipient: x400;
     /I=j/S=nosuchuser/OU=dle/O=cambridge/PRMD=DGC/ADMD=GOLD 400/C=GB/
   Action: failure
   Status: 5.1.1
   Diagnostic-Code: x400; Reason 1 (Unable-To-Transfer);
       Diagnostic 0 (Unrecognised-ORName)
   X400-Supplementary-Info: "DG 21187: (CEO POA) Unknown addressee."
   X400-Originally-Specified-Recipient-Number: 1

   --boundary-1--


5.3.9.  Probe

   This is an MTS internal issue.  Any probe shall be serviced by the
   gateway, as there is no equivalent RFC 822 functionality.  The value
   of the reply is dependent on whether the gateway could service an MTS
   Message with the values specified in the probe.  The reply shall make
   use of MTS.SupplementaryInformation to indicate that the probe was
   serviced by the gateway.


























Kille                       Standards Track                   [Page 113]

RFC 2156                         MIXER                      January 1998


Appendix A - Mappings Specific to SMTP

   This Appendix is specific to the Simple Mail Transfer  Protocol (RFC
   821).  It describes specific changes in the context of this protocol.
   When MIXER is used with SMTP, conformance to this appendix is
   mandatory.

   1.  Probes

   When servicing a probe, as described in section 5.3.9, use may be
   made of the SMTP VRFY command to increase the accuracy of information
   contained in the delivery report.

   2.  Long Lines

   SMTP is a text oriented protocol, and is required to support a line
   length of at least 1000 characters.   Some implementations do not
   support line lengths greater than 1000 characters.   This can cause
   problems.  Where body parts have long lines, it is recommended to use
   a MIME encoding that folds lines (quoted printable).

   3.  SMTP Extensions

   There are several RFCs that specify extensions to SMTP. Most of these
   are not relevant to MIXER.  The NOTARY work to support delivery
   report defines extensions which are relevant [29].  Use of these
   extensions by a MIXER gateway is optional.  If these extensions are
   used, they shall be used in the manner described below.

   3.1.  SMTP Extension mapping to X.400

   Mappings are defined for the following extensions:

   NOTIFY
      This is used to set the report and non-delivery bits of
      MTA.PerRecipientMessageTransferFields.per-recipient-indicators.
      If the value is NEVER, both bits are zero.  If SUCCESS is present,
      the report bit is set.  Otherwise, the non-delivery-report bit is
      set.  If the gateway uses the NOTIFY command, it shall perform
      this mapping in all cases.

   ORCPT
      If the address type of the original recipient is "x400" or
      "rfc822", this may be used at the MTS level, to generate an
      element of redirection history, with the redirection date being
      the date of conversion and the reason set to "alias".





Kille                       Standards Track                   [Page 114]

RFC 2156                         MIXER                      January 1998


   ENVID
      If present, this may be used to generate a content correlator.
      This is used rather than the MTS Identifier, as the ENVID is
      unique for the UA only and is likely to be too large to map to an
      MTS identifier. The content correlator is encoded as an IA5 String
      containing the ENVID and prefixed by the string:

                            "SMTP/NOTARY ENVID: "

      If the ENVID starts with the string "X400-MTS-Identifier: ", then
      this ENVID was generated from an X.400 MTS Identifier.  The
      reverse mapping defined in Section 3.2 of Appendix A shall not be
      used, as this may cause problems in certain situations (e.g.,
      where the message was expanded by an Internet mailing list).

   3.2.  X.400 Mapping to SMTP Extensions

   The following extensions may be used as a part of the MIXER mapping:

   NOTIFY
      The originator-report and originator-non-delivery-report bits of
      MTA.PerRecipientMessageTransferFields.per-recipient-indicators
      determine how this is used.   If both bits are zero, the parameter
      is NEVER.  If the report bit is set, SUCCESS is used.   Otherwise,
      FAILURE is used.  If this is done, the gateway shall not generate
      a delivery report for this recipient, unless this is needed in the
      case where the originating MTA service report requirements differ
      from the user requirements.   Additional originating MTA
      requrirements are satisfied by the gateway.

   ORCPT
      If the MTS.perRecipientDeliveryFields.originally-intended-
      recipient-name is present, the ORCPT command may be used to carry
      this value, using the "x400" syntax.

   ENVID
      This may be generated, with the value taken from the
      MTS.MessageDeliveryEnvelope.message-delivery-identifer.  If this
      is done, it shall be encoded as EBNF.mts-msg-id, preceded by the
      string "X400-MTS-Identifier: ".

   RET
      If MTA.PerMessageTransferFields.per-message-indicators.content-
      return-request is set to FALSE, the parameter RET may be set to
      HDRS, to specify return of headers only.






Kille                       Standards Track                   [Page 115]

RFC 2156                         MIXER                      January 1998


Appendix B - Mapping with X.400(1984)

   This appendix defines modifications to the  mapping for use with
   X.400(1984).

   The X.400(1984) protocols are a proper subset of X.400(1988).  When
   mapping from X.400(1984) to RFC 822, no changes to this specification
   are needed.

   When mapping from RFC 822 to X.400(1984), no use can be made of 1988
   specific features.   No use of such features is made at the MTS
   level.  The heading extension feature is used at the IPMS level, and
   this shall be replaced by the RFC 987 approach.  All header
   information which would usually be mapped into the rfc-822-heading-
   list extension is mapped into a single IA5 body part, which is the
   first body part in the message.  This body part will start with the
   string "RFC-822-Headers:" as the first line.  The headers then follow
   this line.  This specification requires correct reverse mapping of
   this format, either from 1988 or 1984.  RFC 822 extended headers
   which could be mapped into X.400(1988) elements, are also mapped to
   the body part.

   In an environment where RFC 822 is of major importance, it may be
   desirable for downgrading to consider the case where the message was
   originated in an RFC 822 system, and mapped according to this
   specification.  The rfc-822-heading-list extension may be mapped
   according to this appendix.

   When parsing std-or, the following restrictions shall be observed:

   -    Only the 84/88 attributes identified in the table in
        Section 4.2 are present.

   -    No teletex encoding is allowed.

   If an address violates this, it shall be treated as an RFC 822
   address, which will usually lead to encoding as a DDA "RFC-822".

   It is possible that attributes of zero length may be present
   in an OR Address.  This is not legal in 1988, except for ADMD
   where the case is explicitly described in Section 4.3.5.
   Attributes of zero length are deprecated (the attribute shall be
   omitted), and will therefore be unusual.  However, some systems
   generate them and rely on them.  Therefore, any null attribute
   shall be enoded using the std-or encoding (e.g., /O=/).






Kille                       Standards Track                   [Page 116]

RFC 2156                         MIXER                      January 1998


   If a non-Teletex Common Name (CN) is present, it shall be
   mapped onto a Domain Defined Attribute "Common".  This is in line
   with RFC 1328 on X.400 1988 to 1984 downgrading [22].

   This specification defines a mapping of the Internet message
   framework to X.400.  Body part mappings are defined in RFC
   2157 [6], which relies on X.400(88) features.   Downgrading to
   X.400(84) for body parts is defined in RFC 1496 (HARPOON), which
   shall be followed in the context of this appendix [5].










































Kille                       Standards Track                   [Page 117]

RFC 2156                         MIXER                      January 1998


Appendix C - RFC 822 Extensions for X.400 access

   This appendix defines a number of optional mappings which may be
   provided to give access from RFC 822 to a number of X.400 services.
   These mappings are beyond the basic scope of this specification.
   There has been a definite demand to use extended RFC 822 as a
   mechanism to access X.400, and these extensions provide access to
   certain features.  If this functionality is provided, this appendix
   shall be followed.  The following headings are defined:

         extended-heading =
             "Prevent-NonDelivery-Report" ":"
             / "Generate-Delivery-Report" ":"
             / "Alternate-Recipient" ":" prohibition
             / "Disclose-Recipients" ":"  prohibition
             / "X400-Content-Return" ":" prohibition

   Prevent-NonDelivery-Report and Generate-Delivery-Report allow setting
   of MTS.PerRecipientSubmissionFields.originator-report-request.  The
   setting will be the same for all recipients.

   Alternate-Recipient, Disclose-Recipients, and X400-Content-Return
   allow for override of the default settings for
   MTS.PerMessageIndicators.

   Use of NOTARY mechanisms is a preferred meachanism for controlling
   these parameters.
























Kille                       Standards Track                   [Page 118]

RFC 2156                         MIXER                      January 1998


Appendix D - Object Identifier Assignment

   The following Object Identifiers shall be used.

   internet ::= OBJECT IDENTIFIER  { iso org(3) dod(6) 1 } -- from RFC
   1155

   mail OBJECT IDENTIFIER ::= { internet 7 }  -- IANA assigned

   mixer OBJECT IDENTIFIER ::= { mail mixer(1) } -- inherited from RFC
   1495
   mixer-core OBJECT IDENTIFIER ::= { mixer core(3) }

   id-rfc-822-field-list OBJECT IDENTIFIER ::= {mixer-core 2}
   id-dsn-header-list OBJECT IDENTIFIER ::= {mixer-core 3}
   id-dsn-field-list OBJECT IDENTIFIER ::= {mixer-core 4}

   eit-mixer OBJECT IDENTIFIER ::= {mixer-core 5}
                  -- the MIXER pseudo-EIT

   This object identifier for id-rfc-822-field-list is different to
   the one assigned in RFC 1327, which was erroneous.





























Kille                       Standards Track                   [Page 119]

RFC 2156                         MIXER                      January 1998


Appendix E - BNF Summary

   boolean = "TRUE" / "FALSE"


   numericstring = *(DIGIT / " ")


   printablestring  = *( ps-char )
   ps-restricted-char      = 1DIGIT /  1ALPHA / " " / "'" / "+"
                             / "," / "-" / "." / "/" / ":" / "=" / "?"
   ps-delim         = "(" / ")"
   ps-char          = ps-delim / ps-restricted-char


   ps-encoded       = *( ps-restricted-char / ps-encoded-char )
   ps-encoded-char  = "(a)"               ; (@)
                      / "(p)"               ; (%)
                      / "(b)"               ; (!)
                      / "(q)"               ; (")
                      / "(u)"               ; (_)
                      / "(l)"               ; "("
                      / "(r)"               ; ")"
                      / "(" 3DIGIT ")"

   teletex-string   = *( ps-char / t61-encoded )
   t61-encoded      = "{" 1* t61-encoded-char "}"
   t61-encoded-char = 3DIGIT


   teletex-and-or-ps = [ printablestring ] [ "*" teletex-string ]


   labelled-integer ::= [ key-string ] "(" numericstring ")"

   labelled-integer-2 ::= [ numericstring ] "(" key-string ")"

   key-string      = *key-char
   key-char        = <a-z, A-Z, 0-9, and "-">

   object-identifier  ::= oid-comp object-identifier
                          | oid-comp

   oid-comp ::= [ key-string ] "(" numericstring ")"


   encoded-info    = 1#encoded-type




Kille                       Standards Track                   [Page 120]

RFC 2156                         MIXER                      January 1998


   encoded-type    = built-in-eit / object-identifier

   built-in-eit    = "Undefined"         ; undefined (0)
                   / "Telex"             ; tLX (1)
                   / "IA5-Text"          ; iA5Text (2)
                   / "G3-Fax"            ; g3Fax (3)
                   / "TIF0"              ; tIF0 (4)
                   / "Teletex"           ; tTX (5)
                   / "Videotex"          ; videotex (6)
                   / "Voice"             ; voice (7)
                   / "SFD"               ; sFD (8)
                   / "TIF1"              ; tIF1 (9)



   encoded-pn      = [ given "." ] *( initial "." ) surname

   given           = 2*<ps-char not including ".">

   initial         = ALPHA

   surname         = printablestring


   std-or-address  = 1*( "/" attribute "=" value ) "/"
   attribute       = standard-type
                   / "RFC-822"
                   / dd-key "." std-printablestring

   std-or-address-input =  [ sep pair ] sep  pair *( sep pair )
                          sep  [ pair sep ]

   sep             = "/" / ";"
   pair            = input-attribute "=" value
   input-attribute = attribute
                   / dd-key ":" std-printablestring


   standard-type   = key-string

   dd-key          = key-string

   value           = std-printablestring

   std-printablestring
                   = *( std-char / std-pair )

   std-char        = <"{", "}", "*", and any ps-char



Kille                       Standards Track                   [Page 121]

RFC 2156                         MIXER                      January 1998


                                                except "/" and "=" >
   std-pair        = "$" ps-char


   global-id = std-or-address



   mta-field       = "X400-Received" ":" x400-trace
                   / "Deferred-Delivery" ":" date-time
                   / "Latest-Delivery-Time" ":" date-time



   x400-trace       = "by" md-and-mta ";"
                    [ "deferred until" date-time ";" ]
                    [ "converted" "(" encoded-info ")" ";" ]
                    [ "attempted" md-or-mta ";"  ]
                       action-list
                       ";" arrival-time


   md-and-mta       = [ "mta" mta "in" ]  global-id
   mta              = word
   arrival-time     = date-time

   md-or-mta        = "MD" global-id
                    / "MTA" mta

   Action-list      = 1#action
   action           = "Redirected"
                    / "Expanded"
                    / "Relayed"
                    / "Rerouted"

   dr-user-info = dr-summary <CRLF>
                  dr-recipients <CRLF>
                  dr-content-return


   dr-content-return = "The Original Message is not available"
        / "The Original Message follows:"

   dr-summary = "This report relates to your message:" <CRLF>
                   content-correlator <CRLF> <CRLF>
                "of" date-time <CRLF> <CRLF>

   dr-recipients = *(dr-recipient <CRLF> <CRLF>)



Kille                       Standards Track                   [Page 122]

RFC 2156                         MIXER                      January 1998


   dr-recipient = dr-recip-success / dr-recip-failure

   dr-recip-success =
                   "Your message was successfully delivered to:"
                    mailbox "at" date-time


   dr-recip-failure = "Your message was not delivered to:"
                           mailbox <CRLF>
                   "for the following reason:" *word

   report-point = [ "mta" mta-name "in" ] global-id
   content-correlator = *word
   mta-name = word

      dr-per-message-fields =
                / "X400-Conversion-Date" ":" date-time
                / "X400-Subject-Submision-Identifier" ":"
                                      mts-msg-id
                / "X400-Content-Identifier" ":" printablestring
                / "X400-Content-Type" ":" mts-content-type
                / "X400-Original-Encoded-Information-Types" ":"
                              encoded-info
                / "X400-Originator-and-DL-Expansion-History" ":"
                              mailbox ";" date-time ";"
                / "X400-Reporting-DL-Name" ":" mailbox
                / "X400-Content-Correlator" ":" content-correlator
                / "X400-Recipient-Info" ":" recipient-info
                / "X400-Subject-Intermediate-Trace-Information" ":"
                                        x400-trace
                / dr-extensions


      dr-per-recipient-fields =
                / "X400-Redirect-Recipient" ":" "x400" ";" std-or
                / "X400-Mapped-Redirect-Recipient" ":" "rfc822" ";"
                      mailbox
                / "X400-Converted-EITs" ":" encoded-info ";"
                / "X400-Delivery-Time" ":" date-time
                / "X400-Type-of-MTS-User" ":" labelled-integer
                / "X400-Last-Trace" ":" [ encoded-info ] date-time
                / "X400-Supplementary-Info" ":"
                      <"> printablestring <"> ";"
                / "X400-Redirection-History" ":" redirect-history-item
                / "X400-Physical-Forwarding-Address" ":" mailbox
                / "X400-Originally-Specified-Recipient-Number" ":"
                      integer
                / dr-extensions



Kille                       Standards Track                   [Page 123]

RFC 2156                         MIXER                      January 1998


      dr-extensions = "X400-Discarded-DR-Extensions" ":"
                        1# (object-identifier / labelled-integer)


      dr-diagnostic = "Reason" labelled-integer-2
                      [ ";" "Diagnostic" labelled-integer-2 ]

      mts-field = "X400-MTS-Identifier" ":" mts-msg-id
                / "X400-Originator" ":" mailbox
                / "X400-Recipients" ":" 1#mailbox
                / "Original-Encoded-Information-Types" ":"
                                encoded-info
                / "X400-Content-Type" ":" mts-content-type
                / "X400-Content-Identifier" ":" printablestring
                / "Priority" ":" priority
                / "Originator-Return-Address" ":" 1#mailbox
                / "DL-Expansion-History" ":" mailbox ";" date-time ";"
                / "Conversion" ":" prohibition
                / "Conversion-With-Loss" ":" prohibition
                / "Delivery-Date" ":" date-time
                / "Discarded-X400-MTS-Extensions" ":"
                             1#( object-identifier / labelled-integer )

      prohibition     = "Prohibited" / "Allowed"

      mts-msg-id       = "[" global-id ";" *text "]"

      mts-content-type = "P2" /  labelled-integer
                      / object-identifier

      priority        = "normal" / "non-urgent" / "urgent"

      ipn-body-format = ipn-description <CRLF>
                      [ ipn-extra-information <CRLF> ]
                      [ ipn-content-return ]

      ipn-description = ipn-receipt / ipn-non-receipt

      ipn-receipt = "Your message to:" preferred-recipient <CRLF>
               "was received at" receipt-time <CRLF> <CRLF>
               "This notification was generated"
                acknowledgement-mode <CRLF>
               "The following extra information was given:" <CRLF>
                ipn-suppl <CRLF>

      ipn-non-receipt = "Your message to:"
              preferred-recipient <CRLF>
              ipn-reason



Kille                       Standards Track                   [Page 124]

RFC 2156                         MIXER                      January 1998


      ipn-reason = ipn-discarded / ipn-auto-forwarded

      ipn-discarded = "was discarded for the following reason:"
                      discard-reason <CRLF>

      ipn-auto-forwarded = "was automatically forwarded." <CRLF>
                      [ "The following comment was made:"
                              auto-comment ]


      ipn-extra-information =
               "The following information types were converted:"
               encoded-info

      ipn-content-return = "The Original Message is not available"
                      / "The Original Message follows:"

      preferred-recipient = mailbox
      receipt-time        = date-time
      auto-comment        = printablestring
      ipn-suppl           = printablestring


      discard-reason     = "Expired" / "Obsoleted" /
                  "User Subscription Terminated" / "IPM Deleted"

      acknowledgement-mode = "Manually" / "Automatically"


      ipms-field = "Supersedes" ":" 1*msg-id
                 / "Expires" ":" date-time
                 / "Reply-By" ":" date-time
                 / "Importance" ":" importance
                 / "Sensitivity" ":" sensitivity
                 / "Autoforwarded" ":" boolean
                 / "Incomplete-Copy" ":"
                 / "Content-Language" ":" 1#language
                 / "Message-Type" ":" message-type
                 / "Discarded-X400-IPMS-Extensions" ":"
                       1#object-identifier
                 / "Autosubmitted" ":" autosubmitted

      importance      = "low" / "normal" / "high"

      sensitivity     = "Personal" / "Private" /
                             "Company-Confidential"

      language        = 2*ALPHA [ "(" language-description ")" ]



Kille                       Standards Track                   [Page 125]

RFC 2156                         MIXER                      January 1998


      language-description = printable-string


      message-type    = "Delivery Report"
                      / "InterPersonal Notification"
                      / "Multiple Part"


      autosubmitted   = "not-auto-submitted"
                      / "auto-generated"
                      / "auto-replied"
                      / "auto-forwarded"

      redirect-comment  = redirect-first *( redirect-subsequent )

      redirect-first = "Originally To:"  mailbox  "Redirected on"
               date-time "To:"  redirection-reason

      redirect-subsequent = mailbox  "Redirected Again on"
               date-time "To:"  redirection-reason

      redirection-history-item = "intended recipient" mailbox
               "redirected to"  redirection-reason
               "on" date-time

      redirection-reason =
               "Recipient Assigned Alternate Recipient"
               / "Originator Requested Alternate Recipient"
               / "Recipient MD Assigned Alternate Recipient"
               / "Directory Look Up"
               / "Alias"

      subject-line  = "Delivery-Report" "(" status ")"
                      [ "for" destination ]

      status        = "success" / "failure" / "success and failures"

      destination   = mailbox / "MTA" word

      extended-heading =
          "Prevent-NonDelivery-Report" ":"
          / "Generate-Delivery-Report" ":"
          / "Alternate-Recipient" ":" prohibition
          / "Disclose-Recipients" ":"  prohibition
          / "X400-Content-Return" ":" prohibition






Kille                       Standards Track                   [Page 126]

RFC 2156                         MIXER                      January 1998


Appendix F - Text format for MCGAM distribution

1.  Text Formats

   This appendix defines text formats for exchange of four types of
   mapping.

   1.   Domain Name Space -> OR Address Space MCGAM

   2.   OR Address Space -> Domain Name Space MCGAM

   3.   Domain Name Space -> OR Address of preferred gateway

   4.   OR Address Space -> Domain Name of preferred gateway

2.  Mechanisms to register and to distribute MCGAMs

   There is a well known set of MCGAM tables.

   The global coordination of the mapping rules is a part of the DANTE
   MailFLOW Project. New mapping rules may be defined by the authority
   responsible for the relevant name space. The rules need to be
   registered with a national mapping registration authority, which in
   turn passes them on to the central mapping registration authority.
   All the collected mapping rules are merged together into the globally
   coordinated mapping tables by the MailFLOW Project Team. The tables
   are available from the national mapping registration authorities.

   To get a contact address of the mapping registration authority for
   the respective country or more information about the MailFLOW Project
   contact:

      SWITCH
      MailFLOW Project Team
      Limmatquai 138
      8001 Zuerich
      Switzerland

      email: mailflow@mailflow.dante.net
             S=MailFLOW;O=MailFLOW;P=DANTE;A=mailnet;C=fi;

      fax:   +41 1 268 15 68
      tel:   +41 1 268 15 20








Kille                       Standards Track                   [Page 127]

RFC 2156                         MIXER                      January 1998


3.  Syntax Definitions

   An address syntax is defined, which is compatible with the syntax
   used for 822.domains.  By representing the OR addresses as domains,
   all lookups can be mechanically implemented as domain -> domain
   mappings.  This syntax defined is initially for use in table format,
   but the syntax is defined in a manner which makes it suitable to be
   adapted for  use with the  Domain Name Service.  This syntax allows
   for a general representation of OR addresses, so that it can be used
   in other applications.  Not all attributes are used in the table
   formats defined.

   To allow the mapping where a level of the hierarchy is omitted, the
   pseudo-value "@" (not a printable string character) is used to
   indicate omission of a level in the hierarchy.  This is distinct from
   the form including the element with no value, although a correct
   X.400 implementation will interpret both in the same manner.

   This syntax is not intended to be handled by users.

      dmn-or-address  = dmn-part *( "." dmn-part )
      mn-part        = dmn-attribute "$" value
      dmn-attribute  = standard-type
                      /  "~" dmn-printablestring
      value           = dmn-printablestring
                      / "@"
      dmn-printablestring =
                      = *( dmn-char / dmn-pair )
      dmn-char        = <"{", "}", "*", and any ps-char
                                              except ".">
      dmn-pair        = "\."

   An example usage:

      ~ROLE$Big\.Chief.ADMD$ATT.C$US
      PRMD$DEC.ADMD$@.C$US

   The first example illustrates quoting of a "." and a domain define
   attribute (ROLE).  The second  example illustrates omission of the
   ADMD level. There shall be a strict ordering of all components in
   this table, with the most significant components on the RHS.   This
   allows the encoding to be treated as a domain.

   Various further restrictions are placed on the usage of dmn-or-
   address in the address space mapping tables.

   a.   Only C, ADMD, PRMD, O, and up to four OUs may be used.




Kille                       Standards Track                   [Page 128]

RFC 2156                         MIXER                      January 1998


   b.   No components shall be omitted from this hierarchy, although
        the hierarchy may terminate at any level.  If the mapping is
        to an omitted component, the "@" syntax is used.

4.  Table Lookups

   When determining a match, there are aspects which apply to all
   lookups.  Matches are always case independent. The key for all three
   tables is a domain. The longest possible match shall be obtained.
   Suppose the table has two entries with the following keys:

      K.L
      J.K.L

      Domain "A.B.C" will not return any matches.  Domain "I.J.K.L"
      will match the entry "J.K.L:.

5.  Domain -> OR Address MCGAM format

   The BNF is:

      domain-syntax "#" dmn-or-address "#"

   EBNF.domain-syntax is defined in Section 4.2. Note that the trailing
   "#" is used for clarity, as the dmn-or-address syntax might lead to
   values with trailing blanks.  Lines starting with "#" are comments.

      For example:
      AC.UK#PRMD$UK\.AC.ADMD$GOLD 400.C$GB#
      XEROX.COM#O$Xerox.ADMD$ATT.C$US#
      GMD.DE#O$@.PRMD$GMD.ADMD$DBP.C$DE#

   A domain is looked up to determine the top levels of an OR Address.
   Components of the domain which are not matched are used to build the
   remainder of the OR address, as described in Section 4.3.4.

6.  OR Address -> Domain MCGAM format

   The syntax of this table is:

      dmn-or-address "#" domain-syntax "#"

      For example:

      #
      # Mapping table
      #
      PRMD$UK\.AC.ADMD$GOLD 400.C$GB#AC.UK#



Kille                       Standards Track                   [Page 129]

RFC 2156                         MIXER                      January 1998


   The OR Address is used to generate a domain key.  It is important to
   order the components correctly, and to fill in missing components in
   the hierarchy.  Use of this mapping is described in Section 4.3.2.

7.  Domain -> OR Address of Preferred Gateway table

   This uses the same format as the domain -> OR address MCGAM table.
   In this case, the restriction to only use C/ADMD/PRMD/O/OU does not
   apply.  Use of this mapping is described in Section 4.3.4. A domain
   cannot appear in this table and in the domain to OR Address table.

8.  OR Addresss -> domain of Preferred Gateway table

   This uses the same format as the OR Address -> domain MCGAM table.
   Use of this mapping is described in Section 4.3.5. An OR Address
   cannot appear in this table and in the OR Address to domain table.



































Kille                       Standards Track                   [Page 130]

RFC 2156                         MIXER                      January 1998


Appendix G - Conformance

   This appendix defines a number of options, which a conforming gateway
   shall specify.  Conformance to this specification shall not be
   claimed if any of the mandatory features are not implemented.  A
   specification of conformance may list the service elements of Chapter
   2, in order to be clear that full conformance is provied.  In
   particular:

   -    Formats for all fields shall be followed.

   -    The gateway shall enable MCGAMs to be used.

   -    Formats for subject lines, delivery reports and IPNs shall
        be followed.   A system which followed the syntax, but
        translated text into a language other than english would be
        conformant.

   -    RFC 1137 shall not be followed when mapping to SMTP.

   -    All mappings of trace shall be implemented.

   -    There shall be a mechanism to access all three global
        mappings.

   -    RFC 2157 shall be followed for mapping body parts.

   -    When it is specified that a MIME format message is
        generated, RFC 2045 shall be followed.

   A gateway shall specify:

   -    Which Interent Message Transport (822-MTS)  protocols are
        supported.  If SMTP is supported, Appendex A of MIXER shall
        be used.

   -    Which X.400 versions  are supported (84, 88, 92).

   -    Which mechanisms (table, X.500, DNS) are supported to access
        MCGAMs.

   -    The mechanism or mechanisms by which the global mapping
        information is accessed.

   The following are optional parts of this specification.  A conforming
   implementation shall specify which of these it supports.

   -    Support for the extension mappings of Appendix C.



Kille                       Standards Track                   [Page 131]

RFC 2156                         MIXER                      January 1998


   -    Support for returning illegal format content in a delivery
        report

   -    Which address interpretation heuristics are supported
        (4.3.4.1)

   -    If RFC 987 generated message ids are handled in a backwards
        compatible manner (4.7.3.6)











































Kille                       Standards Track                   [Page 132]

RFC 2156                         MIXER                      January 1998


Appendix H - Change History: RFC 987, 1026, 1138, 1148

   RFC 987 was the original document, and contained the key elements of
   this specification.  It was specific to X.400(1984).  RFC 1026
   specified a small number of necessary changes to RFC 987.

   RFC 1138 was based on the RFC 987 work.  It contained an editorial
   error, and was reissued a few months later as RFC 1148.  RFC 1148
   will be referred to here, as it is the document which is widely
   referred to elsewhere. The major goal of RFC 1148 was to upgrade RFC
   987 to X.400(1988).  It did this, but did not obsolete RFC 987, which
   was recommended for use with X.400(1984).  This appendix summarises
   the changes made in going from RFC 987 to RFC 1148.

   RFC 1148 noted the following about its upgrade from RFC 987:
   Unnecessary change is usually a bad idea.  Changes on the RFC 822
   side are avoided as far as possible,  so that RFC 822 users do not
   see arbitrary differences between systems conforming to this
   specification, and those following RFC 987.  Changes on the X.400
   side are minimised, but are more  acceptable, due to the mapping onto
   a new set of services and protocols.

1.  Introduction

   The model has shifted from a protocol based mapping to a service
   based mapping.  This has increased the generality of the
   specification, and improved the model.  This change affects the
   entire document.

   A restriction on scope has been added.

2.  Service Elements

   -    The new service elements of X.400 are dealt with.

   -    A clear distinction is made between origination and
        reception

3.  Basic Mappings

   -    Add teletex support

   -    Add object identifier support

   -    Add labelled integer support

   -    Make PrintableString <-> ASCII mapping reversible




Kille                       Standards Track                   [Page 133]

RFC 2156                         MIXER                      January 1998


   -    The printable string mapping is aligned to the NBS mapping
        derived from RFC 987.

4.  Addressing

   -    Support for new addressing attributes

   -    The message ID mapping is changed to not be table driven

5.  Detailed Mappings


   -    Define extended IPM Header, and use instead of second body
        part for RFC 822 extensions

   -    Realignment of element names

   -    New syntax for reports, simplifying the header and
        introducing a mandatory body format (the RFC 987 header
        format was unusable)

   -    Drop complex autoforwarded mapping

   -    Add full mapping for IP Notifications, defining a body
        format

   -    Adopt an MTS Identifier syntax in line with the OR Address
        syntax

   -    A new format for X400 Trace representation on the RFC 822
        side

6.  Appendices

   -    Move Appendix on restricted 822 mappings to a separate RFC

   -    Delete Phonenet and SMTP Appendixes














Kille                       Standards Track                   [Page 134]

RFC 2156                         MIXER                      January 1998


Appendix I - Change History: RFC 1148 to RFC 1327

1.  General

   -    The scope of the document was changed to cover X.400(1984),
        and so obsolete RFC 987.

   -    Changes were made to allow usage to connect RFC 822 networks
        using X.400

   -    Text was tightened to be clear about optional and mandatory
        aspects

   -    A good deal of clarification

   -    A number of minor EBNF errors

   -    Better examples are given

   -    Further X.400 upper bounds are handled correctly

2.  Basic Mappings

   -    The encoding of object identifier is changed slightly

3.  Addressing

   -    A global mapping of domain to preferred gateway is
        introduced.

   -    An overflow mechanism is defined for RFC 822 addresses of
        greater than 128 bytes

   -    Changes were made to improve compatibility with the PDAM on
        writing OR Addresses.

   +         The PD and Terminal Type keywords were aligned to the
             PDAM.  It is believed that minimal use has been made of
             the RFC 1148 keywords.

   +         P and A are allowed as alternate keys for PRMD and ADMD

   +         Where keywords are different, the PDAM keywords are
             alternatives on input.  This is mandatory.

4.  Detailed Mappings

   -    The format of the Subject: lines is defined.



Kille                       Standards Track                   [Page 135]

RFC 2156                         MIXER                      January 1998


   -    Illegal use (repetition) of the heading EXTENSION is
        corrected, and a new object identifier assigned.

   -    The Delivery Report format is extensively revised in light
        of operational experience.

   -    The handling of redirects is significantly changed, as the
        previous mechanism did not work.

5.  Appendices

   -    An SMTP appendix is added, allowing optional use of the VRFY
        command to improve probe information.

   -    Handling of JNT Mail Acknowledge-To is changed slightly.

   -    A DDA JNT-MAIL is allowed on input.

   -    The format definitions of Appendix F are explained further,
        and a third table definition added.

   -    An appendix on use with X.400(1984) is added.

   -    Optional extensions are defined to give RFC 822 access to
        further X.400 facilities.

   -    An appendix on conformance is added.
























Kille                       Standards Track                   [Page 136]

RFC 2156                         MIXER                      January 1998


Appendix J - Change History: RFC 1327 to this Document

1.  General

   This update is primarily for stability, and to fold in compatibility
   for MIME and to add support for the new NOTARY delivery status
   notifications.  Other general changes:

   -    Various editorial updates

   -    Minor EBNF errors

   -    Reference to mapping table support by DNS and X.500.

   -    Alignment to X.400(92)

   -    Assignment of a new object identifier

   -    Removal of specification relating to body mapping, which is
        now defined in RFC 2157.

2.  Service Elements

   -    Support of Auto-Submitted service

3.  Basic Mappings

   -    Comments shall not be used in new headers, to remove parsing
        ambiguity

   -    RFC 1522 encoding may be used as an alternative to X.408
        downgrade, where appropriate.

   -    Correct handling of RFC 822 four year dates.

4.  Addressing

   -    Replaced the mandatory global address mapping with MCGAMs.

   -    Add codes and add a heuristic to align to the standard X.400
        form of writing OR Addresses.

   -    Improved text on ordering heuristic

   -    Leading "/" interpretation added






Kille                       Standards Track                   [Page 137]

RFC 2156                         MIXER                      January 1998


   -    All bar one of the address mapping heuristics made
        mandatory.

   -    Interpretation of domain defined attribute "RFC-822" made
        mandatory in all cases

   -    Make report request comments optional

5.  Detailed Mappings

   -    Comments no longer maps to separate body part

   -    Allow Languages to be multi-valued

   -    Change Content-Identifier to X400-Content-Identifier, in
        order to avoid confusion with MIME.

   -    Reverse mapping of MIXER defined fields made mandatory

   -    "Expiry-Date:" changed to "Expires:".

   -    "Obsoletes:" changed to "Supersedes:".

   -    Define correct handling when "Resent-Date:" is present.

6.  Appendices

   -    Change "Content-Return" to "X400-Content-Return" in Appendix
        C.

   -    Relaxation of restrictions on mapping 3 in Appendix F.

   -    Add linkage to HARPOON in Appendix B.

   -    RFC 2157 added to the conformance statement of Appendix
         G.

   -    Added Appendix L, with ASN. Summary.













Kille                       Standards Track                   [Page 138]

RFC 2156                         MIXER                      January 1998


Appendix L - ASN.1 Summary

   MIXER Definitions { iso org(3) dod(6) internet(1) mail(7)
         mixer(1) mixer-core(3) definitions(1) }

   DEFINITIONS IMPLICIT TAGS ::=

   BEGIN

   -- exports everything

   IMPORTS

   EXTENSION FROM
     MTSAbstractService {join-iso-ccit mhs-motis(6) mts(3)
           modules(0) mts-abstract-service(1) }

     HEADING-EXTENSION FROM
       IPMSAbstractService {join-iso-ccit mhs-motis(6) ipms(1)
             modules(0) abstract-service(3) }



   rfc-822-field HEADING-EXTENSION
           VALUE RFC822FieldList
           ::= id-rfc-822-field-list


   RFC822FieldList ::= SEQUENCE OF RFC822Field

   RFC822Field ::= IA5String



   dsn-header-list EXTENSION
      RFC822FieldList
      ::= id-dsn-header-list

   dsn-field-list EXTENSION
      RFC822FieldList
      ::= id-dsn-field-list

   internet ::= OBJECT IDENTIFIER  { iso org(3) dod(6) 1 } -- from RFC
   1155

   mail OBJECT IDENTIFIER ::= { internet 7 }  -- IANA assigned





Kille                       Standards Track                   [Page 139]

RFC 2156                         MIXER                      January 1998


   mixer OBJECT IDENTIFIER ::= { mail mixer(1) } -- inherited from RFC
   1495
   mixer-core OBJECT IDENTIFIER ::= { mixer core(3) }

   id-rfc-822-field-list OBJECT IDENTIFIER ::= {mixer-core 2}
   id-dsn-header-list OBJECT IDENTIFIER ::= {mixer-core 3}
   id-dsn-field-list OBJECT IDENTIFIER ::= {mixer-core 4}

   eit-mixer OBJECT IDENTIFIER ::= {mixer-core 5}
                   -- the MIXER pseudo-EIT


   END -- MIXER ASN.1






































Kille                       Standards Track                   [Page 140]

RFC 2156                         MIXER                      January 1998


SECURITY CONSIDERATIONS

   Security issues are not discussed in this memo.

AUTHOR'S ADDRESS

   Steve Kille
   Isode Ltd
   The Dome
   The Square
   Richmond
   TW9 1DT
   England

   Phone: +44-181-332-9091
   Internet EMail: S.Kille@ISODE.COM

   X.400 Email: I=S; S=Kille; P=Isode; A=Mailnet; C=FI;

   UFN:  S.Kille, Isode, GB

References

   1.   CCITT , "Recommendations X.400", Message Handling Systems:
        System Model - Service Elements, October 1984.

   2.   Allocchio, C., "MaXIM11 - Mapping between X.400 / Internet
        Mail and Mail-11 mail", RFC 2162, January 1998.

   3.   Allocchio, C., "Using the Internet DNS to Distribute MIXER
        Conformant Global Address Mapping (MCGAM)", RFC 2163,
        January 1998.

   4.   Alvestrand, H., Kille, S., Miles, R., Rose, M., and S.
        Thompson, "Mapping between X.400 and RFC-822 Message
        Bodies", RFC 1495, August 1993.

   5.   Alvestrand, H., Romaguera, J., and K. Jordan, "Rules for
        Downgrading Messages for X.400(88) to X.400(84) When MIME
        Content-Types are Present in the Messages (Harpoon)", RFC
        1496, August 1993.

   6.   Alvestrand, H., and S. Thompson, "Equivalences between X.400
        and RFC-822 Message Bodies", RFC 1494, August 1993.

   7.   Alvestrand, H., "Tags for the Identification of Languages",
        RFC 1766, March 1995.




Kille                       Standards Track                   [Page 141]

RFC 2156                         MIXER                      January 1998


   8.   Alvestrand, H., "Mapping between X.400 and RFC-822/MIME
        Message Bodies", RFC 2157, January 1998.

   9.   Freed, N., and N. Borenstein, "Multipurpose Internet
        Mail Extensions (MIME) Part One: Format of Internet Message
        Bodies", RFC 2045, November 1996.

   10.  Braden, R., "Requirements for Internet Hosts -- Application
        and Support", STD 3, RFC 1123, October 1989.

   11.  CCITT/ISO, "CCITT Recommendations X.420/ ISO/IEC 10021-7,"
        Message Handling Systems: Interpersonal Messaging System,
        Dec 1988.

   12.  CCITT/ISO, "CCITT Recommendations X.411/ ISO/IEC 10021-4,"
        Message Handling Systems: Message Transfer System: Abstract
        Service Definition and Procedures, Dec 1988.

   13.  CCITT/ISO, "CCITT Recommendations X.400/ ISO/IEC 10021-1,"
        Message Handling: System and Service Overview , Dec 1988.

   14.  CCITT/ISO, "Specification of Abstract Syntax Notation One
        (ASN.1)," CCITT Recommendation X.208 / ISO/IEC 8824, Dec
        1988.

   15.  CCITT/ISO, "CCITT Recommendations X.400/ ISO/IEC 10021-1,"
        Message Handling: System and Service Overview , Dec 1992.

   16.  Crocker, D., "Standard of the Format of ARPA Internet Text
        Messages", STD 11, RFC 822, August 1982.

   17.  Kille, S., "Mapping Between X.400 and RFC 822", UK Academic
        Community Report (MG.19) / RFC 987, June 1986.

   18.  Kille, S., "Addendum to RFC 987", UK Academic Community
        Report (MG.23) / RFC 1026, August 1987.

   19.  Kille, S., "Mapping Between X.400(1988) / ISO 10021 and RFC
        822", RFC 1138, October 1989.

   20.  Kille, S., "Mapping Between X.400(1988) / ISO 10021 and RFC
        822", RFC 1148, March 1990.

   21.  Kille, S., "Mapping Between X.400(1988) / ISO 10021 and RFC
        822", RFC 1327, May 1992.

   22.  Kille, S., "X.400 1988 to 1984 downgrading", RFC 1328, May
        1992.



Kille                       Standards Track                   [Page 142]

RFC 2156                         MIXER                      January 1998


   23.  Kille, S., "A String Encoding of Presentation Address", RFC
        1278, November 1992.

   24.  Kille, S., "A String Representation of Distinguished Name",
        RFC 1485, January 1992.

   25.  Kille, S., "Using the OSI Directory to achieve User
        Friendly Naming", RFC 1484, January 1992.

   26.  Kille, S., "Use of an X.500/LDAP directory to support MIXER
        address mapping", RFC 2164, January 1998.

   27.  Koorland, N., "Message Attachmment Work Group (MAWG): MAWG
        Feasibility Project Guide," EMA Report, Version 1.5, Nov
        1995.

   28.  Moore, K., and G. Vaudreuil, "An Extensible Message Format for
        Delivery Status Notifications", RFC 1894, January 1996.

   29.  Moore, K., "SMTP Service Extensions for Delivery Status
        Notifications", RFC 1891, Januaty 1996.

   30.  Postel, J., "SIMPLE MAIL TRANSFER PROTOCOL", STD 10, RFC 821,
        August 1982.



























Kille                       Standards Track                   [Page 143]

RFC 2156                         MIXER                      January 1998


Full Copyright Statement

   Copyright (C) The Internet Society (1998).  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.
























Kille                       Standards Track                   [Page 144]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/