rfc1327.txt

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   Services).  Going from X.400 to RFC 822, an RFC 822 message and the
   associated 822-MTS information may be derived from:

   1.   A Report (MTA, and MTS Services)

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

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

   Probes (MTA Service) must 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 should be
   rejected at the gateway.

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.



Hardcastle-Kille                                                [Page 7]

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   Where an X.400 messages 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.  X.400 (1984)

   Much of this work 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.  A basic decision is that the mapping
   defined in this document is to 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.
   This is important, to give good support to communities which will
   utilise full X.400 at an early date.   To interwork with 1984
   systems, Appendix G 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 G.

1.7.  Compatibility with previous versions

   The changes between this and older versions of the document are given
   in Appendices I and J.    These are RFCs 987, 1026, 1138, and 1148.
   This document is a revision of RFC 1148 [Kille90a].  As far as
   possible, changes have been made in a compatible fashion.

1.8.  Aspects not covered

   There have been a number of cases where RFC 987 was 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

   -    Mapping X.400 to extended versions of RFC 822, with support
        for multimedia content.

   The first two of 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



Hardcastle-Kille                                                [Page 8]

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   all X.400 services.  Thus, it will be a poor choice for X.400
   implementors to use RFC 987(88) as 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 H.

1.9.  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 strongly
   discouraged.

1.10.  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 O/R
        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 eleven 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.

1.11.  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.



Hardcastle-Kille                                                [Page 9]

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   A number of comments from people on RFC 1148 lead to this document.
   In particular, there were comments and suggestions from:  Maurice
   Abraham (HP); Harald Alvestrand (Sintef); Peter Cowen (X-Tel); Jim
   Craigie (JNT); Ella Gardener (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).

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:

   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



Hardcastle-Kille                                               [Page 10]

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   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
   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).



Hardcastle-Kille                                               [Page 11]

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   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.

   Keywords:
        Supported by use of a heading extension.

   Subject:
        Supported.

   Comments:
        Supported by use of an extra body part.



Hardcastle-Kille                                               [Page 12]

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   Encrypted:
        Supported by use of a heading extension.

   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.

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:

   Message-Id:

   In-Reply-To:

   References:

   Subject: