Network Working Group                                      H. Alvestrand
Request for Comments: 1496                                  SINTEF DELAB
Updates: 1328                                               J. Romaguera
                                                           NetConsult AG
                                                               K. Jordan
                                              Control Data Systems, Inc.
                                                             August 1993

     Rules for Downgrading Messages from X.400/88 to X.400/84 When
             MIME Content-Types are Present in the Messages

Status of this Memo

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

Table of Contents

   1.  Introduction ...............................................    1
   2.  Basic approach .............................................    2
   3.  Conversion rules ...........................................    3
   3.1  EBP conversions to Basic ..................................    3
   3.2  Encapsulation format ......................................    3
   4.  Implications ...............................................    4
   5.  Security Considerations ....................................    4
   6.  Authors' Addresses .........................................    4
   7.  References .................................................    5

1. Introduction

   Interworking between X.400(88) and MIME is achieved by [4], which
   complements RFC-1327 [2], which in turn specifies the interworking
   between X.400(88) and RFC-822 based mail.

   Interworking between X.400(88) and X.400 (84) is achieved by RFC-1328
   [3]. That document does not describe what to do in the case where
   body parts arrive at the gateway that cannot be adequately
   represented in the X.400(84) system.

   This document describes how RFC-1328 must be modified in order to
   provide adequate support for the scenarios:

      SMTP(MIME) -> X.400(84)

      X.400(84) -> SMTP(MIME)

   It replaces chapter 6 of RFC-1328. The rest of RFC-1328 is NOT
   obsoleted.

   NOTE: A desireable side-effect of HARPOON is that a standardized
   method for the identification and transmission of multimedia and
   binary data (like spreadsheets) between X.400/84 UAs is achieved.

   While this method is not compatible with current proprietary
   approaches, we believe that it requires less invasive changes to
   current UAs than other possible methods.

   This memo updates RFC 1328.  HARPOON is a pure name, and has no
   meaning.  Please send comments to the MIME-MHS mailing list
   <mime-mhs@surnet.nl>.

2.  Basic approach

   The approach is to imagine that the connection X.400(84) <->
   SMTP(MIME) never happens. This, of course, is an illusion, but can be
   a very useful illusion.

   All mail will therefore go on one of the paths

      X.400(84) -> X.400(88) -> SMTP(MIME)

      SMTP(MIME) -> X.400(88) -> X.400(84)

   when it goes between a MIME user and an X.400(84) user.

   The approach at the interface between X.400(88) and X.400(84) is:

      o  Convert what you can

      o  Encapsulate what you have to

      o  Never drop a message

   Of course, for X.400/88 body parts that are already defined in
   X.400/84, no downgrading should be done. In particular, multi-body
   messages should remain multi-body messages, IA5 messages including
   IA5 messages encoded as Extended Body Parts) should remain IA5
   messages, and G3Fax messages should remain G3Fax messages.

3.  Conversion rules

3.1.  EBP conversions to Basic

   Some body parts are defined by X.400(88) as having both a Basic form
   and an Extended form. These are listed in Annex B of X.420.

   For all of these, the transformation from the Extended Body Part to
   the Basic Body Part takes the form of putting the PARAMETERS and the
   DATA members together in a SEQUENCE.

   This transformation should be applied by the gateway in order to
   allow (for example) X.400(88) systems that use the Extended form of
   the IA5 body part to communicate with X.400(84) systems.

3.2.  Encapsulation format

   For any body part that cannot be used directly in X.400(84), the
   following IA5Text body part is made:

   -  Content = IA5String

   -  First bytes of content: (the description is in USASCII, with C
      escape sequences used to represent control characters):

       MIME-version: <version>\r\n
           Content-type: <the proper MIME content type>\r\n
         Content-transfer-encoding: <quoted-printable or base64>\r\n
         <Possibly other Content headings here, terminated by\r\n>
         \r\n

      <Here follows the bytes of the content, as per [4], encoded in the
      proper encoding>

   All implementations MUST place the MIME-version: header first in the
   body part. Headers that are placed by [2] and [4] into other parts of
   the message MUST NOT be placed in the MIME body part.

   This includes RFC-822 headings carried as heading-extensions, which
   must be placed in a new IA5 body part starting with the string "RFC-
   822-HEADERS", as specified in [2], Appendix G.

   Other heading-extensions are still handled as described in chapter 5
   of RFC 1328: They are dropped.

   Since all X.400(88) body parts can be represented in MIME by using
   the x400-bp MIME content-type, this conversion will never fail.

   In the reverse direction, any IA5 body part that starts with the
   token "MIME-Version:" will be subjected to conversion according to
   [4] before including the body part into an X.400(88) message.

4.  Implications

   The implications are several:

   - People with X.400(84) readers who have the ability to save messages
     to file will now be able to save MIME multimedia messages

   - People who can use features like the "Mailcaps" file to identify
     what to do about a bodypart can now grab implementations of MIME
     that can run as subprograms and achieve at least some multimedia
     functionality

5.  Security Considerations

   The security considerations in [1] and [4] (beware of trojans that
   can hit you if your UA automagically starts programs for you) are now
   relevant also for X.400(84) systems.

6.  Authors' Addresses

   Harald Tveit Alvestrand
   SINTEF DELAB
   N-7034 Trondheim
   NORWAY

   EMail: Harald.T.Alvestrand@delab.sintef.no


   Kevin E. Jordan, ARH215
   Control Data Systems, Inc.
   4201 Lexington Avenue N
   Arden Hills, MN  55126
   USA

   EMail: Kevin.E.Jordan@mercury.oss.arh.cpg.cdc.com


   James A. Romaguera
   NetConsult AG
   Mettlendwaldweg 20a
   3037 Herrenschwanden
   Switzerland

   EMail: Romaguera@netconsult.ch

7.  References

   [1]  Borenstein, N, and N. Freed, "MIME: Mechanisms for Specifying
        and Describing the Format of Internet Message Bodies", RFC 1341,
        Bellcore, Innosoft, June 1992.

   [2]  Hardcastle-Kille, S., "Mapping between X.400(1988) / ISO 10021
        and RFC-822", RFC 1327, University College London, May 1992.

   [3]  Hardcastle-Kille, S., "X.400 1988 to 1984 downgrading", RFC
        1328, University College London, May 1992.

   [4]  Alvestrand, H., Kille, S., Miles, R., Rose, M., and S. Thompson,
        "Mapping between X.400 and RFC-822 Message Bodies", RFC 1494,
        SINTEF DELAB, ISODE Consortium, Soft*Switch, Inc, Dover Beach
        Consulting, Inc., Soft*Switch, Inc., August 1993.