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RFC 2157 - Mapping between X.400 and RFC-822/MIME Message Bodies


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Network Working Group                                      H. Alvestrand
Request for Comments: 2157                                       UNINETT
Category: Standards Track                                   January 1998

         Mapping between X.400 and RFC-822/MIME Message Bodies

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 Introduction ...........................................    2
   1.1 Glossary .............................................    3
   2 Basic rules for body part conversion ...................    4
   2.1 Generating the IPM Body from MIME ....................    5
   2.2 Generating the MIME Body from the IPMS.Body ..........    6
   2.3 Mapping the EMA FTBP parameters ......................    7
   2.3.1 Mapping GraphicStrings .............................    7
   2.3.2 Mapping specific parameters ........................    7
   2.3.3 Summary of FTBP elements generated .................   10
   2.4 Information that is lost when mapping ................   11
   3 Encapsulation of body parts ............................   11
   3.1 Encapsulation of MIME in X.400 .......................   12
   3.1.1 FTBP encapsulating body part .......................   12
   3.1.2 BP15 encapsulating body part .......................   13
   3.1.3 Encapsulation using IA5 (HARPOON) ..................   15
   3.1.4 Content passing using BP14 .........................   16
   3.2 Encapsulating X.400 Body Parts in MIME ...............   16
   3.3 Encapsulating FTBP body parts in MIME ................   17
   4 User control over the gateway choice ...................   18
   4.1 Conversion from MIME to X.400 ........................   18
   4.2 Conversion from X.400 to MIME ........................   20
   5 The equivalence registry ...............................   21
   5.1 What information one must give about a mapping
        .....................................................   21
   5.2 Equivalence summary for known X.400  and  MIME
       Types ................................................   22
   5.3 MIME to X.400 Table ..................................   23

   5.4 X.400 to MIME Table ..................................   23
   5.5 Use of OBJECT IDENTIFIERs and ASN.1 MACROS ...........   24
   6 Defined Equivalences ...................................   26
   6.1 IA5Text - text/plain .................................   26
   6.2 GeneralText - text/plain (ISO-8859) ..................   27
   6.3 BilaterallyDefined - application/octet-stream
       ......................................................   29
   6.4 FTBP  EMA  Unknown  Attachment   -
       application/octet-stream .............................   29
   6.5 MessageBodyPart - message/RFC822 .....................   30
   6.6 MessageBodyPart - multipart/* ........................   31
   6.7 Teletex - Text/Plain (Teletex) .......................   32
   7 Body parts where encapsulation is recommended ..........   33
   7.1 message/external-body ................................   34
   7.2 message/partial ......................................   35
   7.3 multipart/signed .....................................   35
   7.4 multipart/encrypted ..................................   36
   8 Conformance requirements ...............................   37
   9 Security Considerations ................................   38
   10 Author's Address ......................................   38
   11 Acknowledgements ......................................   38
    References ..............................................   38
    APPENDIXES ..............................................   41
    Appendix A: Escape code normalization ...................   41
    Appendix B: OID Assignments .............................   44
    Appendix  C:  Registration information for the
        Teletex character set ...............................   46
    Appendix  D:  IANA Registration form for new
    mappings ................................................   48
    Full Copyright Statement .................................  49

1.  Introduction

   This document is a companion to [MIXER], which defines the principles
   and translation of headers for interworking between MIME-based RFC-
   822 mail and X.400 mail.

   This document defines how to map body parts of X.400 messages into
   MIME entities and vice versa, including the handling of multipart
   messages and forwarded messages.

1.1.  Glossary

   The following terms are defined in this document:

   Body part
      Part of a message that has a unique type. This term comes from
      X.400; the corresponding term in MIME (RFC 2046) is limited to use
      in parts of a multipart message; the term "body" may correspond
      better.

   Content-type
      Type information indicating what the content of a body part
      actually is. This term comes from MIME; the corresponding X.400
      term is "body part type".

   Mapping
      (noun): A description of how to transform an X.400 body part into
      a MIME body part, or how to transform a MIME body part into an
      X.400 body part.

   Equivalence
      A set of two mappings that taken together provide a lossless
      conversion between an X.400 body part and a MIME body part

   Encapsulation
      The process of wrapping something from one of the mail systems in
      such a way that it can be carried inside the other mail system.
      When encapsulating, it is not expected that the other mail system
      can make reasonable sense of the body part, but a gateway back
      into the first system will always be able to convert the body part
      without loss back to its original format.

   HARPOON encapsulation
      The encapsulating of a MIME body part by putting it inside an IA5
      body with all headers and encoding intact. First described in RFC
      1496 [HARPOON].

   Tunneling
      What happens when one gateway encapsulates a message and sends it
      to another gateway that decapsulates it.  The hope is that this
      will cause minimal damage to the message in transit.

   DISCUSSION
      At many points in this document, the author has found it useful to
      include material that explains part of the reasoning behind the
      specification. These sections all start with DISCUSSION: and
      continue to the next numbered section heading; they do not dictate
      any additional requirements on a gateway.

   The words MUST, SHOULD and MAY, when capitalized, are used as defined
   in RFC 2119 [MUST].

2.  Basic rules for body part conversion

   The basic approach for translating body parts is described in section
   2.1 and 2.2.

   Chapter 3 gives details on "encapsulation", which allows you to be
   certain that no information is lost even when unknown types are
   encountered.

   Chapter 6 gives the core mappings for various body parts.

   The conformance requirements in chapter 8 describe what the minimum
   conformance for a MIXER gateway is with respect to body part
   conversion.

   DISCUSSION:

   At the moment both the MIME and the X.400 worlds seem to be in a
   stable state of flux with regards to carrying around stuff that is
   not text.  In such a situation, there is little chance of defining a
   mapping between them that is the best for all people, all of the
   time.  For this reason, this specification allows a gateway
   considerable latitude in deciding exactly what conversion to apply.

   The decision taken by the gateway may be based on various information
   sources:

   (1)   If the gateway knows what body parts or content
         types the recipient is able to handle, or has
         registered a particular set of preferences for a
         user, and knows how to convert the message
         reasonably to those body parts, the gateway may
         choose to convert body parts in the message to
         those types only.

   (2)   If the gateway gets indications (via special
         headers or heading-extensions defined for the
         purpose) that the sender wanted a particular
         representation on the "other side", and the gateway
         is able to satisfy the request, it may do so. Such
         a mechanism is defined in chapter 4 of this
         document.

   (3)   If the gateway gets a message that might be
         appropriate to send as one out of several types,
         but where the typing information does not tell you
         which one to use (like an X.400 BP14, FTAM "just a
         file", or MIME application/octet-stream), it may
         apply heuristics like looking at content or looking
         at filenames to figure out how to deal with the
         message.

   (4)   If the gateway knows that the next hop for the
         message has limited capabilities (like X.400/84),
         it may choose to perform conversions appropriate
         for that medium.

   (5)   Where no mapping is known by  the  gateway,  it
         may  choose  to  drop the body part, reject the
         message, or encapsulate the body  part  as
         described  in  chapter 3.  The choice may be
         configurable, but a conformant MIXER gateway  MUST
         be able to be configured for encapsulation.

   In many cases, a message that goes SMTP->X.400->SMTP will arrive
   without loss of information.

   In some cases, the reverse translation may not be possible, or two
   gateways may choose to apply different translations, based on the
   criteria above, leading to an apparently inconsistent service.

   In addition, service will vary because some gateways will have
   implemented conversions not implemented by other gateways.

   This is believed to be unavoidable.

2.1.  Generating the IPM Body from MIME

   When converting the body of a message from MIME to X.400, the
   following steps are taken:

   If the header does not contain a 822.MIME-Version field, then
   generate an IPMS.Body with a single IPMS.BodyPart of type
   IPMS.IA5TextBodyPart containing the body of the RFC 822 message with
   IPMS.IA5TextBodyPart.parameters.repertoire set to the default (IA5).

   If 822.MIME-Version is present, the body is analyzed as a MIME
   message and the body is converted according to the mappings
   configured and implemented in the gateway.

2.2.  Generating the MIME Body from the IPMS.Body

   When converting the body of a message from X.400 to MIME, the
   following steps are taken:

   If there is more than one body part, and the first body part is IA5
   and starts with the string "RFC-822-Headers:"  as the first line,
   then the remainder of this body part shall be appended to the RFC 822
   header.  This relies upon the theory that this body part has been
   generated according to Appendix B of MIXER.  A gateway shall check
   the consistency and syntax of this body part, to ensure that the
   resulting message is conformant with RFC 822.

   If the remaining IPMS.Body consists of a single IPMS.Bodypart, there
   are three possibilities.

   (1)   If it is of type IPMS.IA5Text, and the first line
         is "MIME-Version: 1.0", it is assumed to be a
         HARPOON-encapsulated body part. The complete body
         content is then appended to the headers; the
         separating blank line is inside the message. If an
         RFC 822 syntax error is discovered inside the
         message, it may be mapped directly as described
         below instead.

   (2)   If it is of type IPMS.IA5Text, then this is mapped
         directly and the default MIME encoding (7bit) is
         used, unless very long lines or non-ASCII or
         control characters are found in the body part, in
         which case Quoted-Printable SHOULD be used.

   (3)   All other types are mapped according to the
         mappings configured and implemented in the gateway.

   If the IPMS.Body contains multiple IPMS.Bodypart fields, then a MIME
   message of content type multipart is generated.  If all of the body
   parts are messages, then this is multipart/digest.  Otherwise it is
   multipart/mixed.  The components of the multipart are generated in
   the same order as in the IPMS.Body.

   Each component is mapped according to the mappings configured and
   implemented in the gateway; any IA5 body parts are checked to see if
   they are HARPOON mappings, as described above.

2.3.  Mapping the EMA FTBP parameters

   DISCUSSION:

   EMA has defined a profile for use of the File Transfer Body Part
   (FTBP). [MAWG]

   New mappings are expected to use this as the mechanism for carrying
   body parts, and since it is important to have a consistent mapping
   for the special FTBP parameters, these are defined here.

   The mapping of the body will depend on the content-type in MIME and
   on the application-reference in FTBP, and is not specified here.

   However, in many cases, we expect that the translation will involve
   simply copying the octets from one format to the other; that is, "no
   conversion".

2.3.1.  Mapping GraphicStrings

   Some parameters of the EMA Profile are encoded as ASN.1
   GraphicStrings, which are troublesome because they can contain any
   ISO registered graphic character set.  To map these to ASCII for use
   in mail headers, the gateway may either:

     (1)   Use the RFC 2047 [MIME-HDR] encoding mechanism to
           create appropriate encoded-words for the headers
           involved. Note that in some cases, such as within
           Content-Disposition filenames, the encoded-words
           must be in quotes, which is not the normal usage of
           encoded-words.

     (2)   Apply the normalization procedure given in Appendix
           A to identify the ASCII characters of the string,
           and replace all non-ASCII characters with the
           question mark (?).

   Both procedures are valid for MIXER gateways; the simplified
   procedure of ignoring escape sequences and bit-stripping the result
   is NOT valid.

2.3.2.  Mapping specific parameters

   The following parameters are mapped in both directions:

   Content-ID

      The mapping of this element is complex.

      The Content-ID is encoded as an IPM.MessageIdentifier and entered
      into the FTBP.FileTransferParameters.related-stored-file.  file-
      identifier.cross-reference.message-reference.

      FTBP.FileTransferParameters.related-stored-file.
      relationship.descriptive-relationship is set to the string
      "Internet MIME Body Part".

      FTBP.FileTransferParameters.related-stored-file.  file-
      identifier.cross-reference.application-crossreference is set to a
      null OCTET STRING.

      The reverse mapping is only performed if the
      FTBP.FileTransferParameters.related-stored-file.
      relationship.descriptive-relationship has the string value
      "Internet MIME Body Part".

   Content-Description

      The value of this field is mapped to and from the first string in
      FTBP.FileTransferParameters.environment.user-visible-string.

   Content-Disposition

      This field is defined in [CDISP]. It has multiple components; the
      handling of each component is given below.

      The "disposition" component is ignored on MIME -> X.400 mapping,
      and is always "attachment" on X.400 -> MIME mapping.

   C-D: filename

      The filename component of the C-D header is mapped to and from
      FileTransferParameters.file-attributes.pathname.

      The EBNF.disposition-type is ignored when creating the FTBP
      pathname, and always set to "attachment" when creating the
      Content-Disposition header.  For example:

         Content-Disposition: attachment; filename=dodo.doc

      or

         Content-Disposition: attachment; filename=/etc/passwd

      The filename will be carried as a single incomplete-pathname
      string.  No special significance is assumed for the characters "/"
      and "\".  Note that normal security precautions MUST be taken in
      using a filename on a local file system; this should be obvious
      from the second example.

      This is done to be conformant with the EMA Profile.

   C-D: Creation-date

      Mapped to and from FileTransferParameters.file-attributes.date-
      and-time-of-creation

      For this and all other date fields, the RFC-822 date format is
      used (822.date-time). Note that the parameter syntax of [CDISP]
      requires that all dates be quoted!

   C-D: Modification-date

      Mapped to and from FileTransferParameters.file-attributes.date-
      and-time-of-last-modification

   C-D: Read-date

      Mapped to and from FileTransferParameters.file-attributes.date-
      and-time-of-last-read-access

   C-D: Size

      Mapped to and from FileTransferParameters.file-attributes.object-
      size.  If the value is "no-value-available", the component is NOT
      generated.

   Other RFC-822 headers

      Mapped to extension in FTBP.FileTransferParameters.extensions
      using the rfc-822-field HEADING-EXTENSION from [MIXER].

   NOTE:
      The set of headers that are mapped will depend on the placement of
      the body part (single body part or multipart).
      When it is the only body of a message, headers starting with
      "content-" SHOULD be put into the FTAM extension, and all other
      headers should be put into the IPMS extension for the message.
      When it is a single bodypart of a multipart, ALL headers on the
      body part are included, since there is nowhere else to put them.
      Note that only headers that start with "content-" have defined
      semantics in this case.

   EMA NOTE

      The EMA profile, version 1.5, specifies that handling of
      extensions is Optional for reception. This means that some non-
      MIXER gateways may not implement handling of this field, and some
      UAs may not have the possibility of showing the content of this
      field to the user.

      An alternative approach using
      FTBP.FileTransferParameters.environment.user-visible-string was
      suggested to EMA, and the EMA MAWG recommended in its April 1996
      conference that the IETF MIXER group should rather choose this
      approach.

2.3.3.  Summary of FTBP elements generated

   This is a summary of the preceding section, and does not add new
   information.

   The following elements of the FTBP parameters are mapped or used (the
   rightmost column gives their status in the EMA profile; M=Mandatory,
   O=Optional, R=Recommended for Origination/Receipt):

FileTransferParameters                                             M/M
  Related-Stored-File                                              O/O
    file-identifier
      cross-reference
        application-crossreference         NULL
        message-reference                  Content-ID
    descriptive-relationship               Used as marker
  contents-type                    Must be unstructured-binary     M/M
  environment                                                      M/M
    application-reference                  Selects mapping         M/M
    user-visible-string                    Content-description     R/M
  file-attributes
    pathname                               C-D: Filename           R/M
    date-and-time-of-creation              C-D: Creation-Date      O/O
    date-and-time-of-last-modification     C-D: Modification-Date  R/M
    date-and-time-of-last-read-access      C-D: Read-Date          O/O
    object-size                            C-D: Size               R/M
  extensions                     Other headers       O/O

   All other elements of the FTBP parameters are discarded.

   NOTE: There is ongoing work on defining a more complete
   mapping between FTBP headers and a set of RFC-822 headers.
   A gateway MAY choose to support the larger set once it is
   available, but MUST support this limited set.

2.4.  Information that is lost when mapping

   MIME defines fields which add information to MIME contents.  Two of
   these are "Content-ID", and "Content-Description", which have special
   rules here, but MIME allows new fields to be defined at any time.

   The possibilities are limited about what one can do with this
   information:

   (1)   When using encapsulation, the information can be
         preserved

   (2)   When using mapping to FTBP, the information can be
         preserved in the FileTransferParameters.extensions
         defined for that purpose.

   (3)   When mapping to a single-body message, the
         information can be preserved as P22 header
         extensions

   (4)   When mapping to other body part types, the
         information must be discarded.

3.  Encapsulation of body parts

   Where no mapping is possible, the gateway may choose any of the
   following alternatives:

   -    Discard the body part, leaving a "marker" saying what
        happened

   -    Reject the message

   -    "Encapsulate" the body part, by wrapping it in a body
        part defined for that purpose in the other mail
        system

   The choice to be made should be configurable in the gateway, and may
   depend on both policy and knowledge of the recipient's capabilities.

3.1.  Encapsulation of MIME in X.400

   Four body parts are defined here to encapsulate MIME body parts in
   X.400.

   This externally-defined body part is backwards compatible with RFC
   1494. The FTBP body part is compatible with the EMA MAWG document
   [MAWG], version 1.5, but has some extensions, in particular the one
   for extra headers.

   The imagined scenarios for each body part are:

   FTBP For use when sending to recipients that can handle
        generic FTBP, and for tunnelling MIME to a MIME UA

   BP15 For use when tunnelling MIME to a MIME UA through an
        X.400(88) network, or to UAs that have been written
        to RFC 1494

   IA5  For use when tunneling MIME to a MIME UA through an
        X.400 network, where some of the links may involve
        X.400(84).

   BP14 For use when the recipient may be an X.400(84) UA
        with BP14 handling capability, and the loss of
        information in headers is not regarded as important.

   but the gateway is free to use any method it finds appropriate in any
   situation.

   FTBP is expected to be the most useful body part in sending to
   X.400(92) systems, while the BP14 content passing is primarily useful
   for sending to X.400(84) systems.

3.1.1.  FTBP encapsulating body part

   This body part utilizes the fundamental assumption in MIME that all
   message content can be legally and completely represented by a single
   octet stream, the "canonical format".

   The FTBP encapsulating body part is defined by the application-
   reference id-mime-ftbp-data; all headers are mapped to the FTBP
   headers, including putting the "Content-type:" header inside the FTBP
   ExtensionsField.

   Translation from the MIME body part is done by:

   -    Undoing the content-transfer-encoding

   -    Setting the "FileTransferData.FTdata.value.octet-
        aligned" to the resulting string of octets

   -    Putting the appropriate parameters into the headers.

   Reversing the translation is done by:

   -    Extracting the headers

   -    Applying an appropriate content-transfer-encoding to
        the body. If this is for some reason different from
        the content-transfer-encoding: header retrieved from
        the headers, the old one must be deleted.

   This mapping is lossless, and therefore counts as "no conversion".

   Note that this mapping does not work with multipart types; the
   multipart must first be mapped to a ForwardedIPMessage.

3.1.2.  BP15 encapsulating body part

   This section defines an extended body part, based on body part 15,
   which may be used to hold any MIME content.

    mime-body-part EXTENDED-BODY-PART-TYPE
          PARAMETERS MimeParameters
                   IDENTIFIED BY id-mime-bp-parameters
          DATA            OCTET STRING
          ::= id-mime-bp-data

    MimeParameters ::=
          SEQUENCE {
                     content-type       IA5String,
                     content-parameters SEQUENCE OF
                                        SEQUENCE {
                                            parameter          IA5String
                                            parameter-value    IA5String
                                        }
                     other-header-fields RFC822FieldList
                 }

   The OBJECT IDENTIFIERS id-mime-bp-parameter and id-mime-bp-data are
   defined in Appendix B.  A MIME content is mapped onto this body part.
   The MIME headers of the body part are mapped as follows:

   RFC822FieldList is defined in Appendix L of [MIXER].

   Content-Type:
        The "type/subtype" string is mapped to
        MimeParameters.content-type.

        For each "parameter=value" string create a
        MimeParameters.content-parameters element. The
        MimeParameters.content-Parameters.parameter field is
        set to the parameter and the MimeParameters.content-
        parameters.parameter-value field is set to the value.

        Quoting is preserved in the parameter-value.

    Other
        Take all other headers and create
        MimeParameters.other-header-fields.
        The MIME-version, content-type and content-transfer-
        encoding fields are NOT copied.

   NOTE:
        The set of headers that are mapped will depend on the
        placement of the body part (single body part or
        multipart).
        When it is the only body of a message, headers
        starting with "content-" SHOULD be put into the
        other-header-fields, and all other headers should be
        put into the IPMS extension for the message.
        When it is a single bodypart of a multipart, ALL
        headers on the body part are included, since there is
        nowhere else to put them. Note that only headers that
        start with "content-" have defined semantics in this
        case.

   The body is mapped as follows:

   Convert the MIME body part into its canonical form, as specified in
   Appendix H of MIME [MIME].  This canonical form is used to generate
   the mime-body-part.data octet string.

   The Parameter mapping may be used independently of the body part
   mapping (e.g., in order to use a different encoding for a mapped MIME
   body part).

   This body part contains all of the MIME information, and so can be
   mapped back to MIME without loss of information.

   The OID id-mime-bp-data is added to the Encoded Information Types of
   the envelope.

   This body part is completely compatible with RFC 1494.

   When converting back to a MIME body part, the gateway is responsible
   for:

   (1)   Selecting an appropriate content-transfer-encoding,
         and deleting any content-transfer-encoding header
         from the other-header-fields

   (2)   Adding quotes to any parameters that need them (but
         not adding quotes to parameters that are already
         quoted)

   (3)   Removing any content-type field that is left in the
         RFC822FieldList of the message that is redundant or
         conflicting with the one from the mime-body-part

   (4)   Make sure that on multipart messages, the boundary
         string actually used is reflected in the boundary-
         parameter of the content-type header, and does not
         occur within the body of the message.

3.1.3.  Encapsulation using IA5 (HARPOON)

   This approach is the one taken in RFC 1496 - HARPOON - for tunneling
   any MIME body part through X.400/84 networks. It has proven rather
   unhelpful for bringing information to X.400 users, but preserves all
   the information of a MIME body part.

   The following IA5Text body part is made:

   -    Content = IA5String

   -    First bytes of content: (the description is in US
        ASCII, 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: <7bit, quoted-printable or base64>\r\n
        <Possibly other Content headings here, terminated by\r\n>
         \r\n
        <Here follows the bytes of the content, encoded
         in the proper encoding>

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

   This encapsulation may also be applied to subtypes of multipart,
   creating a single IA5 body part that contains a single multipart/*,
   which in turn may contain multiple MIME body parts.

3.1.4.  Content passing using BP14

   This is described in this section because it is at the same
   conceptual level as encapsulation. It is a lossy transformation; it
   is impossible to reconstruct the MIME type information from it.

   Nevertheless, there is a demand for such functionality.

   This "encapsulation" simply strips off all headers, undoes the
   content-transfer-encoding, and creates a BilaterallyDefined body part
   (BP14) from the resulting octet stream.

   No reverse translation is defined; when a BP14 arrives at a MIXER
   gateway, it will be turned into an application/octet-stream according
   to chap. 6.3

3.2.  Encapsulating X.400 Body Parts in MIME

   This section specifies a generic mechanism to map X.400 body parts to
   a MIME content.  This allows for the body part to be tunneled through
   MIME.   It may also be used directly by an appropriately configured
   MIME UA.

   This content-type is defined to carry any X.400 extended body part.
   The mapping of all standard X.400 body parts is defined in this
   document.  The content-type field is "application/x400-bp".  The
   parameter is defined by the EBNF:

       mime-parameter =  "bp-type=" ( object-identifier / 1*DIGIT=

   If the body is a basic body part, the bp-type parameter is set to the
   number of the body part's context-specific tag, that is, the tag of
   the IPMS.Body.BodyPart component.

   If the body is an Extended Body Part, the EBNF.object-dentifier is
   set to the OBJECT IDENTIFIER from IPMS.body.externally-
   defined.data.direct-reference.

   For example, a basic VideotexBodyPart will have

      Content-type=application/x400-bp; bp-type=6

   whilst a Extended Videotex body part will have

      Content-type=application/x400-bp; bp-type=2.6.1.4.5

   The body contains the raw ASN.1 IPM body octet stream, that is, the
   BER encoding of the IPM.Body.BodyPart, including the initial tag
   octet.  The content may use a content-transfer-encoding of either
   base64 or quoted-printable when carried in 7-bit MIME.  It is
   recommended to use the one which gives the more compact encoding of
   the data.  If this cannot be determined, Base64 is recommended.  No
   attempt is made to turn the parameters of Extended Body Parts into
   MIME parameters, as this cannot be done in a general manner.

   For extended body parts, the

3.3.  Encapsulating FTBP body parts in MIME

   The File Transfer Body Part is believed to be important in the future
   as "the" means of carrying well-identified data in X.400 networks.

   They also share the property (at lest when limited to the EMA MAWG
   functional profile) of having a well-defined data part that is always
   representable as a sequence of bytes.

   This conversion will have to fail, and the x400-bp encapsulation used
   instead, if:

   -    FileTransferData has more than one element

   -    Contents-type is not unstructured-binary

   -    Parameters that are not mappable, but important, are
        present (like Compression, which EMA doesn't
        recommend).

   Otherwise, it can be encapsulated in MIME by:

   -    Creating the "content-type" value by forming the
        string "application/x-ftbp." and appending the
        numbers of the OID found in
        FileTransferParameters.environment.application-
        reference.registered-identifier

   -    Mapping all other parameters according to the
        standard FTBP parameter mapping

   -    Applying an appropriate content-transfer-encoding to
        the data contained in FileTransferData.value.encoding

   DISCUSSION:

   The choice of the somewhat strange, and by necessity unregistered,
   MIME type "application/x-ftbp.n.n.n.n" is because for any concrete
   example of this usage, it will be easy to configure any MIME reader
   to take advantage of the identification. If the MIME type
   registration rules are ever changed to allow the registration of a
   namespace, rather than just of names, the "x-" can be deleted, and
   the types can be "application/ftbp.n.n.n.n".

4.  User control over the gateway choice

   In some cases, the gateway may make an inappropriate choice when
   deciding what to do about a particular body part.

   To allow an escape clause, this chapter defines a way in which the
   user can signal the gateway what action it finds most appropriate.

   The headers given here override any "conversion prohibited" and
   "conversion with loss prohibited" on the message.

   It is still the gateway's responsibility that the generated messages
   conform to the destination domain's syntax rules.

   DISCUSSION:

   The intent of this mechanism is to allow the sender to efficiently
   get a message through to a single recipient when the sender has
   information about the recipient that the gateway does not have.

   It is not a part of the minimum functionality listed in chapter 8; a
   gateway does not have to implement this spec to be MIXER conformant,
   but if implemented, it should be done like this.

   The additional complexity, both in user interface and in protocol, of
   making this field selectable per recipient was not thought
   worthwhile;

4.1.  Conversion from MIME to X.400

   The header field described below specifies explicit MIXER conversion.
   Comments are allowed within the field according to the usual RFC 822
   convention.

   If "x400-object-id" is omitted, "tunnel" is assumed.

      mime-to-x400 = "Wanted-X400-Conversion" ":"
                      [ mime-from ]  [ x400-object-id ]

                      "in" x400-encoding

      x400-object-id =  "to" ( object-identifier-2 / "tunnel" )
      x400-encoding = "bp14" / "bp15" / "ftbp" / "ia5"
      mime-from = "from" mime-type
      mime-type = word

   There is no way to ask for a different conversion based on MIME
   parameters or bodypart content.

   Examples:

   Wanted-X400-Conversion: from application/msword
                   to 1.2.840.113556.4.2 (Microsoft defined ms-word)
                   in ftbp

   This uses the MAWG definitions, and leads to an FTBP encoding.

   Wanted-X400-Conversion: from application/msword
                  to tunnel in bp14

   This leads to a Body Part 14 encoding for all body parts of type
   application/msword.

   Wanted-X400-Conversion: in bp14

   This requests that this specific body part be encoded in Body Part
   14.

   This field may be used in two places:

      (1)   In the heading of an unstructured MIME body part.
            In this case the EBNF.mime-from is omitted, and the
            requested conversion applies to the body part.

      (2)   In a multipart. In this case, the body part type to
            which the conversion applies is defined by
            EBNF.mime-from, and the conversion applies to all
            body parts of this MIME type contained in the
            multipart, including those contained in nested
            messages and multiparts. If a contained body part
            has its own heading, this takes precedence. Note
            that the "from" parameter is mandatory when used in
            a multipart.

   The EBNF.x400-object-id shall be present when "bp15" or
   "ftbp" encoding is selected.

   The value "tunnel" implies encapsulation as defined in
   Chapter 3.

   The "object identifier" used below is:

   -    For BP 15, it is the value of the EXTENDED-BODY-PART-
        TYPE macro that defines the body part, which is found
        in ExternallyDefinedBodyPart.data.direct-reference.

   -    For FTBP, it is the value of the
        Environment.application-reference.

4.2.  Conversion from X.400 to MIME

   The IPM heading defined here shall be present in the heading of a
   message. It defines the mapping for all body parts of the specified
   types, including those in nested messages.

   wanted-MIME-conversion HEADING-EXTENSION
           VALUE WantedMIMEConversions
           ::= id-wanted-MIME-conversions

   WantedMIMEConversions ::= SEQUENCE OF X400toMIMEConversion

   X400toMIMEConversion ::= SEQUENCE {
           x400-type X400Type,
           mime-type MIMEType }

   X400Type ::= CHOICE {
           standard [0] INTEGER,           -- standard body part
           extended [1] OBJECT IDENTIFIER,  -- BP 15
           ftbp     [2] OBJECT IDENTIFIER}     -- FTBP
                                               -- application-reference

   MIMEType ::= SEQUENCE {
           type IA5String,         -- type (e.g., application/ms-word)
           encoding [1] IA5String OPTIONAl -- e.g. quoted-printable
           parameters [2] IA5String OPTIONAL }     -- MIME Parameters

   The heading extension includes all requested conversions, with
   explicit information as to how each body part type is encoded in
   MIME.

   FTBP is identified as a separate body part type, as there will be a
   need for different encodings, dependent on what is being carried.

   Encapsulation is requested by asking for "application/x400-bp" or
   "application/ftbp" as the destination type.

   For FTAM body parts, the parameters will survive the gatewaying
   process. For other body parts, there are three alternatives:

      (1)   The gateway knows a defined mapping for this
           particular body part and destination type. It will be used,
           and parameters mapped accordingly.

      (2)   The gateway knows how to extract an OCTET STRING
           from the body part, and the destination is a simple MIME body
           part. All information outside the OCTET STRING is lost. (This
           may be the case for a BP14 that should end up in an
           application/xyzzy, for instance).

      (3)   The gateway knows of no relevant mapping, and does
           not know how to simplify the X.400 body part. The gateway
           will then proceed as if the mapping control field had not
           been present.

5.  The equivalence registry

5.1.  What information one must give about a mapping

   The following information MUST be supplied when describing an
   equivalence or a mapping:

   MIME type name (which must be preregistered)

   X.400 body part (often BP15 or FTAM Body Part)

   If BP15 is used, the following information must be given:

      (1)   Object Identifier for X.400 BP15 Data

      (2)   Object Identifier for X.400 BP15 Parameters

      (3)   X.400 ASN.1 Syntax (must be an EXTENDED-BODY-PART-
           TYPE macro)

   If FTBP is used, the following information must be given:

      <1)   Object Identifier for the FTAM Environment.application-
      reference

      <2)   Object Identifier for the FTAM Contents-type, if
           unstructured-binary is not used

      (3)   Any other special considerations

   In all cases, the following must be given:

   Conversion algorithms. The expected effect of "Conversion prohibited"
   and "Conversion with loss prohibited" should be noted.

   The conversion must be specified with enough detail to permit
   independent implementation; literature references are acceptable.

   An equivalence can be registered with IANA using the form at the end
   of this document. The purpose of the registration is to achieve a
   greater uniformity among gateways implementing the same translation;
   there is no requirement that a gateway must support all of the
   translations that are registered with IANA, and there is no
   requirement that all conversions supported by a gateway are
   registered with IANA. Specific conformance requirements for MIXER are
   given at the end of this document.

   Anyone can register an equivalence with IANA, and may update the
   registered equivalence at any time, or reassign the right to update
   the registry entry at any time.  However, the IESG has the power to
   "lock" a registration, so that changing it requires IESG approval,
   and to update such a "locked" registration. All registered
   equivalences defined in standards-track documents (including this
   one) are locked.

5.2.  Equivalence summary for known X.400 and MIME Types

   This section itemizes the equivalences for all currently known MIME
   content-types and X.400 body parts.

   For each MIME content-type/X.400 body part pair, the equivalence
   table contains an entry with the following sections:

      X.400 Body Part
         This section identifies the X.400 Body Part governed by this
         Table entry. It includes any OBJECT IDENTIFIERs or other
         parameters necessary to uniquely identify the Body Part.

      MIME Content-Type
         This section identifies the MIME content-type governed by this
         Table entry.  The MIME content-type named here must be
         registered with the IANA.

      Section/document reference
         Reference to section of this document, or to the other document
         that describes this mapping.

   The initial Equivalence Table entries in this document are described
   using this convention.

   Further registrations of equivalences should be submitted to the IANA
   after a public review, using the example form given at the end of
   this document.

5.3.  MIME to X.400 Table

   MIME content-type          X.400 Body Part             Section
   -----------------          ------------------          -------
   text/plain
     charset=us-ascii         ia5-text                     6.1
     charset=ISO-8859-x       EBP - GeneralText            6.2
   text/richtext              no mapping defined           Encap
   application/oda            EBP - ODA                    [ODA]
   application/octet-stream   bilaterally-defined or       6.3
                              FTBP unknown attachment      6.4
   application/postscript     EBP - mime-postscript-body   [POSTSCRIPT]
   image/g3fax                g3-facsimile                 [IMAGES]
   image/jpeg                 EBP - mime-jpeg-body         [IMAGES]
   image/gif                  EBP - mime-gif-body          [IMAGES]
   audio/basic                no mapping defined           Encap
   video/mpeg                 no mapping defined           Encap
   message/RFC822             ForwardedIPMessage           6.5
   multipart/*                ForwardedIPMessage           6.6
   multipart/signed           HARPOON encap                7.3
   multipart/encrypted        HARPOON encap                7.4

   Abbreviation: EBP - Extended Body Part

5.4.  X.400 to MIME Table

                             Basic Body Parts

   X.400 Basic Body Part      MIME content-type           Section
   ---------------------      --------------------        -------
   ia5-text                   text/plain;charset=us-ascii 6.1
   voice                      No Mapping Defined          Encap
   g3-facsimile               image/g3fax                 [IMAGES]
   g4-class1                  no mapping defined          Encap
   teletex                    text/plain;charset=teletex  6.7
   videotex                   no mapping defined          Encap
   encrypted                  no mapping defined          Encap
   bilaterally-defined        application/octet-stream    6.3
   nationally-defined         no mapping defined          Encap
   externally-defined         See Extended Body Parts below

   ForwardedIPMessage         message/RFC822 or multipart 6.5,6.6

   X.400 Extended Body Part   MIME content-type             Section
   -------------------------  --------------------          -------
   GeneralText                text/plain;charset=ISO-8859-x  6.2
   ODA                        application/oda               [ODA]
   mime-postscript-body       application/postscript        [POSTSCRIPT]
   mime-jpeg-body             image/jpeg                    [IMAGES]
   mime-gif-body              image/gif                     [IMAGES]
   FTAM                       various                       2.3,6.4

   FTAM application ID        MIME content type              Section
   -------------------        -----------------              -------
   ema-unknown-attachment     application/octet-stream       6.4

5.5.  Use of OBJECT IDENTIFIERs and ASN.1 MACROS

   When one wants to define new BP15 body parts for use with
   equivalences, it is important to know that X.420 dictates that
   Extended Body Parts shall:

   (1)   use OBJECT IDENTIFIERs (OIDs) to uniquely identify
         the contents, and

   (2)   be defined by using the ASN.1 Macro:

              EXTENDED-BODY-PART-TYPE MACRO::=
              BEGIN
                 TYPE NOTATION  ::= Parameters Data
                 VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)

                 Parameters     ::=  "PARAMETERS" type "IDENTIFIED"
                                     "BY" value(OBJECT IDENTIFIER)
                                   | empty;
                 Data           ::= "DATA" type
              END

   To meet these requirements, this document uses the OID

      mixer

   defined in [MIXER], as the root OID for X.400 Extended Body Parts
   defined for MIME interworking.

   Each Extended Body Part contains Data and optional Parameters, each
   being named by an OID.  To this end, two OID subtrees are defined
   under mixer-bodies, one for Data, and the other for Parameters:

      mixer-bp-data  OBJECT IDENTIFIER ::=
                     { mixer 1 }

      mixer-bp-parameter OBJECT IDENTIFIER ::=
                     { mixer 2 }

   All definitions of extended X.400 body parts submitted to the IANA
   for registration with a mapping must use the Extended Body Part Type
   macro for the definition.  See [IMAGES] for an example.

   Lastly, the IANA will use the mixer-bp-data and mixer-bp-parameter
   OIDs as root OIDs for any new MIME content-type/subtypes that aren't
   otherwise registered in the Equivalence Table.

   NOTE: The ASN.1 for an ExternallyDefinedBodyPart is

      ExternallyDefinedBodyPart ::= SEQUENCE {
        parameters [0] ExternallyDefinedParameters OPTIONAL,
        data           ExternallyDefinedData }

      ExternallyDefinedParameters ::= EXTERNAL

      ExternallyDefinedData ::= EXTERNAL

   The ASN.1 for EXTERNAL is (from X.208):

      EXTERNAL ::= [UNIVERSAL 8] IMPLICIT SEQUENCE
      {direct-reference     OBJECT IDENTIFIER OPTIONAL,
      indirect-reference    INTEGER OPTIONAL,
      data-value-descriptor ObjectDescriptor OPTIONAL,
      encoding CHOICE
        {single-ASN1-type  [0] ANY,
         octet-aligned     [1] IMPLICIT OCTET STRING,
         arbitrary         [2] IMPLICIT BIT STRING}}

      ObjectDescriptor ::= [UNIVERSAL 7] IMPLICIT GraphicString

   There are a bit too many choices here; the common X.400 usage for
   BP15 encoding is to:

   (1)   Always use direct-reference

   (2)   Omit indirect-reference and data-value-descriptor

   (3)   Use the single-ASN1-type encoding only

   Unfortunately, some implementations have chosen to use the octet-
   aligned choice when constructing values where the ASN.1 type is OCTET
   STRING, which of course caused interoperability problems.

   An attempt to specify that X.420 only allowed the single-ASN1-type
   choice in the 1996 versions is still (Sept 1995) being debated in
   ISO; the end result seems to be that all agree in principle that
   single-ASN1-type should be used, but that one has to allow the
   generation of the octet-aligned choice as being conformant.

6.  Defined Equivalences

6.1.  IA5Text - text/plain

   X.400 Body Part: IA5Text MIME Content-type: text/plain; charset=US-
   ASCII Conversion Type: No conversion Comments:

   When mapping from X.400 to MIME, the "repertoire" parameter is
   ignored.

   When mapping from MIME to X.400, the "repertoire" parameter is set to
   IA5 (5).

   NOTE: The MIME Content-type headers are omitted, when mapping from
   X.400 to MIME, if and only if the IA5Text body part is the only body
   part in the IPMS.Body sequence.

   NOTE: IA5Text specifies the "currency" symbol in position 2/4. This
   is converted without comment to the "dollar" symbol, since the author
   of this document has seen many documents in which the position was
   intended to indicate "dollar" while he has not yet seen one in which
   the "currency" symbol is intended.

   (For reference: The T.50 (1988) recommendation, which defines IA5,
   talks about ISO registered set number 2, while ASCII, using the
   "dollar" symbol, is ISO registered set number 6. There are no other
   differences.)

   NOTE: It is not uncommon, though it is a violation of the standard,
   to use 8-bit character sets inside an IA5 body part. Gateways that
   can expect to encounter this situation should consider implementing
   something like the guidance given in RFC 1428 [MIMETRANS],
   "Transition of Internet Mail from just-send-8 to 8-bit SMTP/MIME",
   and generate appropriate charset parameters for the MIME messages
   they generate. This behavior is not required for MIXER conformance,
   since it is only needed when the base standards are violated.

6.2.  GeneralText - text/plain (ISO-8859)

   X.400 Body Part: GeneralText; CharacterSets in
                   6, 14, 42, 87, 100,101,109,110,126,127,138,144,148
   MIME Content-Type: text/plain; charset=ISO-8859-(1-9)
                               or iso-2022-jp
   Conversion Type: Text conversion without character change When
   mapping from X.400 to MIME, the character-set is chosen from the
   table below according to the value of Parameters.CharacterSets. If no
   match is found, and the gateway does not support a conversion, the
   character set shall be encoded as x-iso-nnn-nnn-nnn, where "nnn" is
   the numbers of the Parameters.CharacterSets, sorted in numeric order.

   When mapping from MIME to X.400, GeneralText is an Extended Body
   Part, hence it requires an OID.  The OID for the GeneralText body is
   defined in [MOTIS], part 8, annex D, as {2 6 1 4 11}. The OID for the
   parameters is {2 6 1 11 11}.

   The Parameters.CharacterSets is set from table below according to the
   value of "charset"

   The following table lists the MIME character sets and the
   corresponding ISO registry numbers. If no correspondence is found,
   this conversion fails, and the generic body part approach is used.

   MIME charset    ISO IR numbers          Comment
   -----------------------------------------------
   ISO-8859-1      6, 100                  West European "8-bit ASCII"
   ISO-8859-2      6, 101                  East European
   ISO-8859-3      6, 109                  <regarded as obsolete>
   ISO-8859-4      6, 110                  <regarded as obsolete>
   ISO-8859-5      6, 144                  Cyrillic
   ISO-8859-6      6, 127                  Arabic
   ISO-8859-7      6, 126                  Greek
   ISO-8859-8      6, 138                  Hebrew
   ISO-8859-9      6, 148                  Other Latin-using languages
   ISO-2022-JP     6, 14, 42, 87           Japanese

   When converting from MIME to X.400, generate the correct OIDs for use
   in the message envelope's Encoded Information Types by looking up the
   ISO IR numbers in the above table, and then appending each to the
   id-cs-eit-authority {1 0 10021 7 1 0} OID, generating 2-4 OIDs.

   Similar procedures can be used with other MIME charsets that map to a
   set of ISO character sets.

   The escape sequences to designate and invoke the relevant character
   sets in their proper positions must be added to the front of the
   GeneralText character string.

   For ISO 8859-1, the relevant escape sequence will be:

   ESC 28 42
         ASCII in G0

   ESC 2D 41
         ISO-IR-100 in G1

   ESC 21 41
         High control character set in C1

   ESC 7E
         Locking shift 1 Right

   These escape sequences are removed when converting from GeneralText
   to text/plain.

   Note that new character sets may be defined on both the Internet side
   and the X.400 side; a gateway MAY choose to implement more
   conversions in the same fashion.

   DISCUSSION:

   The conversion of text is a problematic one, and one in which it is
   likely that gateways should be given wide latitude to make decisions
   based upon their knowledge of the user's preferences. The text given
   below is thought to give the best approximation to a gateway
   conforming to current and anticipated usage in the MIME and X.400
   worlds, and is the way recommended when no knowledge of the
   recipient's capabilities exists.

   The lossless changes, such as normalizing escape sequences, can be
   done even when "conversion-prohibited" is set. If "conversion-with-
   loss-prohibited" is set, translation to a character set that is not
   able to encode all characters cannot be done, and the message should
   be non-delivered with an appropriate non-delivery reason.

   The common use of character sets in MIME is somewhat different from
   the rules given by X.400; in particular, it is common in MIME to
   assume that the character sets follow strict rules. For the ISO-
   8859-x character sets, it is assumed that they are designated and
   invoked at the beginning of the text, and that no designation or
   invocation sequences occur within the body of the text.

   The rules for ISO-2022-JP are given in RFC 1468 [2022-JP], and are
   even more particular, using a pure 7-bit encoding in which each line
   of text starts in ASCII.

   Therefore, the text must be "normalized" by going through the whole
   message, using a state machine or similar device to remove or rewrite
   all escape and shift sequences.

   Appendix A gives pseudocode for such a conversion.

   NOTE: In 1988, the GeneralText body part was defined in ISO 10021-8
   [MOTIS], and NOT in the corresponding CCITT recommendation; this was
   added later.  Also, the parameters have been heavily modified; they
   should be a SET OF INTEGER in the currently valid text.  Use the
   latest version of the standard that you can get hold of.

6.3.  BilaterallyDefined - application/octet-stream

   X.400 Body Part: BilaterallyDefined
   MIME Content-Type: Application/Octet-Stream (no parameters)
   Conversion Type: No conversion

   When mapping from MIME to X.400, if there are parameters present in
   the Content-Type: header field, they are removed.

   DISCUSSION:

   The parameters "name" "type" and "conversions" are advisory; name and
   conversions are depreciated in RFC 2046.

   The parameter "padding" changes the interpretation of the last byte
   of the data, but it is deemed better by the WG to delete this
   information than to non-deliver the body part. The "padding"
   parameter is rarely used with MIME.

   Use of BilaterallyDefined Body Parts is specifically deprecated in
   both 1988 and 1992 X.400.  It is retained solely for backward
   compatibility with 1984 systems, and because it is in common use.

6.4.  FTBP EMA Unknown Attachment - application/octet-stream

   X.400 Body Part: FTBP EMA Unknown Attachment
   MIME Content-Type: Application/Octet-Stream
   Conversion Type: No conversion

   The OID for the Unknown Attachment is { joint-iso-ccitt(2)
   country(16) us(840) organization(1) ema(113694) objects(2)
   messaging(2) attachments(1) unknown(1) }, or
   2.16.840.1.113694.2.2.1.1 for short.

   NOTE: Previous EMA drafts gave it as { iso(1) countries(2) usa(840)
   organization (1) ema (113694) objects(2) messaging(2) attachments(1)
   unknown (1)}, or 1.2.840.1.113694.2.2.1.1 for short.

   The parameters for this type must be mapped according to chapter 2.3,
   with the following extensions for the parameters of the
   application/octet-stream:

      If there is no Content-Disposition parameter with a filename, and
      there is a name parameter, the FTBP.FileTransferParameters.File-
      attributes.pathname is generated from this parameter. Note that
      RFC 2046 recommends not using the "name" parameter.

   The "type", "conversions" and "padding" attributes are ignored;
   "type" is for human consumption; "conversions" are discouraged in RFC
   2046.

   The body mapping is just copying the bytes in both directions.

6.5.  MessageBodyPart - message/RFC822

   X.400 body part: MessageBodyPart
   MIME Content-Type: message/RFC822
   Conversion Type: Special

   NOTE: If the headers of the X.400 MessageBodyPart contains the
   "multipart-message" heading extension with the isAMessage bit set
   (either explicitly or implicitly), the mapping should be to
   multipart/* according to section 6.6, below.

   To map an IPMS.MessageBodyPart, the full X.400 -> RFC 822 mapping  is
   recursively applied, to generate an RFC 822 Message.  If present, the
   IPMS.MessageBodyPart.parameters.delivery-envelope is used for the MTS
   Abstract Service Mappings.  If present, the
   IPMS.MessageBodyPart.parameters.delivery-time is mapped to the
   extended RFC 822 field "Delivery-Date:".

   When a message/RFC822 is contained within a MIME message, it is
   mapped to an IPMS.MessageBodyPart according to MIXER.  specification.
   Any mappings that would have been made to the MTS Abstract Service
   are placed in IPMS.MessageBodyPart.parameters.delivery-envelope.

6.6.  MessageBodyPart - multipart/*

   X.400 body part: MessageBodyPart
   MIME Content-Type: multipart/*
   Conversion Type: Special

   NOTE: If the headers of the X.400 MessageBodyPart do not contain the
   "multipart-message" heading extension with the "isAMessage" flag
   FALSE=, the mapping should be to message/RFC822.

   A MIME multipart is a set of content-types and not a message with a
   set of content types. When the multipart is at the outermost MIME
   header, elements of the multipart are mapped directly onto
   IPMS.Bodypart.

   When the MIME multipart is not at the outermost level, it is mapped
   to an IPMS.MessageBodyPart containing an IPMS.Bodypart for each
   element of the multipart.

   When a nested IPMS.Message is generated from a multipart, an
   IPMS.heading shall always be generated.  The only mandatory field is
   the IPMS.Heading.this-IPM message id, which shall be generated by the
   gateway.  An IPMS.Heading.subject field shall also be generated, in
   order to provide useful information to non-MIME capable X.400(88) UAs
   and to all X.400(84) UAs.  The subject field is set as follows
   according to the multipart subtype:

   mixed:
      "Multipart Message"

   alternative:
      "Alternative Body Parts containing the same information"

   digest:
      "Message Digest"

   parallel:
      "Body Parts interpreted in parallel"

   other:
      "Multipart Message (<subtype>)"

   For other types of multipart, the multipart subtype shall be included
   in the subject line.

   For each multipart, the following IPMS.HeadingExtension shall be
   generated, with the value set according to the subtype.

   If the multipart is the outermost multipart, and the subtype is
   "mixed", it may be omitted.

           multipart-message HEADING-EXTENSION
                   VALUE MultipartType
                    ::= id-hex-multipart-message-v2

           MultipartType ::= SEQUENCE {
                         subtype IA5String,
                         isAMessage BOOLEAN DEFAULT TRUE }

   The MultipartType contains the subtype, for example "digest".  If
   this heading is present when mapping from X.400 to MIME, the
   appropriate multipart may be generated.

   The isAMessage flag is needed because of the case where a message
   contains a ForwardedIPMessage, which itself was generated from a MIME
   message that was a Multipart; it is set whenever the multipart is the
   outermost level of nesting inside a Message/RFC822.

   NOTE:
      When downgrading to X.400/84, the content-type SHOULD be
      regenerated from this heading-extension and put into the RFC-822-
      HEADERS extra body part.

   NOTE:
      This definition is different from the one in RFC 1494, because the
      RFC 1494 definition turned out to be insufficient when new
      subtypes of Multipart (like Signed or Related) were defined. That
      is the reason for the "-v2" part of the name of the OID.

      If both the old and the new heading extensions occur on a message,
      a MIXER gateway should give preference to the new one.

6.7.  Teletex - Text/Plain (Teletex)

   X.400 Body Part: Teletex
   MIME Content-Type: text/plain; charset=Teletex
   Conversion Type: Text conversion

   From X.400 to RFC-822, the conversion shall take the bytes
   of all the pages in the "data" part of the
   TeletexBodyPart, add a FF character (0x0C, control-L) to
   each part that does not already end in one, and
   concatenate them together to form the body of the
   Text/Plain.

   The character set shall be "Teletex", which is especially
   registered for this purpose. Its definition is shown in an
   appendix.

   The parameters are discarded.

   From RFC-822 to X.400, the conversion shall split the
   content at each occurrence of the FF character (0x0C),
   delete the character and construct the Teletex body part
   as a SEQUENCE OF TeletexString, as described in X.420(88),
   section 7.3.5

   The TeletexParameters may, but need not, contain the
   number-of-pages component.

   NOTE: It is recommended, but not mandated, that the data
   be converted into a more widespread character set like
   ISO-8859-1 or ISO-2022-JP (if applicable) if possible.
   This will result in the reverse translation giving a
   GeneralText body part, which will have to be dealt with
   appropriately at the X.400/88 to X.400/84 downgrading
   boundary, if possible, but will give a much greater chance
   that the MIME recipient can actually read the message.

   DISCUSSION:

   The Teletex body part is frequently used in X.400(84) to
   send around text with slightly extended character sets
   beyond ASCII.

   Its body consists of a series of "pages", separated by
   ASN.1 representation.  It is important to many people to
   have this mapped into something that is readable to most
   end-users; therefore, it is recommended to map this onto
   Text/Plain; however, since this is not plain text, the
   conversion must be specified.

   Note that the definition of Text/Plain permits only CRLF as a line
   separator; the sequences "CR FF" and "CR LF LF LF.." permitted in
   Teletex must be encoded as Quoted-Printable.

7.  Body parts where encapsulation is recommended

   Some body parts are MIME constructs, and their functionality will be
   severely damaged if they are coerced into an X.400 framework.

   Special care needs to be taken with these; they are described below.

7.1.  message/external-body

   The gateway MUST support the encapsulation of this body part using
   the HARPOON encapsulation (IA5).

   It MAY support some kind of retrieval of the referred object.

   DISCUSSION:

   The message/external-body part points to an object that can be
   retrieved using Internet protocols.

   There are three cases to consider for the recipient's capabilities:

   (1)   The user has no Internet access. In this case, the
         user might be grateful if the gateway fetches the body part and
         inserts it into the message. If the body part is large or
         dynamic, it might not be appropriate.

   (2)   The user has Internet access, but no UA support for
         fetching external-body objects.

   (3)   The user has Internet access and UA support for
         fetching external-body objects, based on an understanding of
         this document.

   Some access-types, like anonymous FTP, are easy to resolve. Others,
   like the Mailserver access-type, are almost impossible to resolve at
   a gateway.

   To support the second case above, the tunneling method chosen is the
   HARPOON encapsulation described in section 3.1.3, using an IA5 body
   part, inserting the string "MIME-Version: 1.0 (generated by gateway)"
   at the beginning of the body part. (The part in parentheses can be
   changed at will).

   This will:

   (1)   Maximize the chance that the user will see the
         message

   (2)   Give the user hints that will enable him to fetch
         the message using other Internet tools

   (3)   Identify the message as a MIME object in a reliable
         fashion, allowing UAs to support the fetching of the object if
         the UA implementor desires.

7.2.  message/partial

   This represents part of a larger message, where it is only possible
   to parse the complete message after getting all the pieces.

   The gateway MUST support the encapsulation of this body part.

   It MAY implement transparent reassembly of the message, but in this
   case, it MUST support a configurable timeout
    for the reassembly, defaulting back to encapsulation.

   DISCUSSION:

   The gateway's choices are:

   (1)   Wait until all the pieces arrive at the gateway,
         reassemble the message, and use normal processing

   (2)   Encapsulate the message, using any encapsulation
         method (BP15, FTAM or HARPOON).

   In some cases, not all pieces will arrive at the gateway; some may
   have been transferred through other gateways due to route changes or
   machine outages; some may have been lost in transit.

7.3.  multipart/signed

   A gateway MUST implement encapsulation of multipart/signed using
   HARPOON.

   The gateway MAY be configured to do other processing, as outlined in
   the discussion below. This is outside the scope of the standard.

   DISCUSSION:

   Gatewaying security is a problem.  The gateway can basically take
   three approaches:

   -    Strip the multipart/signed, leaving the bare body
        part unsecured, possibly with a comment that the signature was
        stripped

   -    Attempt to check the signature and re-signing the
        message using X.400 security functions, then stripping as above

   -    Encapsulate the message. This is the only approach
        that allows end to end security, but requires MIME functionality
        at the recipient.

   -    Replace the message content with multiple body parts,
        containing first an unsecured body part and then the
        encapsulated multipart/signed.

   All these are valid options for a MIXER gateway.

   Note that the encapsulation must use HARPOON, as the signature is
   computed on the ENCODED body part, not on the canonical
   representation, and HARPOON is the only encapsulation that preserves
   the content transfer encoding of the message.

   Note also that all methods except for encapsulation break end-to-end
   security; the recipient can place no more trust in the integrity of
   the message than he can place in the security of the gateway.

7.4.  multipart/encrypted

   A gateway MUST implement encapsulation of multipart/encrypted using
   HARPOON.

   If the implementor chooses to allow other processing at the gateway,
   as outlined below, he/she is advised that there are grave security
   concerns with such a solution, since it violates the general rule of
   keeping decryption keys as close to the user as possible.

   DISCUSSION:

   There are two basic cases for a gateway:

   -    The gateway is trusted with the user's keys. In this
        case, the gateway can decrypt the message, possibly add a note
        that it has done so, and gateway the unencrypted form, possibly
        applying X.400 security functions, and possibly attaching a copy
        of the original, encrypted material for reference.  This does
        nothing to protect the transfer from gateway to recipient,
        unless suitable X.400-native security is applied. It also means
        that the gateway must be part of the user's trusted environment.

   -    The gateway is not trusted with the recipient's keys.
        In this case, encapsulation is the only approach that preserves
        any information at all.

   The valid options for a MIXER gateway are therefore:

   -    Decrypt the body part

   -    Encapsulate the body part

   -    Drop the body part

   The MIXER WG has shown strong preference for the encapsulation
   alternative, and urges anyone who thinks of buying or implementing
   gateway decryption to carefully evaluate this choice in light of the
   company's general security policy.

8.  Conformance requirements

   In order to be called MIXER conformant, a gateway must implement:

   -    Encapsulation of MIME content in the FTBP body part

   -    Encapsulation of X.400 body parts in the x400-bp body
        part

   -    Encapsulation of FTBP body parts in the
        application/x-ftbp.oid body part

   -    Encapsulation of security multiparts using HARPOON

   -    Text/plain <-> IA5Text

   -    Text/plain; charset=iso-8859-* <-> GeneralText

   -    Multipart/* <-> ForwardedIPMessage

   -    message/RFC822 <-> ForwardedIPMessage

   -    application/octet-stream <-> FTBP unknown

   -    application/octet-stream <-> BilaterallyDefined

   -    A configuration choice of which application/octet-
        stream translation to use

   All other parts of this specification MAY be implemented by the
   gateway. If they are implemented at all, they MUST be implemented
   conformant to this specification.

   In this context, a feature is "implemented" in a product if it is
   possible to configure the product in such a way that this feature is
   used. This specification does not restrict the product to only be
   configured in such a fashion.

9.  Security Considerations

   The security issues identified in this memo are:

   (1)   Security implications of using filenames that
        arrive in body part headers (section 2.3.2)

   (2)   Security implications of letting a gateway handle
        encrypted and/or signed content (section 7.3 and 7.4)

   If a gateway fetches message/external-body on behalf of the
   recipient, as described in section 7.1, it may be tricked into
   performing inappropriate actions by malicious senders.

   In addition, all the normal caveats that apply to sending data that
   may contain executable code apply to UAs on both sides of the
   gateway.

10.  Author's Address

   Harald Tveit Alvestrand
   UNINETT
   P.O.box 6883 Elgeseter
   N-7002 Trondheim
   NORWAY

   EMail: Harald.T.Alvestrand@uninett.no

11.  Acknowledgements

   The author wishes to thank all the members of the MIXER WG for their
   valuable input, and in particular (in no particular order):

   Steve Kille, Peter Sylvester, Ned Freed, Julian Onions, Ruth Moulton,
   Keith Moore, Alain Zahm, Urs Eppenberger, Kevin Jordan, Jeroen
   Houttuin, Claudio Allocchio, Colin Robbins, Steven Thomson, Jim
   Craigie, Efifiom Edem, David Wilson, and many others who have been
   active over the long lifetime of this document.

References

   [RFC-822]
      Crocker, D., "Standard for the Format of ARPA Internet Text
      Messages", STD 11, RFC 822, August, 1982.

   [MIME]
      Freed, N. and  N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part Two:  Media Types", RFC 2046, November
      1996.

   [MIME-HDR]
        Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
        Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
        November 1996.

   [HARPOON]
        Alvestrand, H., Romaguera, J., and K. Jordan, "Rules for
        downgrading messages from X.400/88 to X.400/84 when MIME
        content-types are present in the messages", RFC 1496, August
        1993.

   [MIMETRANS]
      Vaudreuil, G., "Transition of Internet Mail from Just-Send-8 to
      8Bit-SMTP/MIME", RFC 1428, February 1993.

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

   [T.4]
      CCITT Recommendation T.4, Standardization of Group 3 Facsimile
      Apparatus for Document Transmission (1988)

   [T.30]
      CCITT Recommendation T.30, Procedures For Document Facsimile
      Transmission in the General Switched Telephone Network (1988)

   [T.411]
      CCITT Recommendation T.411 (1988), Open Document Architecture
      (ODA) and Interchange Format, Introduction and General Principles

   [MOTIS]
      ISO/IEC International Standard 10021, Information technology -
      Text Communication - Message-Oriented Text Interchange Systems
      (MOTIS) (Parts 1 to 8)

   [X.400]
      CCITT, Data Communication Networks - Message Handling Systems -
      Recommendations X.400 - X.420 (1988 version)

   [X.420]
      CCITT Recommendation X.420 (1988), Interpersonal Messaging System

   [RFC-X400USE]
      Alvestrand, H., "X.400 use of extended Character Sets", RFC 1502,
      August 1993.

   [MAWG]
      Electronic Messaging Association Message Attachment Working Group
      (MAWG): File Transfer Body Part Feasibility Project Guide -
      version 1.5 - September 1995

   [CDISP]
      Troost, R., and S. Dorner, "Communicating Presentation Information
      in Internet Messages: The Content-Disposition Header", RFC 1806,
      June 1995.

   [POSTSCRIPT]
      Alvestrand, H., "Carrying PostScript in X.400 and MIME", RFC 2160,
      June 1997.

   [IMAGES]
      Alvestrand, H., "X.400 Image Body Parts", RFC 2158, June 1997.

   [ODA]
      Alvestrand, H., "A MIME Body Part for ODA", RFC 2161, June 1997.

   [ISO 2022]
      ISO/IEC 2022:1994(E): Information technology - Character code
      structure and extension techniques

   [ISO 8859]
      ISO 8859: Information processing -- 8-bit single-byte coded
      graphic character sets (various parts)

   [2022-JP]
      Murai, J., Crispin, M., and E. van der Poel, "Japanese Character
      Encoding for Internet Messages", RFC 1468, June 1993.

   [MUST]
      Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", RFC 2119, March 1997.

APPENDIXES

Appendix A: Escape code normalization

   The algorithm given here in pseudocode will reduce a GeneralString
   ISO-2022 unlimited use of shifts sequence to a pure 8-bit sequence
   that does not use shift sequences, if possible.

   Some error conditions, like EOF, are not tested for. It crashes if
   asked to do something it cannot.  Control character set switching is
   missing.

   A similar routine, albeit more complex, can be written for
   normalizing to the ISO-2022-JP character set.

   BEGIN: (from X.209)
     g0 = 6 (should be 2, but ignore the difference)
     g1 = NULL
     g2 = NULL
     g3 = NULL
     c0 = 1 (ASCII control)
     c1 = NULL
     leftset = &g0 (current input set, low)
     rightset = &g1 (current input set, high)
     lowset = 6 (output set, low)
     highset = NULL (output set, high)
     charset = US-ASCII

     (Init for the set tables)
     chartoid[{2D,2E,2F}, 41] = 100
     .....
     idtoname[100] = "ISO-8859-1"
     .....

   WHILE (more data)
     CASE head of input
       {These are the locking shift sequences}
       INCASE "00/14": (LS0, SO)
           leftset = &g0;
       INCASE "00/15": (LS1, SI)
           leftset = &g
       INCASE "ESC 07/14": (LS1R)
           rightset = &g1;
       INCASE "ESC 07/13": (LS2R)
           rightset = &g2;
       INCASE "ESC 07/12": (LS3R)
           rightset = &g3;
       {There is missing code for handling the single shift function}

       {These are the changes of graphic character sets}
       {Note that G0 can contain only 94-character charsets}
       INCASE "ESC 28"
           g0 = chartoid[lastchar, next character]
           sethiset(g0)
       INCASE "ESC 2D", "ESC 29"
           g1 = chartoid[lastchar, next character]
           sethiset(g1)
       INCASE "ESC 2E", "ESC 2A"
           g2 = chartoid[lastchar, next character]
           sethiset(g2)
       INCASE "ESC 2F", "ESC 2B"
           g3 = chartoid[lastchar, next character]
           sethiset(g3)
       {control characters. There is missing code for changing these}
       INCASE 00/00-01/15 {normal control}
           write(char)
       INCASE 08/00-09/15 {upper control}
           write(char)
       {Normal characters}
       INCASE 02/00-07/15 (Left)
           IF (*leftset == lowset)
               write(char)
           ELSIF (*leftset == highset)
               write(char+80)
           ELSE
               ERROR "Shift error"
           ENDIF
       INCASE 10/00-15/15
           IF (*rightset == highset)
               write(char)
           ELSIF (*rightset == lowset)
               write(char-80)
           ELSE
               ERROR "Shift error"
           ENDIF
     ENDCASE
   ENDWHILE

    SUBROUTINE sethighset(g1)

           IF (highset == NULL)
               charset = idtoname[g1]
               highset = g1
           ELSIF (highset == g1)
               (it's OK)
           ELSE
               ERROR "Too many charsets encountered"

           ENDIF

   ENDROUTINE

Appendix B: OID Assignments

   MIXER-MAPPINGS DEFINITIONS ::= BEGIN
   EXPORTS -- everything --;

   IMPORTS

      mixer -- { iso(1) org(3) dod(6) internet(1) mail(7) mixer(1) }
           FROM MIXER --Companion RFC--;

   mixer-headings OBJECT IDENTIFIER ::=
           { mixer 1 } -- called mime-mhs-headings in RFC 1495 --

   mixer-bodies OBJECT IDENTIFIER ::=
           { mixer 2 } -- called mime-mhs-bodies in RFC 1495 --

   -- mixer-core is defined as { mixer core(3) } in [MIXER]

   mixer-bp-data OBJECT IDENTIFIER ::=
           { mixer-bodies 1 }; -- called mime-mhs-bp-data in RFC 1494 --

   mixer-bp-parameter OBJECT IDENTIFIER ::=
           { mixer-bodies 2 };

   id-mime-bp-data OBJECT IDENTIFIER ::=
           { mixer-bp-data 1 };
   -- for debugging: mixer-bp-data is 1.3.6.1.7.1.2.1.1

   id-mime-bp-parameters OBJECT IDENTIFIER ::=
           { mixer-bp-parameter 1 };

   -- the following assignments were done in RFC 1494, using
   -- slightly different names, but the same numbers.
   -- their defining text is now is now in other documents
      id-mime-postscript-body OBJECT IDENTIFIER ::=
                     { mixer-bp-data 2 }

      id-mime-jpeg-body OBJECT IDENTIFIER ::=
                     { mixer-bp-data 3 }

      id-mime-gif-body OBJECT IDENTIFIER ::=
                     { mixer-bp-data 4 }

   -- This is a new definition, and defines an FTAM application
   reference,
   -- not a BP15 data OID.
   id-mime-ftbp-data OBJECT IDENTIFIER ::=
                      { mixer-bp-data 5 }

   -- The following heading extensions are defined
   id-hex-partial-message OBJECT IDENTIFIER ::=
              { mixer-headings 1 }

   id-hex-multipart-message OBJECT IDENTIFIER ::=
              { mixer-headings 2 } -- from RFC 1495; obsolete

   id-hex-multipart-message-v2 OBJECT IDENTIFIER ::=
           { mixer-headings 3 }

   END

Appendix C: Registration information for the Teletex
            character set

   The Teletex character set is a character set in which the ISO 2022
   character set switching mechanism may be used to switch between the
   following registered ISO character sets:

   ISO-IR-87 - JIS_C6226-1983; a 16-bit Japanese character set
   ISO-IR-102 - a fairly standard US-ASCII variant
   ISO-IR-103 - Latin characters using non-spacing accents
   ISO-IR-106 - Control characters for C0 use; CR, LF, FF and a few more.
   ISO-IR-107 - Control characters for C1 use

   Its intended use of this character set is to represent data that
   comes from ISO protocols that use the ASN.1 construct "TeletexString"
   or "T61string" without conversion.

   The set of allowed character sets can be found in CCITT
   recommendation X.208(1988), chapter 31.2 and Table 6/X.208.

   The rules for encoding the data type can be found in CCITT
   recommendation X.209(1988), chapter 23. It states that at the
   beginning of the string, G0 is always ISO-IR-102, C0 is ISO-IR-106,
   and C1 is ISO-IR-107.

   The specification seems somehow to have missed the implicit
   assumption that ISO-IR-103 is designated and invoked as G1 and
   shifted into the upper half of the character set which seems to be
   assumed at least by the X.400 and X.500 software that uses
   TeletexStrings; implementors should act as if the sequence ESC 2/9
   7/6 LS1R is always present at the beginning of the data; however,
   when generating Teletex strings, implementors should include the
   sequence ESC 2/9 7/6 within the string before the first occurence of
   a character from ISO-IR-103.

   The rules for interpreting T.61 data are found (I believe) in CCITT
   recommendations T.51, T.52 and T.53 (data from the ITU WWW server):

      T.51 (09/92) [Rev.1] [26 pp.] [Publ.: May.93]
        Latin based coded character sets for telematic services
      T.52 (1993) [New] [88 pp.] [Publ.: Apr.94]
        Non-Latin coded character sets for telematic services
      T.53 (04/94) [New] [68 pp.] [Publ.: Jan.95]
        Character coded control functions for telematic services

   The Teletex character set is closely related to (but not identical
   with) that specified in ISO 6937.

   No further restrictions are imposed by this registration; in
   particular, character set switching can occur anywhere, and there is
   no guarantee that the character sets will be switched "back" at the
   end.

   Appendix D: IANA Registration form for new mappings

   To: IANA@isi.edu
   Subject: Registration of new X.400/MIME content type mapping

   MIME type name:

   (this must have been registered previously with IANA)

   X.400 body part:

   IF BP15:

   - X.400 Object Identifier for Data:

   (If left empty, an OID will be assigned by IANA  under mixer-bp-data)

   - X.400 Object Identifier for Parameters:

   (If left empty, an OID will be assigned by IANA under mixer-bp-
   parameter.  If it is not used, fill in the words NOT USED.)

   X.400 ASN.1 Syntax:

   (must be an EXTENDED-BODY-PART-TYPE macro, or reference to a Basic
   body part type)

   IF FTBP:

   - FTAM Object Identifier for application-reference:

   - FTAM Object Identifier for contents-type:

   (if left empty, unstructured-binary is assumed)

   Conversion algorithm:

   (must be defined completely enough for independent implementation. It
   may be defined by reference to RFCs).
   Person & email address to contact for further information:

   INFORMATION TO THE SUBMITTER:

   The accepted registrations will be listed in the "Assigned Numbers"
   series of RFCs.  The information in the registration form is freely
   distributable.

Full Copyright Statement

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