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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

Alvestrand Standards Track [Page 1] RFC 2157 X.400/MIME Body Mapping January 1998

 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.

Alvestrand Standards Track [Page 2] RFC 2157 X.400/MIME Body Mapping January 1998

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

Alvestrand Standards Track [Page 3] RFC 2157 X.400/MIME Body Mapping January 1998

 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
 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
 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
 (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

Alvestrand Standards Track [Page 4] RFC 2157 X.400/MIME Body Mapping January 1998

 (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
 (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.

Alvestrand Standards Track [Page 5] RFC 2157 X.400/MIME Body Mapping January 1998

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.

Alvestrand Standards Track [Page 6] RFC 2157 X.400/MIME Body Mapping January 1998

2.3. Mapping the EMA FTBP parameters

 EMA has defined a profile for use of the File Transfer Body Part
 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

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
   (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:
    The mapping of this element is complex.

Alvestrand Standards Track [Page 7] RFC 2157 X.400/MIME Body Mapping January 1998

    The Content-ID is encoded as an IPM.MessageIdentifier and entered
    into the FTBP.FileTransferParameters.related-stored-file.  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
    relationship.descriptive-relationship has the string value
    "Internet MIME Body Part".
    The value of this field is mapped to and from the first string in
    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
    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
       Content-Disposition: attachment; filename=/etc/passwd

Alvestrand Standards Track [Page 8] RFC 2157 X.400/MIME Body Mapping January 1998

    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
    For this and all other date fields, the RFC-822 date format is
    used ( Note that the parameter syntax of [CDISP]
    requires that all dates be quoted!
 C-D: Modification-date
    Mapped to and from
 C-D: Read-date
    Mapped to and from
 C-D: Size
    Mapped to and from FileTransferParameters.file-attributes.object-
    size.  If the value is "no-value-available", the component is NOT
 Other RFC-822 headers
    Mapped to extension in FTBP.FileTransferParameters.extensions
    using the rfc-822-field HEADING-EXTENSION from [MIXER].
    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.

Alvestrand Standards Track [Page 9] RFC 2157 X.400/MIME Body Mapping January 1998

    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

2.3.3. Summary of FTBP elements generated

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

Alvestrand Standards Track [Page 10] RFC 2157 X.400/MIME Body Mapping January 1998

 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
 (1)   When using encapsulation, the information can be
 (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
 (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:
  1. Discard the body part, leaving a "marker" saying what


  1. Reject the message
  1. "Encapsulate" the body part, by wrapping it in a body

part defined for that purpose in the other mail

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

Alvestrand Standards Track [Page 11] RFC 2157 X.400/MIME Body Mapping January 1998

3.1. Encapsulation of MIME in X.400

 Four body parts are defined here to encapsulate MIME body parts in
 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
 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
 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
 Translation from the MIME body part is done by:
  1. Undoing the content-transfer-encoding

Alvestrand Standards Track [Page 12] RFC 2157 X.400/MIME Body Mapping January 1998

  1. Setting the "FileTransferData.FTdata.value.octet-

aligned" to the resulting string of octets

  1. Putting the appropriate parameters into the headers.
 Reversing the translation is done by:
  1. Extracting the headers
  1. 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].

Alvestrand Standards Track [Page 13] RFC 2157 X.400/MIME Body Mapping January 1998

      The "type/subtype" string is mapped to
      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.
      Take all other headers and create
      The MIME-version, content-type and content-transfer-
      encoding fields are NOT copied.
      The set of headers that are mapped will depend on the
      placement of the body part (single body part or
      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
 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 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.

Alvestrand Standards Track [Page 14] RFC 2157 X.400/MIME Body Mapping January 1998

 This body part is completely compatible with RFC 1494.
 When converting back to a MIME body part, the gateway is responsible
 (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
 (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:
  1. Content = IA5String
  1. 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>
      <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.

Alvestrand Standards Track [Page 15] RFC 2157 X.400/MIME Body Mapping January 1998

 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
 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-
 For example, a basic VideotexBodyPart will have
    Content-type=application/x400-bp; bp-type=6
 whilst a Extended Videotex body part will have

Alvestrand Standards Track [Page 16] RFC 2157 X.400/MIME Body Mapping January 1998

    Content-type=application/x400-bp; bp-type=
 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:
  1. FileTransferData has more than one element
  1. Contents-type is not unstructured-binary
  1. Parameters that are not mappable, but important, are

present (like Compression, which EMA doesn't

 Otherwise, it can be encapsulated in MIME by:
  1. Creating the "content-type" value by forming the

string "application/x-ftbp." and appending the

      numbers of the OID found in
  1. Mapping all other parameters according to the

standard FTBP parameter mapping

  1. Applying an appropriate content-transfer-encoding to

the data contained in FileTransferData.value.encoding

Alvestrand Standards Track [Page 17] RFC 2157 X.400/MIME Body Mapping January 1998

 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.
 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

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
 If "x400-object-id" is omitted, "tunnel" is assumed.
    mime-to-x400 = "Wanted-X400-Conversion" ":"
                    [ mime-from ]  [ x400-object-id ]

Alvestrand Standards Track [Page 18] RFC 2157 X.400/MIME Body Mapping January 1998

                    "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.
 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
 Wanted-X400-Conversion: in bp14
 This requests that this specific body part be encoded in Body Part
 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.

Alvestrand Standards Track [Page 19] RFC 2157 X.400/MIME Body Mapping January 1998

 The value "tunnel" implies encapsulation as defined in
 Chapter 3.
 The "object identifier" used below is:
  1. For BP 15, it is the value of the EXTENDED-BODY-PART-

TYPE macro that defines the body part, which is found

  1. For FTBP, it is the value of the


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

Alvestrand Standards Track [Page 20] RFC 2157 X.400/MIME Body Mapping January 1998

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

Alvestrand Standards Track [Page 21] RFC 2157 X.400/MIME Body Mapping January 1998

    (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.

Alvestrand Standards Track [Page 22] RFC 2157 X.400/MIME Body Mapping January 1998

 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
 -----------------          ------------------          -------
   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

Alvestrand Standards Track [Page 23] RFC 2157 X.400/MIME Body Mapping January 1998

 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


 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:
               TYPE NOTATION  ::= Parameters Data
               Parameters     ::=  "PARAMETERS" type "IDENTIFIED"
                                   "BY" value(OBJECT IDENTIFIER)
                                 | empty;
               Data           ::= "DATA" type
 To meet these requirements, this document uses the OID
 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:

Alvestrand Standards Track [Page 24] RFC 2157 X.400/MIME Body Mapping January 1998

    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):
    {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

Alvestrand Standards Track [Page 25] RFC 2157 X.400/MIME Body Mapping January 1998

 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
 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
 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.

Alvestrand Standards Track [Page 26] RFC 2157 X.400/MIME Body Mapping January 1998

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.

Alvestrand Standards Track [Page 27] RFC 2157 X.400/MIME Body Mapping January 1998

 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
       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.
 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.

Alvestrand Standards Track [Page 28] RFC 2157 X.400/MIME Body Mapping January 1998

 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.
 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

Alvestrand Standards Track [Page 29] RFC 2157 X.400/MIME Body Mapping January 1998

 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. 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. for short.
 The parameters for this type must be mapped according to chapter 2.3,
 with the following extensions for the parameters of the
    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
 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 is used for the MTS
 Abstract Service Mappings.  If present, the 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

Alvestrand Standards Track [Page 30] RFC 2157 X.400/MIME Body Mapping January 1998

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
 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:
    "Multipart Message"
    "Alternative Body Parts containing the same information"
    "Message Digest"
    "Body Parts interpreted in parallel"
    "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.

Alvestrand Standards Track [Page 31] RFC 2157 X.400/MIME Body Mapping January 1998

 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.
    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.
    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

Alvestrand Standards Track [Page 32] RFC 2157 X.400/MIME Body Mapping January 1998

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

Alvestrand Standards Track [Page 33] RFC 2157 X.400/MIME Body Mapping January 1998

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.
 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
 (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.

Alvestrand Standards Track [Page 34] RFC 2157 X.400/MIME Body Mapping January 1998

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.
 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
 The gateway MAY be configured to do other processing, as outlined in
 the discussion below. This is outside the scope of the standard.
 Gatewaying security is a problem.  The gateway can basically take
 three approaches:
  1. Strip the multipart/signed, leaving the bare body

part unsecured, possibly with a comment that the signature was

  1. Attempt to check the signature and re-signing the

message using X.400 security functions, then stripping as above

  1. Encapsulate the message. This is the only approach

that allows end to end security, but requires MIME functionality

      at the recipient.

Alvestrand Standards Track [Page 35] RFC 2157 X.400/MIME Body Mapping January 1998

  1. 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
 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.
 There are two basic cases for a gateway:
  1. 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.
  1. 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:
  1. Decrypt the body part
  1. Encapsulate the body part

Alvestrand Standards Track [Page 36] RFC 2157 X.400/MIME Body Mapping January 1998

  1. 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:
  1. Encapsulation of MIME content in the FTBP body part
  1. Encapsulation of X.400 body parts in the x400-bp body


  1. Encapsulation of FTBP body parts in the

application/x-ftbp.oid body part

  1. Encapsulation of security multiparts using HARPOON
  1. Text/plain ↔ IA5Text
  1. Text/plain; charset=iso-8859-* ↔ GeneralText
  1. Multipart/* ↔ ForwardedIPMessage
  1. message/RFC822 ↔ ForwardedIPMessage
  1. application/octet-stream ↔ FTBP unknown
  1. application/octet-stream ↔ BilaterallyDefined
  1. 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.

Alvestrand Standards Track [Page 37] RFC 2157 X.400/MIME Body Mapping January 1998

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

10. Author's Address

 Harald Tveit Alvestrand
 UNINETT 6883 Elgeseter
 N-7002 Trondheim

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.


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

Alvestrand Standards Track [Page 38] RFC 2157 X.400/MIME Body Mapping January 1998

    Freed, N. and  N. Borenstein, "Multipurpose Internet Mail
    Extensions (MIME) Part Two:  Media Types", RFC 2046, November
      Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
      Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
      November 1996.
      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
    Vaudreuil, G., "Transition of Internet Mail from Just-Send-8 to
    8Bit-SMTP/MIME", RFC 1428, February 1993.
    Kille, S., "Mapping between X.400(1988) / ISO 10021 and RFC-822",
    RFC 1327, May 1992.
    CCITT Recommendation T.4, Standardization of Group 3 Facsimile
    Apparatus for Document Transmission (1988)
    CCITT Recommendation T.30, Procedures For Document Facsimile
    Transmission in the General Switched Telephone Network (1988)
    CCITT Recommendation T.411 (1988), Open Document Architecture
    (ODA) and Interchange Format, Introduction and General Principles
    ISO/IEC International Standard 10021, Information technology -
    Text Communication - Message-Oriented Text Interchange Systems
    (MOTIS) (Parts 1 to 8)
    CCITT, Data Communication Networks - Message Handling Systems -
    Recommendations X.400 - X.420 (1988 version)

Alvestrand Standards Track [Page 39] RFC 2157 X.400/MIME Body Mapping January 1998

    CCITT Recommendation X.420 (1988), Interpersonal Messaging System
    Alvestrand, H., "X.400 use of extended Character Sets", RFC 1502,
    August 1993.
    Electronic Messaging Association Message Attachment Working Group
    (MAWG): File Transfer Body Part Feasibility Project Guide -
    version 1.5 - September 1995
    Troost, R., and S. Dorner, "Communicating Presentation Information
    in Internet Messages: The Content-Disposition Header", RFC 1806,
    June 1995.
    Alvestrand, H., "Carrying PostScript in X.400 and MIME", RFC 2160,
    June 1997.
    Alvestrand, H., "X.400 Image Body Parts", RFC 2158, June 1997.
    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)
    Murai, J., Crispin, M., and E. van der Poel, "Japanese Character
    Encoding for Internet Messages", RFC 1468, June 1993.
    Bradner, S., "Key words for use in RFCs to Indicate Requirement
    Levels", RFC 2119, March 1997.

Alvestrand Standards Track [Page 40] RFC 2157 X.400/MIME Body Mapping January 1998


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
 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}

Alvestrand Standards Track [Page 41] RFC 2157 X.400/MIME Body Mapping January 1998

     {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]
     INCASE "ESC 2D", "ESC 29"
         g1 = chartoid[lastchar, next character]
     INCASE "ESC 2E", "ESC 2A"
         g2 = chartoid[lastchar, next character]
     INCASE "ESC 2F", "ESC 2B"
         g3 = chartoid[lastchar, next character]
     {control characters. There is missing code for changing these}
     INCASE 00/00-01/15 {normal control}
     INCASE 08/00-09/15 {upper control}
     {Normal characters}
     INCASE 02/00-07/15 (Left)
         IF (*leftset == lowset)
         ELSIF (*leftset == highset)
             ERROR "Shift error"
     INCASE 10/00-15/15
         IF (*rightset == highset)
         ELSIF (*rightset == lowset)
             ERROR "Shift error"
  SUBROUTINE sethighset(g1)
         IF (highset == NULL)
             charset = idtoname[g1]
             highset = g1
         ELSIF (highset == g1)
             (it's OK)
             ERROR "Too many charsets encountered"

Alvestrand Standards Track [Page 42] RFC 2157 X.400/MIME Body Mapping January 1998


Alvestrand Standards Track [Page 43] RFC 2157 X.400/MIME Body Mapping January 1998

Appendix B: OID Assignments

 EXPORTS -- everything --;
    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 --
  1. - 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
 id-mime-bp-parameters OBJECT IDENTIFIER ::=
         { mixer-bp-parameter 1 };
  1. - the following assignments were done in RFC 1494, using
  2. - slightly different names, but the same numbers.
  3. - 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 }
  1. - This is a new definition, and defines an FTAM application


  1. - not a BP15 data OID.

id-mime-ftbp-data OBJECT IDENTIFIER ::=

                    { mixer-bp-data 5 }

Alvestrand Standards Track [Page 44] RFC 2157 X.400/MIME Body Mapping January 1998

  1. - 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 }

Alvestrand Standards Track [Page 45] RFC 2157 X.400/MIME Body Mapping January 1998

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.

Alvestrand Standards Track [Page 46] RFC 2157 X.400/MIME Body Mapping January 1998

 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

Alvestrand Standards Track [Page 47] RFC 2157 X.400/MIME Body Mapping January 1998

 Appendix D: IANA Registration form for new mappings
 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:
  1. X.400 Object Identifier for Data:
 (If left empty, an OID will be assigned by IANA  under mixer-bp-data)
  1. 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)
  1. FTAM Object Identifier for application-reference:
  1. 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:
 The accepted registrations will be listed in the "Assigned Numbers"
 series of RFCs.  The information in the registration form is freely

Alvestrand Standards Track [Page 48] RFC 2157 X.400/MIME Body Mapping January 1998

Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an

Alvestrand Standards Track [Page 49]

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