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RFC 1872 - The MIME Multipart/Related Content-type

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Network Working Group                                        E. Levinson
Request for Comments: 1872            Accurate Information Systems, Inc.
Category: Experimental                                     December 1995

                The MIME Multipart/Related Content-type

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.


   The Multipart/Related content-type provides a common mechanism for
   representing objects that are aggregates of related MIME body parts.
   This document defines the Multipart/Related content-type and provides
   examples of its use.

1.  Introduction

   Several applications of MIME, including MIME-PEM, and MIME-Macintosh
   and other proposals, require multiple body parts that make sense only
   in the aggregate.  The present approach to these compound objects has
   been to define specific multipart subtypes for each new object.  In
   keeping with the MIME philosophy of having one mechanism to achieve
   the same goal for different purposes, this document describes a
   single mechanism for such aggregate or compound objects.

   The Multipart/Related content-type addresses the MIME representation
   of compound objects.  The object is categorized by a "type"
   parameter.  Additional parameters are provided to indicate a specific
   starting body part or root and auxiliary information which may be
   required when unpacking or processing the object.

   Responsibility for the display or processing of a Multipart/Related's
   constituent entities rests with the application that handles the
   compound object.

2.  Multipart/Related Registration Information

   The following form is copied from RFC 1590, Appendix A.

   To:  IANA@isi.edu
   Subject:  Registration of new Media Type content-type/subtype

   Media Type name:           Multipart

   Media subtype name:        Related

   Required parameters:       Type, a media type/subtype.

   Optional parameters:       Start, a content-id.
                              Start-info, a string or content-id list.

   Encoding considerations:   Multipart content-types cannot have

   Security considerations:   Depends solely on the referenced type.

   Published specification:   This document.

   Person & email address to contact for further information:
                              Edward Levinson
                              Accurate Information Systems, Inc.
                              2 Industrial Way
                              Eatontown, NJ 07724
                              +1 908 389 5550
                              +1 908 389 5556 (fax)

3.  Intended usage

   The Multipart/Related media type is intended for compound objects
   consisting of several inter-related body parts.  For a
   Multipart/Related object, proper display cannot be achieved by
   individually displaying the constituent body parts.  The content-type
   of the Multipart/Related object is specified by the type parameter.
   The "start" parameter, if given, points, via a content-ID, to the
   body part that contains the object root.  The default root is the
   first body part within the Multipart/Related body.

   The relationships among the body parts of a compound object
   distinguishes it from other object types.  These relationships are
   often represented by links internal to the object's components that
   reference the other components.  Within a single operating
   environment the links are often file names, such links may be

   represented within a MIME message using content-IDs or the value of
   some other "Content-" header.

3.1.  The Type Parameter

   The type parameter must be specified and its value is the MIME media
   type of the root body part.  It permits a MIME user agent to
   determine the content-type without reference to the enclosed body
   part.  If the value of the type parameter and the root body part's
   content-type differ then the User Agent's behavior is undefined.

   Note: Constraining the "type" parameter's value to an existing media
   type allows the appropriate processing to be identified without
   creating yet another hierarchy of registered types.  A possible
   default action would have the MIME mail User Agent (MUA) to display
   the "start" entity alone when it could process the media type as a
   basic type but not as Multipart/Related.

3.2.  The Start Parameter

   The start parameter, if given, is the content-ID of the compound
   object's root.  If not present the root is the first body part in the
   Multipart/Related entity.  The root is the element the application
   processes first.

   In the case of a Multipart/Alternative body part containing several
   entities with identical content-IDs the start entity should be
   selected using the Multipart/Alternative rules.

   Note: The "start" parameter allows for types in which the root
   element gets generated by the sending application, perhaps on the
   fly.  Such an application can create the "start" content-id when
   processing begins and then insert the body part when it is complete.

3.3.  The Start-Info Parameter

   Additional information can be provided to an application by the
   start-info parameter.  It contains either a string or points, via a
   content-ID, to another MIME entity in the message.  A typical use
   might be to provide additional command line parameters or a MIME
   entity giving auxiliary information for processing the compound

   Applications that use Multipart/Related must specify the
   interpretation of start-info.  User Agents shall provide the
   parameter's value to the processing application.  Processes can
   distinguish a start-info reference from a token or quoted-string by
   examining the first non-white-space character, "<" indicates a

   content-id reference.

3.4.  Syntax

     related-param    := [ ";" "start" "=" cid ]
                         [ ";" "start-info"  "="
                           ( cid-list / value ) ]
                         [ ";" "type"  "=" type "/" subtype ]
                         ; order independent

     cid-list        := cid cid-list

     cid             := msg-id     ; c.f. [822]

     value           := token / quoted-string    ; c.f. [MIME]
                             ; value cannot begin with "<"

   Note that the parameter values will usually require quoting.  Msg-id
   contains the special characters "<", ">", "@", and perhaps other
   special characters.  If msg-id contains quoted-strings, those quote
   marks must be escaped.  Similarly, the type parameter contains the
   special character "/".

4.  Examples

4.1 Application/X-FixedRecord

   The X-FixedRecord content-type consists of one or more octet- streams
   and a list of the lengths of each record.  The root, which lists the
   record lengths of each record within the streams.  The record length
   list, type Application/X-FixedRecord, consists of a set of INTEGERs
   in ASCII format, one per line.  Each INTEGER gives the number of
   octets from the octet-stream body part that constitute the next

   The example below, uses a single data block which the sender
   processes on the fly to generate the record length list.
   Consequently the list appears after the data.

     Content-Type: Multipart/Related; boundary=example-1
             start-info="-o ps"

     Content-Type: Application/octet-stream
     Content-Description: The fixed length records
     Content-Transfer-Encoding: base64

     Content-ID: <950120.aaCB@XIson.com>

     Content-Type: Application/X-FixedRecord
     Content-ID: <950120.aaCC@XIson.com>


4.2 Text/X-Okie

The Text/X-Okie is an invented markup language, similar to
HTML, that permits the inclusion of images with text.  A
feature of this example is the inclusion of two additional
body parts, both picture. They are referred to internally by
the encapsulated document via each picture's body part
content-ID.  Usage of "cid:", as in this example, may be
useful for a variety of compound objects.  It is not, however,
a part of the Multipart/Related specification.

     Content-Type: Multipart/Related; boundary=example-2;

     Content-Type: Text/x-Okie; charset=iso-8859-1;
     Content-ID: <950118.AEBH@XIson.com>
     Content-Description: Document

     This picture was taken by an automatic camera mounted ...
     {image file=cid:950118.AECB@XIson.com}
     Now this is an enlargement of the area ...

     {image file=cid:950118.AFDH@XIson.com}
     Content-Type: image/jpeg
     Content-ID: <950118.AFDH@XIson.com>
     Content-Transfer-Encoding: BASE64
     Content-Description: Picture A

     [encoded jpeg image]
     Content-Type: image/jpeg
     Content-ID: <950118.AECB@XIson.com>
     Content-Transfer-Encoding: BASE64
     Content-Description: Picture B

     [encoded jpeg image]

5.  User Agent Requirements

   User agents that do not recognize Multipart/Related shall, in
   accordance with [MIME], treat the entire entity as Multipart/Mixed.
   MIME User Agents that recognize Multipart/Related entities but are
   unable to process the given type shall either suppress the entire
   Multipart/Related body part or process the root alone.  In either
   case the user should be notified of the MUA's action.

   Handling Multipart/Related differs from other media types in that
   processing cannot be reduced to handling the individual entities.
   Existing media types are handled by MIME-capable MUAs handle in a
   straightforward manner.  For basic media types (e.g., text, image,
   etc.) the body of the entity can be directly passed to a display
   process.  Composite media types can be reduced to handing one or more
   discrete types.

   Multipart/Related provides an irreducible composite media type.

   The following sections discuss what information the processing
   application requires.

   It is possible that an application specific "receiving agent" will
   manipulate the entities, after initial processing by the MIME User
   Agent, prior to invoking actual application process.  From the
   viewpoint of the MUA, the receiving agent is the application.  Okie,
   above, demonstrates this; it may need a receiving agent to parse the
   document and substitute local file names for the originator's file
   names.  Other applications may just require a table showing the
   correspondence between the local file names and the originator's.

   The receiving agent takes responsibility any for such processing.

5.1 Data Requirements

   MIME-capable MUAs are required to provide the application:

   (a)  the bodies of the MIME entities and the entity Content-*

   (b)  the parameters of the Multipart/Related Content-type
        header, and

   (c)  the correspondence between each body's local file name,
        that body's header data, and, if present, the body part's

5.2 Storing Multipart/Related Entities

   The Multipart/Related media type will be used for objects that have
   internal linkages between the body parts.  When the objects are
   stored the linkages may require processing by the application or its
   receiving agent.

5.3 Recursion

   MIME is a recursive structure.  Hence one must expect a
   Multipart/Related entity to contain other Multipart/Related entities.
   When a Multipart/Related entity is being processed for display or
   storage, any enclosed Multipart/Related entities shall be processed
   as though they were being stored.  It shall be the responsibility of
   the application handling the outermost Multipart/Related to insure
   the appropriate processing of embedded Multipart/Related entities.

5.5 Configuration Considerations

   It is suggested that MUAs that use configuration mechanisms, see
   [CFG] for an example, refer to Multipart/Related as
   Multipart/Related/<type>, were <type> is the value of the "type"

6.  Security Considerations

   Security considerations relevant to Multipart/Related are identical
   to those of the underlying content-type.

7.  Acknowledgments

   This proposal is the result of conversations the author has had with
   many people.  In particular, similar work was described by Harald A.
   Alvestrand (early drafts of Multipart/Related), Dave Crocker
   (Multipart/Families), and Keith Moore (Multipart/References). In
   addition, James Clark, Charles Goldfarb, Gary Houston, Ned Freed, Ray
   Moody, and Don Stinchfield, provided both encouragement and
   invaluable help.  The author, however, take full responsibility for
   all errors contained in this document.

8.  References

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

   [CFG]       Borenstein, N., "A User Agent Configuration
               Mechanism For Multimedia Mail Format Information",
               RFC 1524, Bellcore, September 1993.

   [MIME]      Borenstein, N. and and N. Freed, "MIME (Multipurpose
               Internet Mail Extensions) Part One: Mechanisms for
               Specifying and Describing the Format of Internet Message
               Bodies", RFC 1521, Bellcore, Innosoft, September 1993.

9.  Author's Address

   Edward Levinson
   Accurate Information Systems, Inc.
   2 Industrial Way
   Eatontown, NJ  07724-2265

   Phone: +1 908 389 5550
   EMail: ELevinson@Accurate.com


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