rfc2112.txt

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Network Working Group                                        E. Levinson
Request for Comments: 2112                                   XIson, Inc.
Category: Standards Track                                     March 1997
Obsoletes: 1872


                The MIME Multipart/Related Content-type

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.

Abstract

   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.

   Multipart/Related MIME entities may contain Content-Disposition
   headers that provide suggestions for the storage and display of a
   body part.  Multipart/Related processing takes precedence over
   Content-Disposition; the interaction between them is discussed in
   section 4.

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



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RFC 2112          MIME Multipart/Related Content-type         March 1997


2.  Multipart/Related Registration Information

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


   To:  [e-mail] 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
                     Start-info

   Encoding considerations:   Multipart content-types cannot have
                              encodings.

   Security considerations:   Depends solely on the referenced type.

   Published specification:   RFC-REL (this document).

   Person & email address to contact for further information:
                     Edward Levinson
                     47 Clive Street
                     Metuchen, NJ 08840-1060
                     [phone]
                     [e-mail]

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.










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RFC 2112          MIME Multipart/Related Content-type         March 1997


   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-" headers.

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.

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
   applications processes first.

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
   object.

   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
   reference.

3.4.  Syntax

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

     cid-list        := cid cid-list



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RFC 2112          MIME Multipart/Related Content-type         March 1997


     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.  Handling Content-Disposition Headers

   Content-Disposition Headers [DISP] suggest presentation styles for
   MIME body parts.  [DISP] describes two presentation styles, called
   the disposition type, INLINE and ATTACHMENT.  These, used within a
   multipart entity, allow the sender to suggest presentation
   information.  [DISP] also provides for an optional storage (file)
   name.  Content-Disposition headers could appear in one or more body
   parts contained within a Multipart/Related entity.

   Using Content-Disposition headers in addition to Multipart/Related
   provides presentation information to User Agents that do not
   recognize Multipart/Related.  They will treat the multipart as
   Multipart/Mixed and they may find the Content-Disposition information
   useful.

   With Multipart/Related however, the application processing the
   compound object determines the presentation style for all the
   contained parts.  In that context the Content-Disposition header
   information is redundant or even misleading.  Hence, User Agents that
   understand Multipart/Related shall ignore the disposition type within
   a Multipart/Related body part.

   It may be possible for a User Agent capable of handling both
   Multipart/Related and Content-Disposition headers to provide the
   invoked application the Content-Disposition header's optional
   filename parameter to the Multipart/Related.  The use of that
   information will depend on the specific application and should be
   specified when describing the handling of the corresponding compound
   object.  Such descriptions would be appropriate in an RFC registering
   that object's media type.









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RFC 2112          MIME Multipart/Related Content-type         March 1997


5.  Examples

5.1 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
   "record".

   The example below, uses a single data block.

     Content-Type: Multipart/Related; boundary=example-1
             start="<[e-mail]>";
             type="Application/X-FixedRecord"
          start-info="-o ps"

     --example-1
     Content-Type: Application/X-FixedRecord
     Content-ID: <[e-mail]>

     25