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rfc:rfc2112

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.

Levinson Standards Track [Page 1] 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:  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
                   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
                   +1 908 494 1606
                   XIson@cnj.digex.net

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.

Levinson Standards Track [Page 2] 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

Levinson Standards Track [Page 3] 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.

Levinson Standards Track [Page 4] 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="<950120.aaCC@XIson.com>";
           type="Application/X-FixedRecord"
        start-info="-o ps"
  1. -example-1

Content-Type: Application/X-FixedRecord

   Content-ID: <950120.aaCC@XIson.com>
   25
   10
   34
   10
   25
   21
   26
   10
   --example-1
   Content-Type: Application/octet-stream
   Content-Description: The fixed length records
   Content-Transfer-Encoding: base64
   Content-ID: <950120.aaCB@XIson.com>
   T2xkIE1hY0RvbmFsZCBoYWQgYSBmYXJtCkUgSS
   BFIEkgTwpBbmQgb24gaGlzIGZhcm0gaGUgaGFk
   IHNvbWUgZHVja3MKRSBJIEUgSSBPCldpdGggYS
   BxdWFjayBxdWFjayBoZXJlLAphIHF1YWNrIHF1
   YWNrIHRoZXJlLApldmVyeSB3aGVyZSBhIHF1YW
   NrIHF1YWNrCkUgSSBFIEkgTwo=
  1. -example-1–

Levinson Standards Track [Page 5] RFC 2112 MIME Multipart/Related Content-type March 1997

5.2 Text/X-Okie

 The Text/X-Okie is an invented markup language permitting 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;
           start="<950118.AEBH@XIson.com>"
        type="Text/x-Okie"
  1. -example-2

Content-Type: Text/x-Okie; charset=iso-8859-1;

           declaration="<950118.AEB0@XIson.com>"
   Content-ID: <950118.AEBH@XIson.com>
   Content-Description: Document
   {doc}
   This picture was taken by an automatic camera mounted ...
   {image file=cid:<950118.AECB@XIson.com>}
   {para}
   Now this is an enlargement of the area ...
   {image file=cid:<950118:AFDH@XIson.com>}
   {/doc}
   --example-2
   Content-Type: image/jpeg
   Content-ID: <950118.AFDH@XIson.com>
   Content-Transfer-Encoding: BASE64
   Content-Description: Picture A
   [encoded jpeg image]
   --example-2
   Content-Type: image/jpeg
   Content-ID: <950118.AECB@XIson.com>
   Content-Transfer-Encoding: BASE64
   Content-Description: Picture B
   [encoded jpeg image]
   --example-2--

Levinson Standards Track [Page 6] RFC 2112 MIME Multipart/Related Content-type March 1997

5.3 Content-Disposition

 In the above example each image body part could also have a Content-
 Disposition header.  For example,
   ...
   --example-2
   Content-Type: image/jpeg
   Content-ID: <950118.AECB@XIson.com>
   Content-Transfer-Encoding: BASE64
   Content-Description: Picture B
   Content-Disposition: INLINE
   [encoded jpeg image]
   --example-2--
 User Agents that recognize Multipart/Related will ignore the
 Content-Disposition header's disposition type.  Other User Agents
 will process the Multipart/Related as Multipart/Mixed and may make
 use of that header's information.

6. 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 do recognize Multipart/Related entities but are
 unable to process the given type should give the user the option of
 suppressing the entire Multipart/Related body part shall be.
 Existing MIME-capable mail user agents (MUAs) handle the existing
 media types in a straightforward manner.  For discrete media types
 (e.g. text, image, etc.) the body of the entity can be directly
 passed to a display process.  Similarly the existing composite
 subtypes can be reduced to handing one or more discrete types.
 Handling Multipart/Related differs in that processing cannot be
 reduced to handling the individual entities.
 The following sections discuss what information the processing
 application requires.
 It is possible that an application specific "receiving agent" will
 manipulate the entities for display prior to invoking actual
 application process.  Okie, above, is an example of 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
 for such processing.

Levinson Standards Track [Page 7] RFC 2112 MIME Multipart/Related Content-type March 1997

6.1 Data Requirements

 MIME-capable mail user agents (MUAs) are required to provide the
 application:
 (a)  the bodies of the MIME entities and the entity Content-*
      headers,
 (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
      content-ID.

6.2 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.

6.3 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.

6.4 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"
 parameter.

7. Security considerations

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

Levinson Standards Track [Page 8] RFC 2112 MIME Multipart/Related Content-type March 1997

8. Acknowledgments

 This proposal is the result of conversations the author has had with
 many people.  In particular, Harald A. Alvestrand, 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.

9. References

 [822]       Crocker, D., "Standard for the Format of ARPA
             Internet Text Messages", August 1982, University
             of Delaware, RFC 822.
 [CID]       E. Levinson, J. Clark, "Message/External-Body
             Content-ID Access Type", 12/26/1995, RFC 1873
             Levinson, E., "Message/External-Body Content-ID
             Access Type", February 1997, RFC 2111.
 [CFG]       Borenstein, N., "A User Agent Configuration
             Mechanism For Multimedia Mail Format
             Information", September 23, 1993, RFC 1524
 [DISP]      R. Troost, S. Dorner, "Communicating Presentation
             Information in Internet Messages:  The Content-
             Disposition Header", June 7, 1995, RFC 1806
 [MIME]      Borenstein, N. and Freed, N., "MIME (Multipurpose
             Internet Mail Extensions): Mechanisms for
             Specifying and Describing the Format of Internet
             Message Bodies", June 1992, RFC 1341.

9. Author's Address

 Edward Levinson
 XIson, Inc.
 47 Clive Street
 Metuchen, NJ  08840-1060
 USA
 +1 908 549 3716
 <XIson@cnj.digex.com>

Levinson Standards Track [Page 9]

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