GENWiki

Network Working Group E. Levinson Request for Comments: 2387 August 1998 Obsoletes: 2112 Category: Standards Track

              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.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

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.

Levinson Standards Track [Page 1] RFC 2387 Multipart/Related August 1998

 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
                              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.

 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

Levinson Standards Track [Page 2] RFC 2387 Multipart/Related August 1998

 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

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

Levinson Standards Track [Page 3] RFC 2387 Multipart/Related August 1998

   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.

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

Levinson Standards Track [Page 4] RFC 2387 Multipart/Related August 1998

 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"

-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=

-example-1–

Levinson Standards Track [Page 5] RFC 2387 Multipart/Related August 1998

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"

-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--

5.3 Content-Disposition

 In the above example each image body part could also have a Content-
 Disposition header.  For example,

Levinson Standards Track [Page 6] RFC 2387 Multipart/Related August 1998

-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.

6.1 Data Requirements

 MIME-capable mail user agents (MUAs) are required to provide the
 application:

Levinson Standards Track [Page 7] RFC 2387 Multipart/Related August 1998

 (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 Multi-
 part/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.

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.

Levinson Standards Track [Page 8] RFC 2387 Multipart/Related August 1998

9. References

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

 [CID]       Levinson, E., and J. Clark, "Message/External-Body
             Content-ID Access Type",  RFC 1873, December 1995,
             Levinson, E., "Message/External-Body Content-ID Access
             Type", Work in Progress.

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

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

 [MIME]      Borenstein, N., and Freed, N., "Multipurpose Internet
             Mail Extensions (MIME) Part One: Format of Internet
             Message Bodies", RFC 2045, November 1996.

9. Author's Address

 Edward Levinson
 47 Clive Street
 Metuchen, NJ  08840-1060
 USA

 Phone: +1 908 494 1606
 EMail: XIson@cnj.digex.com

10. Changes from previous draft (RFC 2112)

 Corrected cid urls to conform to RFC 2111; the angle brackets were
 removed.

Levinson Standards Track [Page 9] RFC 2387 Multipart/Related August 1998

11. 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
 English.

 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Levinson Standards Track [Page 10]