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rfc:rfc1872

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.

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

Levinson Experimental [Page 1] RFC 1872 Multipart/Related December 1995

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
                            encodings.
 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)
                            ELevinson@Accurate.com

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

Levinson Experimental [Page 2] RFC 1872 Multipart/Related December 1995

 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
 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

Levinson Experimental [Page 3] RFC 1872 Multipart/Related December 1995

 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
 "record".
 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="<950120.aaCC@XIson.com>";
           type="Application/X-FixedRecord"
           start-info="-o ps"
  1. -example-1

Content-Type: Application/octet-stream

   Content-Description: The fixed length records
   Content-Transfer-Encoding: base64

Levinson Experimental [Page 4] RFC 1872 Multipart/Related December 1995

   Content-ID: <950120.aaCB@XIson.com>
   T2xkIE1hY0RvbmFsZCBoYWQgYSBmYXJtCkUgSS
   BFIEkgTwpBbmQgb24gaGlzIGZhcm0gaGUgaGFk
   IHNvbWUgZHVja3MKRSBJIEUgSSBPCldpdGggYS
   BxdWFjayBxdWFjayBoZXJlLAphIHF1YWNrIHF1
   YWNrIHRoZXJlLApldmVyeSB3aGVyZSBhIHF1YW
   NrIHF1YWNrCkUgSSBFIEkgTwo=
   --example-1
   Content-Type: Application/X-FixedRecord
   Content-ID: <950120.aaCC@XIson.com>
   25
   10
   34
   10
   25
   21
   26
   10
   --example-1--

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;
           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 ...

Levinson Experimental [Page 5] RFC 1872 Multipart/Related December 1995

   {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-1--

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.

Levinson Experimental [Page 6] RFC 1872 Multipart/Related December 1995

 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-*
      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.

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"
 parameter.

6. Security Considerations

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

Levinson Experimental [Page 7] RFC 1872 Multipart/Related December 1995

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

Levinson Experimental [Page 8]

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