The MIME Multipart/Related Content-type
draft-ietf-mhtml-related-00

MIMESGML Working Group                                       E. Levinson
Internet Draft: Multipart/Related                            XIson, Inc.
<draft-ietf-mhtml-related-00.txt>                            13 May 1996

                The MIME Multipart/Related Content-type

This draft document is being circulated for comment.  Please send your
comments to the author or to the ietf-types and mhtml mail lists <ietf-
types@uninett.no> and <mhtml@segate.sunet.se>.  If consensus is reached,
this document may be submitted to the RFC editor as a Proposed Standard
protocol specification for use with MIME.

Status of this Memo

This document is an Internet Draft; Internet Drafts are working docu-
ments of the Internet Engineering Task Force (IETF) its Areas, and Work-
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Abstract

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

0.  Changes from RFC 1872

Described the interaction between Multipart/Related and the Content-
Disposition headers.  This document will obsolete RFC 1872.

Behavior for unknown types changed from "shall suppress" to "should give
user the option ...".

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

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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 com-
pound 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
                  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 con-
tent-type of the Multipart/Related object is specified by the

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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 Multi-
part/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 compo-
nents 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 com-
pound 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 mes-
sage.  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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     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 per-
haps 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 Mul-
tipart/Related entity.

Using Content-Disposition headers in addition to Multi-
part/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 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

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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--

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 specifica-
tion.

     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;

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             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,

     ...
     --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 Multi-
part/Mixed.  MIME User Agents that do recognize Multi-
part/Related entities but are unable to process the given type

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should give the user the option of suppressing the entire Mul-
tipart/Related body part shall be.

Existing MIME-capable mail user agents (MUAs) handle the
existing media types in a straightforward manner.  For dis-
crete 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 corre-
spondence 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:

(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.

5.1 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 Multi-
part/Related entity to contain other Multipart/Related enti-
ties.  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.

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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 responsibil-
ity 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 Con-
            tent-ID Access Type", 12/26/1995, RFC 1873 Levin-
            son, E., "Message/External-Body Content-ID Access
            Type", work in progress,
            ftp://ds.internic.net/internet-drafts/draft-ietf-
            mimesgml-access-cid-01.txt.

[CFG]       Borenstein, N., "A User Agent Configuration Mecha-
            nism 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 Specify-
            ing and Describing the Format of Internet Message
            Bodies", June 1992, RFC 1341.

9.  Author's address

Edward Levinson
XIson, Inc.
47 Clive Street

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Metuchen, NJ  08840-1060
USA
+1 908 549 3716
<XIson@cnj.digex.com>

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