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.

2. Multipart/Related Registration Information

The following form is copied from RFC1590, 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
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]

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 RFCregistering
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 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 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,

...
--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:

(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.

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, RFC822.

[CID] E. Levinson, J. Clark, "Message/External-Body
Content-ID Access Type", 12/26/1995, RFC1873
Levinson, E., "Message/External-Body Content-ID
Access Type", February 1997, RFC2111.

[CFG] Borenstein, N., "A User Agent Configuration
Mechanism For Multimedia Mail Format
Information", September 23, 1993, RFC1524

[DISP] R. Troost, S. Dorner, "Communicating Presentation
Information in Internet Messages: The Content-
Disposition Header", June 7, 1995, RFC1806

[MIME] Borenstein, N. and Freed, N., "MIME (Multipurpose
Internet Mail Extensions): Mechanisms for
Specifying and Describing the Format of Internet
Message Bodies", June 1992, RFC1341.

9. Author's Address

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