MIMESGML Working Group E. Levinson
Internet Draft: Multipart/Related ACCURATE Info. Sys.
<draft-ietf-mimesgml-related-03.txt> October 11, 1995
The MIME Multipart/Related Content-type
This draft document is being circulated for comment. Please
send your comments to the authors or to the ietf-822 and
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internet@ebt.com>, respecively.
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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.
0. Change History
0.1 Changes From Previous Version (02, July 7, 1995)
Required the "type" parameter and content-type of start body
part to agree. Added a note to explain the reasoning behind
this approach. Also added note to provide the reasoning
behind the start parameter.
Removed content reference and replaced with recommendations
for handling ambiguities when referring to body parts with
the same content-ID.
Eliminated the term "unpacker" in favor of "receiving
agent". The receiving agent is described in section 5.
Clarification of text and elimination of typographic errors
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were also made.
0.2 Changes From Versions 00 and 01
Introduced content reference as formal pointer to another
body part. This handles a potentially conflicting use and
ambiguity with content-IDs. See section 4.
Added Content-Reference header and description (see above).
Added Start-Info parameter which allow "command line" or
other data to be passed to the invoked process.
Changed Start parameter to be a single content reference.
Multiple content references were thought to be needed for
SGML but, it turns out are not needed.
Changed Type parameter to be required and gave it precedence
over the root body part's media-type. This enables user
agents that must process messages on the fly to avoid
searching ahead.
Revised introductory text in section 3 describing the
parameters and eliminated comment about parameters on the
start body part.
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.
2. Multipart/Related Registration Information
The following form is copied from RFC 1590, Appendix A.
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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 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
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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 applications processes first.
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 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]
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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"
--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
Content-Type: Application/X-FixedRecord
Content-ID: <950120.aaCC@XIson.com>
25
10
34
10
25
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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"
--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-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
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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. 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
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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 enti-
ties 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 process-
ing 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.
7. 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.
8. References
[822] Crocker, D., "Standard for the Format of ARPA
Internet Text Messages", August 1982, University
of Delaware, RFC 822.
[CFG] Borenstein, N., "A User Agent Configuration
Mechanism For Multimedia Mail Format Information",
September 23, 1993, RFC 1524
[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
Accurate Information Systems, Inc.
2 Industrial Way
Eatontown, NJ 07724-2265
USA
+1 908 389 5550
<ELevinson@Accurate.com>
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