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RFC2387 - The MIME Multipart/Related Content-type

王朝other·作者佚名  2008-05-31
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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.

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 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.

9. References

[822] Crocker, D., "Standard for the Format of ARPA Internet

Text Messages", STD 11, RFC822, August 1982.

[CID] Levinson, E., and J. Clark, "Message/External-Body

Content-ID Access Type", RFC1873, 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", RFC1524, September

1993.

[DISP] Troost, R., and S. Dorner, "Communicating Presentation

Information in Internet Messages: The Content-

Disposition Header", RFC1806, June 1995.

[MIME] Borenstein, N., and Freed, N., "Multipurpose Internet

Mail Extensions (MIME) Part One: Format of Internet

Message Bodies", RFC2045, 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 (RFC2112)

Corrected cid urls to conform to RFC2111; the angle brackets were

removed.

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.

 
 
 
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