SGML/MIME: Message/External-Body Content-ID Access Type
MIMESGML Working Group E. Levinson
Internet Draft: Access-type=Content-ID J. Clark
<draft-ietf-mimesgml-access-cid-01.txt> May 25, 1995
Message/External-Body Content-ID Access Type
This draft document is being circulated for comment. Please
send your comments to the authors or to the ietf-types mail
list <ietf-types@cs.utk.edu>. If consensus is reached, this
Access Type may be registered with IANA and this document
may be submitted to the RFC editor as an Informational
protocol specification.
Status of this Memo
This document is an Internet Draft; Internet Drafts are
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Abstract
When using MIME [MIME] to encapsulate a structured object
that consist of many elements, for example an SGML [SGML]
document, a single element may occur several times. An
encapsulation normally maps each of the structured objects
elements to a MIME entity. It is useful to include elements
that occur multiple time exactly once. To accomplish that
and to preserve the object structure it is desirable to
unambiguously refer to another body part of the same
message.
The exsisting MIME Content-Type Message/External-Body
access-types allow a MIME entity (body-part) to refer to an
object that is not in the message by specifying how to
access that object. The Content-ID access method described
in this document provides the capability to refer to an
object within the message.
1. Introduction
Consider a MIME multipart entity several of whose body parts
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contain the same data (body) but different parameters or
Content-* headers. Representing those body parts without
duplicating the data in each one promotes efficient use of
resources (bandwidth and storage space). To achieve these
benefits an access-type is defined that permits one message
part to refer to another one in the same message.
2. The Content-ID Access Type
2.1 Registration Information
MIME access-type name: content-id
Required parameters: none
Optional parameters: none
Published specification: this document
Person & email address
to contact for further
information: Ed Levinson <ELevinson@accurate.com>
Additional requirements:
The content-id header of the access-type=content-id MIME
entity must match (be identical to) exactly one content-id
in the same message, excluding other access-type=content-id
entities. Thus, the content-id access type can only occur
within a multipart message and can refer to another body
part anywhere in the same message.
A MIME User Agent (MUA) constructs the resultant MIME body
part as described below. We call the access-type=content-id
MIME entity the referring body part and the MIME body part
to which it refers, the one with the matching content-id,
the referenced body part. The MIME entity that results from
content-id access type consists of:
(a) the referenced body part's content-type header,
(b) the referring body part's headers except its content-type header,
(c) any headers in the referenced body part not in the referring one,
(d) the line separating the headers from the body, and
(e) the referenced body part's body.
2.2 Example Usage
The following example shows a message that consists of two identical
images.
MIME-Version: 1.0
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Content-Type: Multipart/Mixed;
boundary=tiger-lily
--tiger-lily
Content-Type: image/jpeg
Content-ID: <950323.1552@XIson.com>
AAAcdb...
--tiger-lily
Content-type: Message/External-Body;
access-type=content-id
Content-ID: <950323.1552@XIson.com>
Content-Description:
This body part is duplicated by reference
--tiger-lily--
The equivalent MIME entity for the second body part is:
--tiger-lily
Content-Type: image/jpeg
Content-ID: <950323.1552@XIson.com>
Content-Description:
This body part is duplicated by reference
AAAcdb...
--tiger-lily
3. Security
The content-id access-type does not impact the security of messages or
systems. The referenced MIME entity may have security implications.
4. References
[822] Crocker, D., Standard for the Format of ARPA Internet Text
Messages, August 1982, University of Delaware, RFC 822.
[SGML] ISO 8879:1988, Information processing -- Text and office
systems -- Standard Generalized Markup Language (SGML).
[MIME] Borenstein, N. and Freed, N., MIME (Multipurpose Internet
Mail Extensions): Mechanisms for Specifying and Describing
the Format of Internet Message Bodies, June 1992, RFC 1341.
5. 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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James Clark
90 Clarendon Road
London W11 2HR
UK
<jjc@jclark.com>
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