Username / Password : Request For Comments

RFC Number : 1873

Title : Message/External-Body Content-ID Access Type.

Network Working Group E. Levinson
Request for Comments: 1873 Accurate Information Systems, Inc.
Category: Experimental J. Clark
December 1995

Message/External-Body Content-ID Access Type

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.


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 existing 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 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.

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RFC 1873 Access Type Content-ID December 1995

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

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

(c) any headers in the referenced body part not in the referring

(d) the line separating the headers from the body, and

(e) the referenced body part's body.

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RFC 1873 Access Type Content-ID December 1995

2.2 Example Usage

The following example shows a message that consists of two identical

MIME-Version: 1.0
Content-Type: Multipart/Mixed;

Content-Type: image/jpeg
Content-ID: <>

Content-type: Message/External-Body;
Content-ID: <>
This body part is duplicated by reference


The equivalent MIME entity for the second body part is:

Content-Type: image/jpeg
Content-ID: <>
This body part is duplicated by reference


3. Security Considerations

The content-id access-type does not impact the security of messages
or systems. The referenced MIME entity may have security

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RFC 1873 Access Type Content-ID December 1995

4. References

[822] Crocker, D., 'Standard for the Format of ARPA Internet
Text Messages', STD 11, RFC 822, UDEL, August 1982.

[SGML] ISO 8879:1988, Information processing -- Text and office
systems -- Standard Generalized Markup Language (SGML).

[MIME] Borenstein, N., and N. Freed, 'MIME (Multipurpose
Internet Mail Extensions) Part One: Mechanisms for
Specifying and Describing the Format of Internet
Message Bodies', RFC 1521, Bellcore, Innosoft,
September 1993.

5. Authors' Addresses

Edward Levinson
Accurate Information Systems, Inc.
2 Industrial Way
Eatontown, NJ 07724-2265

Phone: +1 908 389 5550

James Clark
90 Clarendon Road
London W11 2HR


Levinson & Clark Experimental [Page 4]

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