RFC1873

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.

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 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.

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.

The Content-ID Access Type

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 <[email protected]>

   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.

Example Usage

The following example shows a message that consists of two identical images.

        MIME-Version: 1.0
        Content-Type: Multipart/Mixed;
           boundary=tiger-lily

        --tiger-lily
        Content-Type: image/jpeg
        Content-ID: <[email protected]>

        AAAcdb...
        --tiger-lily
        Content-type: Message/External-Body;
           access-type=content-id
        Content-ID: <[email protected]>
        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: <[email protected]>
        Content-Description:
           This body part is duplicated by reference

        AAAcdb...
        --tiger-lily

Security Considerations

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

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.

Authors' Addresses

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

Phone: +1 908 389 5550 EMail: <[email protected]>

James Clark 90 Clarendon Road London W11 2HR UK

EMail: <[email protected]>