GENWiki

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.

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.

Levinson & Clark Experimental [Page 1]  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 <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.

Levinson & Clark Experimental [Page 2]  RFC 1873 Access Type Content-ID December 1995

2.2 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

1. -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

1. -tiger-lily–

     The equivalent MIME entity for the second body part is:

1. -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 Considerations

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

Levinson & Clark Experimental [Page 3]  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 USA

Phone: +1 908 389 5550 EMail: ELevinson@Accurate.com

James Clark 90 Clarendon Road London W11 2HR UK

EMail: jjc@jclark.com

Levinson & Clark Experimental [Page 4]