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.












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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 <[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.








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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

           --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

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.












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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: <[email protected]>


James Clark
90 Clarendon Road
London W11 2HR
UK

EMail: <[email protected]>


















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