SGML/MIME: Message/External-Body Content-ID Access Type

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 <>.  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
       working documents of the Internet Engineering Task Force
       (IETF) its Areas, and Working Groups.  Note that other
       groups may also distribute working documents as Internet

       Internet Drafts are draft documents valid for a maximum of
       six months.  They may be updated, replaced, or obsoleted by
       other documents at any time.  It is not appropriate to use
       Internet Drafts as reference material or to cite them other
       than as a "working draft" or  "work in progress".

       Please check the abstract listing in each Internet Draft
       directory for the current status of this or any other
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       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

       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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       Internet Draft                                    Access Type Content-ID

       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 <>

       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

            MIME-Version: 1.0

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       Internet Draft                                    Access Type Content-ID

            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

       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
       +1 908 389 5550

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       Internet Draft                                    Access Type Content-ID

       James Clark
       90 Clarendon Road
       London W11 2HR

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