Message/External-Body Content-ID Access Type
RFC 1873

Document Type RFC - Experimental (December 1995; No errata)
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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.

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 <>

       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.

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

            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

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
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