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Network Working Group                                        E. LevinsonRequest for Comments: 1873            Accurate Information Systems, Inc.Category: Experimental                                          J. Clark                                                           December 1995Message/External-Body Content-ID Access TypeStatus 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 19952. The Content-ID Access Type2.1 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 19952.2 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: <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            --tiger-lily--       The equivalent MIME entity for the second body part is:            --tiger-lily            Content-Type: image/jpeg            Content-ID: <950323.1552@XIson.com>            Content-Description:               This body part is duplicated by reference            AAAcdb...            --tiger-lily3. 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 19954. 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' AddressesEdward LevinsonAccurate Information Systems, Inc.2 Industrial WayEatontown, NJ  07724-2265USAPhone: +1 908 389 5550EMail: <ELevinson@Accurate.com>James Clark90 Clarendon RoadLondon W11 2HRUKEMail: <jjc@jclark.com>Levinson & Clark              Experimental                      [Page 4]