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Versions: 02 03 RFC 1872

       MIMESGML Working Group                           E. Levinson
       Internet Draft: Multipart/Related        ACCURATE Info. Sys.
       <draft-ietf-mimesgml-related-03.txt>        October 11, 1995
       
                 The MIME Multipart/Related Content-type
       
       This draft document is being circulated for comment.  Please
       send your comments to the authors or to the ietf-822 and
       mimesgml mail lists, <ietf-822@list.cren.net> and <sgml-
       internet@ebt.com>, respecively.
       
       Status of this Memo
       
       This document is an Internet Draft; Internet Drafts are
       working documents of the Internet Engineering Task Force
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       Internet Drafts are draft documents valid for a maximum of
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       Please check the abstract listing in each Internet Draft
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       Abstract
       
       The Multipart/Related content-type provides a common
       mechanism for representing objects that are aggregates of
       related MIME body parts.  This document defines the
       Multipart/Related content-type and provides examples of its
       use.
       
       0.  Change History
       
       0.1 Changes From Previous Version (02, July 7, 1995)
       
       Required the "type" parameter and content-type of start body
       part to agree.  Added a note to explain the reasoning behind
       this approach. Also added note to provide the reasoning
       behind the start parameter.
       
       Removed content reference and replaced with recommendations
       for handling ambiguities when referring to body parts with
       the same content-ID.
       
       Eliminated the term "unpacker" in favor of "receiving
       agent".  The receiving agent is described in section 5.
       
       Clarification of text and elimination of typographic errors
       
       
       
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       were also made.
       
       0.2 Changes From Versions 00 and 01
       
       Introduced content reference as formal pointer to another
       body part.  This handles a potentially conflicting use and
       ambiguity with content-IDs.  See section 4.
       
       Added Content-Reference header and description (see above).
       
       Added Start-Info parameter which allow "command line" or
       other data to be passed to the invoked process.
       
       Changed Start parameter to be a single content reference.
       Multiple content references were thought to be needed for
       SGML but, it turns out are not needed.
       
       Changed Type parameter to be required and gave it precedence
       over the root body part's media-type.  This enables user
       agents that must process messages on the fly to avoid
       searching ahead.
       
       Revised introductory text in section 3 describing the
       parameters and eliminated comment about parameters on the
       start body part.
       
       1.  Introduction
       
       Several applications of MIME, including MIME-PEM, and MIME-
       Macintosh and other proposals, require multiple body parts
       that make sense only in the aggregate.  The present approach
       to these compound objects has been to define specific
       multipart subtypes for each new object.  In keeping with the
       MIME philosophy of having one mechanism to achieve the same
       goal for different purposes, this document describes a
       single mechanism for such aggregate or compound objects.
       
       The Multipart/Related content-type addresses the MIME
       representation of compound objects.  The object is
       categorized by a "type" parameter.  Additional parameters
       are provided to indicate a specific starting body part or
       root and auxiliary information which may be required when
       unpacking or processing the object.
       
       Responsibility for the display or processing of a
       Multipart/Related's constituent entities rests with the
       application that handles the compound object.
       
       2.  Multipart/Related Registration Information
       
       The following form is copied from RFC 1590, Appendix A.
       
       
       
       
       
       
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       To:  IANA@isi.edu
       Subject:  Registration of new Media Type content-type/subtype
       
       Media Type name:           Multipart
       
       Media subtype name:        Related
       
       Required parameters:       Type, a media type/subtype.
       
       Optional parameters:       Start, a content-id.
                                  Start-info, a string or content-id list.
       
       Encoding considerations:   Multipart content-types cannot have
                                  encodings.
       
       Security considerations:   Depends solely on the referenced type.
       
       Published specification:   This document.
       
       Person & email address to contact for further information:
                                  Edward Levinson
                                  Accurate Information Systems, Inc.
                                  2 Industrial Way
                                  Eatontown, NJ 07724
                                  +1 908 389 5550
                                  +1 908 389 5556 (fax)
                                  ELevinson@Accurate.com
       
       3.  Intended usage
       
       The Multipart/Related media type is intended for compound
       objects consisting of several inter-related body parts.  For
       a Multipart/Related object, proper display cannot be
       achieved by individually displaying the constituent body
       parts.  The content-type of the Multipart/Related object is
       specified by the type parameter.  The "start" parameter, if
       given, points, via a content-ID, to the body part that
       contains the object root.  The default root is the first
       body part within the Multipart/Related body.
       
       The relationships among the body parts of a compound object
       distinguishes it from other object types.  These
       relationships are often represented by links internal to the
       object's components that reference the other components.
       Within a single operating environment the links are often
       file names, such links may be represented within a MIME
       message using content-IDs or the value of some other
       "Content-" header.
       
       3.1.  The Type Parameter
       
       The type parameter must be specified and its value is the
       MIME media type of the root body part.  It permits a MIME
       
       
       
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       user agent to determine the content-type without reference
       to the enclosed body part.  If the value of the type
       parameter and the root body part's content-type differ then
       the User Agent's behavior is undefined.
       
       Note: Constraining the "type" parameter's value to an
       existing media type allows the appropriate processing to be
       identified without creating yet another hierarchy of
       registered types.  A possible default action would have the
       MIME mail User Agent (MUA) to display the "start" entity
       alone when it could process the media type as a basic type
       but not as Multipart/Related.
       
       3.2.  The Start Parameter
       
       The start parameter, if given, is the content-ID of the
       compound object's root.  If not present the root is the
       first body part in the Multipart/Related entity.  The root
       is the element the applications processes first.
       
       Note: The "start" parameter allows for types in which the
       root element gets generated by the sending application,
       perhaps on the fly.  Such an application can create the
       "start" content-id when processing begins and then insert
       the body part when it is complete.
       
       3.3.  The Start-Info Parameter
       
       Additional information can be provided to an application by
       the start-info parameter.  It contains either a string or
       points, via a content-ID, to another MIME entity in the
       message.  A typical use might be to provide additional
       command line parameters or a MIME entity giving auxiliary
       information for processing the compound object.
       
       Applications that use Multipart/Related must specify the
       interpretation of start-info.  User Agents shall provide the
       parameter's value to the processing application.  Processes
       can distinguish a start-info reference from a token or
       quoted-string by examining the first non-white-space
       character, "<" indicates a content-id reference.
       
       3.4.  Syntax
       
            related-param    := [ ";" "start" "=" cid ]
                                [ ";" "start-info"  "="
                                  ( cid-list / value ) ]
                                [ ";" "type"  "=" type "/" subtype ]
                                ; order independent
       
            cid-list        := cid cid-list
       
            cid             := msg-id     ; c.f. [822]
       
       
       
       
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            value           := token / quoted-string    ; c.f. [MIME]
                                    ; value cannot begin with "<"
       
       Note that the parameter values will usually require quoting.
       Msg-id contains the special characters "<", ">", "@", and
       perhaps other special characters.  If msg-id contains
       quoted-strings, those quote marks must be escaped.
       Similarly, the type parameter contains the special character
       "/".
       
       4.  Examples
       
       4.1 Application/X-FixedRecord
       
       The X-FixedRecord content-type consists of one or more
       octet-streams and a list of the lengths of each record.  The
       root, which lists the record lengths of each record within
       the streams.  The record length list, type Application/X-
       FixedRecord, consists of a set of INTEGERs in ASCII format,
       one per line.  Each INTEGER gives the number of octets from
       the octet-stream body part that constitute the next
       "record".
       
       The example below, uses a single data block which the sender
       processes on the fly to generate the record length list.
       Consequently the list appears after the data.
       
       
            Content-Type: Multipart/Related; boundary=example-1
                    start="<950120.aaCC@XIson.com>";
                    type="Application/X-FixedRecord"
                    start-info="-o ps"
       
            --example-1
            Content-Type: Application/octet-stream
            Content-Description: The fixed length records
            Content-Transfer-Encoding: base64
            Content-ID: <950120.aaCB@XIson.com>
       
            T2xkIE1hY0RvbmFsZCBoYWQgYSBmYXJtCkUgSS
            BFIEkgTwpBbmQgb24gaGlzIGZhcm0gaGUgaGFk
            IHNvbWUgZHVja3MKRSBJIEUgSSBPCldpdGggYS
            BxdWFjayBxdWFjayBoZXJlLAphIHF1YWNrIHF1
            YWNrIHRoZXJlLApldmVyeSB3aGVyZSBhIHF1YW
            NrIHF1YWNrCkUgSSBFIEkgTwo=
            --example-1
            Content-Type: Application/X-FixedRecord
            Content-ID: <950120.aaCC@XIson.com>
       
            25
            10
            34
            10
            25
       
       
       
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            21
            26
            10
            --example-1--
       
       4.2 Text/X-Okie
       
       The Text/X-Okie is an invented markup language, similar to
       HTML, that permits the inclusion of images with text.  A
       feature of this example is the inclusion of two additional
       body parts, both picture. They are referred to internally by
       the encapsulated document via each picture's body part
       content-ID.  Usage of "cid:", as in this example, may be
       useful for a variety of compound objects.  It is not,
       however, a part of the Multipart/Related specification.
       
            Content-Type: Multipart/Related; boundary=example-2;
                    start="<950118.AEBH@XIson.com>"
                    type="Text/x-Okie"
       
            --example-2
            Content-Type: Text/x-Okie; charset=iso-8859-1;
                    declaration="<950118.AEB0@XIson.com>"
            Content-ID: <950118.AEBH@XIson.com>
            Content-Description: Document
       
            {doc}
            This picture was taken by an automatic camera mounted ...
            {image file=cid:<950118.AECB@XIson.com>}
            {para}
            Now this is an enlargement of the area ...
            {image file=cid:<950118.AFDH@XIson.com>}
            {/doc}
            --example-2
            Content-Type: image/jpeg
            Content-ID: <950118.AFDH@XIson.com>
            Content-Transfer-Encoding: BASE64
            Content-Description: Picture A
       
            [encoded jpeg image]
            --example-2
            Content-Type: image/jpeg
            Content-ID: <950118.AECB@XIson.com>
            Content-Transfer-Encoding: BASE64
            Content-Description: Picture B
       
            [encoded jpeg image]
            --example-1--
       
       5.  User Agent Requirements
       
       User agents that do not recognize Multipart/Related shall,
       in accordance with [MIME], treat the entire entity as
       Multipart/Mixed.  MIME User Agents that recognize
       
       
       
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       Multipart/Related entities but are unable to process the
       given type shall either suppress the entire
       Multipart/Related body part or process the root alone.  In
       either case the user should be notified of the MUA's action.
       
       Handling Multipart/Related differs from other media types in
       that processing cannot be reduced to handling the individual
       entities.  Existing media types are handled by MIME-capable
       MUAs handle in a straightforward manner.  For basic media
       types (e.g. text, image, etc.) the body of the entity can be
       directly passed to a display process.  Composite media types
       can be reduced to handing one or more discrete types.
       Multipart/Related provides an irreducible composite media
       type.
       
       The following sections discuss what information the
       processing application requires.
       
       It is possible that an application specific "receiving
       agent" will manipulate the entities, after initial
       processing by the MIME User Agent, prior to invoking actual
       application process.  From the viewpoint of the MUA, the
       receiving agent is the application.  Okie, above,
       demonstrates this; it may need a receiving agent to parse
       the document and substitute local file names for the
       originator's file names.  Other applications may just
       require a table showing the correspondence between the local
       file names and the originator's.  The receiving agent takes
       responsibility any for such processing.
       
       5.1 Data Requirements
       
       MIME-capable MUAs are required to provide the application:
       
       (a)  the bodies of the MIME entities and the entity Content-*
            headers,
       
       (b)  the parameters of the Multipart/Related Content-type
            header, and
       
       (c)  the correspondence between each body's local file name,
            that body's header data, and, if present, the body part's
            content-ID.
       
       5.2 Storing Multipart/Related Entities
       
       The Multipart/Related media type will be used for objects that
       have internal linkages between the body parts.  When the
       objects are stored the linkages may require processing by the
       application or its receiving agent.
       
       5.3 Recursion
       
       MIME is a recursive structure.  Hence one must expect a
       
       
       
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       Multipart/Related entity to contain other Multipart/Related
       entities.  When a Multipart/Related entity is being processed
       for display or storage, any enclosed Multipart/Related enti-
       ties shall be processed as though they were being stored.  It
       shall be the responsibility of the application handling the
       outermost Multipart/Related to insure the appropriate process-
       ing of embedded Multipart/Related entities.
       
       5.5 Configuration Considerations
       
       It is suggested that MUAs that use configuration mechanisms,
       see [CFG] for an example, refer to Multipart/Related as
       Multipart/Related/<type>, were <type> is the value of the
       "type" parameter.
       
       6.  Security considerations
       
       Security considerations relevant to Multipart/Related are
       identical to those of the underlying content-type.
       
       7.  Acknowledgments
       
       This proposal is the result of conversations the author has
       had with many people.  In particular, Harald A. Alvestrand,
       James Clark, Charles Goldfarb, Gary Houston, Ned Freed, Ray
       Moody, and Don Stinchfield, provided both encouragement and
       invaluable help.  The author, however, take full responsibil-
       ity for all errors contained in this document.
       
       8.  References
       
       
       [822]       Crocker, D., "Standard for the Format of ARPA
                   Internet Text Messages", August 1982, University
                   of Delaware, RFC 822.
       
       [CFG]       Borenstein, N., "A User Agent Configuration
                   Mechanism For Multimedia Mail Format Information",
                   September 23, 1993, RFC 1524
       
       [MIME]      Borenstein, N. and Freed, N., "MIME (Multipurpose
                   Internet Mail Extensions): Mechanisms for Specify-
                   ing and Describing the Format of Internet Message
                   Bodies", June 1992, RFC 1341.
       
       9.  Author's address
       
       Edward Levinson
       Accurate Information Systems, Inc.
       2 Industrial Way
       Eatontown, NJ  07724-2265
       USA
       +1 908 389 5550
       <ELevinson@Accurate.com>
       
       
       
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