    Network Working Group                                           G. Klyne
    Request for Comments: 2913                          Content Technologies
    Category: Standards Track                                 September 2000

                MIME Content Types in Media Feature Expressions

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2000).  All Rights Reserved.

    Abstract

       In "A Syntax for Describing Media Feature Sets", an expression format
       is presented for describing media feature capabilities using simple
       media feature tags.

       This memo defines a media feature tag whose value is a Multipurpose
       Internet Mail Extensions (MIME) content type.  This allows the
       construction of feature expressions that take account of the MIME
       content type of the corresponding data.

    Table of Contents

       1. Introduction .................................................. 2
          1.1 Terminology and document conventions ...................... 2
       2. Motivation and goals .......................................... 3
       3. MIME content type feature tag ................................. 3
       4. Examples ...................................................... 4
          4.1 Simple text ............................................... 4
          4.2 Fax image ................................................. 4
          4.3 Voice message ............................................. 4
          4.4 Web browser capabilities .................................. 5
       5. IANA Considerations ........................................... 5
       6. Security Considerations ....................................... 5
       7. Acknowledgements .............................................. 5
       8. References .................................................... 6
       9. Author's Address .............................................. 6
       Appendix A: 'Type' feature tag registration ...................... 7
       Full Copyright Statement ......................................... 9

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

    1. Introduction

       In "A Syntax for Describing Media Feature Sets" [1], an expression
       format is presented for describing media feature capabilities as a
       combination of simple media feature tags, registered according to
       "Media Feature Tag Registration Procedure" [2].  This provides a
       format for message handling agents to describe the media feature
       content of messages that they can handle.

       This memo defines a media feature tag whose value is a MIME content
       type.  This allows the construction of feature expressions that take
       account of the MIME content type of the corresponding data.

       Note that a content type feature value may contain parameters, but
       this is discouraged.  See section 3 and appendix A, "Summary of the
       media features indicated" for discussion of this point.

    1.1 Terminology and document conventions

       This section defines a number of terms and other document
       conventions, which are used with specific meaning in this memo.

       media feature
             information that indicates facilities assumed to be available
             for the message content to be properly rendered or otherwise
             presented.  Media features are not intended to include
             information that affects message transmission.

       feature set
             some set of media features described by a media feature
             assertion, as described in "A Syntax for Describing Media
             Feature Sets" [1].  (See that memo for a more formal definition
             of this term.)

       feature set expression
             a string that describes some feature set, formulated according
             to the rules in "A Syntax for Describing Media Feature Sets"
             [1] (and possibly extended by other specifications).

       This specification uses syntax notation and conventions described in
       RFC 2234, "Augmented BNF for Syntax Specifications: ABNF" [3].

          NOTE:  Comments like this provide additional nonessential
          information about the rationale behind this document.  Such
          information is not needed for building a conformant
          implementation, but may help those who wish to understand the
          design in greater depth.

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

    2. Motivation and goals

       The media feature expression syntax [1] and feature tags [2] were
       designed with a view to providing content media information that
       augments basic MIME content type information.  There are some
       situations where it is useful to be able include that content type
       information in a media feature expression:

       o  Media feature details may depend upon the content type being used.
          The media feature combining algebra and syntax [1] cannot apply to
          content type information unless it appears in the feature
          expression.

          For example, in HTTP 1.1 [4] with Transparent Content Negotiation
          (TCN) [5] acceptable content types and other media features are
          indicated in different request headers, with no clear way to
          indicate that they may be acceptable only in certain combinations.

       o  It is sometimes useful for all media capability information to be
          included in a single expression.  For example, DSN and MDN
          extensions [6] that allow a recipient to indicate media
          capabilities provide a single field for conveying this
          information.

       o  When media features are used to describe a message content, they
          may refer to inner parts of a MIME composite; e.g. the component
          parts of a 'multipart', files in a compressed archive, or
          encrypted message data.

    3. MIME content type feature tag

       Feature tag name    Legal values
       ----------------    ------------
       type                <string>
                           containing a MIME content-type value.

       Reference: this document, appendix A.

       The 'type' feature tag indicates a MIME media content type (i.e.
       that appears in a 'Content-type:' header of the corresponding MIME-
       formatted data).  It must be a string of the form "type/subtype",
       where 'type' and 'subtype' are defined by the MIME specification [7].
       Only lower-case letters should be used.

       The content type must be given without any content-type parameter
       values.

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

       To include information in media feature expressions that is otherwise
       conveyed in a MIME content-type parameter, a separate media feature
       tag should be registered [2] and used in the media feature
       expression.  This is illustrated by the use of 'charset' in the
       example at 4.1 below -- the 'charset' tag is defined by a separate
       registration [10].

          NOTE:  Allowing content-type parameters to be part of a type tag
          value was considered, but rejected because of concerns about
          canonicalization, ordering, case sensitivity, etc.  Only exact,
          case-sensitive, character matching is defined for media feature
          expressions [1].

    4. Examples

    4.1 Simple text

          (& (type="text/plain") (charset=US-ASCII)
             (color=binary) (paper-size=A4) )

    4.2 Fax image

          (& (type="image/tiff")
             (color=binary)
             (image-file-structure=TIFF-S)
             (dpi=200)
             (dpi-xyratio=[200/100,200/200])
             (paper-size=A4)
             (image-coding=MH) (MRC-mode=0)
             (ua-media=stationery) )

    4.3 Voice message

          (& (type="multipart/voice-message")
             (VPIM-version="3.0")
             (audio-codec=[G726-32,GSM-610])
             (audio-file-structure=[None,WAV])
             (ua-terminal=mobile-handset)
             (audio-channels=1) )

             NOTE:  in this case, some media features apply to MIME parts
             contained within the declared 'multipart/voice- message'
             content type.  The goal here is not so much to mirror the MIME
             structure as to convey useful information about the (possible)
             message content.

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

    4.4 Web browser capabilities

          (& (pix-x<=800) (pix-y<=600)
             (| (& (type="text/html") (charset=iso-8859-1)
                   (color=limited) )
                (& (type="text/plain") (charset=US-ASCII) )
                (& (type="image/gif") (color=mapped))
                (& (type="image/jpeg") (color=full) ) ) )

       This example describes an HTML viewer that can deal with a limited
       number of color text tags, a gif viewer that supports mapped color,
       and a jpeg viewer that supports color.

    5. IANA Considerations

       Appendix A of this document calls for registration of a feature tag
       in the "IETF tree", as defined in section 3.1.1 of "Media Feature Tag
       Registration Procedure" [2] (i.e. these feature tags are subject to
       the "IETF Consensus" policies described in RFC 2434 [9]).

       ASN.1 identifier 1.3.6.1.8.1.30 has been assigned by the IANA for
       this registered feature tag and has been placed in the body of the
       registration.

    6. Security Considerations

       This memo is not believed to introduce any security considerations
       that are not already inherent in the use of media feature tags and
       expressions [1,2].

    7. Acknowledgements

       This proposal draws from discussions in the IETF 'conneg' working
       group.  The voice message example is based on some ideas by Glen
       Parsons.

       The author would like to thank the following people who offered
       comments that led to significant improvements:  Ted Hardie, Larry
       Masinter, Paul Hoffman, Jacob Palme, Ned Freed.

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

    8. References

       [1]  Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
            2533, March 1999.

       [2]  Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
            Registration Procedure", RFC 2506, March 1999.

       [3]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 2234, November 1997.

       [4]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
            Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
            2068, January 1997.

       [5]  Holtman, K. and A. Mutz, "Transparent Content Negotiation in
            HTTP", RFC 2295, March 1998.

       [6]  Wing, D., "Indicating Supported Media Features Using Extensions
            to DSN and MDN", RFC 2530, March 1999.

       [7]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message Bodies",
            RFC 2045, November 1996.

       [8]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Two: Media Types", RFC 2046, November
            1996.

       [9]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
            Considerations Section in RFCs", RFC 2434, October 1998.

      [10]  Hoffman, P., "Registration of Charset and Languages Media
            Features Tags", Work in Progress.

    9. Author's Address

       Graham Klyne
       Content Technologies Ltd.
       1220 Parkview,
       Arlington Business Park
       Theale
       Reading, RG7 4SA
       United Kingdom

       Phone: +44 118 930 1300
       Fax:   +44 118 930 1301
       EMail: GK@ACM.ORG

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

    Appendix A: 'Type' feature tag registration

       -  Media Feature tag name(s):

             Type

       -  ASN.1 identifier associated with this feature tag:

             1.3.6.1.8.1.30

       -  Summary of the media features indicated:

             This feature tag indicates a MIME content type that a message
             agent is capable of handling, or that is contained within some
             message data.

             The content type consists of the MIME media type and subtype,
             presented using all lower case letters and with any whitespace
             characters removed.

       -  Values appropriate for use with this feature tag:

             String

       -  The feature tag is intended primarily for use in the following
          applications, protocols, services, or negotiation mechanisms:

             Any application that wishes to convey MIME content type
             information in a media feature expression.

       -  Examples of typical use:

             (type="image/tiff")

             (& (type="text/plain") (charset=US-ASCII) )

       -  Related standards or documents:

             MIME, RFC 2045 [7]

             MIME, RFC 2046 [8]

             Registration of Charset and Languages Media Features Tags [10]

       -  Considerations particular to use in individual applications,
          protocols, services, or negotiation mechanisms:

             (N/A)

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

       -  Interoperability considerations:

             String feature matching is case sensitive, so consistent use of
             case for content type values and parameters is essential if
             content type value matching is to be achieved in a fashion
             consistent with MIME content type matching.

             Similarly, white space must be used consistently.

             This registration specifies a canonical form to be used for
             content type values (lower case letters and remove all
             whitespace).

       -  Related feature tags:

             (N/A)

       -  Intended usage:

             Common

       -  Author/Change controller:

             IETF

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    RFC 2913       MIME Content in Media Feature Expressions  September 2000

    Full Copyright Statement

       Copyright (C) The Internet Society (2000).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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