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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



Klyne                       Standards Track                     [Page 1]

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.




Klyne                       Standards Track                     [Page 2]

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.





Klyne                       Standards Track                     [Page 3]

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.






Klyne                       Standards Track                     [Page 4]

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.












Klyne                       Standards Track                     [Page 5]

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



Klyne                       Standards Track                     [Page 6]

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)



Klyne                       Standards Track                     [Page 7]

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



























Klyne                       Standards Track                     [Page 8]

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
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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



















Klyne                       Standards Track                     [Page 9]