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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

  1. Media Feature tag name(s):
       Type
  1. ASN.1 identifier associated with this feature tag:
       1.3.6.1.8.1.30
  1. 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.
  1. Values appropriate for use with this feature tag:
       String
  1. 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.
  1. Examples of typical use:
       (type="image/tiff")
       (& (type="text/plain") (charset=US-ASCII) )
  1. Related standards or documents:
       MIME, RFC 2045 [7]
       MIME, RFC 2046 [8]
       Registration of Charset and Languages Media Features Tags [10]
  1. 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

  1. 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).
  1. Related feature tags:
       (N/A)
  1. Intended usage:
       Common
  1. 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]

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