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IETF conneg working group
Internet draft
Category: Work-in-progress
Graham Klyne
Content Technologies
4 April 2000
Expires: October 2000

MIME content types in media feature expressions
  <draft-ietf-conneg-feature-type-03.txt>

Status of this memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress."

To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html.

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 MIME content type. This allows the construction of feature expressions that take account of the MIME content type of the corresponding data.

Internet draft         MIME content types in media feature expressions
4 April 2000

Table of contents

  1. Introduction ............................................2
     1.1 Terminology and document conventions                 2
     1.2 Discussion of this document                          3
  2. Motivation and goals ....................................4
  3. MIME content type feature tag ...........................4
  4. Examples ................................................5
     4.1 Simple text                                          5
     4.2 Fax image                                            5
     4.3 Voice message                                        5
     4.4 Web browser capabilities                             6
  5. IANA considerations .....................................6
  6. Security considerations .................................6
  7. Acknowledgements ........................................6
  8. References ..............................................6
  9. Author's address ........................................8
  Appendix A: 'Type' feature tag registration ................8
  Full copyright statement ...................................10
  Revision history ...........................................11

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.
Internet draft         MIME content types in media feature expressions
4 April 2000
  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.

1.2 Discussion of this document

Discussion of this document should take place on the content negotiation and media feature registration mailing list hosted by the Internet Mail Consortium (IMC):

Please send comments regarding this document to:

      ietf-medfree@imc.org

To subscribe to this list, send a message with the body 'subscribe' to "ietf-medfree-request@imc.org".

To see what has gone on before you subscribed, please see the mailing list archive at:

      http://www.imc.org/ietf-medfree/
Internet draft         MIME content types in media feature expressions
4 April 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.

Internet draft         MIME content types in media feature expressions
4 April 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.
Internet draft         MIME content types in media feature expressions
4 April 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]).

An ASN.1 identifier should be assigned for this registered feature tag and replaced 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.

8. References

[1]  RFC 2533, "A syntax for describing media feature sets"
     Graham Klyne, 5GM/Content Technologies
     March 1999.
Internet draft         MIME content types in media feature expressions
4 April 2000
[2]  RFC 2506, "Media Feature Tag Registration Procedure"
     Koen Holtman, TUE
     Andrew Mutz, Hewlett-Packard
     Ted Hardie, NASA
     March 1999.
[3]  RFC 2234, "Augmented BNF for Syntax Specifications: ABNF"
     D. Crocker (editor), Internet Mail Consortium
     P. Overell, Demon Internet Ltd.
     November 1997.
[4]  RFC 2068, "Hyptertext Transfer Protocol -- HTTP/1.1"
     R. Fielding, UC Irvine
     J. Gettys,
     J. Mogul, DEC
     H. Frytyk,
     T. Berners-Lee, MIT/LCS
     January 1997.
     (Non-normative)
[5]  RFC 2295, "Transparent Content Negotiation in HTTP"
     Koen Holtman, TUE
     Andrew Mutz, Hewlett Packard
     March 1998.
     (Non-normative)
[6]  RFC 2530, "Indicating Supported Media Features Using Extensions
     to DSN and MDN"
     Dan Wing, Cisco Systems
     March 1999.
     (Non-normative)
[7]  RFC 2045, "Multipurpose Internet Mail Extensions (MIME)
     Part 1: Format of Internet message bodies"
     N. Freed, Innosoft
     N. Borenstein, First Virtual
     November 1996.
[8]  RFC 2046, "Multipurpose Internet Mail Extensions (MIME)
     Part 2: Media types"
     N. Freed, Innosoft
     N. Borenstein, First Virtual
     November 1996.
[9]  RFC 2434, "Guidelines for Writing an IANA Considerations Section
     in RFCs"
     T. Narten, IBM
     H. Alvestrand, Maxware
     October 1998.
Internet draft         MIME content types in media feature expressions
4 April 2000
[10] "Registration of Charset and Languages Media Features Tags"
     Paul Hoffman, IMC
     Internet draft: <draft-hoffman-char-lang-media-01.txt>
     Work in progress, July 1999.
     (Non-normative)

9. Author's address

  Graham Klyne
  Content Technologies Ltd.
  1220 Parkview,
  Arlington Business Park
  Theale
  Reading, RG7 4SA
  United Kingdom.
  Telephone: +44 118 930 1300
  Facsimile: +44 118 930 1301
  E-mail:    GK@ACM.ORG

Appendix A: 'Type' feature tag registration

- Media Feature tag name(s):
     Type

- ASN.1 identifier associated with this feature tag:

     [[[New assignment by IANA]]]

- 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.
Internet draft         MIME content types in media feature expressions
4 April 2000

- 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)

- 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
Internet draft         MIME content types in media feature expressions
4 April 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.

Internet draft         MIME content types in media feature expressions
4 April 2000

Revision history

[[[RFC editor: please reove this section on publication]]]

00a 16-Feb-1999 Initial draft.

01a 16-Feb-1999 Added pointers to mailing list for discussion.

01b 04-Mar-1999 Various editorial improvements.

01c 29-Apr-1999 Improved web browser example.

  02a  29-Apr-1999  Highlight and forward reference the content-type
                    parameter issue in the introduction.
  02b  20-Jul-1999  Incorporate review comments.  Also cite charset
                    feature type registration work-in-progress.

02c 14-Nov-1999 Fix RFC 2533 number in references.

02d 30-Nov-1999 Add note about 'charset' feature tag to registration template. Moved copyright notice to end of document text.
  03a  04-Apr-2000  Restrict type tag value to content-type, without
                    parameters.  Add text explaining how to deal with
                    content-type parameters in media feature
                    expressions.