IETF conneg working group Graham Klyne
Internet draft Content Technologies
Category: Work-in-progress 4 April 2000
Expires: October 2000
MIME content types in media feature expressions
<draft-ietf-conneg-feature-type-03.txt>
Status of this memo
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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.
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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.
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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/
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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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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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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.
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[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.
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[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.
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- 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
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Full copyright statement
Copyright (C) The Internet Society 2000. All Rights Reserved.
This document and translations of it may be copied and furnished to
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it or assist in its implementation may be prepared, copied,
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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.
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