IETF conneg working group Graham Klyne
Internet draft 5GM/Content Technologies
Category: Work-in-progress 16 February 1999
Expires: August 1999
MIME content types in media feature expressions
<draft-ietf-conneg-feature-type-00.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 1999. 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.
Klyne Work-in-progress [Page 1]
Internet draft MIME content types in media feature expressions
16 February 1999
Table of contents
1. Introduction ............................................2
1.1 Terminology and document conventions 2
2. Motivation and goals ....................................3
3. MIME content type feature tag ...........................4
4. Examples ................................................4
4.1 Simple text 4
4.2 Fax image 4
4.3 Voice message 5
4.4 Web browser capabilities 5
5. IANA considerations .....................................5
6. Security considerations .................................5
7. Full copyright statement ................................6
8. Acknowledgements ........................................6
9. References ..............................................6
10. Author's address .......................................8
Appendix A: 'Type' feature tag registration ................8
Appendix B: Revision history ...............................10
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.
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.
The terms are listed in alphabetical order.
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.)
Klyne Work-in-progress [Page 2]
Internet draft MIME content types in media feature expressions
16 February 1999
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).
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.
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.
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
be 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.
Klyne Work-in-progress [Page 3]
Internet draft MIME content types in media feature expressions
16 February 1999
3. MIME content type feature tag
Feature tag name Legal values
---------------- ------------
type <string>
containing any 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 in the corresponding MIME-
formatted data).
The media type should be given without any parameter values. The
intention here is that information that is conveyed in MIME content
type parameters is more usefully handled by separate feature tags
in a media feature expression.
NOTE: content type parameters in a 'type' value are not
prohibited but, for the purposes of feature set matching
[1], content types with and without parameters are
treated as completely distinct values and may lead to
unexpected results.
4. Examples
4.1 Simple text
(& (type="text/plain") (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) )
Klyne Work-in-progress [Page 4]
Internet draft MIME content types in media feature expressions
16 February 1999
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.
4.4 Web browser capabilities
(| (& (type=["text/plain","text/html"])
(color=limited) (paper-size=A4) )
(& (type=["image/gif","image/jpeg"])
(color=mapped) (pix-x<=800) (pix-y<=600) ) )
5. IANA considerations
Appendix A of this document calls for registrations of feature tags
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 identifiers should be assigned for each of these registered
feature tags 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].
Klyne Work-in-progress [Page 5]
Internet draft MIME content types in media feature expressions
16 February 1999
7. Full copyright statement
Copyright (C) The Internet Society 1999. 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.
8. Acknowledgements
This proposal draws from discussions in the IETF 'conneg' working
group. The voice message example is based on some ideas by Glen
Parsons.
9. References
[1] "A syntax for describing media feature sets"
Graham Klyne, 5GM/Content Technologies
Internet draft: <draft-ietf-conneg-feature-syntax-00.txt>"
Work in progress, September 1998.
[2] "Media Feature Tag Registration Procedure"
Koen Holtman, TUE
Andrew Mutz, Hewlett-Packard
Ted Hardie, NASA
Internet draft: <draft-ietf-conneg-feature-reg-03.txt>
Work in progress, July 1998.
Klyne Work-in-progress [Page 6]
Internet draft MIME content types in media feature expressions
16 February 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.
[5] RFC 2295, "Transparent Content Negotiation in HTTP"
Koen Holtman, TUE
Andrew Mutz, Hewlett Packard
March 1998.
[6] "Indicating Supported Media Features Using Extensions to DSN and
MDN"
Dan Wing, Cisco Systems
Internet draft: <draft-ietf-fax-reporting-extensions-05.txt>
Work in progress, January 1999.
[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.
Klyne Work-in-progress [Page 7]
Internet draft MIME content types in media feature expressions
16 February 1999
10. Author's address
Graham Klyne
5th Generation Messaging Ltd. Content Technologies Ltd.
5 Watlington Street Forum 1, Station Road
Nettlebed Theale
Henley-on-Thames, RG9 5AB Reading, RG7 4RA
United Kingdom United Kingdom.
Telephone: +44 1491 641 641 +44 118 930 1300
Facsimile: +44 1491 641 611 +44 118 930 1301
E-mail: GK@ACM.ORG
Appendix A: 'Type' feature tag registration
- Media Feature tag name(s):
Type
- ASN.1 identifiers associated with this feature tag:
[[[New assignments 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 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.
In exceptional cases, content type parameters may be included, in
which case the parameter name is also presented in lower case
letters, with all whitespace surrounding the ';' and '=' removed.
The parameter value should be presented in some canonical form.
- 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.
Klyne Work-in-progress [Page 8]
Internet draft MIME content types in media feature expressions
16 February 1999
- Examples of typical use:
(type="text/plain")
(type="text/plain;charset=iso-8859-1")
The second example is not a recommended form. But note that all
spaces around the 'charset' parameter are removed, and the name
and value are presented in lower case.
- Related standards or documents:
MIME, RFC 2045 [7]
MIME, RFC 2046 [8]
- 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 contains specifies a canonical form to be used
for content type values (lower case letters and remove all
whitespace). If content type parameters are introduced, all
letters and whitespace that are not part of the parameter value
are treated similarly. The canonical form for parameter values
must be appropriate to the equivalence rules for that value.
- Related feature tags:
(N/A)
- Intended usage:
Common
- Author/Change controller:
IETF
Klyne Work-in-progress [Page 9]
Internet draft MIME content types in media feature expressions
16 February 1999
Appendix B: Revision history
00a 16-Feb-1999 Initial draft.
TODO:
o Beef up web browser example
o Discuss: should the feature syntax be extended to allow content
types to be "unstringed", hence providing more relaxed matching
rules?
Klyne Work-in-progress [Page 10]
