MIME type feature tag: MIME Content Types in Media Feature Expressions
A media feature tag called type that lets a capability expression say which MIME content types it covers.
Klyne, G., "MIME Content Types in Media Feature Expressions", RFC 2913, DOI 10.17487/RFC2913, September 2000, <https://www.rfc-editor.org/info/rfc2913>.
@misc{rfc2913,
series = {Request for Comments},
number = 2913,
howpublished = {RFC 2913},
publisher = {RFC Editor},
doi = {10.17487/RFC2913},
url = {https://www.rfc-editor.org/info/rfc2913},
author = {G. Klyne},
title = {{MIME Content Types in Media Feature Expressions}},
pagetotal = 9,
year = 2000,
month = sep,
} RFC text
Contents
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
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.
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.
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.
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.
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
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)
- 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
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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Acknowledgement
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Internet Society.
AI ELI5
On this page
§1Introduction
RFC 2913 builds on two earlier documents: RFC 2533, which defines an expression format for describing media feature capabilities, and RFC 2506, which defines how simple media feature tags are registered. A media feature is information about facilities assumed to be available for message content to be rendered or presented. It is not meant to cover information that affects message transmission.
This memo adds one new tag whose value is a MIME content type. That lets a feature expression take account of the content type of the data it describes. The memo uses ABNF from RFC 2234.
There is already a way to write down what a device can show. This memo adds one more word to that way of writing: the kind of file.
§2Why Content Type Belongs in the Expression
The expression syntax was designed to add detail on top of basic MIME content type information. The RFC gives three situations where it helps to put the content type inside the expression itself.
- Media feature details may depend on the content type. The combining algebra and syntax cannot apply to content type information unless it appears in the expression. In HTTP 1.1 with Transparent Content Negotiation, acceptable types and other features travel in different request headers, with no clear way to say they are acceptable only in certain combinations.
- Sometimes all capability information should be in a single expression. The DSN and MDN extensions of RFC 2530 offer a single field for it.
- When features describe message content, they may refer to inner parts of a MIME composite, such as the parts of a multipart, files in a compressed archive, or encrypted data.
If you say 'I can show color' without saying which kind of file, it is unclear what you mean. Putting the file kind in the same sentence fixes that.
§3The type Feature Tag
The tag is named type and its legal value is a string containing a MIME content-type value, the kind that appears in a Content-type: header. The string must have the form type/subtype, as defined by MIME (RFC 2045), and only lower-case letters should be used.
The content type must be given without any parameters. If information normally carried in a content-type parameter is needed, a separate feature tag should be registered and used. The RFC's example is charset, which has its own registration. The RFC notes that allowing parameters was considered and rejected because of worries about canonicalization, ordering and case sensitivity, since only exact, case-sensitive character matching is defined for media feature expressions.
The registration in Appendix A says the canonical form is the media type and subtype in lower case with all whitespace removed. Because string matching is case sensitive and whitespace sensitive, using the canonical form consistently is essential if matching is to agree with MIME content type matching.
Section 1 also says that a content type value may contain parameters but this is discouraged. Section 3 is stricter: the type must be given without them.
You write the file kind in one fixed way, all small letters and no spaces, so two people writing it always get exactly the same words.
§4Worked Examples
The RFC shows four expressions. A simple text one combines type="text/plain" with charset, color and paper-size. A fax image one uses type="image/tiff" along with tags such as dpi and image-coding.
A voice message uses type="multipart/voice-message" with tags such as audio-codec and audio-channels. Some of those features apply to MIME parts inside the multipart. The RFC says the goal is not to mirror the MIME structure but to convey useful information about the possible message content.
The last example describes a web browser. It limits screen size with pix-x<=800 and pix-y<=600, then lists alternatives joined by |: HTML with limited color, plain text, GIF with mapped color, and JPEG with full color.
The examples are like checklists. One says 'plain text, in this alphabet, on this paper', and another says 'a fax picture, this sharp'. The browser one says 'I can show web pages, text and two kinds of pictures, on a small screen'.
(& (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) ) ) )§5Registration
Appendix A asks for registration of the feature tag in the IETF tree defined in RFC 2506, which falls under the IETF Consensus policy of RFC 2434. IANA has assigned the ASN.1 identifier 1.3.6.1.8.1.30 to the tag.
The registration says the tag indicates a MIME content type that a message agent can handle or that is contained within some message data. Its value type is String, its intended usage is Common, and the change controller is the IETF.
The new word was written into an official list, with a number of its own, so everyone uses it the same way.
§6Security Considerations
The memo is not believed to introduce any security considerations beyond those already inherent in using media feature tags and expressions as defined in RFC 2533 and RFC 2506.
Adding this one word does not open any new doors for trouble beyond what the existing system already has.