A Syntax for Describing Media Feature Sets
RFC 2533
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RFC - Proposed Standard
(March 1999; No errata)
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RFC 2533 (Proposed Standard)
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Network Working Group G. Klyne
Request for Comments: 2533 Content Technologies/5GM
Category: Standards Track March 1999
A Syntax for Describing Media Feature Sets
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 (1999). All Rights Reserved.
Abstract
A number of Internet application protocols have a need to provide
content negotiation for the resources with which they interact [1].
A framework for such negotiation is described in [2], part of which
is a way to describe the range of media features which can be handled
by the sender, recipient or document transmission format of a
message. A format for a vocabulary of individual media features and
procedures for feature registration are presented in [3].
This document introduces and describes a syntax that can be used to
define feature sets which are formed from combinations and relations
involving individual media features. Such feature sets are used to
describe the media feature handling capabilities of message senders,
recipients and file formats.
An algorithm for feature set matching is also described here.
Table of Contents
1. Introduction.............................................3
1.1 Structure of this document ...........................3
1.2 Document terminology and conventions .................4
1.3 Discussion of this document ..........................4
2. Content feature terminology and definitions..............4
3. Media feature combinations and capabilities..............5
3.1 Media features .......................................5
3.2 Media feature collections and sets ...................5
3.3 Media feature set descriptions .......................6
3.4 Media feature combination scenario ...................7
Klyne Standards Track [Page 1]
RFC 2533 A Syntax for Describing Media Feature Sets March 1999
3.4.1 Data resource options............................7
3.4.2 Recipient capabilities...........................7
3.4.3 Combined options.................................7
3.5 Feature set predicates ...............................8
3.5.1 Comparison with directory search filters.........8
3.6 Describing preferences ...............................9
3.7 Combining preferences ...............................10
4. Feature set representation..............................11
4.1 Textual representation of predicates ................11
4.2 Interpretation of feature predicate syntax ..........12
4.2.1 Filter syntax...................................12
4.2.2 Feature comparison..............................13
4.2.3 Feature tags....................................13
4.2.4 Feature values..................................14
4.2.4.1 Boolean values 14
4.2.4.2 Numeric values 14
4.2.4.3 Token values 15
4.2.4.4 String values 15
4.2.5 Notational conveniences.........................15
4.3 Feature set definition example ......................16
5. Matching feature sets...................................16
5.1 Feature set matching strategy .......................18
5.2 Formulating the goal predicate ......................19
5.3 Replace set expressions .............................19
5.4 Move logical negations inwards ......................20
5.5 Replace comparisons and logical negations ...........20
5.6 Conversion to canonical form ........................21
5.7 Grouping of feature predicates ......................22
5.8 Merge single-feature constraints ....................22
5.8.1 Rules for simplifying ordered values............23
5.8.2 Rules for simplifying unordered values..........23
6. Other features and issues...............................24
6.1 Named and auxiliary predicates ......................24
6.1.1 Defining a named predicate......................24
6.1.2 Invoking named predicates.......................25
6.1.3 Auxiliary predicates in a filter................25
6.1.4 Feature matching with named predicates..........25
6.1.5 Example.........................................26
6.2 Unit designations ...................................26
6.3 Unknown feature value data types ....................27
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