Identifying Composite Media Features
RFC 2938

Document Type RFC - Proposed Standard (September 2000; Errata)
Updates RFC 2533
Last updated 2013-03-02
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Network Working Group                                            G. Klyne
Request for Comments: 2938                           Content Technologies
Updates: 2533                                                 L. Masinter
Category: Standards Track                                            AT&T
                                                           September 2000

                  Identifying Composite Media Features

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 RFC 2533, an expression format is presented for describing media
   feature capabilities as a combination of simple media feature tags.

   This document describes an abbreviated format for a composite media
   feature set, based upon a hash of the feature expression describing
   that composite.

Table of Contents

   1.    Introduction ................................................2
   1.1   Organization of this document ...............................2
   1.2   Terminology and document conventions ........................2
   2.    Motivation and goals ........................................3
   3.    Composite feature representation ............................4
   3.1   Feature set hashed reference format .........................5
   3.1.1 Hash value calculation ......................................6
   3.1.2 Base-32 value representation ................................7
   3.2   Resolving feature set identifiers ...........................8
   3.2.1 Query protocol ..............................................8
   3.2.2 Inline feature set details ..................................9
   4.    Examples ...................................................10
   5.    Internationalization Considerations ........................12
   6.    Security Considerations ....................................13
   7.    Acknowledgements ...........................................13
   8.    References .................................................13

Klyne & Masinter            Standards Track                     [Page 1]
RFC 2938          Identifying Composite Media Features    September 2000

   9.    Authors' Addresses .........................................15
   10.   Appendix A: The birthday paradox ...........................16
   11.   Full Copyright Statement ...................................18

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 [2].

   This document proposes an abbreviated format for a composite media
   feature set, based upon a hash of the feature expression describing
   that composite.

   This memo extends and builds upon the expression syntax described in
   RFC 2533 [1], and it is assumed that the reader is familiar with the
   interpretation of feature set expressions described there.

1.1 Organization of this document

   Section 2 sets out some of the background and goals for feature set
   references.

   Section 3 presents a syntax for feature set references, and describes
   how they are related to feature set expressions.

1.2 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.

   dereference
            the act of replacing a feature set reference with its
            corresponding feature set expression.  Also called
            "resolution".

   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).

Klyne & Masinter            Standards Track                     [Page 2]
RFC 2938          Identifying Composite Media Features    September 2000

   feature set reference
            a brief construct that references some feature set.  (See
            also: "dereference".)

   feature set tag
            a name that conforms to the syntax of a feature tag [2] that
            is used to denote a feature set rather than a single
            feature.

   resolution
            (See "dereference").

   This specification uses syntax notation and conventions described
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