Corrections to "A Syntax for Describing Media Feature Sets"
RFC 2738

Document Type RFC - Proposed Standard (December 1999; No errata)
Updates RFC 2533
Last updated 2013-03-02
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Network Working Group                                         G. Klyne
Request for Comments: 2738                        Content Technologies
Updates: 2533                                            December 1999
Category: Standards Track

      Corrections to "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

   In RFC 2533, "A Syntax for Describing Media Feature Sets", an
   expression format is presented for describing media feature
   capabilities using simple media feature tags.

   This memo contains two corrections to that specification:  one fixes
   an error in the formal syntax specification, and the other fixes an
   error in the rules for reducing feature comparison predicates.

Table of Contents

   1. Introduction ............................................2
      1.1 Terminology and document conventions                 2
      1.2 Discussion of this document                          2
   2. Correction to feature expression syntax .................3
   3. Correction to feature set matching reduction rules ......3
   4. Security Considerations .................................4
   5. References ..............................................4
   6. Author's Address ........................................4
   7. Full Copyright Statement ................................5

Klyne                       Standards Track                     [Page 1]
RFC 2738        Corrections to Syntax for Media Features   December 1999

1. Introduction

   In RFC 2533, "A Syntax for Describing Media Feature Sets" [1], an
   expression format is presented for describing media feature
   capabilities using simple media feature tags.  This provides a format
   for message handling agents to describe the media feature content of
   messages that they can handle.  That memo also describes an algorithm
   for finding the common capabilities expressed by two different
   feature expressions.

   This memo contains two corrections to that specification:  one fixes
   an error in the formal syntax specification, and the other fixes an
   error in the feature set matching algorithm, in the rules for
   reducing feature comparison predicates.

   The first of these corrections affects the normative content of RFC
   2533;  the second affects non-normative content.

1.1 Terminology and document conventions

   This specification uses syntax notation and conventions described in
   RFC 2234, "Augmented BNF for Syntax Specifications: ABNF" [2].

       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/

Klyne                       Standards Track                     [Page 2]
RFC 2738        Corrections to Syntax for Media Features   December 1999

2. Correction to feature expression syntax

   In section 4.1, RFC 2533 defines the syntax for a "set" expression as
   follows:

       set        =  attr "=" "[" setentry *( "," setentry ) "]"
       setentry   =  value "/" range

   The production for 'setentry' should read:

       setentry   =  value / range

   That is: the '/' character is not a character literal, but separates
   two alternative forms for 'setentry'.  This corrected syntax allows
   the set expression examples given in section 4.2.5 of RFC 2533, such
   as:

       ( width=[3,4,6..17/2] )

3. Correction to feature set matching reduction rules

   In section 5.8.2, "Rules for simplifying unordered values", RFC 2533
   lists the following rewriting rules for simplifying feature tag
   comparisons with unordered values:

      (LE f a)  (LE f b)      -->  (LE f a),   a=b
                                   FALSE,      otherwise
      (LE f a)  (GE f b)      -->  FALSE,      a!=b
      (LE f a)  (NL f b)      -->  (LE f a)    a!=b
                                   FALSE,      otherwise
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