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

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/

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

   The second of these rules is incomplete, and should read:

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

        NOTE:  implementation experience with these rules has
        suggested a revised feaure set matching algorithm with a
        more useful set of simplification rules.  Apart from the
        change noted above, the algorithm given in RFC 2533 has
        been implemented and shown to work as intended, but the
        results generated are not always in the most convenient
        form.  It is planned to test and publish a revised
        algorithm at a future date.

4. Security Considerations

   Security considerations are discussed in RFC 2533 [1] and related
   documents.

5. References

   [1]  Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
        2533, March 1999.

   [2]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

6. 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

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
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.