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rfc:rfc2738

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
    (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.

Klyne Standards Track [Page 3] RFC 2738 Corrections to Syntax for Media Features December 1999

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

Klyne Standards Track [Page 4] RFC 2738 Corrections to Syntax for Media Features December 1999

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
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Klyne Standards Track [Page 5]

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