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Versions: 00 RFC 2738

IETF conneg working group                                 Graham Klyne
Internet draft                                5GM/Content Technologies
Category: Work-in-progress                               27 April 1999
                                                  Expires: August 1999


     Corrections to "A syntax for describing media feature sets"
             <draft-ietf-conneg-feature-syntax-er-00.txt>


Status of this memo

  This document is an Internet-Draft and is in full conformance with
  all provisions of Section 10 of RFC 2026.

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF), its areas, and its working groups.  Note that
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  To view the list Internet-Draft Shadow Directories, see
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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.









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Internet draft                Corrections to Syntax for Media Features
27 April 1999



Table of contents

  1. Introduction ............................................2
     1.1 Terminology and document conventions                 2
     1.2 Discussion of this document                          3
  2. Correction to feature expression syntax .................3
  3. Correction to feature set matching reduction rules ......3
  4. Security considerations .................................4
  5. Full copyright statement ................................4
  6. References ..............................................5
  7. Author's address ........................................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).




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Internet draft                Corrections to Syntax for Media Features
27 April 1999


  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



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Internet draft                Corrections to Syntax for Media Features
27 April 1999


       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. 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.











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Internet draft                Corrections to Syntax for Media Features
27 April 1999


6. References

[1]  RFC 2253, "A syntax for describing media feature sets"
     Graham Klyne, 5GM/Content Technologies
     March 1999.

[2]  RFC 2234, "Augmented BNF for Syntax Specifications: ABNF"
     D. Crocker (editor), Internet Mail Consortium
     P. Overell, Demon Internet Ltd.
     November 1997.

7. Author's address

  Graham Klyne
  5th Generation Messaging Ltd.    Content Technologies Ltd.
  5 Watlington Street              Forum 1, Station Road
  Nettlebed                        Theale
  Henley-on-Thames, RG9 5AB        Reading, RG7 4RA
  United Kingdom                   United Kingdom.
  Telephone: +44 1491 641 641      +44 118 930 1300
  Facsimile: +44 1491 641 611      +44 118 930 1301
  E-mail: GK@ACM.ORG






























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