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 © 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 © 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.