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 (C) The Internet Society (1999). All Rights Reserved.
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
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 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" .
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:
To subscribe to this list, send a message with the body 'subscribe' to "firstname.lastname@example.org".
To see what has gone on before you subscribed, please see the mailing list archive at:
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  and related documents.
 Klyne, G., "A Syntax for Describing Media Feature Sets", RFC 2533, March 1999.
 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
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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.
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