RFC2738 - Corrections to A Syntax for Describing Media Feature Sets

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　　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 RFC2533, "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 RFC2533, "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
RFC2234, "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, RFC2533 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 RFC2533, 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", RFC2533
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 RFC2533 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 RFC2533 [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", RFC2234, 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
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 RFCEditor function is currently provided by the
Internet Society.

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