2738
PROPOSED STANDARD
Corrections to "A Syntax for Describing Media Feature Sets"
Authors: G. Klyne
Date: December 1999
Area: app
Working Group: conneg
Stream: IETF
Updates:
RFC 2533
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. [STANDARDS-TRACK]
RFC 2738
PROPOSED STANDARD
Network Working Group G. Klyne
Request for Comments: 2738 Content Technologies
Updates: <a href="./rfc2533">2533</a> December 1999
Category: Standards Track
<span class="h1">Corrections to "A Syntax for Describing Media Feature Sets"</span>
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 <a href="./rfc2533">RFC 2533</a>, "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
<a href="#section-1">1</a>. Introduction ............................................<a href="#page-2">2</a>
1.1 Terminology and document conventions 2
1.2 Discussion of this document 2
<a href="#section-2">2</a>. Correction to feature expression syntax .................<a href="#page-3">3</a>
<a href="#section-3">3</a>. Correction to feature set matching reduction rules ......<a href="#page-3">3</a>
<a href="#section-4">4</a>. Security Considerations .................................<a href="#page-4">4</a>
<a href="#section-5">5</a>. References ..............................................<a href="#page-4">4</a>
<a href="#section-6">6</a>. Author's Address ........................................<a href="#page-4">4</a>
<a href="#section-7">7</a>. Full Copyright Statement ................................<a href="#page-5">5</a>
<span class="grey">Klyne Standards Track [Page 1]</span><span id="page-2" ></span>
<span class="grey"><a href="./rfc2738">RFC 2738</a> Corrections to Syntax for Media Features December 1999</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
In <a href="./rfc2533">RFC 2533</a>, "A Syntax for Describing Media Feature Sets" [<a href="#ref-1" title=""A Syntax for Describing Media Feature Sets"">1</a>], 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 <a href="./rfc2533">RFC</a>
<a href="./rfc2533">2533</a>; the second affects non-normative content.
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a> Terminology and document conventions</span>
This specification uses syntax notation and conventions described in
<a href="./rfc2234">RFC 2234</a>, "Augmented BNF for Syntax Specifications: ABNF" [<a href="#ref-2" title=""Augmented BNF for Syntax Specifications: ABNF"">2</a>].
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.
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a> Discussion of this document</span>
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:
[email protected]
To subscribe to this list, send a message with the body 'subscribe'
to "[email protected]".
To see what has gone on before you subscribed, please see the mailing
list archive at:
<a href="http://www.imc.org/ietf-medfree/">http://www.imc.org/ietf-medfree/</a>
<span class="grey">Klyne Standards Track [Page 2]</span><span id="page-3" ></span>
<span class="grey"><a href="./rfc2738">RFC 2738</a> Corrections to Syntax for Media Features December 1999</span>
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Correction to feature expression syntax</span>
In <a href="#section-4.1">section 4.1</a>, <a href="./rfc2533">RFC 2533</a> 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 <a href="./rfc2533#section-4.2.5">section 4.2.5 of RFC 2533</a>, such
as:
( width=[3,4,6..17/2] )
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Correction to feature set matching reduction rules</span>
In <a href="#section-5.8.2">section 5.8.2</a>, "Rules for simplifying unordered values", <a href="./rfc2533">RFC 2533</a>
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 <a href="./rfc2533">RFC 2533</a> 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.
<span class="grey">Klyne Standards Track [Page 3]</span><span id="page-4" ></span>
<span class="grey"><a href="./rfc2738">RFC 2738</a> Corrections to Syntax for Media Features December 1999</span>
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Security Considerations</span>
Security considerations are discussed in <a href="./rfc2533">RFC 2533</a> [<a href="#ref-1" title=""A Syntax for Describing Media Feature Sets"">1</a>] and related
documents.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. References</span>
[<a id="ref-1">1</a>] Klyne, G., "A Syntax for Describing Media Feature Sets", <a href="./rfc2533">RFC</a>
<a href="./rfc2533">2533</a>, March 1999.
[<a id="ref-2">2</a>] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", <a href="./rfc2234">RFC 2234</a>, November 1997.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Author's Address</span>
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: [email protected]
<span class="grey">Klyne Standards Track [Page 4]</span><span id="page-5" ></span>
<span class="grey"><a href="./rfc2738">RFC 2738</a> Corrections to Syntax for Media Features December 1999</span>
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Full Copyright Statement</span>
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]
Annotations
Select text to annotate