RFC2738 - Corrections to A Syntax for Describing Media Feature Sets

王朝other·作者佚名 2008-05-31

宽屏版字体: 小|中|大|超大

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.

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

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.