RFC 2738

Corrections to "A Syntax for Describing Media Feature Sets"

📅 published December 1999, from ietf

RFC 2738 💖 Corrections to "A Syntax for Describing Media Feature Sets" 🎀

Author: G. Klyne (Content Technologies) ✨ Date: December 1999 🗓️ Category: Standards Track 👑 Updates: RFC 2533 🔥 Obsoletes: nothing, this is a patch, not a replacement 🩹

Status of this Memo 📣

okay so this is an Internet standards track protocol doc for the Internet community, and it's basically asking everyone to discuss it and suggest improvements 🫶 for the official standardization state of this protocol, check the current edition of "Internet Official Protocol Standards" (STD 1). distribution of this memo is unlimited, so share away 🦋

Copyright Notice 🌸

Abstract ✨

so RFC 2533, "A Syntax for Describing Media Feature Sets", gives us an expression format for describing media feature capabilities using simple media feature tags. this memo is two corrections to that spec 🧚 one fixes a bug in the formal syntax, and the other fixes a bug in the rules for reducing feature comparison predicates. bestie, it's a tidy-up 🧹

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 🎀

okay so RFC 2533, "A Syntax for Describing Media Feature Sets" [1], presents an expression format for describing media feature capabilities using simple media feature tags 💅 it gives message handling agents a way to describe the media feature content of messages they can handle. that memo also has an algorithm for finding the common capabilities shared by two different feature expressions, which is giving main character energy 👑

this memo is two corrections to that spec: one fixes an error in the formal syntax, and the other fixes an error in the feature set matching algorithm, specifically the rules for reducing feature comparison predicates 🔥

the first correction changes the normative content of RFC 2533, and the second changes non-normative content, so you know which one is the real rule-change 🙈

1.1 Terminology and document conventions 📚

this spec uses the syntax notation and conventions from 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 doc should happen on the content negotiation and media feature registration mailing list hosted by the Internet Mail Consortium (IMC) 🫶

send your comments to:

ietf-medfree@imc.org

to subscribe, send a message with the body 'subscribe' to:

ietf-medfree-request@imc.org

to catch up on what happened before you joined, check 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 like this:

set        =  attr "=" "[" setentry *( "," setentry ) "]"
setentry   =  value "/" range

the production for 'setentry' should read:

setentry   =  value / range

so the '/' character is not a character literal. it separates two alternative forms for 'setentry' 🦄 this fixed syntax makes the set expression examples in section 4.2.5 of RFC 2533 work, like:

( 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 these 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 feature 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. go read those, they matter 🙏

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 💅

you can copy and share this document and translations of it, and you can write derivative works that comment on or explain it or help with implementing it. you can prepare, copy, publish and distribute those, in whole or in part, with no restrictions of any kind, as long as the above copyright notice and this paragraph are on all copies and derivative works. the 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. the exceptions are when needed to develop Internet standards, in which case the procedures for copyrights defined in the Internet Standards process must be followed, or when translating it into languages other than English. 🌍

the permissions above are perpetual and will not be revoked by the Internet Society or its successors or assigns ♾️

this document and the information in it are 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 ✨

📦 where this page comes from

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