IETF conneg working group Graham Klyne Internet draft 5GM/Content Technologies Category: Work-in-progress 27 April 1999 Expires: August 1999 Corrections to "A syntax for describing media feature sets" Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society 1999. All Rights Reserved. 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. Klyne Work-in-progress [Page 1] Internet draft Corrections to Syntax for Media Features 27 April 1999 Table of contents 1. Introduction ............................................2 1.1 Terminology and document conventions 2 1.2 Discussion of this document 3 2. Correction to feature expression syntax .................3 3. Correction to feature set matching reduction rules ......3 4. Security considerations .................................4 5. Full copyright statement ................................4 6. References ..............................................5 7. Author's address ........................................5 1. Introduction In RFC 2533, "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 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 document should take place on the content negotiation and media feature registration mailing list hosted by the Internet Mail Consortium (IMC). Klyne Work-in-progress [Page 2] Internet draft Corrections to Syntax for Media Features 27 April 1999 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, 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 Klyne Work-in-progress [Page 3] Internet draft Corrections to Syntax for Media Features 27 April 1999 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. 4. Security considerations Security considerations are discussed in RFC 2533 [1] and related documents. 5. 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. Klyne Work-in-progress [Page 4] Internet draft Corrections to Syntax for Media Features 27 April 1999 6. References [1] RFC 2253, "A syntax for describing media feature sets" Graham Klyne, 5GM/Content Technologies March 1999. [2] RFC 2234, "Augmented BNF for Syntax Specifications: ABNF" D. Crocker (editor), Internet Mail Consortium P. Overell, Demon Internet Ltd. November 1997. 7. Author's address Graham Klyne 5th Generation Messaging Ltd. Content Technologies Ltd. 5 Watlington Street Forum 1, Station Road Nettlebed Theale Henley-on-Thames, RG9 5AB Reading, RG7 4RA United Kingdom United Kingdom. Telephone: +44 1491 641 641 +44 118 930 1300 Facsimile: +44 1491 641 611 +44 118 930 1301 E-mail: GK@ACM.ORG Klyne Work-in-progress [Page 5]