Network Working GroupG. Klyne
Internet-DraftMIMEsweeper Group
Expires: March 19, 2002Sep 18, 2001

A URN sub-namespace for media feature tags

Status of this Memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.

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Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.


1 This specification defines a form of URN to be used for media feature tags registered according to RFC 2506. The URN namespace used is 'ietf', defined by RFC???? and extended by RFC????


Table of Contents


1. Introduction

2 This specification defines a form of URN to be used for media feature tags registered according to RFC 2506 [3]. The URN namespace used is 'IETF', defined by RFC 2648 [3] and extended by RFC???? [10] to include names for IETF protocol parameters.

1.1 Background

3 RFC 2506 [3] defines a registry for feature tags used to describe media features of systems and documents. These tags can be used to describe the capabilities and preferences of systems that send or receive data, and also the features used by specific data instances. A companion document, RFC 2533 [4], describes a format for combining these tags into expressions that describe the combined media features of a system or document.

4 W3C have also been working on an XML [6] format for expressing client capabilities called CC/PP [9]. It is based on RDF [8], a generic XML metadata format that uses URIs to identify objects and the relationships between them.

5 It has for some time been a goal that RFC2506-registered media feature tags will be usable by CC/PP profiles, providing some basis for interworking between IETF and W3C content negotiation schemes. By defining a URI form based on the urn: scheme, this specification satsifies that goal. Thus, any new media feature tag registered will also have a defined for of URI for use in CC/PP, or any other scheme that uses URIs for identification od media features.


2. Registration template

6 The URN sub-namespace for media feature tags is defined as follows.

Registry name:
7 Media-features
8 RFC 2506
Index value:
10 The feature tag name is the registry index value. RFC2506 allows this tag name to contain uppercase letters, lowercase letters, digits, colon (":"), slash ("/"), dot ("."), percent ("%"), and dash ("-"). Feature tags are case-insensitive.
URN formation:
11 The URN for a media feature tag is formed as: "urn:ietf:params:media-features: <tag-name>", where <tag-name> is the media feature registry index value.
RFC 2141 [1] defines the format of URNs. Allowable characters include all of those noted above, with the following exceptions:
  • 12 '%' is used to introduced an escape sequence and if present in the feature tag name.
  • 13 '/' is reserved for future use and is recommended to be escaped.

These characters, if they appear as part of a media feature tag name should be replaced by their %-encoded equivalent (per URI specification [2]); i.e. '%' is represented as '%25', and '/' as '%2f'.

URNs are defined by RFC 2141 [1] as lexically equivalent if they are identical following case normalization of the urn scheme name, the namespace name and any %-escaping used. Media feature tags are defined such that upper- and lower-case ASCII characters are not distinguished. Thus, in forming a URN, all ASCII characters in the media feature tag must be expressed in lower case.


3. Examples

14 This table lists some media feature tags, and the corresponding urn: URIs.

15 urn:ietf:params:dpi
16 urn:ietf:params:pix-x
17 urn:ietf:params:jbig-stripe-size
18 urn:ietf:params:g.acme.special%2fparam


4. IANA considerations

19 This document calls for the creation of a new IETF sub-namespace, per RFC???? [10]. Registration details are in the preceding section.


5. Security considerations

20 No security considerations are introduced by the specification beyond those already inherrent in use of media feature tags.



[1] Moats, R., "URN Syntax", RFC 2141, May 1997.
[2] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[3] Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag Registration Procedure", BCP 31, RFC 2506, March 1999.
[4] Klyne, G., "A Syntax for Describing Media Feature Sets", RFC 2533, March 1999.
[5] Moats, R., "A URN Namespace for IETF Documents", RFC 2648, August 1999.
[6] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C Recommendation REC-xml, October 2000.
[7] Bray, T., Hollander, D. and A. Layman, "Namespaces in XML", W3C Recommendation xml-names, January 1999.
[8] Lassila, O. and R. Swick, "Resource Description Framework (RDF) Model and Syntax Specification", W3C Recommendation rdf-syntax, February 1999.
[9] Klyne, G., Reynolds, F., Woodrow, C. and H. Ohta, "Composite Capability/Preference Profiles (CC/PP): Structure and Vocabularies", W3C Working Draft CCPP-struct-vocab, March 2001.
[10] Mealling, M., Masinter, L., Hardie, T. and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", August 2001.


Author's Address

  Graham Klyne
  MIMEsweeper Group
  1310 Waterside
  Arlington Business Park
  Theale, Reading RG7 4SA
Phone:  +44 118 903 8000
Fax:  +44 118 903 9000


Appendix A. Revision history

A.1 draft-klyne-urn-ietf-conneg-00


33 17-Sep-2001: document initially created.
34 18-Sep-2001: added examples, revision history.


Appendix B. Todo



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