tech-invite   World Map     

IETF     RFCs     Groups     SIP     ABNFs    |    3GPP     Specs     Gloss.     Arch.     IMS     UICC    |    Misc.    |    search     info

RFC 2506

BCP 31
Pages: 12
Top     in Index     Prev     Next
in Group Index     No Prev: Lowest Number in Group     Next in Group     Group: CONNEG

Media Feature Tag Registration Procedure


Top       ToC       Page 1 
Network Working Group                                           K. Holtman
Request for Comments: 2506                                             TUE
BCP: 31                                                            A. Mutz
Category: Best Current Practice                            Hewlett-Packard
                                                                 T. Hardie
                                                                March 1999

                Media Feature Tag Registration Procedure

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   Recent Internet applications, such as the World Wide Web, tie
   together a great diversity in data formats, client and server
   platforms, and communities.  This has created a need for media
   feature descriptions and negotiation mechanisms in order to identify
   and reconcile the form of information to the capabilities and
   preferences of the parties involved.

   Extensible media feature identification and negotiation mechanisms
   require a common vocabulary in order to positively identify media
   features.  A registration process and authority for media features is
   defined with the intent of sharing this vocabulary between
   communicating parties. In addition, a URI tree is defined to enable
   sharing of media feature definitions without registration.

   This document defines a registration procedure which uses the
   Internet Assigned Numbers Authority (IANA) as a central registry for
   the media feature vocabulary.

   Please send comments to the CONNEG working group at <ietf->.  Discussions of the working group are archived at

Top       Page 2 

   1 Introduction .................................................  2
   2 Media feature tag definitions ................................  3
    2.1 Media feature tag purpose .................................  3
    2.2 Media feature tag syntax ..................................  4
    2.3 Media feature tag values ..................................  4
    2.4  ASN.1 identifiers for media feature tags .................  5
   3 Media feature tag registration ...............................  5
    3.1 Registration trees ........................................  6
    3.1.1 IETF tree ...............................................  6
    3.1.2 Global tree .............................................  6
    3.1.3 URL tree ................................................  6
    3.1.4 Additional registration trees ...........................  7
    3.2 Location of registered media feature tag list .............  7
    3.3 IANA procedures for registering media feature tags ........  7
    3.4 Registration template .....................................  7
   4 Security Considerations ...................................... 10
   5 Acknowledgments .............................................. 10
   6 References ................................................... 10
   7 Authors' Addresses ........................................... 11
   8 Full Copyright Statement ..................................... 12

1 Introduction

   Recent Internet applications, such as the World Wide Web, tie
   together a great diversity in data formats, client and server
   platforms, and communities.  This has created a need for media
   feature descriptions and negotiation mechanisms in order to identify
   and reconcile the form of information to the capabilities and
   preferences of the parties involved.

   Extensible media feature identification and negotiation mechanisms
   require a common vocabulary in order to positively identify media
   features.  A registration process and authority for media features is
   defined with the intent of sharing this vocabulary between
   communicating parties. In addition, a URI tree is defined to enable
   sharing of media feature definitions without registration.

   This document defines a registration procedure which uses the
   Internet Assigned Numbers Authority (IANA) as a central registry for
   the media feature vocabulary.

   This document uses the terms MUST, MUST NOT, SHOULD, SHOULD NOT and
   MAY according to usage described in [8].

Top      ToC       Page 3 
2 Media feature tag definitions

2.1 Media feature tag purpose

   Media feature tags represent individual and simple characteristics
   related to media capabilities or properties associated with the
   resource to which they are applied.  Examples of such features are:

   * the color depth of the screen on which something is to be displayed
   * the type of paper available in a printer
   * the support of the `floating 5 dimensional tables' feature
   * the fonts which are available to the recipient
   * the capability to display graphical content

   Each media feature tag identifies a single characteristic. Values
   associated with a specific tag must use the data type defined for
   that tag.  The list of allowed data types is presented below, in
   section 2.3.

   Examples of media feature tags with values are:

   * the width of a display in pixels per centimeter represented as an
   integer value.
   * a font available to a recipient, selected from an enumerated list.
   * the version of a protocol composed of integers "i.j.k", defined as
   either a value in an enumerated list or with a defined mapping to
   make the value isomorphic to a subset of integers (e.g. i*100 + j*10
   +k, assuming j<=9 and k<=9).

   Further examples of media feature tags are defined in detail
   elsewhere [4].

   Feature collections may be composed using a number of individual
   feature tags [2]. Composition of feature collections is described
   elsewhere [2].  Examples of feature collections requiring multiple
   media feature tags are:

   * the set of all fonts used by a document
   * the width and height of a display
   * the combination of color depth and resolution a display can support

   This registry presumes the availability of the MIME media type
   registry, and MIME media types MUST NOT be re-registered as media
   feature tags.  Media feature tags which are currently in use by
   individual protocols or applications MAY be registered with this
   registry if they might be applied outside of their current domain.

Top      ToC       Page 4 
   The media feature tag namespace is not bound to a particular
   transport protocol or capability exchange mechanism.  The registry is
   limited, however, to feature tags which express a capability or
   preference related to how content is presented.  Feature tags related
   to other axes of negotiation are not appropriate for this registry.
   Capability exchange mechanisms may, of course, be used to express a
   variety of capabilities or preferences.

2.2 Media feature tag syntax

   A media feature tag is a string consisting of one or more of the
   following US-ASCII characters: uppercase letters, lowercase letters,
   digits, colon (":"), slash ("/"), dot (".") percent ("%"), and dash
   ("-"). Feature tags are case-insensitive.  Dots are understood to
   potentially imply hierarchy; a feature can be subtyped by describing
   it as tree.feature.subfeature and by indicating this in the
   registration.  Tags should begin with an alphabetic character.

   In ABNF [6], this may be represented as:

   Feature-tag = ALPHA *( ALPHA / DIGIT / ":" / "/" / "." / "-" /"%" )

   Registrants should take care to avoid creating tags which might
   conflict with the creation of new registration trees; in general this
   means avoiding tags which begin with an alphabetic character followed
   by a dot.  The current registration trees are described in section 3

2.3 Media feature tag values

   The registry will initially support the use of the following data
   types as tag values:

      - signed integers
      - rational numbers
      - tokens, with equality relationship
      - tokens, with defined ordering relationship
      - strings, with standard (octet-by-octet) equality relationship
      - strings, with defined equality and/or comparison relationship

   "Token" here means the token data type as defined by [7], which may
   be summarized as:

Top      ToC       Page 5 
      token          = 1*<any CHAR except CTLs or tspecials>

      tspecials      = "(" / ")" / "<" / ">" / "@"
                     / "," / ";" / ":" / "\" / <">
                     / "/" / "[" / "]" / "?" / "="
                     / "{" / "}" / SP / HT

   At the time of registration, each tag must be associated with a
   single data type.  If that data type implies a defined comparison or
   an ordering, the registrant must define the ordering or comparison.
   For ordered tokens, this may be by full enumeration of the tokens and
   their order or by reference to an ordering mechanism.  For defined
   comparisons, a full description of the rules for comparison must be
   provided or included by reference.

   Media feature tags related to spatial or temporal characteristics
   must be registered with a single canonical unit.  It is strongly
   preferred that units be in the SI system; where current practice has
   defined units in other systems (such as pixels per inch), a
   conversion method to SI units must be provided.  Conversion methods
   should include a defined rounding practice.

2.4  ASN.1 identifiers for media feature tags

   Certain protocols use ASN.1 identifiers rather than human-readable
   representations for capability exchange.  In order to allow both
   systems to interoperate, registrants may provide an ASN.1 identifier
   or ask that IANA assign an ASN.1 identifier during registration.
   These identifiers are not required for registration, but may provide
   assistance to those building gateways or other cross-protocol
   systems.  Note that ASN.1 identifiers assigned by IANA will be
   treated as tokens, not as elements from which sub-delegated
   identifiers may be created or derived.

3 Media feature tag registration

   Media feature tags can be registered in several different
   registration trees, with different requirements as discussed below.
   The vocabulary for these requirements is taken from [5]. In general,
   a feature tag registration proposal is circulated and reviewed in a
   fashion appropriate to the tree involved.  The feature tag is then
   registered if the proposal is accepted.

   Review of a feature tag in the URI tree is not required.

Top      ToC       Page 6 
3.1 Registration trees

   The following subsections define registration "trees", distinguished
   by the use of faceted names (e.g., names of the form "tree.feature-

3.1.1 IETF tree

   The IETF tree is intended for media feature tags of general interest
   to the Internet Community, and proposals for these tags must meet the
   "IETF Consensus" policies described in [5].

   Registration in the IETF tree requires approval by the IESG and
   publication of the feature tag specification as an RFC.  Submissions
   for feature tag registration in the IETF tree can originate in any WG
   of the IETF or as an individual submission to the IESG.

   Feature tags in the IETF tree normally have names that are not
   explicitly faceted, i.e., do not contain period (".", full stop)

3.1.2 Global tree

   Tags in the global tree will be distinguished by the leading facet
   "g.".  An organization may propose either a designation indicative of
   the feature, (e.g., "g.blinktags") or a faceted designation including
   the organization name (e.g., "g.organization.blinktags").
   Organizations which have registered media types under the MIME vendor
   tree should use the same organizational name for media feature tags
   if they propose a faceted designation. The acceptance of the proposed
   designation is at the discretion of the IANA. If the IANA believes
   that a designation needs clarification it may request a new proposal
   from the proposing organization or otherwise coordinate the
   development of an appropriate designation.

   Registrations of feature tags in the global tree must meet the
   "Expert Review" policies described in [5].  In this case, a
   designated area expert will review the proposed tag, consulting with
   the members of a related mailing list.  A registration may be
   proposed for the global tree by anyone who has the need to allow for
   communication on a particular capability or preference.

3.1.3 URI tree

   A feature tag may be defined as a URI using the restricted character
   set defined above. Feature tags in the URI tree are identified by the
   leading facet "u.". The leading facet u. is followed by a URI [9]
   which conforms to the character limitations specified in this

Top      ToC       Page 7 
   document.  The author of the URI is assumed to be registration
   authority regarding features defined and described by the content of
   the URI.  These tags are considered unregistered for the purpose of
   this document.

3.1.4 Additional registration trees

   From time to time and as required by the community, the IANA may,
   with the advice and consent of the IESG, create new top-level
   registration trees. These trees may be created for external
   registration and management by (for example) well-known permanent
   bodies, such as scientific societies for media feature types specific
   to the sciences they cover.  Establishment of these new trees will be
   announced through RFC publication approved by the IESG.

3.2 Location of registered feature tag list

   Feature tag registrations will be posted in the anonymous FTP
   directory:  "
   feature-tags/" and all registered feature tags will be listed in the
   periodically issued "Assigned Numbers" RFC [currently STD 2, RFC-
   1700].  The feature tag description and other supporting material may
   also be published as an Informational RFC by sending it to "rfc-".

3.3 IANA procedures for registering feature tags

   The IANA will only register feature tags in the IETF tree in response
   to a communication from the IESG stating that a given registration
   has been approved.

   Global tags will be registered by the IANA after review by a
   designated expert.  That review will serve to ensure that the tag
   meets the technical requirements of this specification.

3.4 Registration template

   To: (Media feature tags mailing list)
   Subject: Registration of media feature tag XXXX

    | Instructions are preceded by `|'.  Some fields are optional.

   Media feature tag name:

   ASN.1 identifier associated with feature tag:       [optional]

    | To have IANA assign an ASN.1 identifier,
    | use the value "New assignment by IANA" here.

Top      ToC       Page 8 
   Summary of the media feature indicated by this feature tag:

    | Include a short (no longer than 4 lines) description or summary
    | Examples:
    |   `Use of the xyzzy feature is indicated by ...'
    |   `Support of color display is indicated by ...'
    |   `Number of colors in a palette which can be defined ...'

   Values appropriate for use with this feature tag:

     [ ] 1. The feature tag is Boolean and may have values of
          TRUE or FALSE.   A value of TRUE indicates an available
          capability.  A value of FALSE indicates the capability
          is not available.

    | If you wish to indicate two mutually exclusive possibilities
    | that cannot be expressed as the availability or lack of a
    | capability, use a two-token list, rather than a Boolean value.

     [ ] 2. The feature has an associated numeric or enumerated value.

   For case 2: Indicate the data type of the value:

      [ ] 2a. Signed Integer
      [ ] 2b. Rational number
      [ ] 2c. Token (equality relationship)
      [ ] 2d. Token (ordered)
      [ ] 2e. String (equality relationship)
      [ ] 2f. String (defined comparison)

    |IMPORTANT: You may only chose one of the above data types.

   (Only for case 2) Detailed description of the feature value meaning,
   and of the format and meaning of the feature tag values for the
   alternative results.

    | If you have selected 2d you must provide the ordering mechanism
    | or a full and ordered enumeration of possible values.  If you
    | have selected 2f, you must provide a definition of the comparison.
    | Definitions by included reference must be to stable and readily
    | available specifications:
    | If the number of alternative results is small, you may
    | enumerate the identifiers of the different results and describe
    | their meaning.
    | If there is a limited useful numeric range of result (2b, 2c),

Top      ToC       Page 9 
    | indicate the range.
    | The identifiers of the alternative results could also be
    | described by referring to another IANA registry, for example
    | the paper sizes enumerated by the Printer MIB.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms:

    | For applications, also specify the number of the first version
    | which will use the tag, if applicable.

   Examples of typical use:                               [optional]

   Related standards or documents:                        [optional]

   Considerations particular to use in individual applications,
   protocols, services, or negotiation mechanisms:        [optional]

   Interoperability considerations:                       [optional]

   Security considerations:

     Privacy concerns, related to exposure of personal information:

     Denial of service concerns related to consequences of specifying
     incorrect values:


   Additional information:                                [optional]

     Keywords:                                            [optional]

     Related feature tags:                                [optional]

     Related media types or data formats:                 [optional]

     Related markup tags:                                 [optional]

   Name(s) & email address(es) of person(s) to contact for
   further information:

   Intended usage:


Top      ToC       Page 10 
   Author/Change controller:

   Requested IANA publication delay:                      [optional]

    | A delay may only be requested for final placement in the global
    | or IETF trees, with a maximum of two months.  Organizations
    | requesting a registration with a publication delay should note
    | that this delays only the official publication of the tag
    | and does not prevent information on it from being disseminated
    | by the members of the relevant mailing list.

   Other information:                                     [optional]

    |  Any other information that the author deems interesting may be
    |  added here.

4 Security Considerations

   Negotiation mechanisms reveal information about one party to other
   parties.  This may raise privacy concerns, and may allow a malicious
   party to make better guesses about the presence of specific security

5 Acknowledgments

   The details of the registration procedure in this document were
   directly adapted from [1].  Much of the text in section 3 was
   directly copied from this source.

   The idea of creating a vocabulary of areas of media features,
   maintained in a central open registry, is due to discussions on
   extensible negotiation mechanisms [3] in the IETF HTTP working group.

   The authors wish to thank Larry Masinter, Graham  Klyne, Al Gilman,
   Dan Wing, Jacob Palme, and Martin Duerst for their contributions to
   discussions about media feature tag registration.

6 References

   [1] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail
       Extensions (MIME) Part Four: Registration Procedures", BCP 13,
       RFC 2048, November 1996.

   [2] Klyne, G., "An algebra for describing media feature sets", Work
       in Progress.

   [3] Holtman, K. and  A. Mutz, "Transparent Content Negotiation in
       HTTP.  RFC 2295, March 1998.

Top      ToC       Page 11 
   [4] Masinter, L., Holtman, K., Mutz, A. and D. Wing, "Media Features
       for Display, Print, and Fax", RFC 2534, March 1999.

   [5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [6] Crocker, D., Ed., "Augmented BNF for Syntax Specifications:
       ABNF", RFC 2234, November 1997.

   [7] Fielding, R., Gettys, J., Mogul, J. Frystyk, H. and T. Berners-
       Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, January

   [8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

   [9] Berners-Lee, T., "Universal Resource Identifiers in WWW," RFC
       1630, June 1994.

7 Authors' Addresses

   Koen Holtman
   Technische Universiteit Eindhoven
   Postbus 513
   Kamer HG 6.57
   5600 MB Eindhoven
   The Netherlands


   Andrew H. Mutz
   Hewlett-Packard Company
   11000 Wolfe Rd. 42UO
   Cupertino CA 95014 USA

   Fax +1 408 447 4439

   Ted Hardie
   901 Marshall Street
   Redwood City, CA 94063 USA


Top      ToC       Page 12 
8 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

   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