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Versions: 00 02 03 rfc2506                                              
HTTP Working Group                                     Koen Holtman, TUE
Internet-Draft                              Andrew Mutz, Hewlett-Packard
Expires: September 11, 1998                             Ted Hardie, NASA
                                                          March 11, 1998

                    Content Feature Tag Registration Procedure

                     draft-ietf-conneg-feature-reg-00.txt


STATUS OF THIS MEMO

This document is an Internet-Draft. Internet-Drafts are working
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Copyright (C) The Internet Society (1998).  All Rights Reserved.


Distribution of this document is unlimited.  Please send comments to the
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ABSTRACT

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

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


TABLE OF CONTENTS

   1 Introduction

   2 Feature tag definitions
    2.1 Feature tag purpose
    2.2 Feature tag syntax

   3 Feature tag registration
    3.1 Registration trees
    3.1.1 IETF tree
    3.1.2 Global tree
    3.1.3 URL tree
    3.1.4 Additional registration trees
    3.2 Location of registered feature tag list
    3.3 IANA procedures for registering feature tags
    3.4 Registration template

   4 Security considerations

   5 Acknowledgments

   6 References

   7 Authors' addresses

   Appendix A: IANA and RFC editor to-do list



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

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


2 Feature tag definitions

2.1 Feature tag purpose

  Content feature tags represent individual and simple dimensions of
  feature capability.  Examples of content features related to media
  are:

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

  A feature tag identifies a single dimension of characteristic. Feature
  tag values should be represented as(and must be representable or
  isomorphic to) boolean, enumerated values, or numeric values.
  Examples of feature tags are defined in detail elsewhere [4].


  Many features are not Boolean and require values to qualify them.
  Examples of feature tags with values are:
  * the width of a display in pixels represented as an integer value.
  * the fonts available to a recipient as an enumerated list.
  * the version of a protocol composed of integers "i.j.k", defined as
  either a value in an enumerated list or isomorphic to the integer
  numeric value ijk.


  Complex features should be composed using a number of individual
  content feature tags [2]. Composition of complex features is described
  elsewhere [2].  Examples of complex features requiring multiple
  feature tags are:

  * the width and height of a display
  * the combination of color depth and resolution a display can support

  Features of content already described and registered (such as MIME
  types) should not be registered again.

  The feature tag namespace is not bound to a particular transport
  protocol or capability exchange mechanism.  There is no restriction on
  the type of content feature which may be identified by a feature tag,
  other than the intent of expressing a capability or a preference
  regarding a presentation-related feature of content.  Feature tags
  indicating political or social context are not appropriate.

2.2 Feature tag syntax

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

  A feature tag value is a string consisting of one or more of the
  following US-ASCII characters: uppercase letters, lowercase letters,
  digits, colon (":"), slash("/"), dot("."), and dash ("-"). Values are
  case-insensitive.  Feature tag values should be simple atomic values
  of either enumerated or numeric form and must be isomorphic with
  either enumerated or numeric values.  The form of feature tag values
  is indicated upon feature registration.



3 Feature tag registration

  Feature tags can be registered in several different registration
  trees, with different requirements as discussed below.  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
  registration in the URI tree is not required.

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-
  name").


3.1.1 IETF tree

  The IETF tree is intended for feature tags of general interest to the
  Internet Community.  Registration in the IETF tree requires approval
  by the IESG and publication of the feature tag specification an RFC.
  Submissions for feature tag registration in the IETF tree can
  originate in any WG of IETF.

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

  The "owner" of a feature tag in the IETF tree is assumed to be the
  IETF itself.  Modification or alteration of the specification requires
  the same level of processing (e.g. standards track) required for the
  initial registration.

3.1.2 Global tree

  Tags in the global tree will be distinguished by the leading facet
  "g.". That may be followed, at the discretion of the registration, by
  either a designation indicative of the feature, (e.g., "g.blinktags")
  or by an IANA-approved designation of the producer's name which is
  then followed by a designation of the feature (e.g.,
  g.bigcompany.obscurefeature).


  Registration of a new content feature tag in the global tree is
  initiated by the submission of a registration proposal to IANA.  The
  global tree is intended for feature tags of general interest to the
  Internet Community.  Unlike registration in the IETF tree,
  registration in the global tree does not imply or require approval by
  the IESG.  A registration may be placed in the global tree by anyone
  who has the need to allow for communication on a particular capability
  or preference.

  If the creator of an Internet service or product introduces a new
  content feature to the Internet Community, and if it is meaningful to
  identify a content feature tag with it, the feature can be associated
  with a feature tag by registration in the global tree.

  Registration of a content feature tag does not in itself imply any
  form of ownership or control of the underlying feature by the
  originator of the registration.

  The owner of "global" content feature tag is the person or entity
  making the registration, or one to whom responsibility has been
  transferred as described below.

  While public exposure and review of feature tags to be registered in
  the global tree is not required, using the ietf-feature-tags@iana.org
  list for review is strongly encouraged to improve the quality of those
  specifications.


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 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 content 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 "ftp://ftp.isi.edu/in-notes/iana/assignments/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-
  editor@isi.edu" (please follow the instructions to RFC authors [RFC-
  1543]).


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 automatically and without
  any formal review as long as the following minimal conditions are met:

  (1)   A feature tag must serve as an actual identifier of an area of
       content feature or capability.

  (2)   A feature tag name must be unique, and must conform to the
       syntax in section 2.

  (3)   An openly available description of the feature or capability is
       minimally required.  The specification of a feature tag must
       state whether the choice in the indicated area is a simple yes/no
       choice, a numeric value, or a choice among enumerated or multiple
       values. If the choice is among multiple values, and a canonical
       format for these values is defined, these values must conform to
       the syntax in section 2.

  (4)   Any security considerations given must not be obviously bogus.
       (It is neither possible nor necessary for the IANA to conduct a
       comprehensive security review of feature tag registrations.
       Nevertheless, the IANA has the authority to identify obviously
       incompetent material and exclude it.)


3.4 Registration template

   To: ietf-feature-tags@iana.org (Feature tags mailing list)
        (or directly to iana@iana.org)
   Subject: Registration of feature tag XXXX


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

   Content feature tag name:

   Summary of the content 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 ...'

   Number of possible values associated with this feature tag:

     [ ] 1. The feature tag is Boolean and the feature tag has no
          associated value. The tag indicates presence (or absence) of
          the feature.
     [ ] 2. The feature has an associated numeric or enumerated value.

   For case 1: describe the nature of the `yes' and `no' alternatives:


   For case 2: How is a single alternative result naturally identified?

     [ ] 2a. With a name, keyword, label, or tag (e.g. a language tag)
     [ ] 2b. With an integer value
     [ ] 2c. With a numeric value of a non-integer type (e.g. float)
     [ ] 2d. Other (must be isomorphic to 2a, 2b, or 2c.)

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

    | If the number of alternative results is small, the description
    | could simply enumerate the identifiers of the different results
    | and describe their meaning.
    |
    | If there is a limited useful numeric range of result (2b, 2c),
    | indicate the range.
    |
    | The identifiers of the alternative results could also be
    | described by referring to another IANA registry, for example
    | the MIME media type registry.

  Expected value or behavior in the absence of the feature tag (if
  applicable):

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

    | 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:

     Other:

   Additional information:                                [optional]

     Keywords:                                            [optional]

     Related feature tags:                                [optional]

     Related media types or data formats:                 [optional]

     Related HTML markup tags:                            [optional]

   Person & email address to contact for further information:

   Intended usage:

    | one of COMMON, LIMITED USE or OBSOLETE

   Author/Change controller:

   Requested IANA publication delay:                      [optional]

    | A delay may only be requested for registration in global or
    | local trees, with a maximum of two months.

   Other information:                                     [optional]

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


4 Security considerations

   When used, negotiation mechanisms usually reveal some information
   about one party to other parties.  This may raise privacy concerns,
   and may allow a malicious party to make more educated guesses about
   the presence of security holes in the other party.


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 content 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 Ted Hardie, Larry Masinter and Graham  Klyne
  for contributing to discussions about feature tag registration.


6 References

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

  [2] G. Klyne, An algebra for describing media feature sets, Internet
     Draft: <draft-ietf-conneg-feature-algebra-00.txt> Work in progress
     March 1998

  [3] Holtman, K., et al, "The Alternates Header Field", Internet-Draft
     draft-ietf-http-alternates-01.txt, Work in progress, November 1997.

  [4] Masinter, L., et al, "Media Features for Display, Print, and Fax",
     Internet-Draft draft-ietf-conneg-media-features-00.txt, Work in
     progress , March 1998.


7 Authors' addresses

   Koen Holtman
   Technische Universiteit Eindhoven
   Postbus 513
   Kamer HG 6.57
   5600 MB Eindhoven (The Netherlands)
   Email: koen@win.tue.nl

   Andrew H. Mutz
   Hewlett-Packard Company
   11000 Wolfe Rd. 42UO
   Cupertino CA 95014 USA
   Fax +1 408 447 4439
   Email: andy_mutz@hp.com

   Edward Hardie
   NASA NIC
   hardie@nic.nasa.gov




Appendix A: IANA and RFC editor to-do list

   VERY IMPORTANT NOTE:  This appendix is intended to communicate
   various editorial and procedural tasks the IANA and the RFC
   Editor should undertake prior to publication of this document
   as an RFC.  This appendix should NOT appear in the actual RFC
   version of this document!

   This document refers to the feature tags mailing list
   ietf-feature-tags@iana.org.  This list does not exist at the
   present time and needs to be created.

   The ftp://ftp.isi.edu/in-notes/iana/assignments/feature-tags/"
   area does not exist at the present time and needs to be created.


Expires: September 11, 1998