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Internet Engineering Task Force                              S. Johnston
Internet-Draft                                     Open Cloud Initiative
Intended status: Experimental                         September 29, 2011
Expires: April 1, 2012


                             Web Categories
                 draft-johnston-http-category-header-02

Abstract

   This document specifies the Category header-field for HyperText
   Transfer Protocol (HTTP), which enables the sending of taxonomy
   information in HTTP headers in a similar fashion to that employed by
   Atom.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 1, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  Categories  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  The Category Header Field . . . . . . . . . . . . . . . . . . . 4
     3.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
     4.1.  Category Header Registration  . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Internationalisation Considerations . . . . . . . . . . . . . . 5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Appendix A.  Notes on use with HTML . . . . . . . . . . . . . . . . 7
   Appendix B.  Notes on use with Atom . . . . . . . . . . . . . . . . 7
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . . . 8
   Appendix D.  Change Log (to be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . . . 8
     D.1.  draft-johnston-http-category-header-00  . . . . . . . . . . 8
     D.2.  draft-johnston-http-category-header-01  . . . . . . . . . . 8
     D.3.  draft-johnston-http-category-header-02  . . . . . . . . . . 8
   Appendix E.  Outstanding Issues . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8



























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1.  Introduction

   A means of indicating categories for resources on the web has been
   defined by Atom [RFC4287].  This document defines a framework for
   exposing category information in the same format via HTTP headers.

   The atom:category element conveys information about a category
   associated with an entry or feed.  A given atom:feed or atom:entry
   element MAY have zero or more categories which MUST have a "term"
   attribute (a string that identifies the category to which the entry
   or feed belongs) and MAY also have a scheme attribute (an IRI that
   identifies a categorization scheme) and/or a label attribute (a
   human-readable label for display in end-user applications).

   Similarly a web resource may be associated with zero or more
   categories as indicated in the Category header-field(s).  These
   categories may be divided into separate vocabularies or "schemes"
   and/or accompanied with human-friendly labels.

   [[ Feedback is welcome on the ietf-http-wg@w3.org mailing list,
   although this is NOT a work item of the HTTPBIS WG. ]]

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119], as
   scoped to those conformance targets.

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [RFC2616], and explicitly includes the following rules from it:
   quoted-string, token.  Additionally, the following rules are included
   from [RFC3986]: URI.


2.  Categories

   In this specification, a category is a grouping of resources by
   'term', from a vocabulary ('scheme') identified by an IRI [RFC3987].
   It is comprised of:

   o  A "term" which is a string that identifies the category to which
      the resource belongs.

   o  A "scheme" which is an IRI that identifies a categorization scheme
      (optional).





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   o  An "label" which is a human-readable label for display in end-user
      applications (optional).

   A category can be viewed as a statement of the form "resource is from
   the {term} category of {scheme}, to be displayed as {label}", for
   example "'Loewchen' is from the 'dog' category of 'animals', to be
   displayed as 'Canine'".


3.  The Category Header Field

   The Category entity-header provides a means for serialising one or
   more categories in HTTP headers.  It is semantically equivalent to
   the atom:category element in Atom [RFC4287].

   Category           = "Category" ":" #category-value
   category-value     = term *( ";" category-param )
   category-param     = ( ( "scheme" "=" <"> scheme <"> )
                      | ( "label" "=" quoted-string )
                      | ( "label*" "=" enc2231-string )
                      | ( category-extension ) )
   category-extension = token [ "=" ( token | quoted-string ) ]
   enc2231-string     = <extended-value, see [RFC2231], Section 7>
   term               = token
   scheme             = URI

   Each category-value conveys exactly one category but there may be
   multiple category-values for each header-field and/or multiple
   header-fields per [RFC2616].

   Note that schemes are REQUIRED to be absolute URLs in Category
   headers, and MUST be quoted if they contain a semicolon (";") or
   comma (",") as these characters are used to separate category-params
   and category-values respectively.

   The "label" parameter is used to label the category such that it can
   be used as a human-readable identifier (e.g. a menu entry).
   Alternately, the "label*" parameter MAY be used encode this label in
   a different character set, and/or contain language information as per
   [RFC2231].  When using the enc2231-string syntax, producers MUST NOT
   use a charset value other than 'ISO-8859-1' or 'UTF-8'.

3.1.  Examples

   NOTE: Non-ASCII characters used in prose for examples are encoded
   using the format "Backslash-U with Delimiters", defined in Section
   5.1 of [RFC5137].




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   For example:
   Category: dog

   indicates that the resource is in the "dog" category.
   Category: dog; label="Canine"; scheme="http://purl.org/net/animals"

   indicates that the resource is in the "dog" category, from the
   "http://purl.org/net/animals" scheme, and should be displayed as
   "Canine".

   The example below shows an instance of the Category header encoding
   multiple categories, and also the use of [RFC2231] encoding to
   represent both non-ASCII characters and language information.
   Category: dog; label="Canine"; scheme="http://purl.org/net/animals",
             lowchen; label*=UTF-8'de'L%c3%b6wchen";
             scheme="http://purl.org/net/animals/dogs"

   Here, the second category has a label encoded in UTF-8, uses the
   German language ("de"), and contains the Unicode code point \u'00F6'
   ("LATIN SMALL LETTER O WITH DIAERESIS").


4.  IANA Considerations

4.1.  Category Header Registration

   This specification adds an entry for "Category" in HTTP to the
   Message Header Registry [RFC3864] referring to this document:
   Header Field Name: Category
   Protocol: http
   Status: standard
   Author/change controller:
       IETF (iesg@ietf.org)
       Internet Engineering Task Force
   Specification document(s):
       [ this document ]


5.  Security Considerations

   The content of the Category header-field is not secure, private or
   integrity-guaranteed, and due caution should be exercised when using
   it.


6.  Internationalisation Considerations

   Category header-fields may be localised depending on the Accept-



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   Language header-field, as defined in section 14.4 of [RFC2616].

   Scheme IRIs in atom:category elements may need to be converted to
   URIs in order to express them in serialisations that do not support
   IRIs, as defined in section 3.1 of [RFC3987].  This includes the
   Category header-field.


7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions:
              Character Sets, Languages, and Continuations", RFC 2231,
              November 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, December 2005.

   [RFC5137]  Klensin, J., "ASCII Escaping of Unicode Characters",
              BCP 137, RFC 5137, February 2008.

7.2.  Informative References

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, January 1997.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.



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   [W3C.REC-html401-19991224]
              Raggett, D., Jacobs, I., and A. Hors, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

   [W3C.WD-html5-20110525]
              Hickson, I., "HTML5", World Wide Web Consortium
              LastCall WD-html5-20110525, May 2011,
              <http://www.w3.org/TR/2011/WD-html5-20110525>.

   [rel-tag-microformat]
              Celik, T., Marks, K., and D. Powazek, "rel="tag"
              Microformat", <http://microformats.org/wiki/rel-tag>.


Appendix A.  Notes on use with HTML

   In the absence of a dedicated category element in HTML 4
   [W3C.REC-html401-19991224] and HTML 5 [W3C.WD-html5-20110525],
   category information (including user supllied folksonomy
   classifications) MAY be exposed using HTML A and/or LINK elements by
   concatenating the scheme and term:
   category-link = scheme term
   scheme        = URI
   term          = token

   These category-links MAY form a resolveable "tag space" in which case
   they SHOULD use the "tag" relation-type per [rel-tag-microformat].

   Alternatively META elements MAY be used:

   o  where the "name" attribute is "keywords" and the "content"
      attribute is a comma-separated list of term(s)

   o  where the "http-equiv" attribute is "Category" and the "content"
      attribute is a comma-separated list of category-value(s)


Appendix B.  Notes on use with Atom

   Where the cardinality is known to be one (for example, when
   retrieving an individual resource) it MAY be preferable to render the
   resource natively over HTTP without Atom structures.  In this case
   the contents of the atom:content element SHOULD be returned as the
   HTTP entity-body and metadata including the type attribute and atom:
   category element(s) via HTTP header-field(s).




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   This approach SHOULD NOT be used where the cardinality is guaranteed
   to be one (for example, search results which MAY return one result).


Appendix C.  Acknowledgements

   The author would like to thank Mark Nottingham for his work on Web
   Linking [RFC5988] (on which this document was based), the authors of
   [RFC2068] for specification of the Link: header-field on which this
   is based and all those who commented upon, encouraged and gave
   feedback to this draft.


Appendix D.  Change Log (to be removed by RFC Editor before publication)

D.1.  draft-johnston-http-category-header-00

   Initial draft based on draft-nottingham-http-link-header-05

D.2.  draft-johnston-http-category-header-01

   Updated references, affiliation and tickled for IETF-78.

D.3.  draft-johnston-http-category-header-02

   Revived draft for further work due to renewed interest, resolved two
   of the issues, updated affiliation.


Appendix E.  Outstanding Issues

   [[ to be removed by the RFC editor should document proceed to
   publication as an RFC. ]]

   The following issues are oustanding and should be addressed:

   1.  Is supporting multi-lingual representations of the same
       category(s) necessary?  If so, what are the risks of doing so?

   2.  Is a mechanism for maintaining Category header-fields required?
       If so, should it use the headers themselves or some other
       mechanism?









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Author's Address

   Sam Johnston
   Open Cloud Initiative

   Email: samj@samj.net













































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