02 July 2009

Organising the Internet with Web Categories

In order to scratch an itch relating to the Open Cloud Computing Interface (OCCI) I submitted my first Internet-Draft to the IETF this week: Web Categories (draft-johnston-http-category-header).

The idea's fairly simple and largely inspired by the work of others (most notably the original HTTP and Atom authors, and a guy down under who's working on another draft). It defines an intuitive mechanism for web servers to express flexible category information for any resource (including opaque/binary/non-HyperText formats) in the HTTP headers, allowing users to categorise web resources into vocabularies or "schemes" and assign human-friendly "labels" in addition to the computer-friendly "terms".

This approach to taxonomies was lifted directly from (and is thus 100% compatible with) Atom and is another step closer to being able to render individual resources natively over HTTP rather than encoded and wrapped in XML (which gets unwieldly when you're dealing with multi-gigabyte virtual machines, as we are with OCCI).

It's anybody's guess where the document will go from here - it's currently marked "Experimental" but with any luck it will pique the interest of the standards and/or semantic web community and go on to live a long and happy life.

Internet Engineering Task Force                              S. Johnston
Internet-Draft                               Australian Online Solutions
Intended status: Experimental                               July 1, 2009
Expires: January 2, 2010


                             Web Categories
                 draft-johnston-http-category-header-00

Status of this Memo

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

   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.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on January 2, 2010.

Copyright Notice

   Copyright (c) 2009 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
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
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Abstract

   This document specifies the Category header-field for HyperText
   Transfer Protocol (HTTP), which enables the sending of taxonomy
   information in HTTP headers.



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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.  Document History . . . . . . . . . . . . . . . . . . . 8
   Appendix E.  Outstanding Issues . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9































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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 [email protected] 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     = 
   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 ([email protected])
       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. and R. Sayre, "The Atom Syndication
              Format", RFC 4287, December 2005.

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

7.2.  Informative References

   [OCCI]     Open Grid Forum (OGF), Edmonds, A., Metsch, T., Johnston,
              S., and A. Richardson, "Open Cloud Computing Interface
              (OCCI)", .

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



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              RFC 2068, January 1997.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification",
              .

   [W3C.WD-html5-20090423]
              Hyatt, D. and I. Hickson, "HTML 5", April 2009,
              .

   [draft-nottingham-http-link-header]
              Nottingham, M., "Web Linking",
              draft-nottingham-http-link-header-05 (work in progress),
              April 2009.

   [rel-tag-microformat]
              Celik, T., Marks, K., and D. Powazek, "rel="tag"
              Microformat", .


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-html5-20090423],
   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



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   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).

   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 [draft-nottingham-http-link-header] (on which this document
   was based) and to the authors of [RFC2068] for specification of the
   Link: header-field on which this is based.

   The author would like to thank members of the OGF's Open Cloud
   Computing Interface [OCCI] working group for their contributions and
   others who commented upon, encouraged and gave feedback to this
   draft.


Appendix D.  Document History

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

      -00

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


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 extensibility of Category headers necessary as is the case for
       Link: headers?  If so, what are the use cases?

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

   3.  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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   4.  Does this proposal conflict with others in the same space?  If
       so, is it an improvement on what exists?


Author's Address

   Sam Johnston
   Australian Online Solutions
   GPO Box 296
   Sydney, NSW  2001

   Email: [email protected]
   URI:   http://samj.net/






































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2 comments:

  1. Sam,

    have you considered using a Link header to just link to a categories document (e.g. via describedby relation?

    IMHO, there is a tendency recently to stuff everything in the HTTP header and I wonder when they just become too large to handle.

    Jan

    ReplyDelete
  2. True but for the application I have in mind (an API) there will be many requests already - doubling it for categories would hurt performance fairly badly.

    The headers aren't compressed and I'm not sure if they are reliably cacheable (or indeed, cacheable at all), nor how to detect when they have been dropped vs when they aren't present.

    Anyway I reckon avoiding multiple requests is a GoodThing™, but in some instances (e.g. when you're not dealing with large and/or binary files) it may make sense to use Atom.

    Sam

    ReplyDelete

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