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-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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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
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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 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 =
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. 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: samj@samj.net
URI: http://samj.net/
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