Internet DRAFT - draft-johnston-http-category-header
draft-johnston-http-category-header
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Johnston Expires April 1, 2012 [Page 1]
Internet-Draft Abbreviated Title September 2011
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
Johnston Expires April 1, 2012 [Page 2]
Internet-Draft Abbreviated Title September 2011
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).
Johnston Expires April 1, 2012 [Page 3]
Internet-Draft Abbreviated Title September 2011
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].
Johnston Expires April 1, 2012 [Page 4]
Internet-Draft Abbreviated Title September 2011
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-
Johnston Expires April 1, 2012 [Page 5]
Internet-Draft Abbreviated Title September 2011
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.
Johnston Expires April 1, 2012 [Page 6]
Internet-Draft Abbreviated Title September 2011
[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).
Johnston Expires April 1, 2012 [Page 7]
Internet-Draft Abbreviated Title September 2011
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?
Johnston Expires April 1, 2012 [Page 8]
Internet-Draft Abbreviated Title September 2011
Author's Address
Sam Johnston
Open Cloud Initiative
Email: samj@samj.net
Johnston Expires April 1, 2012 [Page 9]
