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Versions: 00 01 02 03 04 05 RFC 6596
Network Working Group M. Ohye
Internet-Draft J. Kupke
Intended status: Informational October 14, 2011
Expires: April 16, 2012
The Canonical Link Relation
draft-ohye-canonical-link-relation-04
Abstract
[RFC5988] specified a way to define relationships between links on
the web. This document describes a new type of such relationship,
"canonical," which designates the preferred URI from a set of
identical or vastly similar ones.
Editorial Note (To be removed by RFC Editor)
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described in the Simplified BSD License.
1. Introduction
The canonical link relation specifies the preferred URI from a set of
URIs that return identical or vastly similar content, making it
possible for references to the context URI to be updated to reference
the target URI.
The most common application of the canonical link relation includes
specifying the preferred version of a URI from duplicate content
pages created with the addition of parameters (e.g. session IDs,
tracking IDs, category, or sort information).
2. Notational Conventions
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 [RFC2119].
3. The Canonical Link Relation
The canonical (target) URI MUST identify content that duplicates, is
extremely similar, or is a superset of the content at the context
(referring) URI. Authors who declare the canonical link relation
ought to anticipate that applications such as search engines can:
o Index content only from the canonical target (i.e. content from
the context URIs will be likely disregarded as duplicative)
o Consolidate URI properties, such as link popularity, to the
canonical
o Display the canonical target as the representative URI
A resource SHOULD NOT specify more than one canonical link relation.
The target/canonical URI MAY:
o Specify a relative URI (see [RFC3986] Section 4.2)
o Be self-referential (context URI identical to target URI)
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o Exist on a different hostname or domain
o Have different scheme names, such as "http" to "https," or
"gopher" to "ftp"
o Be a superset of the content of the context URI
* For example, "page1" of a multi-page article may specify the
canonical target as the "view-all" URI because "view-all" is a
superset of page1's content. However, "page2" SHOULD NOT
designate "page1" as the canonical because the content of page1
is not inclusive of page2.
o Be the source URI of a temporary redirect. For HTTP, this refers
to status codes 302, 303, or 307 (Sections 10.3.3, 10.3.4, and
10.3.8, respectively, of [RFC2616]).
The target/canonical URI SHOULD NOT designate:
o The source URI of a permanent redirect (for HTTP, this refers to
300 and 301 response codes, defined in Sections 10.3.1 and 10.3.2
of [RFC2616])
o A URI that also specifies a canonical link relation to a URI other
than itself
o A URI that returns an error code, such as 4xx response in HTTP
(Section 10.4 of [RFC2616])
o The first page of a multi-page article or multi-page listing of
items (since the first page is not a duplicate or a superset of
the context URI). For example, page2 and page3 of an article
SHOULD NOT specify page1 as the canonical.
4. Examples
The following example illustrates:
o Three URIs that serve nearly the exact same content
o One URI which is the canonical or "preferred version"
o Two URIs with additional query parameters, making them the non-
preferred version of the content (duplicates). The canonical link
relation is therefore specified on these duplicates.
If the preferred version of a URI and its content exists at:
http://www.example.com/page.php?item=purse
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Then duplicate content URIs such as:
http://www.example.com/page.php?item=purse&category=bags
http://www.example.com/page.php?item=purse&category=bags&sid=1234
may designate the canonical link relation in HTML as specified in
[REC-html401-19991224]:
<link rel="canonical"
href="http://www.example.com/page.php?item=purse">
or as a relative URI:
<link rel="canonical" href="page.php?item=purse">
or alternatively, in the HTTP header field as specified in Section 5
of [RFC5988]:
Link: <http://www.example.com/page.php?item=purse>; rel="canonical"
This signals to automated programs, such as search engines, that
these are duplicates of the canonical URI:
http://www.example.com/page.php?item=purse.
Automated programs may then select the canonical value as the display
URI (such as in search results), and additional URI properties such
as indexing and ranking signals, can be transferred as well.
5. Recommendations
Before adding the canonical link relation, verification of the
following is recommended:
1. The content of the context URI is identical with, similar to, or
a subset of the content of the canonical.
2. For HTTP, Permanent HTTP redirects (Section 10.3.2 of [RFC2616]),
the traditional strong indicator that a URI's content has been
permanently moved, could not be implemented in place of the
canonical link relation.
3. In the case where the canonical target is a superset of content
from the context URI (e.g. page1 or page2 to view-all), that the
user experience is strongly taken into consideration, both in
regard to possible increased load time and potential complexity
in navigation.
6. IANA Considerations
IANA is asked to register the Canonical Link Relation below as per
[RFC5988].
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Relation Name:
CANONICAL
Description:
Designates the preferred version of a resource (the URI and its
contents).
Reference:
This specification.
Notes:
None.
Application Data:
None.
7. Security Considerations
When a site is compromised, the canonical link relation can be
implemented with malicious intent to designate the attacker's URI as
the preferred version of the content. While this technique is
largely unnoticeable to humans, automated programs may cluster the
compromised resource as duplicative of the attacker's designated
canonical, transferring properties such as link popularity away from
the resource to the attacker's URI.
8. Internationalisation Considerations
In designating a canonical URI, please see [RFC3986] for information
on URI encoding.
9. Normative References
[REC-html401-19991224] Le Hors, A., Raggett, D., and I. Jacobs,
"HTML 4.01 Specification", W3C
Recommendation REC-html401-19991224,
December 1999, <http://www.w3.org/TR/1999/
REC-html401-19991224>.
Latest version available at
<http://www.w3.org/TR/html401>.
[RFC2119] Bradner, S., "Key words for use in RFCs to
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Indicate Requirement Levels", BCP 14,
RFC 2119, March 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.
[RFC3986] Berners-Lee, T., Fielding, R., and L.
Masinter, "Uniform Resource Identifier (URI):
Generic Syntax", STD 66, RFC 3986,
January 2005.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
October 2010.
Appendix A. Implementations
Automated programs that implement functionality with regard for the
canonical link relation include:
o Google, canonical link relation HTML and HTTP header support,
within the same domain and across domains:
* <http://googlewebmastercentral.blogspot.com/2009/02/
specify-your-canonical.html>
* <http://googlewebmastercentral.blogspot.com/2011/06/
supporting-relcanonical-http-headers.html>
* <http://googlewebmastercentral.blogspot.com/2009/12/
handling-legitimate-cross-domain.html>
o Yahoo, canonical link relation HTML support within the same
domain:
* <http://www.ysearchblog.com/2009/02/12/
fighting-duplication-adding-more-arrows-to-your-quiver/>
o Bing, canonical link relation HTML support within the same domain:
* <http://www.bing.com/community/site_blogs/b/webmaster/archive/
2009/02/12/
partnering-to-help-solve-duplicate-content-issues.aspx>
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Authors' Addresses
Maile Ohye
EMail: maileohye@gmail.com
URI: http://maileohye.com/
Joachim Kupke
EMail: joachim@kupke.za.net
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