Network Working Group M. Ohye
Internet-Draft J. Kupke
Intended status: Informational July 2011
Expires: January 02, 2012

The Canonical Link Relation
draft-ohye-canonical-link-relation-01

Abstract

The canonical link relation, developed from [RFC5988] which indicates relationships between Internet links, specifies the preferred URI from a set of identical or vastly similar content accessible on multiple URIs.

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1. Introduction

The canonical link relation specifies the preferred URI from a set of identical or vastly similar content accessible on multiple URIs, making it possible for references to the context URI to be updated to reference the designated 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:

A resource SHOULD NOT specify more than one canonical link relation.

The target/canonical URI MAY:

The target/canonical URI SHOULD NOT designate:

4. Examples

The following example illustrates:

If the preferred version of a URI and its content exists at:

http://www.example.com/page.php?item=purse

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, extremely similar, or a subset of the canonical.
  2. 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].

Relation Name:

Description:

Reference:

Notes:

Application Data:

7. Security Considerations

When a site is compromised, the canonical link relation can be implemented with malicious intent to designate the hacker'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 hacker's designated canonical, transferring properties such as link popularity away from the resource to the hacker's URI.

8. Internationalisation Considerations

In designating a canonical URI, please see [RFC3986] for information on URI encoding.

9. References

[REC-html401-19991224] Le Hors, A., Raggett, D. and I. Jacobs, "HTML 4.01 Specification", W3C Recommendation REC-html401-19991224, December 1999.

Latest version available at

[RFC2119] Bradner, S., "Key words for use in RFCs to 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 functionalty with regard for the canonical link relation include:

Authors' Addresses

Maile Ohye EMail: maileohye@gmail.com URI: http://maileohye.com/
Joachim Kupke EMail: joachim@kupke.za.net

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