The Canonical Link Relation
RFC 6596

Document Type RFC - Informational (April 2012; No errata)
Was draft-ohye-canonical-link-relation (individual in app area)
Authors Maile Ohye  , Joachim Kupke 
Last updated 2015-10-14
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IESG IESG state RFC 6596 (Informational)
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Internet Engineering Task Force (IETF)                           M. Ohye
Request for Comments: 6596                                      J. Kupke
Category: Informational                                       April 2012
ISSN: 2070-1721

                      The Canonical Link Relation


   RFC 5988 specifies a way to define relationships between links on the
   web.  This document describes a new type of such a relationship,
   "canonical", to designate an Internationalized Resource Identifier
   (IRI) as preferred over resources with duplicative content.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2012 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.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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.

Ohye & Kupke                  Informational                     [Page 1]
RFC 6596               The Canonical Link Relation            April 2012

1.  Introduction

   The canonical link relation specifies the preferred IRI from
   resources with duplicative content.  Common implementations of the
   canonical link relation are to specify the preferred version of an
   IRI from duplicate pages created with the addition of IRI parameters
   (e.g., session IDs) or to specify the single-page version as
   preferred over the same content separated on multiple component

   In regard to the link relation type, "canonical" can be described
   informally as the author's preferred version of a resource.  More
   formally, the canonical link relation specifies the preferred IRI
   from a set of resources that return the context IRI's content in
   duplicated form.  Once specified, applications such as search engines
   can focus processing on the canonical, and references to the context
   (referring) IRI can be updated to reference the target (canonical)

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  The Canonical Link Relation

   The target (canonical) IRI MUST identify content that is either
   duplicative or a superset of the content at the context (referring)
   IRI.  Authors who declare the canonical link relation ought to
   anticipate that applications such as search engines can:

   o  Index content only from the target IRI (i.e., content from the
      context IRIs will be likely disregarded as duplicative).

   o  Consolidate IRI properties, such as link popularity, to the target

   o  Display the target IRI as the representative IRI.

   The target (canonical) IRI MAY:

   o  Specify a relative IRI (see [RFC3986], Section 4.2).

   o  Be self-referential (context IRI identical to target IRI).

   o  Exist on a different hostname or domain.

Ohye & Kupke                  Informational                     [Page 2]
RFC 6596               The Canonical Link Relation            April 2012

   o  Have different scheme names, such as "http" to "https" or "gopher"
      to "ftp".

   o  Be a superset of the content at the context IRI.

      *  As an example, each component page (e.g., page-1.html, page-
         2.html) of a multi-page article MAY specify the "view-all"
         version (e.g., page-all.html), the superset of their content,
         as the target IRI.  This is because the content from each
         component page is contained within the view-all version.  Given
         this implementation, applications can mark page-1.html and
         page-2.html as duplicates of page-all.html, process content
         only from page-all.html, and disregard the component pages.
         All references can then be made to the view-all version (page-
         all.html, the target IRI), and no content will have been lost
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