Network Working Group H. Van de Sompel
Internet-Draft Data Archiving and Networked Services
Intended status: Informational M. Nelson
Expires: June 21, 2019 Old Dominion University
G. Bilder
J. Kunze
California Digital Library
S. Warner
Cornell University
December 18, 2018

cite-as: A Link Relation to Convey a Preferred URI for Referencing


A web resource is routinely referenced by means of the URI with which it is directly accessed. But cases exist where referencing a resource by means of a URI, different than that access URI, is preferred. This specification defines a link relation type that can be used to convey such a preference.

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Table of Contents

1. Introduction

A web resource is routinely referenced (e.g. linked, bookmarked) by means of the URI with which it is directly accessed. But cases exist where referencing a resource by means of a different URI is preferred, for example because the latter URI is intended to be more persistent over time. Currently, there is no link relation type to convey such alternative referencing preference; this specification addresses this deficit by introducing a link relation type intended for that purpose.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

This specification uses the terms "link context" and "link target" as defined in [RFC8288]. These terms respectively correspond with "Context IRI" and "Target IRI" as used in [RFC5988]. Although defined as IRIs, in common scenarios they are also URIs.

Additionally, this specification uses the following terms:

By interacting with the access URI, the user agent may discover typed links. For such links, the access URI is the link context.

3. Scenarios

3.1. Persistent Identifiers

Despite sound advice regarding the design of Cool URIs [CoolURIs], link rot ("HTTP 404 Not Found") is a common phenomena when following links on the web. Certain communities of practice (see examples, below) have introduced solutions to combat this problem that typically consist of:

This approach is, for example, used by:

In order for the investments in infrastructure involved in these approaches to pay off, and hence for links to effectively remain operational as intended, it is crucial that a resource be referenced by means of its reference URI. However, the access URI is where a user agent actually accesses the resource (e.g., it is the URI in the browser's address bar). As such, there is a considerable risk that the access URI instead of the reference URI is used for referencing [PIDs-must-be-used].

The link relation type defined in this document makes it possible for user agents to differentiate the reference URI from the access URI.

3.2. Version Identifiers

Resource versioning systems often use a naming approach whereby:

For example, Wikipedia uses generic URIs of the form and version URIs of the form

While the current version of a resource is accessed at the generic URI, some versioning systems adhere to a policy that favors linking and referencing a specific version URI. To express this using the terminology of Section 2, these policies intend that the generic URI is the access URI, and that the version URI is the reference URI. These policies are informed by the understanding that the content at the generic URI is likely to evolve over time, and that accurate links or references should lead to the content as it was at the time of referencing. To that end, Wikipedia's "Permanent link" and "Cite this page" functionalities promote the version URI, not the generic URI.

The link relation type defined in this document makes it possible for user agents to differentiate the version URI from the generic URI.

3.3. Preferred Social Identifier

A web user commonly has multiple profiles on the web, for example, one per social network, a personal homepage, a professional homepage, a FOAF (Friend Of A Friend) profile [FOAF], etc. Each of these profiles is accessible at a distinct URI. But the user may have a preference for one of those profiles, for example, because it is most complete, kept up-to-date, or expected to be long-lived. As an example, the first author of this document has, among others, the following profile URIs:

Of these, from the perspective of the person described by these profiles, the first URI may be the preferred profile URI for the purpose of referencing because the domain is not under the custodianship of a third party. When an agent accesses another profile URI, such as, this preference for referencing by means of the first URI could be expressed.

The link relation type defined in this specification makes it possible for user agents to differentiate the preferred profile URI from the accessed profile URI.

3.4. Multi-Resource Publications

When publishing on the web, it is not uncommon to make distinct components of a publication available as different web resources, each with their own URI. For example:

While each of these components is accessible at its distinct URI - the access URI - they often also share a URI assigned to the intellectual publication of which they are components - the reference URI.

The link relation type defined in this document makes it possible for user agents to differentiate the URI of the intellectual publication from the access URI of a component of the publication.

4. The "cite-as" Relation Type for Expressing a Preferred URI for the Purpose of Referencing

A link with the "cite-as" relation type indicates that, for referencing the link context, use of the URI of the link target is preferred over use of the URI of the link context. It allows the resource identified by the access URI (link context) to unambiguously link to its corresponding reference URI (link target), thereby expressing that the link target is preferred over the link context for the purpose of permanent citation.

The link target of a "cite-as" link SHOULD support protocol-based access as a means to ensure that applications that store them can effectively re-use them for access.

The link target of a "cite-as" link SHOULD provide the ability for a user agent to follow its nose back to the context of the link, e.g. by following redirects and/or links. This helps a user agent to establish trust in the target URI.

Because a link with the "cite-as" relation type expresses a preferred URI for the purpose of referencing, the access URI SHOULD only provide one link with that relation type. If more than one "cite-as" link is provided, the user agent may decide to select one (e.g. an HTTP URI over a mailto URI), for example, based on the purpose that the reference URI will serve.

Providing a link with the "cite-as" relation type does not prevent using the access URI for the purpose of referencing if such specificity is needed for the application at hand. For example, in the case of scenario Section 3.4 the access URI is likely required for the purpose of annotating a specific component of an intellectual publication. Yet, the annotation application may also want to appropriately include the reference URI in the annotation.

Applications can leverage the information provided by a "cite-as" link in a variety of ways, for example:

5. Distinction with Other Relation Types

Some existing IANA-registered relationships intuitively resemble the relationship that "cite-as" is intended to convey. But a closer inspection of these candidates provided in the blog posts [identifier-blog], [canonical-blog], and [bookmark-blog] shows that they are not appropriate for various reasons and that a new relation type is required. The remainder of this section provides a summary of the detailed explanations provided in the referenced blog posts.

It can readily be seen that the following relation types do not address the requirements described in Section 3:

Two existing IANA-registered relationships deserve closer attention and are discussed in the remainder of this section.

5.1. bookmark

"bookmark" [W3C.REC-html5-20151028]: The link target provides a URI for the purpose of bookmarking the link context.

The intent of "bookmark" is closest to that of "cite-as" in that the link target is intended to be a permalink for the link context, for bookmarking purposes. The relation type dates back to the earliest days of news syndication, before blogs and news feeds had permalinks to identify individual resources that were aggregated into a single page. As such, its intent is to provide permalinks for different sections of an HTML document. It was originally used with HTML elements such as <div>, <h1>, <h2>, etc. and, more recently, HTML5 revised it to be exclusively used with the <article> element. Moreover, it is explictly excluded from use in the <link> element in HTML <head>, and, as a consequence, in the HTTP Link header that is semantically equivalent. For these technical and semantic reasons, the use of "bookmark" to convey the relationship intented by "cite-as" is not appropriate.

A more detailed justification regarding the inappropriatenss of "bookmark", including a thorough overview of its turbulent history, is provided in [bookmark-blog].

5.2. canonical

"canonical" [RFC6596]: The meaning of "canonical" is commonly misunderstood on the basis of its brief definition as being "the preferred version of a resource." The description in the abstract of [RFC6596] is more helpful and states that "canonical" is intended to link to a resource that is preferred over resources with duplicative content. A more detailed reading of [RFC6596] clarifies that the intended meaning is preferred for the purpose of content indexing. A typical use case is linking from each page in a multi-page magazine article to a single page version of the article provided for indexing by search engines: the former pages provide content that is duplicative to the superset content that is available at the latter page.

The semantics intended by "canonical" as preferred for the purpose of content indexing differ from the semantics intended by "cite-as" as preferred for the purpose of referencing. A further exploration of the various scenarios shows that the use of "canonical" is not appropriate to convey the semantics intended by "cite-as":

A more detailed justification regarding how the use of "canonical" is inappropriate to address the requirements described in this document, including examples, is provided in [canonical-blog].

6. Examples

Sections Section 6.1 through Section 6.4 show examples of the use of links with the "cite-as" relation type. They illustrate how the typed links can be used in a response header and/or response body.

6.1. Persistent HTTP URI

PLOS ONE is one of many scholarly publishers that assigns DOIs to the articles it publishes. For example, is the persistent identifier for such an article. Via the DOI resolver, this persistent identifier redirects to in the domain. This URI itself redirects to, which delivers the actual article in HTML.

The HTML article contains a <link> element with the "canonical" relation type pointing at itself, As per Section 5.2, this indicates that the article content at that URI should be indexed by search engines.

PLOS ONE can additionally provide a link with the "cite-as" relation type pointing at the persistent identifier to indicate it is the preferred URI for permanent citation of the article. Figure 1 shows the addition of the "cite-as" link both in the HTTP header and the HTML that results from an HTTP GET on the article URI

HTTP/1.1 200 OK
Link: <> ; rel="cite-as"
Content-Type: text/html;charset=utf-8

  <link rel="cite-as" href="" />
  <link rel="canonical"
   href="" />


Figure 1: Response to HTTP GET on the URI of a scholarly article

6.2. Version URIs

The preprint server has a versioning approach like the one described in Section 3.2:

A reader who accessed between 10 November 2017 and 23 January 2018, obtained the first version of the preprint. Starting 24 January 2018, the second version was served at that URI. In order to support accurate referencing, could implement the "cite-as" link to point from the generic URI to the most recent version URI. In doing so, assuming the existence of reference manager tools that consume "cite-as" links:

Figure 2 shows the header that would have returned in the first case, in response to a HTTP HEAD on the generic URI

HTTP/1.1 200 OK
Date: Sun, 24 Dec 2017 16:12:43 GMT
Content-Type: text/html; charset=utf-8
Link: <> ; rel="cite-as"
Vary: Accept-Encoding,User-Agent


Figure 2: Response to HTTP HEAD on the generic URI of the landing page of an preprint

6.3. Preferred Profile URI

If the access URI is the home page of John Doe, John can add a link with the "cite-as" relation type to it, as a means to convey that he would preferably be referenced by means of the URI of his FOAF profile. Figure 3 shows the response to an HTTP GET on the URI of John's home page.

HTTP/1.1 200 OK
Content-Type: text/html;charset=utf-8

  <link rel="cite-as" href="" 


Figure 3: Response to HTTP GET on the URI of John Doe's home page

6.4. Multi-Resource Publication

The Dryad Digital Repository at specializes in hosting and preserving scientific datasets. Each dataset typically consists of multiple resources. For example, the dataset "Data from: Climate, demography, and lek stability in an Amazonian bird" consists of an Excel spreadsheet, a csv file, and a zip file. Each of these resources have different content and are accessible at their respective URIs. In addition, the dataset has a landing page at

Each of these resources should be permanently cited by means of the persistent identifier that was assigned to the entire dataset as an intellectual publication, i.e. To that end, the Dryad Digital Repository can add "cite-as" links pointing from the URIs of each of these resources to This is shown in Figure 4 for the csv file that is a component resource of the dataset, through use of the HTTP Link header.

HTTP/1.1 200 OK
Date: Tue, 12 Jun 2018 19:19:22 GMT
Last-Modified: Wed, 17 Feb 2016 18:37:02 GMT
Content-Type: text/csv;charset=ISO-8859-1
Content-Length: 25414
Link: <> ; rel="cite-as"



Figure 4: Response to HTTP GET on the URI of a csv file that is a component of a scientfic dataset

7. IANA Considerations

7.1. Link Relation Type: cite-as

The link relation type below has been registered by IANA per Section 2.1.1 of [RFC8288]:

8. Security Considerations

In cases where there is no way for the agent to automatically verify the correctness of the reference URI (cf. Section 4), out-of-band mechanisms might be required to establish trust.

If a trusted site is compromised, the "cite-as" link relation could be used with malicious intent to supply misleading URIs for referencing. Use of these links might direct user agents to an attacker's site, break the referencing record they are intended to support, or corrupt algorithmic interpretation of referencing data.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication Format", RFC 4287, DOI 10.17487/RFC4287, December 2005.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010.
[RFC6249] Bryan, A., McNab, N., Tsujikawa, T., Poeml, P. and H. Nordstrom, "Metalink/HTTP: Mirrors and Hashes", RFC 6249, DOI 10.17487/RFC6249, June 2011.
[RFC6596] Ohye, M. and J. Kupke, "The Canonical Link Relation", RFC 6596, DOI 10.17487/RFC6596, April 2012.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017.
[W3C.REC-html5-20151028] Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Doyle Navara, E., O'Connor, E. and S. Pfeiffer, "HTML5", World Wide Web Consortium Recommendation REC-HTML5-20141028, October 2014.

9.2. Informative References

[bookmark-blog] Nelson, M. and H. Van de Sompel, "rel=bookmark also does not mean what you think it means", August 2017.
[canonical-blog] Nelson, M. and H. Van de Sompel, "rel=canonical does not mean what you think it means", August 2017.
[CoolURIs] Berners-Lee, T., "Cool URIs don't change", World Wide Web Consortium Style, 1998.
[DOI-URLs] Hendricks, G., "Display guidelines for Crossref DOIs", June 2017.
[DOIs] "Information and documentation - Digital object identifier system", ISO 26324:2012(en), 2012.
[draft-kunze-ark-18] Kunze, J. and R. Rodgers, "The ARK Identifier Scheme", Internet Draft draft-kunze-ark-18, April 2013.
[FOAF] Brickley, D. and L. Miller, "FOAF Vocabulary Specification 0.99", January 2014.
[identifier-blog] Nelson, M. and H. Van de Sompel, "Linking to Persistent Identifiers with rel=identifier", July 2016.
[PIDs-must-be-used] Van de Sompel, H., Klein, M. and S. Jones, "Persistent URIs Must Be Used To Be Persistent", February 2016.
[PURLs] "Persistent uniform resource locator", April 2017.

Appendix A. Acknowledgements

Thanks for comments and suggestions provided by Martin Klein, Harihar Shankar, Peter Williams, John Howard, Mark Nottingham, Graham Klyne.

Authors' Addresses

Herbert Van de Sompel Data Archiving and Networked Services EMail: URI:
Michael Nelson Old Dominion University EMail: URI:
Geoffrey Bilder Crossref EMail: URI:
John Kunze California Digital Library EMail: URI:
Simeon Warner Cornell University EMail: URI: