Network Working Group H. Van de Sompel
Internet-Draft Los Alamos National Laboratory
Intended status: Informational M. Nelson
Expires: February 3, 2018 Old Dominion University
G. Bilder
J. Kunze
California Digital Library
S. Warner
Cornell University
August 2, 2017

Identifier: A Link Relation to Convey a Preferred URI for Referencing


This specification defines a link relation type that is intended to convey that a URI, other than the URI that provides a link with the relation type, is preferred for the purpose of referencing.

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Table of Contents

1. Introduction

A web resource is routinely referenced (e.g. linked, bookmarked) by means of the URI where 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", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

This specification uses the terms "link context" and "link target" as defined in [I-D.nottingham-rfc5988bis]. 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 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 identifying 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 identifying URI is used for referencing [PIDs-must-be-used].

The link relation type defined in this specification allows to convey to user agents that the identifying URI is the preferred URI for referencing. Applications such as bookmarking tools, citation managers, and webometrics applications can take this preference into account when recording a 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 by means of the version URI that was minted for the current version. 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 identifying 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 use the version URI, not the generic URI.

The link relation type defined in this specification allows to convey to user agents that the version URI is preferred over the generic URI for referencing.

3.3. Preferred Social Identifier

A web user commonly has multiple profiles on the web, for example, one per social network she takes part in, a personal homepage, a professional homepage, a FOAF 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.

The link relation type defined in this specification allows to convey to user agents that a profile URI - the identifying URI - other than the one the agent is accessing - the access URI - is preferred for referencing.

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 its own URI. For example:

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

The link relation type defined in this specification allows to convey to user agents that, for the purpose of referencing, the identifying URI of the intellectual publication is preferred over an access URI of a component of the publication.

4. The "identifier" Relation Type for Expressing a Preferred URI for the Purpose of Referencing

A link with the "identifier" relation type indicates that the link target - the identifying URI - is preferred over the link context for the purpose of referencing.

An identifying URI SHOULD support protocol-based access as a means to ensure that applications that store identifying URIs can effectively re-use them for access.

An identifying URI SHOULD provide the ability for a user agent to follow its nose back to the access URI, e.g. by following redirects and/or links. This helps a user agent to establish trust in the identifying URI.

Because a link with the "identifier" 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 "identifier" 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 identifying URI will serve.

Providing a link with the "identifier" 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 identifying URI in the annotation.

5. Distinction with Other Relation Types

The following existing IANA-registered relationships are similar to the relationship that "identifier" is intended to convey, but are not appropriate for various reasons:

A closer inspection of these candidates [identifier-blog] shows that they are not appropriate and that a new relation type is required.

In the scenario of Section 3.1 there is no content available at the identifying URI as it merely redirects to the access URI. In the scenario of Section 3.3, the content at the identifying URI is a profile that is different than the profile at the access URI. In the scenario of Section 3.4 the content at the identifying URI, if any, would typically be a sort of table of contents with links to component resources and possibly a summary. These considerations exclude "alternate", "canonical", and "duplicate" as possible relation types.

The intent of "bookmark" is closest to that of "identifier" in that the link target of a link with this relation type is intended for bookmarking, which is a case of referencing. However, "bookmark" is specifically defined for use in conjunction with the HTML <article> element and is explictly excluded from use in the <link> element in HTML <head>. Since a link in <link> and a link in the HTTP Link header are semantically equivalent, "bookmark" is also excluded from use in HTTP Link.

While "related" could be used, its semantics are too vague to convey the specific nature of "identifier" as a means to convey a URI for the purpose of referencing.

6. Examples

Sections Section 6.1 and Section 6.2 show examples of the use of links with the "identifier" relation type. One example shows its use in a response header and body, the other in a response body only.

6.1. Persistent HTTP URI

If the access URI is a landing page for a scholarly article for which the persistent HTTP URI <> was minted, then the response to an HTTP GET on the landing page's URI could be as shown in Figure 1.

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

  <link rel="identifier" href="" />


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

6.2. Preferred Profile URI

If the access URI is the home page of John Doe, John can add a link with the "identifier" 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 2 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="identifier" href="" type="text/ttl"/>


Figure 2: Response to HTTP GET on the URI of John Doe's home page

7. IANA Considerations

7.1. Link Relation Type: identifier

The link relation type below has been registered by IANA per Section 2.1.1 of [I-D.nottingham-rfc5988bis]:

8. Security Considerations

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

If a trusted site is compromised, the "identifier" 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

[I-D.nottingham-rfc5988bis] Nottingham, M., "Web Linking", Internet-Draft draft-nottingham-rfc5988bis-06, June 2017.
[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.
[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

[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., "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.

Authors' Addresses

Herbert Van de Sompel Los Alamos National Laboratory 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: