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rfc:rfc8574

Independent Submission H. Van de Sompel Request for Comments: 8574 Data Archiving and Networked Services Category: Informational M. Nelson ISSN: 2070-1721 Old Dominion University

                                                             G. Bilder
                                                              Crossref
                                                              J. Kunze
                                            California Digital Library
                                                             S. Warner
                                                    Cornell University
                                                            April 2019
 cite-as: A Link Relation to Convey a Preferred URI for Referencing

Abstract

 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 different URI is preferred.  This
 specification defines a link relation type that can be used to convey
 such a preference.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This is a contribution to the RFC Series, independently of any other
 RFC stream.  The RFC Editor has chosen to publish this document at
 its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not candidates for any level of Internet Standard;
 see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8574.

Van de Sompel, et al. Informational [Page 1] RFC 8574 The "cite-as" Link Relation Type April 2019

Copyright Notice

 Copyright (c) 2019 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
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
 3.  Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.1.  Persistent Identifiers  . . . . . . . . . . . . . . . . .   3
   3.2.  Version Identifiers . . . . . . . . . . . . . . . . . . .   5
   3.3.  Preferred Social Identifier . . . . . . . . . . . . . . .   5
   3.4.  Multi-resource Publications . . . . . . . . . . . . . . .   6
 4.  The "cite-as" Relation Type for Expressing a Preferred URI
     for the Purpose of Referencing  . . . . . . . . . . . . . . .   6
 5.  Distinction with Other Relation Types . . . . . . . . . . . .   8
   5.1.  The "bookmark" Relation Type  . . . . . . . . . . . . . .   9
   5.2.  The "canonical" Relation Type . . . . . . . . . . . . . .   9
 6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.1.  Persistent HTTP URI . . . . . . . . . . . . . . . . . . .  11
   6.2.  Version URIs  . . . . . . . . . . . . . . . . . . . . . .  12
   6.3.  Preferred Profile URI . . . . . . . . . . . . . . . . . .  13
   6.4.  Multi-resource Publication  . . . . . . . . . . . . . . .  13
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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1. Introduction

 A web resource is routinely referenced (e.g., linked or 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 an 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 correspond with "Context IRI" and
 "Target IRI", respectively, as used in [RFC5988].  Although defined
 as IRIs (Internationalized Resource Identifiers), they are also URIs
 in common scenarios.
 Additionally, this specification uses the following terms:
 o  "access URI": A URI at which a user agent accesses a web resource.
 o  "reference URI": A URI, other than the access URI, that should
    preferentially be used for referencing.
 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.  These
 solutions typically consist of:
 o  Accepting the reality that the web location of a resource -- the
    access URI -- may change over time.

Van de Sompel, et al. Informational [Page 3] RFC 8574 The "cite-as" Link Relation Type April 2019

 o  Minting an additional URI for the resource -- the reference URI --
    that is specifically intended to remain persistent over time.
 o  Redirecting (typically with "HTTP 301 Moved Permanently", "HTTP
    302 Found", or "HTTP 303 See Other") from the reference URI to the
    access URI.
 o  Committing, as a community of practice, to adjust that redirection
    whenever the access URI changes over time.
 This approach is, for example, used by:
 o  Scholarly publishers that use DOIs (Digital Object Identifiers)
    [DOIs] to identify articles and DOI URLs [DOI-URLs] as a means to
    keep cross-publisher article-to-article links operational, even
    when the journals in which the articles are published change hands
    from one publisher to another, for example, as a result of an
    acquisition.
 o  Authors of controlled vocabularies that use PURLs (Persistent
    Uniform Resource Locators) [PURLs] for vocabulary terms to ensure
    that the URIs they assign to vocabulary terms remain stable even
    if management of the vocabulary is transferred to a new custodian.
 o  A variety of organizations (including libraries, archives, and
    museums) that assign ARK (Archival Resource Key) URLs [ARK] to
    information objects in order to support long-term access.
 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.

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3.2. Version Identifiers

 Resource versioning systems often use a naming approach whereby:
 o  The most recent version of a resource is always available at the
    same, generic URI.
 o  Each version of the resource -- including the most recent one --
    has a distinct version URI.
 For example, Wikipedia uses generic URIs of the form
 <https://en.wikipedia.org/wiki/John_Doe> and version URIs of the form
 <https://en.wikipedia.org/w/index.php?title=John_Doe&oldid=
 776253882>.
 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:
 o  <https://hvdsomp.info>
 o  <https://twitter.com/hvdsomp>
 o  <https://www.linkedin.com/in/herbertvandesompel/>
 o  <https://orcid.org/0000-0002-0715-6126>

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 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 <https://orcid.org/0000-0002-0715-6126>, 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:
 o  Contemporary scholarly publications routinely consists of a
    traditional article as well as additional materials that are
    considered an integral part of the publication such as
    supplementary information, high-resolution images, or a video
    recording of an experiment.
 o  Scientific or governmental open data sets frequently consist of
    multiple files.
 o  Online books typically consist of multiple chapters.
 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" Link 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.

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 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 reuse 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) 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 the scenario in 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:
 o  Bookmarking tools and citation managers can take this preference
    into account when recording a URI.
 o  Webometrics applications that trace URIs can trace both the access
    URI and the reference URI.
 o  Discovery tools can support lookup by means of both the access and
    the reference URI.  This includes web archives that typically make
    archived versions of web resources discoverable by means of the
    original access URI of the archived resource; they can
    additionally make these archived resources discoverable by means
    of the associated reference URI.

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5. Distinction with Other Link 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 link
 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 link relation types do not
 address the requirements described in Section 3:
 o  "alternate" [RFC4287]: The link target provides an alternate
    version of the content at the link context.  These are typically
    variants according to dimensions that are subject to content
    negotiation, for example, the same content with varying Content-
    Type (e.g., application/pdf vs. text/html) and/or Content-Language
    (e.g., en vs. fr).  The representations provided by the context
    URIs and target URIs in the scenarios in Sections 3.1 through 3.4
    are not variants in the sense intended by [RFC4287], and, as such,
    the use of "alternate" is not appropriate.
 o  "duplicate" [RFC6249]: The link target is a resource whose
    available representations are byte-for-byte identical with the
    corresponding representations of the link context, for example, an
    identical file on a mirror site.  In none of the scenarios
    described in Sections 3.1 through 3.4 do the link context and the
    link target provide identical content.  As such, the use of
    "duplicate" is not appropriate.
 o  "related" [RFC4287]: The link target is a resource that is related
    to the link context.  While "related" could be used in all of the
    scenarios described in Sections 3.1 through 3.4, its semantics are
    too vague to convey the specific semantics intended by "cite-as".
 Two existing IANA-registered relationships deserve closer attention
 and are discussed in the remainder of this section.

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5.1. The "bookmark" Link Relation Type

 "bookmark" [W3C.REC-html52-20171214]: 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 link 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.; more recently, HTML5
 revised it to be exclusively used with the <article> element.
 Moreover, it is explicitly 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 intended by "cite-
 as" is not appropriate.
 A more detailed justification regarding the inappropriateness of
 "bookmark", including a thorough overview of its turbulent history,
 is provided in [bookmark-blog].

5.2. The "canonical" Link Relation Type

 "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 that "canonical" 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":

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 o  Scenario of Section 3.1: The reference URI that is intended to be
    persistent over time does not serve content that needs to be
    indexed; it merely redirects to the access URI.  Since the meaning
    intended by "canonical" is that it is preferred for the purpose of
    content indexing, it is not appropriate to point at the reference
    URI (persistent identifier) using the "canonical" link relation
    type.  Moreover, Section 6.1 shows that scholarly publishers that
    assign persistent identifiers already use the "canonical" link
    relation type for search engine optimization; it also shows how
    that use contrasts with the intended use of "cite-as".
 o  Scenario of Section 3.2: In most common cases, custodians of
    resource versioning systems want search engines to index the most
    recent version of a page and hence would use a "canonical" link to
    point from version URIs of a resource to the associated generic
    URI.  Wikipedia effectively does this.  However, for some resource
    versioning systems, including Wikipedia, version URIs are
    preferred for the purpose of referencing.  As such, a "cite-as"
    link would point from the generic URI to the most recent version
    URI (that is, in the opposite direction of the "canonical" link).
 o  Scenario of Section 3.3: The content at the link target and the
    link context are different profiles for a same person.  Each
    profile, not just a preferred one, should be indexed.  But a
    single one could be preferred for referencing.
 o  Scenario of Section 3.4: The content at the link target, if any,
    would typically be a landing page that includes descriptive
    metadata pertaining to the multi-resource publication and links to
    its component resources.  Each component resource provides content
    that is different, not duplicative, to the landing page.
 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 6.1 through 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.

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6.1. Persistent HTTP URI

 PLOS ONE is one of many scholarly publishers that assigns DOIs to the
 articles it publishes.  For example, <https://doi.org/10.1371/
 journal.pone.0171057> is the persistent identifier for such an
 article.  Via the DOI resolver, this persistent identifier redirects
 to <https://journals.plos.org/plosone/doi?id=10.1371/
 journal.pone.0171057> in the plos.org domain.  This URI itself
 redirects to <https://journals.plos.org/plosone/article?id=10.1371/
 journal.pone.0171057>, which delivers the actual article in HTML.
 The HTML article contains a <link> element with the "canonical" link
 relation type pointing at itself, <https://journals.plos.org/plosone/
 article?id=10.1371/journal.pone.0167475>.  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 in both the HTTP header and the
 HTML that results from an HTTP GET on the article URI
 <https://journals.plos.org/plosone/article?id=10.1371/
 journal.pone.0167475>.
 HTTP/1.1 200 OK
 Link: <https://doi.org/10.1371/journal.pone.0171057> ; rel="cite-as"
 Content-Type: text/html;charset=utf-8
 <html>
  <head>
  ...
   <link rel="cite-as"
         href="https://doi.org/10.1371/journal.pone.0171057" />
   <link rel="canonical"
         href="https://journals.plos.org/plosone/article?
                            id=10.1371/journal.pone.0167475" />
  ...
  </head>
  <body>
   ...
  </body>
 </html>
   Figure 1: Response to HTTP GET on the URI of a Scholarly Article

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6.2. Version URIs

 The preprint server arXiv.org has a versioning approach like the one
 described in Section 3.2:
 o  The most recent version of a preprint is always available at the
    same, generic URI.  Consider the preprint with generic URI
    <https://arxiv.org/abs/1711.03787>.
 o  Each version of the preprint -- including the most recent one --
    has a distinct version URI.  The considered preprint has two
    versions with respective version URIs: <https://arxiv.org/
    abs/1711.03787v1> (published 10 November 2017) and
    <https://arxiv.org/abs/1711.03787v2> (published 24 January 2018).
 A reader who accessed <https://arxiv.org/abs/1711.03787> 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, arXiv.org 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:
 o  The reader who accesses <https://arxiv.org/abs/1711.03787> between
    10 November 2017 and 23 January 2018 would reference
    <https://arxiv.org/abs/1711.03787v1>.
 o  The reader who accesses <https://arxiv.org/abs/1711.03787>
    starting 24 January 2018 would reference <https://arxiv.org/
    abs/1711.03787v2>.
 Figure 2 shows the header that arXiv.org would have returned in the
 first case, in response to a HTTP HEAD on the generic URI
 <https://arxiv.org/abs/1711.03787>.
 HTTP/1.1 200 OK
 Date: Sun, 24 Dec 2017 16:12:43 GMT
 Content-Type: text/html; charset=utf-8
 Link: <https://arxiv.org/abs/1711.03787v1> ; rel="cite-as"
 Vary: Accept-Encoding,User-Agent
   Figure 2: Response to HTTP HEAD on the Generic URI of the Landing
                     Page of an arXiv.org Preprint

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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, in order to convey that he
 would prefer to 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
 <html>
  <head>
  ...
   <link rel="cite-as" href="http://johndoe.example.com/foaf"
         type="text/ttl"/>
  ...
  </head>
  <body>
   ...
  </body>
 </html>
   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 datadryad.org 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
 <https://datadryad.org/resource/doi:10.5061/dryad.5d23f>.
 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., <https://doi.org/10.5061/
 dryad.5d23f>.  To that end, the Dryad Digital Repository can add
 "cite-as" links pointing from the URIs of each of these resources to
 <https://doi.org/10.5061/dryad.5d23f>.  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.

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 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: <https://doi.org/10.5061/dryad.5d23f> ; rel="cite-as"
 DATE,Year,PLOT/TRAIL,LOCATION,SPECIES CODE,BAND NUM,COLOR,SEX,AGE,
  TAIL,WING,TARSUS,NARES,DEPTH,WIDTH,WEIGHT
 6/26/02,2002,DANTA,325,PIPFIL,969,B/O,M,AHY,80,63,16,7.3,3.9,4.1,
  14.4
 ...
 2/3/13,2013,LAGO,,PIPFIL,BR-5095,O/YPI,M,SCB,78,65.5,14.2,7.5,3.8,
  3.7,14.3
   Figure 4: Response to HTTP GET on the URI of a csv File That Is a
                   Component of a Scientific Dataset

7. IANA Considerations

 The link relation type has been registered by IANA per Section 2.1.1
 of [RFC8288] as follows:
    Relation Name: cite-as
    Description: Indicates that the link target is preferred over the
    link context for the purpose of permanent citation.
    Reference: RFC 8574

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.

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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,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
            Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
            December 2005, <https://www.rfc-editor.org/info/rfc4287>.
 [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
            DOI 10.17487/RFC5988, October 2010,
            <https://www.rfc-editor.org/info/rfc5988>.
 [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,
            <https://www.rfc-editor.org/info/rfc6249>.
 [RFC6596]  Ohye, M. and J. Kupke, "The Canonical Link Relation",
            RFC 6596, DOI 10.17487/RFC6596, April 2012,
            <https://www.rfc-editor.org/info/rfc6596>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
            DOI 10.17487/RFC8288, October 2017,
            <https://www.rfc-editor.org/info/rfc8288>.
 [W3C.REC-html52-20171214]
            Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and
            S. Moon, "HTML 5.2", World Wide Web
            Consortium Recommendation REC-html52-20171214, December
            2017, <https://www.w3.org/TR/2017/REC-html52-20171214/>.

9.2. Informative References

 [ARK]      Kunze, J. and R. Rodgers, "The ARK Identifier Scheme",
            Work in Progress, draft-kunze-ark-18, April 2013.

Van de Sompel, et al. Informational [Page 15] RFC 8574 The "cite-as" Link Relation Type April 2019

 [bookmark-blog]
            Nelson, M. and H. Van de Sompel, "rel=bookmark also does
            not mean what you think it means", August 2017,
            <http://ws-dl.blogspot.com/2017/08/
            2017-08-26-relbookmark-also-does-not.html>.
 [canonical-blog]
            Nelson, M. and H. Van de Sompel, "rel=canonical does not
            mean what you think it means", August 2017,
            <http://ws-dl.blogspot.nl/2017/08/
            2017-08-07-relcanonical-does-not-mean.html>.
 [CoolURIs]
            Berners-Lee, T., "Cool URIs don't change", World Wide Web
            Consortium Style, 1998,
            <https://www.w3.org/Provider/Style/URI.html>.
 [DOI-URLs]
            Hendricks, G., "Display guidelines for Crossref DOIs",
            March 2017,
            <https://blog.crossref.org/display-guidelines/>.
 [DOIs]     ISO, "Information and documentation - Digital object
            identifier system", ISO 26324:2012(en), 2012,
            <https://www.iso.org/obp/ui/
            #iso:std:iso:26324:ed-1:v1:en>.
 [FOAF]     Brickley, D. and L. Miller, "FOAF Vocabulary Specification
            0.99", January 2014, <http://xmlns.com/foaf/spec/>.
 [identifier-blog]
            Nelson, M. and H. Van de Sompel, "Linking to Persistent
            Identifiers with rel=identifier", November 2016,
            <http://ws-dl.blogspot.com/2016/11/
            2016-11-07-linking-to-persistent.html>.
 [PIDs-must-be-used]
            Van de Sompel, H., Klein, M., and S. Jones, "Persistent
            URIs Must Be Used To Be Persistent", February 2016,
            <https://arxiv.org/abs/1602.09102>.
 [PURLs]    Wikipedia, "Persistent uniform resource locator",
            September 2018, <https://en.wikipedia.org/w/index.php?titl
            e=Persistent_uniform_resource_locator&oldid=858558072>.

Van de Sompel, et al. Informational [Page 16] RFC 8574 The "cite-as" Link Relation Type April 2019

Acknowledgements

 The authors would like to thank the following individuals for their
 comments and suggestions: Martin Klein, Harihar Shankar, Peter
 Williams, John Howard, Mark Nottingham, and Graham Klyne.

Authors' Addresses

 Herbert Van de Sompel
 Data Archiving and Networked Services
 Email: herbert.van.de.sompel@dans.knaw.nl
 URI:   https://orcid.org/0000-0002-0715-6126
 Michael Nelson
 Old Dominion University
 Email: mln@cs.odu.edu
 URI:   http://www.cs.odu.edu/~mln/
 Geoffrey Bilder
 Crossref
 Email: gbilder@crossref.org
 URI:   https://www.crossref.org/authors/geoffrey-bilder/
 John Kunze
 California Digital Library
 Email: jak@ucop.edu
 URI:   https://orcid.org/0000-0001-7604-8041
 Simeon Warner
 Cornell University
 Email: simeon.warner@cornell.edu
 URI:   https://orcid.org/0000-0002-7970-7855

Van de Sompel, et al. Informational [Page 17]

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