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Versions: 00 draft-vandesompel-citeas

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
                                                                Crossref
                                                                J. Kunze
                                              California Digital Library
                                                               S. Warner
                                                      Cornell University
                                                          August 2, 2017


 Identifier: A Link Relation to Convey a Preferred URI for Referencing
                    draft-vandesompel-identifier-00

Abstract

   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.

Note to Readers

   Please discuss this draft on the ART mailing list
   (<https://www.ietf.org/mailman/listinfo/art>).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 3, 2018.








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Copyright Notice

   Copyright (c) 2017 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Persistent Identifiers  . . . . . . . . . . . . . . . . .   3
     3.2.  Version Identifiers . . . . . . . . . . . . . . . . . . .   4
     3.3.  Preferred Social Identifier . . . . . . . . . . . . . . .   5
     3.4.  Multi-Resource Publications . . . . . . . . . . . . . . .   5
   4.  The "identifier" Relation Type for Expressing a Preferred URI
       for the Purpose of Referencing  . . . . . . . . . . . . . . .   6
   5.  Distinction with Other Relation Types . . . . . . . . . . . .   6
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Persistent HTTP URI . . . . . . . . . . . . . . . . . . .   8
     6.2.  Preferred Profile URI . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Link Relation Type: identifier  . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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




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   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:

   o  "access URI": A URI at which a user agent accesses a web resource.

   o  "identifying 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 have introduced
   solutions to combat this problem that typically consist of:

   o  Accepting the reality that the web location of a resource - the
      access URI - may change over time.

   o  Minting an additional URI for the resource - the identifying URI -
      that is specifically intended to remain persistent over time.

   o  Redirecting (typically "HTTP 301 Moved Permanently", "HTTP 302
      Found", or "HTTP 303 See Other") from the identifying URI to the
      access URI.

   o  As a community, committing to adjust that redirection whenever the
      access URI changes over time.

   This approach is, for example, used by:




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   o  Scholarly publishers that use DOIs [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 [PURLs] for
      vocabulary terms to ensure that the term URIs remain stable even
      if management of the vocabulary is transfered to a new custodian.

   o  A variety of organizations, including libraries, archives, and
      museums that assign ARK URLs [draft-kunze-ark-18] 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 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:

   o  the most recent version of a resource is at any time 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
   <http://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 by means of the version URI that was minted for the
   current version.  To express this using the terminology of Section 2,



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   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:

   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, 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 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.




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   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:

   o  "alternate" [RFC4287], used to link to an alternate version of the
      content at the link context, 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).





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   o  "bookmark" [W3C.REC-html5-20151028], used to convey a permanent
      link to use for bookmarking purposes.

   o  "canonical" [RFC6596], used to identify content that is either
      duplicative or a superset of the content at the link context, for
      example a single page version of a magazine article, provided for
      indexing by search engines, of an article that is spread over
      several pages for human use.

   o  "duplicate" [RFC6249], used to link to 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.

   o  "related" [RFC4287], used to link to a related resource.

   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.




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6.1.  Persistent HTTP URI

   If the access URI is a landing page for a scholarly article for which
   the persistent HTTP URI <http://persistence.example.org/738207472>
   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: <http://persistence.example.org/738207472> ; rel="identifier"
Content-Type: text/html;charset=utf-8

<html>
 <head>
 ...
  <link rel="identifier" href="http://persistence.example.org/738207472" />
 ...
 </head>
 <body>
  ...
 </body>
</html>


    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.


















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

<html>
 <head>
 ...
  <link rel="identifier" href="http://johndoe.example.com/foaf" type="text/ttl"/>
 ...
 </head>
 <body>
  ...
 </body>
</html>


     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]:

      Relation Name: identifier

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

      Reference: [[ This document ]]

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.








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9.  References

9.1.  Normative References

   [I-D.nottingham-rfc5988bis]
              Nottingham, M., "Web Linking", draft-nottingham-
              rfc5988bis-06 (work in progress), 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,
              <http://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, <http://www.rfc-editor.org/info/rfc4287>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6249>.

   [RFC6596]  Ohye, M. and J. Kupke, "The Canonical Link Relation",
              RFC 6596, DOI 10.17487/RFC6596, April 2012,
              <http://www.rfc-editor.org/info/rfc6596>.

   [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, <https://www.w3.org/TR/2014/REC-
              html5-20141028/>.

9.2.  Informative References

   [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",
              June 2017, <https://blog.crossref.org/display-
              guidelines/>.



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   [DOIs]     "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>.

   [draft-kunze-ark-18]
              Kunze, J. and R. Rodgers, "The ARK Identifier Scheme",
              Internet Draft draft-kunze-ark-18, April 2013,
              <https://datatracker.ietf.org/doc/html/draft-kunze-ark>.

   [FOAF]     Brickley, D. and L. Miller, "FOAF Vocabulary Specification
              0.99", January 2014, <http://xmlns.com/foaf/spec/>.

   [identifier-blog]
              Nelson, M., "Linking to Persistent Identifiers with
              rel="identifier"", July 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]    "Persistent uniform resource locator", April 2017,
              <https://en.wikipedia.org/wiki/
              Persistent_uniform_resource_locator>.

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: herbertv@lanl.gov
   URI:   http://public.lanl.gov/herbertv/


   Michael Nelson
   Old Dominion University

   Email: mln@cs.odu.edu
   URI:   http://www.cs.odu.edu/~mln/




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   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:   http://www.cdlib.org/contact/staff_directory/jkunze.html


   Simeon Warner
   Cornell University

   Email: simeon.warner@cornell.edu
   URI:   https://orcid.org/0000-0002-7970-7855
































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