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Versions: (draft-wilde-linkset-link-rel) 00 01 02 03 04

Network Working Group                                           E. Wilde
Internet-Draft
Intended status: Informational                          H. Van de Sompel
Expires: February 29, 2020         Data Archiving and Networked Services
                                                         August 28, 2019


      Linkset: Media Types and a Link Relation Type for Link Sets
                         draft-wilde-linkset-04

Abstract

   This specification defines two media types and a link relation type
   for sets of links.  The media types can be used to represents links
   in a standalone fashion, in one case in the native format as used in
   the HTTP Link header, and in the other case in a JSON-based format.
   The link relation type can be used by a resource to point at another
   resource that provides a set of links, including links in which the
   former resource participates.  One typical scenario is when the
   number of links to put in an HTTP Link header becomes too big, and
   thus these links should be provided in another way.

Note to Readers

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

   Online access to all versions and files is available on GitHub
   (<https://github.com/dret/I-D/tree/master/linkset>).

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 29, 2020.





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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.  Code Components extracted from this document must
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Third-Party Links . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Challenges Writing to HTTP Link Header Field  . . . . . .   4
     3.3.  Large Number of Links . . . . . . . . . . . . . . . . . .   4
   4.  Document Formats for Sets of Links  . . . . . . . . . . . . .   5
     4.1.  HTTP Link Document Format: application/linkset  . . . . .   5
     4.2.  JSON Document Format: application/linkset+json  . . . . .   6
       4.2.1.  Set of Links  . . . . . . . . . . . . . . . . . . . .   6
       4.2.2.  Link Context Object . . . . . . . . . . . . . . . . .   7
       4.2.3.  Link Target Object  . . . . . . . . . . . . . . . . .   7
       4.2.4.  Link Target Attributes  . . . . . . . . . . . . . . .   9
   5.  The "linkset" Relation Type for Linking to a Set of Links . .  13
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Set of Links Provided By Value as application/linkset . .  14
     6.2.  Set of Links Provided By Reference as
           application/linkset+json  . . . . . . . . . . . . . . . .  15
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Open Journal Systems (OJS)  . . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Link Relation Type: linkset . . . . . . . . . . . . . . .  18
     8.2.  Media Type: application/linkset . . . . . . . . . . . . .  19
       8.2.1.  IANA Considerations . . . . . . . . . . . . . . . . .  19
     8.3.  Media Type: application/linkset+json  . . . . . . . . . .  20
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  21
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  22
   Appendix B.  JSON-LD Context  . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23




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1.  Introduction

   Resources on the Web often convey typed Web Links [RFC8288] as a part
   of resource representations, for example, using the <link> element
   for HTML representations, or the "Link" header field in HTTP response
   headers for representations of any media type.  In some cases,
   however, providing links in this manner is impractical or impossible.

   To that end, this specification defines two document formats and
   associated media types to represent a set of links.  It also defines
   the "linkset" relation type that a resource can use to support
   discovery of another resource that conveys links, including links in
   which the former resource participates.

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 RFC 5988 [RFC5988].
   Although defined as IRIs, in common scenarios they are also URIs.

3.  Scenarios

   The following sections outline scenarios in which providing links by
   means of a standalone document instead of via an HTTP Link header or
   via <link> elements in HTML is advantageous or necessary.  It is
   important to keep in mind that, when providing links by means of a
   standalone, other links can still be provided by value.

3.1.  Third-Party Links

   In some cases, it is useful that links pertaining to a resource are
   provided by a server other than the one that hosts the resource.  For
   example, this allows:

   o  Providing links in which the resource is involved not just as link
      context but also as link target.

   o  Providing links pertaining to the resource that the server hosting
      that resource is not aware of.





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   o  External management of links pertaining to the resource in a
      special-purpose link management service.

   In such cases, a third-party resource can provide a set of links
   pertaining to the resource.  This third-party resource may be managed
   by the same or by another custodian as the resource.  For clients
   intent on consuming links pertaining to the resource, it would be
   beneficial if:

   o  The resource would be able to unambiguously point at the third-
      party resource that provides the links using a registered link
      relation type.

   o  The third-party resource would provide the links in a document
      that is compliant with a registered media type.

3.2.  Challenges Writing to HTTP Link Header Field

   In some cases, it is not straightforward to write links to the HTTP
   Link header field from an application.  This can, for example, be the
   case because not all required link information is available to the
   application or because the application does not have the capability
   to directly write HTTP headers.  In such cases, providing links by
   means of a standalone document can be a solution.  Making the third-
   party resource that provides these links discoverable can be achieved
   by means of an unambiguously typed link, which can typically
   straightforwardly be added by means of a web server rewrite rule that
   leverages the resource's URI.

3.3.  Large Number of Links

   When conveying links in the HTTP "Link" header, it is possible for
   the size of the HTTP response header to become unpredictable.  This
   can be the case when links are determined dynamically dependent on a
   range of contextual factors.  It is possible to statically configure
   a web server to correctly handle large HTTP response headers by
   specifying an upper boundary for their size.  But when the number of
   links is unpredictable, estimating a reliable upper boundary is
   challenging.

   HTTP [RFC7231] defines error codes related to excess communication by
   the user agent ("413 Request Entity Too Large" and "414 Request-URI
   Too Long"), but no specific error codes are defined to indicate that
   response header content exceeds the upper boundary that can be
   handled by the server, and thus it has been truncated.  As a result,
   applications take counter measures aimed at controlling the size of
   the HTTP "Link" header, for example by limiting the links they
   provide to those with select relation types, thereby limiting the



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   value of the HTTP "Link" header to clients.  Providing links by means
   of a standalone document overcomes challenges related to the
   unpredictable nature of the extent of HTTP "Link" headers.

   In more extreme scenarios it is conceivable that the number of links
   to be conveyed becomes so large that even a standalone document would
   become too large.  This could be the case for highly popular
   resources, and when links are provided in which such resources
   participates as both link context and link target.  In such cases,
   the links could be delivered incrementally, for example, by means of
   a paged resource model that uses links with the "next" link relation
   type.

4.  Document Formats for Sets of Links

   A set of links can be represented by serializations that support
   conveying both link contexts and link targets.  This section
   specifies two document formats based on the abstract model specified
   in Section 2 of [RFC8288] that defines a link as consisting of a
   "link context", a "link relation type", a "link target", and optional
   "target attributes".

   The format defined in Section 4.1 is identical to the payload of the
   HTTP Link header as specified in [RFC8288].

   The format defined in Section 4.2 is based on JSON and thus does not
   have the character encoding limitations of HTTP header fields.

4.1.  HTTP Link Document Format: application/linkset

   This document format is identical to the payload of the HTTP Link
   header as defined in Section 3 of [RFC8288], more specifically by its
   ABNF production rule for "Link" and subsequent ones.

   The assigned media type for this format is "application/linkset".

   In order to support use cases where "application/linkset" documents
   are re-used outside the context of an HTTP interaction, it is
   RECOMMENDED to make them self-contained by adhering to the following
   guidelines:

   o  For every link provided in the set of links, explicitly provide
      the link context using the "anchor" attribute.

   o  For link context ("anchor" attribute) and link target ("href"
      attribute), use absolute URIs (as defined in Section 4.3 of
      [RFC3986]).




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4.2.  JSON Document Format: application/linkset+json

   This document format uses JSON [RFC8259] as the syntax to represent a
   set of links.  It is described in this section.

   The assigned media type for this format is "application/
   linkset+json".

   In order to support use cases where "application/linkset+json"
   documents are re-used outside the context of an HTTP interaction, it
   is RECOMMENDED to make them self-contained by adhering to the
   following guidelines:

   o  For every link provided in the set of links, explicitly provide
      the link context using the "anchor" member.

   o  For link context ("anchor" member) and link target ("href"
      member), use absolute URIs (as defined in Section 4.3 of
      [RFC3986]).

   The "application/linkset+json" serialization was designed such that
   it can directly be used as the content of a JSON-LD serialization by
   adding an appropriate context.  Appendix B shows an example of a
   possible context that, when added to the a JSON serialization, allows
   it to be interpreted as RDF.

4.2.1.  Set of Links

   In the JSON representation of a set of links:

   o  A set of links MUST be represented as a JSON object, which MUST
      have "linkset" as its sole member.

   o  The "linkset" member is an array in which a distinct JSON object -
      the "link context object" (see Section 4.2.2) - MUST be used to
      represent links that have the same link context.

   o  If necessary, the "linkset" member MAY contain information in
      addition to link context objects, in which case that information
      MUST NOT change the semantics of the links provided by those link
      context objects.

   o  Even if there is only one link context object, it MUST be wrapped
      in an array.  Members other than link context objects MUST NOT be
      included in this array.






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4.2.2.  Link Context Object

   In the JSON representation, one or more links that have the same link
   context, are represented by a JSON object, the link context object.
   A link context object adheres to the following rules:

   o  Each link context object MUST have an "anchor" member with a value
      that represents the link context.  This value SHOULD be an
      absolute URI as defined in Section 4.3 of [RFC3986].  Cases
      whereby no value is to be provided for the "anchor" member (i.e.
      the resource providing the set of links is the link context for
      each link in the link context object), MUST be handled by
      providing an "anchor" member with null value ("anchor":"").

   o  For each distinct relation type that the link context has with
      link targets, a link context object MUST have an additional
      member.  This member is an array in which a distinct JSON object -
      the " link target object" (see Section 4.2.3) - MUST be used for
      each link target for which the relationship with the link context
      (value of the encompassing anchor member) applies.  The name of
      this member expresses the relation type of the link as follows:

   o

      *  For registered relation types [RFC8288], the name of this
         member is the registered name of the relation type.

      *  For extension relation types [RFC8288], the name of this member
         is the URI that uniquely represents the relation type.

   o  Even if there is only one link target object, it MUST be wrapped
      in an array.  Members other than link target objects MUST NOT be
      included in this array.

4.2.3.  Link Target Object

   In the JSON representation, a link target is represented by a JSON
   object, the link target object.  A link target object adheres to the
   following rules:

   o  Each link target object MUST have an "href" member with a value
      that represents the link target.  This value SHOULD be an absolute
      URI as defined in Section 4.3 of [RFC3986].  Cases whereby no
      value is to be provided for the "href" member (i.e. the resource
      providing the set of links is the target of the link in the link
      target object), MUST be handled by providing an "href" member with
      null value ("href":"").




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   o  In many cases, a link target is further qualified by attributes,
      either defined by the serialization of [RFC8288] or extension
      attributes defined and used by communities.  These attributes are
      conveyed as additional members of the link target object, as
      detailed in Section 4.2.4.1, Section 4.2.4.2, and Section 4.2.4.3.

   Note that the JSON representation does not use the "rel" or "rev"
   constructs used by [RFC8288] and the above specified HTTP Link
   document format.  Rather:

   o  A link that uses the "rel" construct in those approaches is
      conveyed using the URI of the link context as the value for
      "anchor" and the URI of the link target as the value for "href".

   o  A link that uses the "rev" construct in those approaches is
      conveyed using the URI of the link target as the value for
      "anchor" and the URI of the link context as the value for "href".

   The following example of a JSON-serialized set of links represents
   one link with its core components: link context, link relation type,
   and link target.

   {
     "linkset":
       [
         { "anchor":"http://example.net/bar",
           "next": [
                 {"href":"http://example.com/foo"}
           ]
         }
       ]
   }

   The following example of a JSON-serialized set of links represents
   two links that share link context and relation type but have
   different link targets.















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   {
     "linkset":
       [
         { "anchor":"http://example.net/bar",
           "item": [
                 {"href":"http://example.com/foo1"},
                 {"href":"http://example.com/foo2"}
           ]
         }
       ]
   }


   The following example shows a set of links that represents two links,
   each with a different link context, link target, and relation type.
   One relation type is registered, the other is an extension relation
   type.

   {
     "linkset":
       [
         { "anchor":"http://example.net/bar",
           "next": [
                 {"href":"http://example.com/foo1"}
           ]
         },
         { "anchor":"http://example.net/boo",
           "http://example.com/relations/baz" : [
                 {"href": "http://example.com/foo2"}
           ]
         }
       ]
   }

4.2.4.  Link Target Attributes

   In many cases, a link is further qualified by target attributes,
   either defined by the serialization of [RFC8288] or extension
   attributes defined and used by communities.

4.2.4.1.  Target Attributes Defined by RFC 8288

   RFC 8288 defines the following target attributes that may be used to
   annotate links: "hreflang", "media", "title", "title*", and "type";
   these target attributes follow different occurrence and value
   patterns.  In the JSON representation, these attributes MUST be
   conveyed as additional members of the link target object as follows:




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   o  "hreflang": The optional and repeatable "hreflang" target
      attribute MUST be represented by an array (even if there only is
      one value to be represented), and each value in that array MUST be
      a string - representing one value of the "hreflang" target
      attribute for a link - which follows the same model as in the
      [RFC8288] syntax.

   o  "media": The optional and not repeatable "media" target attribute
      MUST be represented by a "media" member in the link target object,
      and its value MUST be a string that follows the same model as in
      the [RFC8288] syntax.

   o  "type": The optional and not repeatable "type" target attribute
      MUST be represented by a "type" member in the link target object,
      and its value MUST be a string that follows the same model as in
      the [RFC8288] syntax.

   o  "title": The optional and not repeatable "title" target attribute
      MUST be represented by a "title" member in the link target object,
      and its value MUST be a string that follows the same model as in
      the [RFC8288] syntax.

   o  "title*": The optional and not repeatable "title*" target
      attribute is motivated by character encoding and language issues
      and follows the model defined in [RFC8187].  The details of the
      JSON representation that applies to title* are described in
      Section 4.2.4.2.

   The following example illustrates how the repeatable "hreflang" and
   the not repeatable "type" target attributes are represented in a link
   target object.

   {
     "linkset":
       [
         { "anchor":"http://example.net/bar",
           "next": [
                 {"href":"http://example.com/foo",
                  "type":"text/html",
                  "hreflang": [ "en" , "de" ]
                 }
           ]
         }
       ]
   }






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4.2.4.2.  Internationalized Target Attributes

   In addition to the target attributes described in Section 4.2.4.1,
   [RFC8288] also supports attributes that follow the content model of
   [RFC8187].  In [RFC8288], these target attributes are recognizable by
   the use of a trailing asterisk in the attribute name, such as
   "title*".  The content model of [RFC8187] uses a string-based
   microsyntax that represents the character encoding, an optional
   language tag, and the escaped attribute value encoded according to
   the specified character encoding.

   The JSON serialization for these target attributes MUST be as
   follows:

   o  An internationalized target attribute is represented as a member
      of the link context object with the same name (including the *) of
      the attribute.

   o  The character encoding information as prescribed by [RFC8187] is
      not preserved; instead, the content of the internationalized
      attribute is represented in the character encoding used for the
      JSON set of links.

   o  The value of the internationalized target attribute is an array
      that contains one or more JSON objects.  The name of the first
      member of such JSON object is "value" and its value is the actual
      content (in its unescaped version) of the internationalized target
      attribute, i.e. the value of the attribute from which the encoding
      and language information are removed.  The name of the optional
      second member of such JSON object is "language" and its value is
      the language tag [RFC5646] for the language in which the attribute
      content is conveyed.

   The following example illustrates how the "title*" target attribute
   defined by [RFC8288] is represented in a link target object.
















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{
  "linkset":
    [
      { "anchor":"http://example.net/bar",
        "next": [
              {"href":"http://example.com/foo",
               "type":"text/html",
               "hreflang": [ "en" , "de" ],
               "title":"Next chapter",
               "title*": [ { "value": "nachstes Kapitel" , "language" : "de" } ]
              }
        ]
      }
    ]
}

   The above example assumes that the German title contains an umlaut
   character (in the native syntax it would be encoded as title*=UTF-
   8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped
   form in the JSON representation.  This is not shown in the above
   example due to the limitations of RFC publication.  Implementations
   MUST properly decode/encode internationalized target attributes that
   follow the model of [RFC8187] when transcoding between the
   "application/linkset" and the "application/linkset+json" formats.

4.2.4.3.  Extension Target Attributes

   Extension target attributes are attributes that are not defined by
   RFC 8288 (as listed in Section 4.2.4.1), but are nevertheless used to
   qualify links.  They can be defined by communities in any way deemed
   necessary, and it is up to them to make sure their usage is
   understood by target applications.  However, lacking standardization,
   there is no interoperable understanding of these extension
   attributes.  One important consequence is that their cardinality is
   unknown to generic applications.  Therefore, in the JSON
   serialization, all extension target attributes are treated as
   repeatable.

   The JSON serialization for these target attributes MUST be as
   follows:

   o  An extension target attribute is represented as a member of the
      link context object with the same name of the attribute, including
      the * if applicable.

   o  The value of an extension attribute MUST be represented by an
      array, even if there only is one value to be represented.




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   o  If the extension target attribute does not have a name with a
      trailing asterisk, then each value in that array MUST be a string
      that represents one value of the attribute.

   o  If the extension attribute has a name with a trailing asterisk (it
      follows the content model of [RFC8187]), then each value in that
      array MUST be a JSON object.  The value of each such JSON object
      MUST be structured as described in Section 4.2.4.2.

   The example shows a link target object with three extension target
   attributes.  The value for each extension target attribute is an
   array.  The two first are regular extension target attributes, with
   the first one ("foo") having only one value and the second one
   ("bar") having two.  The last extension target attribute ("baz*")
   follows the naming rule of [RFC8187] and therefore is encoded
   according to the serialization described in Section 4.2.4.2.

  {
    "linkset":
      [
        { "anchor":"http://example.net/bar",
          "next": [
                {"href":"http://example.com/foo",
                 "type":"text/html",
                 "foo": [ "foovalue" ],
                 "bar": [ "barone", "bartwo" ],
                 "baz*": [ { "value": "bazvalue" , "language" : "en" } ]
                }
          ]
        }
      ]
  }

5.  The "linkset" Relation Type for Linking to a Set of Links

   The target of a link with the "linkset" relation type provides a set
   of links, including links in which the resource that is the link
   context participates.

   A link with the "linkset" relation type MAY be provided in the header
   and/or the body of a resource's representation.  It may also be
   discovered by other means, such as through client-side information.

   A resource MAY provide more than one link with a "linkset" relation
   type.  Multiple such links can refer to the same set of links
   expressed using different media types, or to different sets of links,
   potentially provided by different third-party services.




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   The use of a link with the "linkset" relation type does not preclude
   the provision of links with other relation types, i.e. the resource
   that is the link context can provide typed links other than a
   "linkset" link.

   A user agent that follows a "linkset" link MUST be aware that the set
   of links provided by the resource that is the target of the link can
   contain links in which the resource that is the context of the link
   does not participate; it MAY decide to ignore those links.

   A user agent that follows a "linkset" link, and obtains links for
   which anchors and targets are not expressed as absolute URIs, MUST
   determine what the context is for these links; it SHOULD ignore links
   for which it is unable to unambiguously make that determination.

   There is no constraint on the target URI of a link with the "linkset"
   relation type; designing and using these URIs is left to the
   discretion of implementers.

6.  Examples

   Section 6.1 and Section 6.2 show examples whereby the set of links
   are provided as "application/linkset" and "application/linkset+json"
   documents, respectively.

6.1.  Set of Links Provided By Value as application/linkset

   Figure 1 shows a client issuing an HTTP GET request against resource
   http://example.org/resource1.

   GET /resource1 HTTP/1.1
   Host: example.org
   Connection: close


                     Figure 1: Client HTTP GET request

   Figure 2 shows the response to the GET request of Figure 1.  The
   response contains a Content-Type header with as value application/
   linkset.  A set of links, including links that pertain to the
   responding resource, is provided in the body of the response.










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   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:35:51 GMT
   Server: Apache-Coyote/1.1
   Content-Length: 729
   Content-Type: application/linkset
   Connection: close

   <http://authors.example.net/johndoe>
      ; rel="author"
      ; type="application/rdf+xml"
      ; anchor="http://example.org/resource1",
    <http://authors.example.net/janedoe>
      ; rel="author"
      ; type="application/rdf+xml"
      ; anchor="http://example.org/resource1",
    <http://example.org/resource1/items/AF48EF.pdf>
      ; rel="item"
      ; type="application/pdf"
      ; anchor="http://example.org/resource1",
    <http://example.org/resource1/items/CB63DA.html>
      ; rel="item"
      ; type="text/html"
      ; anchor="http://example.org/resource1",
    <http://example.org/resource1>
      ; rel="latest-version"
      ; anchor="http://example.org/resource41/",
    <http://example.org/resource40>
      ; rel="prev"
      ; anchor="http://example.org/resource41/"


          Figure 2: Response to HTTP GET includes a set of links

6.2.  Set of Links Provided By Reference as application/linkset+json

   Figure 3 shows a client issuing an HTTP HEAD request against resource
   http://example.org/article/view/7507

   HEAD article/view/7507 HTTP/1.1
   Host: example.org
   Connection: close


                    Figure 3: Client HTTP HEAD request

   Figure 4 shows the response to the HEAD request of Figure 3.  The
   response contains a Link header with a link that has the "linkset"
   relation type.  It indicates that a set of links is provided by



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   resource http://example.com/links/article/7507, which provides a
   representation with media type application/linkset+json.

   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:45:54 GMT
   Server: Apache-Coyote/1.1
   Link: <http://example.com/links/article/7507>
         ; rel="linkset"
         ; type="application/linkset+json"
   Content-Length: 236
   Content-Type: text/html;charset=utf-8
   Connection: close


                  Figure 4: Response to HTTP HEAD request

   Figure 5 shows the client issuing an HTTP GET request against the
   target IRI of the "linkset" link provided in Figure 4.  In the
   request, the client uses an Accept header to indicate it requests a
   response in "application/linkset+json" format.

   GET links/article/7507 HTTP/1.1
   Host: example.com
   Accept: application/linkset+json
   Connection: close

          Figure 5: Client issues HTTP GET to obtain set of links

   Figure 6 shows the response to the HTTP GET request of Figure 5.  The
   set of links is serialized according to the media type "application/
   linkset+json" and includes links in which the Context IRI of the
   "linkset" link of Figure 4 participates.  It also includes a link in
   which it does not.  That link actually points at an alternate
   representation of the set of links.

















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   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:46:22 GMT
   Server: Apache-Coyote/1.1
   Content-Type: application/linkset+json
   Content-Length: 802

   {
     "linkset": [
       {
         "anchor": "https://example.org/article/view/7507",
         "author": [
           {
             "href": "https://orcid.org/0000-0002-4311-0897",
             "type": "rdf/xml"
           }
         ],
         "item": [
           {
             "href": "https://example.org/article/7507/item/1",
             "type": "application/pdf"
           },
           {
             "href": "https://example.org/article/7507/item/2",
             "type": "text/csv"
           }
         ],
         "cite-as": [
           {
             "href": "https://doi.org/10.841/zk2557"
           }
         ]
       },
       {
         "anchor": "https://example.com/links/article/7507",
         "alternate": [
           {
             "href": "https://mirror.example.com/links/article/7507",
             "type": "application/linkset"
           }
         ]
       }
     ]
   }

      Figure 6: Response to the client's request for the set of links






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7.  Implementation Status

   Note to RFC Editor: Please remove this section before publication.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 6982
   [RFC6982].  The description of implementations in this section is
   intended to assist the IETF in its decision processes in progressing
   drafts to RFCs.  Please note that the listing of any individual
   implementation here does not imply endorsement by the IETF.
   Furthermore, no effort has been spent to verify the information
   presented here that was supplied by IETF contributors.  This is not
   intended as, and must not be construed to be, a catalog of available
   implementations or their features.  Readers are advised to note that
   other implementations may exist.

   According to RFC 6982, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

7.1.  Open Journal Systems (OJS)

   Open Journal Systems (OJS) is an open-source software for the
   management of peer-reviewed academic journals, and is created by the
   Public Knowledge Project (PKP), released under the GNU General Public
   License.  Open Journal Systems (OJS) is a journal management and
   publishing system that has been developed by PKP through its
   federally funded efforts to expand and improve access to research.

   The OJS platform has implemented "linkset" support as an alternative
   way to provide links when there are more than a configured limit
   (they consider using about 10 as a good default, for testing purpose
   it is currently set to 8).

8.  IANA Considerations

8.1.  Link Relation Type: linkset

   The link relation type below has been registered by IANA per
   Section 6.2.1 of Web Linking [RFC8288]:

      Relation Name: linkset





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      Description: The Target IRI of a link with the "linkset" relation
      type provides a set of links, including links in which the Context
      IRI of the link participates.

      Reference: [[ This document ]]

8.2.  Media Type: application/linkset

8.2.1.  IANA Considerations

   The Internet media type [RFC6838] for a natively encoded link set is
   application/linkset.

      Type name: application

      Subtype name: linkset

      Required parameters: none

      Optional parameters: none

      Encoding considerations: ...

      Security considerations: ...

      Interoperability considerations: ...

      Published specification: [[ This document ]]

      Applications that use this media type: ...

      Additional information:

         Magic number(s): N/A

         File extension(s): This media type does not propose a specific
         extension.

         Macintosh file type code(s): TEXT

      Person & email address to contact for further information: Erik
      Wilde <erik.wilde@dret.net>

      Intended usage: COMMON

      Restrictions on usage: none

      Author: Erik Wilde <erik.wilde@dret.net>



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      Change controller: IETF

8.3.  Media Type: application/linkset+json

   The Internet media type [RFC6838] for a JSON-encoded link set is
   application/linkset+json.

      Type name: application

      Subtype name: linkset+json

      Required parameters: none

      Optional parameters: none

      Encoding considerations: ...

      Security considerations: ...

      Interoperability considerations: ...

      Published specification: [[ This document ]]

      Applications that use this media type: ...

      Additional information:

         Magic number(s): N/A

         File extension(s): JSON documents often use ".json" as the file
         extension, and this media type does not propose a specific
         extension other than this generic one.

         Macintosh file type code(s): TEXT

      Person & email address to contact for further information: Erik
      Wilde <erik.wilde@dret.net>

      Intended usage: COMMON

      Restrictions on usage: none

      Author: Erik Wilde <erik.wilde@dret.net>

      Change controller: IETF






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9.  Security Considerations

   ...

10.  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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <https://www.rfc-editor.org/info/rfc5988>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              DOI 10.17487/RFC6982, July 2013,
              <https://www.rfc-editor.org/info/rfc6982>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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>.

   [RFC8187]  Reschke, J., "Indicating Character Encoding and Language
              for HTTP Header Field Parameters", RFC 8187,
              DOI 10.17487/RFC8187, September 2017,
              <https://www.rfc-editor.org/info/rfc8187>.




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   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

Appendix A.  Acknowledgements

   Thanks for comments and suggestions provided by Mark Nottingham,
   Stian Soiland-Reyes, Sarven Capadisli, ...

Appendix B.  JSON-LD Context

   When adding the below context to the representation of a link set
   rendered according to the JSON serialization defined in Section 4.2,
   it can be interpreted as RDF.
































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   {
     "@context": [
       {
         "@vocab": "https://www.iana.org/assignments/link-relations/",
         "anchor": "@id",
         "href": "@id",
         "dct": "http://purl.org/dc/terms/",
         "link": "https://www.iana.org/assignments/link-relations#",
         "title": {
           "@id": "http://purl.org/dc/terms/title"
         },
         "title*": {
           "@id": "http://purl.org/dc/terms/title"
         },
         "type": {
           "@id": "dct:format",
           "@type": "@vocab"
         }
       },
       {
         "language": "@language",
         "value": "@value",
         "hreflang": {
           "@id": "https://www.w3.org/ns/activitystreams#hreflang",
           "@container": "@set"
         }
       }
     ]
   }

      Figure 7: Context for JSON-LD that could be used with the JSON
                               serialization

Authors' Addresses

   Erik Wilde

   Email: erik.wilde@dret.net
   URI:   http://dret.net/netdret/


   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





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