draft-ietf-httpapi-linkset-02.txt

Network Working Group                                           E. Wilde
Internet-Draft                                                     Axway
Intended status: Informational                          H. Van de Sompel
Expires: December 6, 2021          Data Archiving and Networked Services
                                                            June 4, 2021


      Linkset: Media Types and a Link Relation Type for Link Sets
                     draft-ietf-httpapi-linkset-02

Abstract

   This specification defines two document formats and respective media
   types for representing sets of links as stand-alone resources.  One
   format is JSON-based, the other aligned with the format for
   representing links in the HTTP "Link" header field.  This
   specification also introduces a link relation type to support
   discovery of sets of links.

Note to Readers

   Please discuss this draft on the "Building Blocks for HTTP APIs"
   mailing list (<https://www.ietf.org/mailman/listinfo/httpapi>).

   Online access to all versions and files is available on GitHub
   (<https://github.com/ietf-wg-httpapi/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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 6, 2021.








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Copyright Notice

   Copyright (c) 2021 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
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   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 . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Challenges Writing to HTTP Link Header Field  . . . . . .   5
     3.3.  Large Number of Links . . . . . . . . . . . . . . . . . .   5
   4.  Document Formats for Sets of Links  . . . . . . . . . . . . .   5
     4.1.  HTTP Link Document Format: application/linkset  . . . . .   6
     4.2.  JSON Document Format: application/linkset+json  . . . . .   6
       4.2.1.  Set of Links  . . . . . . . . . . . . . . . . . . . .   7
       4.2.2.  Link Context Object . . . . . . . . . . . . . . . . .   8
       4.2.3.  Link Target Object  . . . . . . . . . . . . . . . . .   8
       4.2.4.  Link Target Attributes  . . . . . . . . . . . . . . .  10
       4.2.5.  JSON Extensibility  . . . . . . . . . . . . . . . . .  14
   5.  The "profile" attribute for media types to Represent Sets of
       Links . . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  The "linkset" Relation Type for Linking to a Set of Links . .  15
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Set of Links Provided as application/linkset  . . . . . .  15
     7.2.  Set of Links Provided as application/linkset+json . . . .  17
     7.3.  Discovering a Link Set via the "linkset" Link Relation
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  20
     8.1.  GS1 . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.2.  FAIR Signposting Profile  . . . . . . . . . . . . . . . .  21
     8.3.  Open Journal Systems (OJS)  . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Link Relation Type: linkset . . . . . . . . . . . . . . .  21
     9.2.  Media Type: application/linkset . . . . . . . . . . . . .  22
     9.3.  Media Type: application/linkset+json  . . . . . . . . . .  23
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24



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   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     11.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  27
   Appendix B.  JSON-LD Context  . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   Resources on the Web often use typed Web Links [RFC8288], either
   embedded in resource representations, for example using the <link>
   element for HTML documents, or conveyed in the HTTP "Link" header for
   documents of any media type.  In some cases, however, providing links
   in this manner is impractical or impossible and delivering a set of
   links as a stand-alone document is preferable.

   Therefore, this specification defines two document formats and
   associated media types to represent sets of links.  It also defines
   the "linkset" relation type that supports discovery of any resource
   that conveys a set of links as a stand-alone document.

2.  Terminology

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

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

   In the examples provided in this document, links in the HTTP "Link"
   header are shown on separate lines in order to improve readability.
   Note, however, that as per Section 3.2 of [RFC7230], line breaks are
   not allowed in values for HTTP headers; only whitespaces and tabs are
   supported as seperators.

3.  Scenarios

   The following sections outline scenarios in which providing links by
   means of a standalone document instead of in an HTTP "Link" header
   field or as links embedded in the resource representation is
   advantageous or necessary.





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   For all scenarios, links could be provided by means of a stand-alone
   document that is formatted according to the JSON-based serialization,
   the serialization aligned with the HTTP "Link" header format, or
   both.  The former serialization is motivated by the widespread use of
   JSON and related tools, which suggests that handling sets of links
   expressed as JSON documents should be attractive to developers.  The
   latter serialization is provided for compatibility with the existing
   serialization used in the HTTP "Link" header and to allow reuse of
   tools created to handle it.

   It is important to keep in mind that when providing links by means of
   a standalone representation, other links can still be provided using
   other approaches, i.e. it is possible combine various mechanisms to
   convey links.

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.

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

   In such cases, links pertaining to a resource can be provided by
   another, specific resource.  That specific resource may be managed by
   the same or by another custodian as the resource to which the links
   pertain.  For clients intent on consuming links provided in that
   manner, it would be beneficial if the following conditions were met:

   o  Links are provided in a document that uses a well-defined media
      type.

   o  The resource to which the provided links pertain is able to link
      to the resource that provides these links using a well-known link
      relation type.

   These requirements are addressed in this specification through the
   definition of two media types and a link relation type, respectively.






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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 resource that provides these links discoverable can be achieved
   by means of a typed link.

3.3.  Large Number of Links

   When conveying links in an HTTP "Link" header field, 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 bound for their size.  But when the
   number of links is unpredictable, estimating a reliable upper bound
   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 bound 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 field, for example by limiting the links they
   provide to those with select relation types, thereby limiting the
   value of the HTTP "Link" header field to clients.  Providing links by
   means of a standalone document overcomes challenges related to the
   unpredictable nature of the size of HTTP "Link" header fields.

4.  Document Formats for Sets of Links

   This section specifies two document formats to convey a set of links.
   Both are based on the abstract model specified in Section 2 of Web
   Linking [RFC8288] that defines a link as consisting of a "link
   context", a "link relation type", a "link target", and optional
   "target attributes":

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

   o  The format defined in Section 4.2 is based on JSON [RFC8259].




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   Note that [RFC8288] deprecates the "rev" construct that was provided
   by [RFC5988] as a means to express links with a directionality that
   is the inverse of direct links that use the "rel" construct.  In both
   serializations for link sets defined here, inverse links SHOULD be
   represented as direct links using the "rel" construct and by
   switching the position of the resources involved in the link.

4.1.  HTTP Link Document Format: application/linkset

   This document format is identical to the payload of the HTTP "Link"
   header field as defined in Section 3 of [RFC8288], more specifically
   by its ABNF production rule for "Link" and subsequent ones.  Whereas
   the HTTP "Link" Header field depends on HTTP and hence on [RFC0822]
   for its encoding, the format specified here is encoded as UTF-8
   [RFC3629].

   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]).

   If these recommendations are not followed, interpretation of links in
   "application/linkset" documents will depend on which URI is used as
   context.

   It should be noted that the "application/linkset" format specified
   here is different than the "application/link-format" format specified
   in [RFC6690] in that the former fully matches the payload of the HTTP
   "Link" header as defined in Section 3 of [RFC8288], whereas the
   latter introduces constraints on that definition to meet requirements
   for Constrained RESTful Environments.

4.2.  JSON Document Format: application/linkset+json

   This document format uses JSON [RFC8259] as the syntax to represent a
   set of links.  The set of links follows the abstract model defined by
   Web Linking [RFC8288].





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   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]).

   If these recommendations are not followed, interpretation of
   "application/linkset+json" will depend on which URI is used as
   context URI.

   The "application/linkset+json" serialization is 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 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 MAY have an "anchor" member with a value
      that represents the link context.  If present, this value SHOULD
      be an absolute URI as defined in Section 4.3 of [RFC3986].  Cases
      where the anchor member is present, but no value is provided for
      it (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 empty string ("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 where the href
      member is present, but no value is provided for it (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 an empty string ("href": "").




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   o  In many cases, a link target is further qualified by target
      attributes.  Various types of attributes exist and they are
      conveyed as additional members of the link target object as
      detailed in Section 4.2.4.

   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.

   {
     "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.










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   {
     "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

   A link may be further qualified by target attributes.  Three types of
   attributes exist:

   o  Attributes defined by the serialization of Web Linking [RFC8288].

   o  Extension attributes defined and used by communities as allowed by
      [RFC8288].

   o  Internationalized versions of the "title" attribute defined by
      [RFC8288] and of extension attributes allowed by [RFC8288].

   The handling of these different types of attributes is described in
   the sections below.

4.2.4.1.  Target Attributes Defined by Web Linking

   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:

   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.





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   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" ]
                 }
           ]
         }
       ]
   }

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




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   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 one 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 another,
      optional, 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.

   {
     "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



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   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
   [RFC8288] (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.

   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.






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   {
     "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" } ]
                 }
           ]
         }
       ]
   }

4.2.5.  JSON Extensibility

   The extensibility of the JSON document format for representing a set
   of links is restricted to the extensibility provided by [RFC8288].
   The Web linking model provides for the use of extension target
   attributes as discussed in Section 4.2.4.3.  Extensions based on the
   JSON syntax MUST NOT be used, and MUST be ignored when found in a
   JSON linkset document.

   This limitation of the JSON format allows to unambiguously round trip
   between links provided in the HTTP "Link" header, sets of links
   serialized according to the "application/linkset" format, and sets of
   links serialized according to the "application/linkset+json" format.

5.  The "profile" attribute for media types to Represent Sets of Links

   As a means to convey specific constraints or conventions (as per
   [RFC6906]) that apply to a link set document, the "profile" attribute
   MAY be used in conjunction with the media types "application/linkset"
   and "application/linkset+json" detailed in Section 4.1 and
   Section 4.2, respectively.  For example, the attribute could be used
   to indicate that a link set uses a specific, limited set of link
   relation types.

   The value of the "profile" attribute MUST be a non-empty list of
   space-separated URIs, each of which identifies specific constraints
   or conventions that apply to the link set document.  Profile URIs MAY
   be registered in the IANA Profile URI Registry in the manner
   specified by [RFC7284].





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   The presence of a "profile" attribute in conjunction with the
   "application/linkset" and "application/linkset+json" media types does
   not change the semantics of a link set.  As such, clients with and
   without knowledge of profile URIs can use the same representation.
   The profile parameter MAY be used by clients to express their
   preferences, and, if a client does so, a server SHOULD return a
   document that honors the profiles it recognizes, and MUST ignore the
   profiles which it does not recognize.

6.  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.

   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
   properly determine what the context is for these links; it SHOULD
   ignore links for which it is unable to unambiguously make that
   determination.

7.  Examples

   Section 7.1 and Section 7.2 show examples whereby a set of links is
   provided as "application/linkset" and "application/linkset+json"
   documents, respectively.  Section 7.3 illustrates the use of the
   "linkset" link relation type to support discovery of sets of links.

7.1.  Set of Links Provided as application/linkset

   Figure 1 shows a client issuing an HTTP GET request against resource
   <https://example.org/links/resource1>.





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   GET /links/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 specifying that the media
   type of the response is "application/linkset".  A set of links,
   revealing authorship and versioning related to resource
   <https://example.org/resource1>, is provided in the response body.
   The HTTP "Link" header indicates the availability of an alternate
   representation of the set of links using media type "application/
   linkset+json".




































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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: 1023
   Content-Type: application/linkset; charset=UTF-8
   Link: <https://example.org/links/resource1> ; rel="alternate"
   ; type="application/linkset+json"
   Connection: close
   <https://authors.example.net/johndoe>
      ; rel="author"
      ; type="application/rdf+xml"
      ; anchor="https://example.org/resource1",
   <https://example.org/resource1?version=3>
      ; rel="latest-version"
      ; type="text/html"
      ; anchor="https://example.org/resource1",
   <https://example.org/resource1?version=2>
      ; rel="predecessor-version"
      ; type="text/html"
      ; anchor="https://example.org/resource1?version=3",
   <https://example.org/resource1?version=1>
      ; rel="predecessor-version"
      ; type="text/html"
      ; anchor="https://example.org/resource1?version=2",
   <https://example.org/resource1?version=1>
      ; rel="memento"
      ; type="text/html"
      ; datetime="Thu, 13 Jun 2019 09:34:33 GMT"
      ; anchor="https://example.org/resource1",
   <https://example.org/resource1?version=2>
      ; rel="memento"
      ; type="text/html"
      ; datetime="Sun, 21 Jul 2019 12:22:04 GMT"
      ; anchor="https://example.org/resource1",
   <https://authors.example.net/alice>
      ; rel="author"
      ; anchor="https://example.org/resource1#comment=1"


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

7.2.  Set of Links Provided as application/linkset+json

   Figure 3 shows the client issuing an HTTP GET request against
   <https://example.org/links/resource1>.  In the request, the client
   uses an "Accept" header to indicate it prefers a response in the
   "application/linkset+json" format.




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   GET links/resource1 HTTP/1.1
   Host: example.org
   Accept: application/linkset+json
   Connection: close

        Figure 3: Client HTTP GET request expressing preference for
                    "application/linkset+json" response

   Figure 4 shows the response to the HTTP GET request of Figure 3.  The
   set of links is serialized according to the media type "application/
   linkset+json".

   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:46:22 GMT
   Server: Apache-Coyote/1.1
   Content-Type: application/linkset+json
   Link: <https://example.org/links/resource1> ; rel="alternate"
   ; type="application/linkset"
   Content-Length: 1349
   {
     "linkset": [
       {
         "anchor": "https://example.org/resource1",
         "author": [
           {
             "href": "https://authors.example.net/johndoe",
             "type": "application/rdf+xml"
           }
         ],
         "memento": [
           {
             "href": "https://example.org/resource1?version=1",
             "type": "text/html",
             "datetime": "Thu, 13 Jun 2019 09:34:33 GMT"
           },
           {
             "href": "https://example.org/resource1?version=2",
             "type": "text/html",
             "datetime": "Sun, 21 Jul 2019 12:22:04 GMT"
           }
         ],
         "latest-version": [
           {
             "href": "https://example.org/resource1?version=3",
             "type": "text/html"
           }
         ]
       },



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       {
         "anchor": "https://example.org/resource1?version=3",
         "predecessor-version": [
           {
             "href": "https://example.org/resource1?version=2",
             "type": "text/html"
           }
         ]
       },
       {
         "anchor": "https://example.org/resource1?version=2",
         "predecessor-version": [
           {
             "href": "https://example.org/resource1?version=1",
             "type": "text/html"
           }
         ]
       },
       {
         "anchor": "https://example.org/resource1#comment=1",
         "author": [
           {
             "href": "https://authors.example.net/alice"
           }
         ]
       }
     ]
   }

      Figure 4: Response to the client's request for the set of links

7.3.  Discovering a Link Set via the "linkset" Link Relation Type

   Figure 5 shows a client issuing an HTTP HEAD request against resource
   <https://example.org/resource1>.

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


                    Figure 5: Client HTTP HEAD request

   Figure 6 shows the response to the HEAD request of Figure 5.  The
   response contains an HTTP "Link" header with a link that has the
   "linkset" relation type.  It indicates that a set of links is
   provided by resource <https://example.org/links/resource1>, which
   provides a representation with media type "application/linkset+json".



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   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:45:54 GMT
   Server: Apache-Coyote/1.1
   Link: <https://example.org/links/resource1>
         ; rel="linkset"
         ; type="application/linkset+json"
   Content-Length: 236
   Content-Type: text/html;charset=utf-8
   Connection: close


                  Figure 6: Response to HTTP HEAD request

   Section 7.2 shows a client obtaining a set of links by issuing an
   HTTP GET on the target of the link with the "linkset" relation type,
   <https://example.org/links/resource1>.

8.  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".

8.1.  GS1

   GS1 is a provider of barcodes (GS1 GTINs and EAN/UPC) for retail
   products and manages an ecology of services and standards to leverage
   them at a global scale.  GS1 has indicated that it will implement
   this "linkset" specification as a means to allow requesting and
   representing links pertaining to products from various retailers.



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   Currently, the GS1 Digital Link specification makes an informative
   reference to version 03 of the "linkset" I-D.  GS1 expresses
   confidence that this will become a normative reference in the next
   iteration of that specification, likely to be ratified as a GS1
   standard around February 2021.

8.2.  FAIR Signposting Profile

   The FAIR Signposting Profile is a community specification aimed at
   improving machine navigation of scholarly objects on the web through
   the use of typed web links pointing at e.g.  web resources that are
   part of a specific object, persistent identifiers for the object and
   its authors, license information pertaining to the object.  The
   specification encourages the use of Linksets and initial
   implementations are ongoing, for example, for the open source
   Dataverse data repository platform that was initiated by Harvard
   University and is meanwhile used by research institutions, worldwide.

8.3.  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).

9.  IANA Considerations

9.1.  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

      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 ]]





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9.2.  Media Type: application/linkset

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

      Type name: application

      Subtype name: linkset

      Required parameters: none

      Optional parameters: profile

      Encoding considerations: Linksets are encoded according to the
      definition of [RFC8288].  The encoding of [RFC8288] is based on
      the general encoding rules of [RFC7230], with the addition of
      allowing indicating character encoding and language for specific
      parameters as defined by [RFC8187].

      Security considerations: The security considerations of [[ This
      document ]] apply.

      Interoperability considerations: The interoperability
      considerations of [RFC7230] apply.

      Published specification: [[ This document ]]

      Applications that use this media type: This media type is not
      specific to any application, as it can be used by any application
      that wants to interchange web links.

      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

9.3.  Media Type: application/linkset+json

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

      Type name: application

      Subtype name: linkset+json

      Required parameters: none

      Optional parameters: profile

      Encoding considerations: The encoding considerations of [RFC8259]
      apply

      Security considerations: The security considerations of [[ This
      document ]] apply.

      Interoperability considerations: The interoperability
      considerations of [RFC8259] apply.

      Published specification: [[ This document ]]

      Applications that use this media type: This media type is not
      specific to any application, as it can be used by any application
      that wants to interchange web links.

      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>



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      Change controller: IETF

10.  Security Considerations

   The security considerations of Web Linking [RFC8288] apply, as long
   as they are not specifically discussing the risks of exposing
   information in HTTP header fields.

   In general, links may cause information leakage when they expose
   information (such as URIs) that can be sensitive or private.  Links
   may expose "hidden URIs" that are not supposed to be openly shared,
   and may not be sufficiently protected.  Ideally, none of the URIs
   exposed in links should be supposed to be "hidden"; instead, if these
   URIs are supposed to be limited to certain users, then technical
   measures should be put in place so that accidentally exposing them
   does not cause any harm.

   For the specific mechanisms defined in this specification, two
   security considerations should be taken into account:

   o  The Web Linking model always has an "implicit context", which is
      the resource of the HTTP interaction.  This original context can
      be lost or can change when self-contained link representations are
      moved.  Changing the context can change the interpretation of
      links when they have no explicit anchor, or when they use relative
      URIs.  Applications may choose to ignore links that have no
      explicit anchor or that use relative URIs when these are exchanged
      in stand-alone resources.

   o  The model introduced in this specification supports "3rd party
      links", where one party can provide links that have another
      party's resource as an anchor.  Depending on the link semantics
      and the application context, it is important to verify that there
      is sufficient trust in that 3rd party to allow it to provide these
      links.  Applications may choose to treat 3rd party links
      differently than cases where a resource and the links for that
      resource are provided by the same party.

11.  References

11.1.  Normative References

   [RFC0822]  Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET
              TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822,
              August 1982, <https://www.rfc-editor.org/info/rfc822>.






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   [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>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [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>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [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>.

   [RFC6906]  Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
              DOI 10.17487/RFC6906, March 2013,
              <https://www.rfc-editor.org/info/rfc6906>.

   [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>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [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>.






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   [RFC7284]  Lanthaler, M., "The Profile URI Registry", RFC 7284,
              DOI 10.17487/RFC7284, June 2014,
              <https://www.rfc-editor.org/info/rfc7284>.

   [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>.

   [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>.

11.2.  Informative References

   [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>.

   [W3C.REC-json-ld-20140116]
              Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0",
              World Wide Web Consortium Recommendation REC-json-ld-
              20140116, January 2014,
              <https://www.w3.org/TR/2014/REC-json-ld-20140116>.

11.3.  URIs

   [1] https://www.w3.org/TR/2014/REC-json-ld-20140116/#interpreting-
       json-as-json-ld

   [2] https://tools.ietf.org/html/rfc8288#appendix-A.2







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Appendix A.  Acknowledgements

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

Appendix B.  JSON-LD Context

   A set of links rendered according to the JSON serialization defined
   in Section 4.2 can be interpreted as RDF triples by adding a JSON-LD
   context [W3C.REC-json-ld-20140116] that maps the JSON keys to
   corresponding Linked Data terms.  And, as per
   [W3C.REC-json-ld-20140116] section 6.8 [1], when delivering a link
   set that is rendered according to the "application/linkset+json"
   media type to a user agent, a server can convey the availability of
   such a JSON-LD context by using a link with the relation type
   "http://www.w3.org/ns/json-ld#context" in the HTTP "Link" header.

   Using the latter approach to support discovery of a JSON-LD Context,
   the response to the GET request of Figure 3 against the URI of a set
   of links would be as shown in Figure 7.






























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   HTTP/1.1 200 OK
   Date: Mon, 12 Aug 2019 10:48:22 GMT
   Server: Apache-Coyote/1.1
   Content-Type: application/linkset+json
   Link: <https://example.org/contexts/linkset.jsonld>
         ; rel="http://www.w3.org/ns/json-ld#context"
         ; type="application/ld+json"
   Content-Length: 846
   {
     "linkset": [
       {
         "anchor": "https://example.org/article/view/7507",
         "author": [
           {
             "href": "https://orcid.org/0000-0002-1825-0097"
           }
         ],
         "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/10.5555/12345680",
             "title": "A Methodology for the Emulation of Architecture"
           }
         ]
       },
       {
         "anchor": "https://example.com/links/article/7507",
         "alternate": [
           {
             "href": "https://mirror.example.com/links/article/7507",
             "type": "application/linkset"
           }
         ]
       }
     ]
   }

      Figure 7: Using a typed link to support discovery of a JSON-LD
                        Context for a Set of Links



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   In order to obtain the JSON-LD Context conveyed by the server, the
   user agent issues an HTTP GET against the link target of the link
   with the "http://www.w3.org/ns/json-ld#context" relation type.  The
   response to this GET is shown in Figure 8.  This particular JSON-LD
   context maps "application/linkset+json" representations of link sets
   to Dublin Core Terms.  It also renders each link relation as an
   absolute URI, inspired by the same approach used for Atom [RFC4287]
   described in [RFC8288] appendix A.2 [2].

   HTTP/1.1 200 OK
   Content-Type: application/ld+json
   Content-Length: 638
   {
     "@context": [
       {
         "@vocab":  "http://www.iana.org/assignments/relation/",
         "anchor":  "@id",
         "href":    "@id",
         "linkset": "@graph",
         "_linkset": "@graph",
         "title":   {
           "@id":    "http://purl.org/dc/terms/title"
         },
         "title*":  {
           "@id":    "http://purl.org/dc/terms/title"
         },
         "type":    {
           "@id":    "http://purl.org/dc/terms/format"
         }
       },
       {
         "language": "@language",
         "value":    "@value",
         "hreflang": {
           "@id":        "http://purl.org/dc/terms/language",
           "@container": "@set"
         }
       }
     ]
   }

   Figure 8: JSON-LD Context mapping to schema.org and IANA assignments

   Applying the JSON-LD context of Figure 8 to the link set of Figure 7
   allows transforming the "application/linkset+json" link set to an RDF
   link set.  Figure 9 shows the latter represented by means of the
   "text/turtle" RDF serialization.




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   <https://example.org/article/view/7507>
       <http://www.iana.org/assignments/relation/author>
       <https://orcid.org/0000-0002-1825-0097> .
   <https://example.org/article/view/7507>
       <http://www.iana.org/assignments/relation/item>
       <https://example.org/article/7507/item/1> .
   <https://example.org/article/7507/item/1>
       <http://purl.org/dc/terms/format>
       "application/pdf" .
   <https://example.org/article/view/7507>
       <http://www.iana.org/assignments/relation/item>
       <https://example.org/article/7507/item/2> .
   <https://example.org/article/7507/item/2>
       <http://purl.org/dc/terms/format>
       "text/csv" .
   <https://example.org/article/view/7507>
       <http://www.iana.org/assignments/relation/cite-as>
       <https://doi.org/10.5555/12345680> .
   <https://doi.org/10.5555/12345680>
       <http://purl.org/dc/terms/title>
       "A Methodology for the Emulation of Architecture" .
   <https://example.com/links/article/7507>
       <http://www.iana.org/assignments/relation/alternate>
       <https://mirror.example.com/links/article/7507> .
   <https://mirror.example.com/links/article/7507>
       <http://purl.org/dc/terms/format>
       "application/linkset" .


    Figure 9: RDF serialization of the link set resulting from applying
                            the JSON-LD context

   Note that the JSON-LD context of Figure 8 does not handle (meta)link
   relations of type ""linkset"" as they are in conflict with the top-
   level JSON key.  A workaround is to rename the top-level key to
   ""_linkset"" in the "application/linkset+json" before transforming a
   link set to JSON-LD.

Authors' Addresses

   Erik Wilde
   Axway

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






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   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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