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Linkset: Media Types and a Link Relation Type for Link Sets
draft-ietf-httpapi-linkset-10

Document Type Active Internet-Draft (httpapi WG)
Authors Erik Wilde , Herbert Van de Sompel
Last updated 2022-06-29 (Latest revision 2022-05-05)
Replaces draft-wilde-linkset
Stream Internet Engineering Task Force (IETF)
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Details
draft-ietf-httpapi-linkset-10
Network Working Group                                           E. Wilde
Internet-Draft                                                     Axway
Intended status: Standards Track                        H. Van de Sompel
Expires: 6 November 2022           Data Archiving and Networked Services
                                                              5 May 2022

      Linkset: Media Types and a Link Relation Type for Link Sets
                     draft-ietf-httpapi-linkset-10

Abstract

   This specification defines two formats and respective media types for
   representing sets of links as stand-alone documents.  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
   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 6 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Cases and Motivation  . . . . . . . . . . . . . . . . . .   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  . . . . .   7
     4.2.  JSON Document Format: application/linkset+json  . . . . .   7
       4.2.1.  Set of Links  . . . . . . . . . . . . . . . . . . . .   8
       4.2.2.  Link Context Object . . . . . . . . . . . . . . . . .   8
       4.2.3.  Link Target Object  . . . . . . . . . . . . . . . . .   9
       4.2.4.  Link Target Attributes  . . . . . . . . . . . . . . .  10
       4.2.5.  JSON Extensibility  . . . . . . . . . . . . . . . . .  15
   5.  The "profile" parameter for media types to Represent Sets of
           Links . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  The "linkset" Relation Type for Linking to a Set of Links . .  16
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Set of Links Provided as application/linkset  . . . . . .  17
     7.2.  Set of Links Provided as application/linkset+json . . . .  18
     7.3.  Discovering a Link Set via the "linkset" Link Relation
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.4.  Link Set Profiles . . . . . . . . . . . . . . . . . . . .  21
       7.4.1.  Using a "profile" Attribute with a "linkset" Link . .  21
       7.4.2.  Using a "profile" Parameter with a Link Set Media
               Type  . . . . . . . . . . . . . . . . . . . . . . . .  21
       7.4.3.  Using a Link with a "profile" Link Relation Type  . .  22
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     8.1.  Link Relation Type: linkset . . . . . . . . . . . . . . .  23
     8.2.  Media Type: application/linkset . . . . . . . . . . . . .  23
     8.3.  Media Type: application/linkset+json  . . . . . . . . . .  24
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  26
   11. Informative References  . . . . . . . . . . . . . . . . . . .  27
   Appendix A.  JSON-LD Context  . . . . . . . . . . . . . . . . . .  28
   Appendix B.  Implementation Status  . . . . . . . . . . . . . . .  33
     B.1.  GS1 . . . . . . . . . . . . . . . . . . . . . . . . . . .  34

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     B.2.  FAIR Signposting Profile  . . . . . . . . . . . . . . . .  34
     B.3.  Open Journal Systems (OJS)  . . . . . . . . . . . . . . .  34
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

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
   field 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 formats for representing
   sets of Web Links and their attributes as stand-alone documents.  One
   serializes links in the same format as used in the HTTP Link header
   field, and the other serializes links in JSON.  It also defines
   associated media types to represent sets of links, and 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" in
   the same manner as [RFC8288].

   In the examples provided in this document, links in the HTTP "Link"
   header field are shown on separate lines in order to improve
   readability.  Note, however, that as per Section 5.5 of
   [I-D.ietf-httpbis-semantics], line breaks are deprecated in values
   for HTTP fields; only whitespaces and tabs are supported as
   separators.

3.  Use Cases and Motivation

   The following sections describe use cases 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" field 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" field 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 to 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:

   *  Providing links in which the resource is involved not just as link
      context but also as link target, with a different resource being
      the link context.

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

   *  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:

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

   *  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 fields.  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 fields to become unpredictable.
   This can be the case when links are determined dynamically in a
   manner dependent on a range of contextual factors.  It is possible to
   statically configure a web server to correctly handle large HTTP
   response fields by specifying an upper bound for their size.  But
   when the number of links is unpredictable, estimating a reliable
   upper bound is challenging.

   Section 15 of HTTP [I-D.ietf-httpbis-semantics] 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 field content
   exceeds the upper bound that can be handled by the server and thus
   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 (to the web server
   implementation) 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":

   *  The format defined in Section 4.1 is nearly identical to the field
      value of the HTTP "Link" header field as specified in Section 3 of
      [RFC8288].

   *  The format defined in Section 4.2 is expressed in JSON [RFC8259].

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   Links provided in the HTTP Link header are intended to be used in the
   context of an HTTP interaction and contextual information that is
   available during an interaction is used to correctly interpret them.
   Links provided in link sets, however, can be re-used outside of an
   HTTP interaction, when no such contextual information is available.
   As a result, implementers of link sets should strive to make them
   self-contained by adhering to the following recommendations.

   For links provided in the HTTP Link header that have no anchor or
   that use relative references, the URI of the resource that delivers
   the links provides the contextual information that is needed for
   their correct interpretation.  In order to support use cases where
   link set 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:

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

   *  For link context ("anchor" attribute) and link target ("href"
      attribute), use URI references that are not relative references
      (as defined in Section 4.1 of [RFC3986]).

   If these recommendations are not followed, interpretation of links in
   link set documents will depend on which URI is used as context.

   For a "title" attribute provided on a link in the HTTP Link header,
   the language in which the title is expressed is provided by the
   Content-Language header of the HTTP interaction with the resource
   that delivers the links.  This does not apply to "title" attributes
   provided for links in link set documents because that would constrain
   all links in a link set to having a single title language and would
   not support determining title languages when a link set is used
   outside of an HTTP interaction.  In order to support use cases where
   link set documents are re-used outside the context of an HTTP
   interaction, it is RECOMMENDED to make them self-contained by using
   the "title*" attribute instead of the "title" attribute because
   "title*" allows expressing the title language as part of its value by
   means of a language tag.  With this regard, note that language tags
   are matched case-insensitively (see Section 2.1.1 of [RFC5646]).  If
   this recommendation is not followed, accurately determining the
   language of titles provided on links in link set documents will not
   be possible.

   Note also that Section 3.3 of [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

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   here, inverse links may be represented as direct links using the
   "rel" construct and by switching the roles of the resources involved
   in the link.

4.1.  HTTP Link Document Format: application/linkset

   This document format is nearly identical to the field value of the
   HTTP "Link" header field as defined in Section 3 of [RFC8288], more
   specifically by its ABNF [RFC5234] production rule for "Link" and
   subsequent ones.  It differs from the format for field values of the
   HTTP "Link" header only in that not only spaces and horizontal tabs
   are allowed as separators but also newline characters as a means to
   improve readability for humans.  The use of non-ASCII characters in
   the field value of the HTTP "Link" Header field is not allowed, and
   as such is also not allowed in "application/linkset" link sets.

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

   When converting an "application/linkset" document to a field value
   for the HTTP "Link" header, newline characters MUST be removed or
   MUST be replaced by white space (SP) in order to comply with
   Section 5.5 of [I-D.ietf-httpbis-semantics].

   Implementers of "application/linkset" link sets should strive to make
   them self-contained by following the recommendations regarding their
   use outside the context of an HTTP interaction provided in Section 4.

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

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 Section 2 of [RFC8288].

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

   In the interests of interoperability "application/linkset+json" link
   sets MUST be encoded using UTF-8 as per Section 8.1 of [RFC8259].

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   Implementers of "application/linkset+json" link sets should strive to
   make them self-contained by following the recommendations regarding
   their use outside the context of an HTTP interaction provided in
   Section 4.

   The "application/linkset+json" serialization allows for OPTIONAL
   support of a JSON-LD serialization.  This can be achieved by adding
   an appropriate context to the "application/linkset+json"
   serialization using the approach described in Section 6.8. of
   [W3C.REC-json-ld-20140116].  Communities of practice can decide which
   context best meets their application needs.  Appendix A shows an
   example of a possible context that, when added to a JSON
   serialization, allows it to be interpreted as Resource Description
   Framework (RDF) [W3C.REC-rdf11-concepts-20140225] data.

4.2.1.  Set of Links

   In the JSON representation of a set of links:

   *  A set of links is represented in JSON as an object which MUST
      contain "linkset" as its sole member.

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

   *  Even if there is only one link context object, it MUST be wrapped
      in an array.

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:

   *  Each link context object MAY contain an "anchor" member with a
      value that represents the link context.  If present, this value
      MUST be a URI reference and SHOULD NOT be a relative reference as
      defined in Section 4.1 of [RFC3986].

   *  For each distinct relation type that the link context has with
      link targets, a link context object MUST contain an additional
      member.  The value of 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:

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      -  For registered relation types (Section 2.1.1 of [RFC8288]), the
         name of this member is the registered name of the relation
         type.

      -  For extension relation types (Section 2.1.2 of [RFC8288]), the
         name of this member is the URI that uniquely represents the
         relation type.

   *  Even if there is only one link target object it MUST be wrapped in
      an 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:

   *  Each link target object MUST contain an "href" member with a value
      that represents the link target.  This value MUST be a URI
      reference and SHOULD NOT be a relative reference as defined in
      Section 4.1 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
      as its value ("href": "").

   *  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": "https://example.net/bar",
         "next": [
           {"href": "https://example.com/foo"}
         ]
       }
     ]
   }

                                  Figure 1

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

                                  Figure 2

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

                                  Figure 3

4.2.4.  Link Target Attributes

   A link may be further qualified by target attributes as defined by
   Section 2 of Web Linking [RFC8288].  Three types of attributes exist:

   *  Serialisation-defined attributes described in Section 3.4.1 of Web
      Linking [RFC8288].

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   *  Extension attributes defined and used by communities as allowed by
      Section 3.4.2 of [RFC8288].

   *  Internationalized versions of the "title" attribute defined by
      [RFC8288] and of extension attributes allowed by Section 3.4 of
      [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

   Section 3.4.1 of [RFC8288] 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:

   *  "hreflang": The "hreflang" target attribute, defined as optional
      and repeatable by [RFC8288], MUST be represented by an "hreflang"
      member, and its value MUST be 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.

   *  "media": The "media" target attribute, defined as optional and not
      repeatable by [RFC8288], 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.

   *  "type": The "type" target attribute, defined as optional and not
      repeatable by [RFC8288], 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.

   *  "title": The "title" target attribute, defined as optional and not
      repeatable by [RFC8288], MUST be represented by a "title" member
      in the link target object, and its value MUST be a JSON string.

   *  "title*": The "title*" target attribute, defined as optional and
      not repeatable by [RFC8288], 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.

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

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

                                  Figure 4

4.2.4.2.  Internationalized Target Attributes

   In addition to the target attributes described in Section 4.2.4.1,
   Section 3.4 of [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:

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

   *  The character encoding information as prescribed by [RFC8187] is
      not preserved; instead, the content of the internationalized
      attribute is represented as a JSON string.

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   *  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 Section 3.4.1 of [RFC8288] is represented in a link target
   object.

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

                                  Figure 5

   The above example assumes that the German title contains an umlaut
   character (in the original 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.  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
   Section 3.4.1 of [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

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   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:

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

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

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

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

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                                  Figure 6

4.2.5.  JSON Extensibility

   The Web linking model ([RFC8288]) provides for the use of extension
   target attributes as discussed in Section 4.2.4.3.  The use of other
   forms of extensions is NOT RECOMMENDED.  Limiting the JSON format in
   this way allows to unambiguously round trip between links provided in
   the HTTP "Link" header field, sets of links serialized according to
   the "application/linkset" format, and sets of links serialized
   according to the "application/linkset+json" format.

   Cases may exist in which the use of extensions other than those of
   Section 4.2.4.3 may be useful.  For example, when a link set
   publisher needs to include descriptive or technical metadata for
   internal consumption.  In case such extensions are used they MUST NOT
   change the semantics of the JSON members defined in this
   specification.  Agents that consume JSON linkset documents can safely
   ignore such extensions.

5.  The "profile" parameter 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" parameter
   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 parameter could be used
   to indicate that a link set uses a specific, limited set of link
   relation types.

   The value of the "profile" parameter 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.  When providing
   multiple profile URIs, care should be taken that the corresponding
   profiles are not conflicting.  Profile URIs MAY be registered in the
   IANA Profile URI Registry in the manner specified by [RFC7284].

   The presence of a "profile" parameter 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.

   Section 7.4.2 shows an example of using the "profile" parameter in
   conjunction with the "application/linkset+json" media type.

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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
   field 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.

   The set of links provided by the resource that is the target of a
   "linkset" link may contain links in which the resource that is the
   context of the "linkset" link does not participate.  User agents MUST
   process each link in the link set independently, including processing
   of link context and link target, and MAY ignore links from the link
   set in which the context of the "linkset" link does not participate.

   A user agent that follows a "linkset" link and obtains links for
   which anchors and targets are expressed as relative references (as
   per Section 4.1 of [RFC3986]) MUST determine what the context is for
   these links; it SHOULD ignore links for which it is unable to
   unambiguously make that determination.

   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 "linkset" link relation type.
   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].  Section 7.4.1 shows an example of using the "profile"
   attribute on a link with the "linkset" relation type, making both the
   link set and the profile(s) to which it complies discoverable.

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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
   and Section 7.4 shows how to convey profile information pertaining to
   a link set.

7.1.  Set of Links Provided as application/linkset

   Figure 7 shows a client issuing an HTTP GET request against resource
   <https://example.org/links/resource1>.

   GET /links/resource1 HTTP/1.1
   Host: example.org

                     Figure 7: Client HTTP GET request

   Figure 8 shows the response to the GET request of Figure 7.  The
   response contains a Content-Type header field 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 field 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
   Link: <https://example.org/links/resource1>
         ; rel="alternate"
         ; type="application/linkset+json"

   <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 8: Response to HTTP GET includes a set of links

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

   Figure 9 shows the client issuing an HTTP GET request against
   <https://example.org/links/resource1>.  In the request, the client
   uses an "Accept" header field 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

        Figure 9: Client HTTP GET request expressing preference for
                    "application/ linkset+json" response

   Figure 10 shows the response to the HTTP GET request of Figure 9.
   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: 1246

   { "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"
           }
         ]
       },
       { "anchor": "https://example.org/resource1?version=3",
         "predecessor-version": [
           { "href": "https://example.org/resource1?version=2",
             "type": "text/html"

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           }
         ]
       },
       { "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 10: Response to the client's request for the set of links

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

   Figure 11 shows a client issuing an HTTP HEAD request against
   resource <https://example.org/resource1>.

   HEAD resource1 HTTP/1.1
   Host: example.org

                    Figure 11: Client HTTP HEAD request

   Figure 12 shows the response to the HEAD request of Figure 11.  The
   response contains an HTTP "Link" header field 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".

   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

                  Figure 12: Response to HTTP HEAD request

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7.4.  Link Set Profiles

   The examples in this section illustrate the use of the "profile"
   attribute for a link with the "linkset" link relation type and the
   "profile" attribute for a link set media type.  The examples are
   inspired by the implementation of link sets by GS1 (the standards
   body behind many of the world's barcodes).

7.4.1.  Using a "profile" Attribute with a "linkset" Link

   Figure 13 shows a client issuing an HTTP HEAD request against trade
   item 09506000134352 at <https://id.gs1.org/01/9506000134352>.

   HEAD /01/9506000134352 HTTP/1.1
   Host: id.gs1.org

                    Figure 13: Client HTTP HEAD request

   Figure 14 shows the server's response to the request of Figure 13,
   including a "linkset" link with a "profile" attribute that has the
   Profile URI <https://www.gs1.org/voc/?show=linktypes> as its value.
   Dereferencing that URI yields a profile document that lists all the
   link relation types that a client can expect when requesting the link
   set made discoverable by the "linkset" link.  The link relation types
   are presented in abbreviated form, e.g. <gs1:activityIdeas>, whereas
   the actual link relation type URIs are available as hyperlinks on the
   abbreviations, e.g. <https://www.gs1.org/voc/activityIdeas>.  For
   posterity that profile document was saved in the Internet Archive at
   <https://web.archive.org/web/20210927160406/https://www.gs1.org/
   voc/?show=linktypes> on 27 September 2021.

   HTTP/1.1 307 Temporary Redirect
   Date:  Mon, 27 Sep 2021 16:03:07 GMT
   Server: nginx
   Link: https://id.gs1.org/01/9506000134352?linkType=all
   ; rel="linkset"
   ; type="application/linkset+json"
   ; profile="https://www.gs1.org/voc/?show=linktypes"
   Location: https://example.com/risotto-rice-with-mushrooms/

        Figure 14: Response to the client's HEAD request including a
                 "profile" attribute for the "linkset" link

7.4.2.  Using a "profile" Parameter with a Link Set Media Type

   Figure 15 shows a client issuing an HTTP HEAD request against the
   link set <https://id.gs1.org/01/9506000134352?linkType=all> that was
   discovered through the HTTP interactions shown in Section 7.4.1.

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   HEAD /01/9506000134352?linkType=all HTTP/1.1
   Host: id.gs1.org

                    Figure 15: Client HTTP HEAD request

   Figure 16 shows the server's response to the request of Figure 15.
   Note the "profile" parameter for the application/linkset+json media
   type, which has as value the same Profile URI <https://www.gs1.org/
   voc/?show=linktypes> as was used in <xref target="Response_pr_at"/>.

   HTTP/1.1 200 OK
   Date:  Mon, 27 Sep 2021 16:03:33 GMT
   Server: nginx
   Content-Type: application/linkset+json;
        profile="https://www.gs1.org/voc/?show=linktypes"
   Content-Length: 396

        Figure 16: Response to the client's HEAD request including a
     "profile" parameter for the "application/linkset+json" media type

7.4.3.  Using a Link with a "profile" Link Relation Type

   Note that the response Figure 16 from the link set resource is
   equivalent to the response shown in Figure 17, which leverages the
   "profile" link relation type defined in [RFC6906].

   HTTP/1.1 200 OK
   Date:  Mon, 27 Sep 2021 16:03:33 GMT
   Server: nginx
   Content-Type: application/linkset+json
   Link: https://www.gs1.org/voc/?show=linktypes; rel="profile"
   Content-Length: 396

        Figure 17: Response to the client's HEAD request including a
                               "profile" link

   A link with a "profile" link relation type as shown in Figure 17 can
   also be conveyed in the link set document itself.  This is
   illustrated by Figure 18.  Following the recommendation that all
   links in a link set document should have an explicit anchor, such a
   link has the URI of the link set itself as anchor and the Profile URI
   as target.  Multiple Profile URIs are handled by using multiple
   "href" members.

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   { "linkset":
     [
       { "anchor": "https://id.gs1.org/01/9506000134352?linkType=all",
         "profile": [
               {"href": "https://www.gs1.org/voc/?show=linktypes"}
         ]
       },
        { "anchor": "https://id.gs1.org/01/9506000134352",
          "https://gs1.org/voc/whatsInTheBox": [
            {"href": "https://example.com/en/packContents/GB"}
          ]
       }
     ]
   }

      Figure 18: A Link Set that declares the profile it complies with
                           using a "profile" link

8.  IANA Considerations

8.1.  Link Relation Type: linkset

   The link relation type below should be registered by IANA in the Link
   Relation Type Registry as per Section 4.2 of Web Linking [RFC8288]:

      Relation Name: linkset

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

      Reference: [[ This document ]]

8.2.  Media Type: application/linkset

   The Internet media type application/linkset for a linkset encoded as
   described in Section 4.1 should be registered by IANA in the Media
   Type Registry as per [RFC6838].

      Type name: application

      Subtype name: linkset

      Required parameters: N/A

      Optional parameters: profile

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      Encoding considerations: Linksets are encoded according to the
      definition of [RFC8288].  The encoding of [RFC8288] is based on
      the general encoding rules of [I-D.ietf-httpbis-semantics], 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: N/A

      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>

      Change controller: IETF

8.3.  Media Type: application/linkset+json

   The Internet media type application/linkset+json for a linkset
   encoded as described in Section 4.2 should be registered by IANA in
   the Media Type Registry as per [RFC6838].

      Type name: application

      Subtype name: linkset+json

      Required parameters: N/A

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      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>

      Change controller: IETF

9.  Security Considerations

   The security considerations of Section 7 of [RFC3986] apply, as well
   as those of Web Linking [RFC8288] as long as the latter 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,

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   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:

   *  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.

   *  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.

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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

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   [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, 16 January 2014,
              <https://www.w3.org/TR/2014/REC-json-ld-20140116>.

   [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>.

   [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>.

   [I-D.ietf-httpbis-semantics]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

11.  Informative References

   [W3C.REC-rdf11-concepts-20140225]
              Cyganiak, R., Wood, D., and M. Lanthaler, "RDF 1.1
              Concepts and Abstract Syntax", World Wide Web Consortium
              Recommendation REC-rdf11-concepts-20140225, 25 February
              2014,
              <https://www.w3.org/TR/2014/REC-rdf11-concepts-20140225>.

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

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

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

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

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [DCMI-TERMS]
              Initiative, D. C. M., "DCMI Metadata Terms", 2020,
              <https://www.dublincore.org/specifications/dublin-core/
              dcmi-terms/>.

Appendix A.  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 (https://www.w3.org/TR/2014/
   REC-json-ld-20140116/#interpreting-json-as-json-ld), 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.

   Figure 19 shows the response to an HTTP GET against the URI of a link
   set resource and illustrates this approach to support discovery of a
   JSON-LD Context.  The example is inspired by the GS1 implementation
   and shows a link set that uses relation types from the GS1 vocabulary
   at <https://www.gs1.org/voc/> that are expressed as HTTP URIs.

   HTTP/1.1 200 OK
   Date: Mon, 11 Oct 2021 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"

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         ; type="application/ld+json"
   Content-Length: 1532

   {
     "linkset": [
       {
         "anchor": "https://id.gs1.org/01/09506000149301",
         "https://gs1.org/voc/pip": [
           {
             "href": "https://example.com/en/defaultPage",
             "hreflang": [
               "en"
             ],
             "type": "text/html",
             "title": "Product information"
           },
           {
             "href": "https://example.com/fr/defaultPage",
             "hreflang": [
               "fr"
             ],
             "title": "Information produit"
           }
         ],
         "https://gs1.org/voc/whatsInTheBox": [
           {
             "href": "https://example.com/en/packContents/GB",
             "hreflang": [
               "en"
             ],
             "title": "What's in the box?"
           },
           {
             "href": "https://example.com/fr/packContents/FR",
             "hreflang": [
               "fr"
             ],
             "title": "Qu'y a-t-il dans la boite?"
           },
           {
             "href": "https://example.com/fr/packContents/CH",
             "hreflang": [
               "fr"
             ],
             "title": "Qu'y a-t-il dans la boite?"
           }
         ],
         "https://gs1.org/voc/relatedVideo": [

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           {
             "href": "https://video.example",
             "hreflang": [
               "en",
               "fr"
             ],
             "title*": [
               {
                 "value": "See it in action!",
                 "language": "en"
               },
               {
                 "value": "Voyez-le en action!",
                 "language": "fr"
               }
             ]
           }
         ]
       }
     ]
   }

      Figure 19: Using a typed link to support discovery of a JSON-LD
                         Context for a Set of Links

   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 20.  This particular JSON-LD
   context maps "application/linkset+json" representations of link sets
   to Dublin Core Terms [DCMI-TERMS].  Note that the "linkset" entry in
   the JSON-LD context is introduced to support links with the "linkset"
   relation type in link sets.

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   HTTP/1.1 200 OK
   Content-Type: application/ld+json
   Content-Length: 658

   {
     "@context": [
       {
         "@version": 1.1,
         "@vocab": "https://gs1.org/voc/",
         "anchor": "@id",
         "href": "@id",
         "linkset": {
           "@id": "@graph",
           "@context": {
             "linkset": "linkset"
           }
         },
         "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 20: JSON-LD Context mapping to Dublin Core Terms

   Applying the JSON-LD context of Figure 20 to the link set of
   Figure 19 allows transforming the "application/linkset+json" link set
   to an RDF link set.  Figure 21 shows the latter represented by means
   of the "text/turtle" RDF serialization.

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   <https://example.com/en/defaultPage>
           <http://purl.org/dc/terms/format>
           "text/html" .
   <https://example.com/en/defaultPage>
           <http://purl.org/dc/terms/language>
           "en" .
   <https://example.com/en/defaultPage>
           <http://purl.org/dc/terms/title>
           "Product information" .
   <https://example.com/en/packContents/GB>
           <http://purl.org/dc/terms/language>
           "en" .
   <https://example.com/en/packContents/GB>
           <http://purl.org/dc/terms/title>
           "What's in the box?" .
   <https://example.com/fr/defaultPage>
           <http://purl.org/dc/terms/language>
           "fr" .
   <https://example.com/fr/defaultPage>
           <http://purl.org/dc/terms/title>
           "Information produit" .
   <https://example.com/fr/packContents/CH>
           <http://purl.org/dc/terms/language>
           "fr" .
   <https://example.com/fr/packContents/CH>
           <http://purl.org/dc/terms/title>
           "Qu'y a-t-il dans la boite?" .
   <https://example.com/fr/packContents/FR>
           <http://purl.org/dc/terms/language>
           "fr" .
   <https://example.com/fr/packContents/FR>
           <http://purl.org/dc/terms/title>
           "Qu'y a-t-il dans la boite?" .
   <https://id.gs1.org/01/09506000149301>
           <https://gs1.org/voc/pip>
           <https://example.com/en/defaultPage> .
   <https://id.gs1.org/01/09506000149301>
           <https://gs1.org/voc/pip>
           <https://example.com/fr/defaultPage> .
   <https://id.gs1.org/01/09506000149301>
           <https://gs1.org/voc/relatedVideo>
           <https://video.example> .
   <https://id.gs1.org/01/09506000149301>
           <https://gs1.org/voc/whatsInTheBox>
           <https://example.com/en/packContents/GB> .
   <https://id.gs1.org/01/09506000149301>
           <https://gs1.org/voc/whatsInTheBox>
           <https://example.com/fr/packContents/CH> .

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   <https://id.gs1.org/01/09506000149301>
           <https://gs1.org/voc/whatsInTheBox>
           <https://example.com/fr/packContents/FR> .
   <https://video.example>
           <http://purl.org/dc/terms/language>
           "en" .
   <https://video.example>
           <http://purl.org/dc/terms/language>
           "fr" .
   <https://video.example>
           <http://purl.org/dc/terms/title>
           "See it in action!"@en .
   <https://video.example>
           <http://purl.org/dc/terms/title>
           "Voyez-le en action!"@fr .

        Figure 21: RDF serialization of the link set resulting from
                        applying the JSON-LD context

Appendix B.  Implementation Status

   This section is to be removed before publishing as an RFC.

   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 7942
   [RFC7942].  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 7942, "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".

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B.1.  GS1

   GS1 is a provider of identifiers, most famously seen in EAN/UPC
   barcodes for retail and healthcare products, and manages an ecology
   of services and standards to leverage them at a global scale.  GS1
   has indicated that it will fully implement this "linkset"
   specification as a means to allow requesting and representing links
   pertaining to products, shipments, assets and locations.  The current
   GS1 Digital Link specification makes an informative reference to
   version 03 of the "linkset" I-D, mentions the formal adoption of that
   I-D by the IETF HTTPAPI Working Group, and indicates it being on
   track to achieve RFC status.  The GS1 Digital Link specification
   adopts the JSON format specified by the I-D and mentions future plans
   to also support the Link header format as well as their respective
   media types, neither of which have changed since version 03.

B.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.

B.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).

Acknowledgements

   Thanks for comments and suggestions provided by Phil Archer,
   Dominique Guinard, Mark Nottingham, Julian Reschke, Rob Sanderson,
   Stian Soiland-Reyes, Sarven Capadisli, and Addison Phillips.

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Authors' Addresses

   Erik Wilde
   Axway
   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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