path: root/doc/rfc/rfc9264.txt

Internet Engineering Task Force (IETF)                          E. Wilde
Request for Comments: 9264                                         Axway
Category: Standards Track                               H. Van de Sompel
ISSN: 2070-1721                    Data Archiving and Networked Services
                                                               July 2022


      Linkset: Media Types and a Link Relation Type for Link Sets

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9264.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   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
   2.  Terminology
   3.  Use Cases and Motivation
     3.1.  Third-Party Links
     3.2.  Challenges Writing to the HTTP "Link" Header Field
     3.3.  Large Number of Links
   4.  Document Formats for Sets of Links
     4.1.  HTTP Link Document Format: application/linkset
     4.2.  JSON Document Format: application/linkset+json
       4.2.1.  Set of Links
       4.2.2.  Link Context Object
       4.2.3.  Link Target Object
       4.2.4.  Link Target Attributes
       4.2.5.  JSON Extensibility
   5.  The "profile" Parameter for Media Types to Represent Sets of
           Links
   6.  The "linkset" Relation Type for Linking to a Set of Links
   7.  Examples
     7.1.  Set of Links Provided as "application/linkset"
     7.2.  Set of Links Provided as "application/linkset+json"
     7.3.  Discovering a Link Set via the "linkset" Link Relation Type
     7.4.  Link Set Profiles
       7.4.1.  Using a "profile" Attribute with a "linkset" Link
       7.4.2.  Using a "profile" Parameter with a Link Set Media Type
       7.4.3.  Using a Link with a "profile" Link Relation Type
   8.  IANA Considerations
     8.1.  Link Relation Type: linkset
     8.2.  Media Type: application/linkset
     8.3.  Media Type: application/linkset+json
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  JSON-LD Context
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Resources on the Web often use typed Web Links [RFC8288], either
   (1) embedded in resource representations -- for example, using the
   <link> element for HTML documents or (2) 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 standalone document is preferable.

   Therefore, this specification defines two formats for representing
   sets of Web Links and their attributes as standalone documents.  One
   serializes links in the same format as the format 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 to support the discovery of any resource that
   conveys a set of links as a standalone 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 that "Web Linking" [RFC8288] uses them.

   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 "HTTP
   Semantics" [RFC9110], 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.

   For all scenarios, links could be provided by means of a standalone
   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 the 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 a
      link context but also as a 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 custodian 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.

3.2.  Challenges Writing to the 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 Semantics" [RFC9110] defines error codes related
   to excess communication by the user agent ("413 Content Too Large"
   and "414 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 countermeasures 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], which 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].

   Links provided in the HTTP "Link" header field 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 reused 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 field 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 reused 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 the 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, the interpretation of
   links in link set documents will depend on which URI is used as the
   context.

   For a "title" attribute provided on a link in the HTTP "Link" header
   field, the language in which the title is expressed is provided by
   the "Content-Language" header field 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 reused 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.  Note that, in this regard, 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
   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 its
   subsequent rules.  It differs from the format for field values of the
   HTTP "Link" header field 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 field, newline characters MUST be removed
   or MUST be replaced by whitespace (SP) in order to comply with
   Section 5.5 of [RFC9110].

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

   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
   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].

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

   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 6.1 of
   [W3C.REC-json-ld].  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) data
   [W3C.REC-rdf11-concepts].

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 that 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:

      -  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: Simple linkset example

   The following example of a JSON-serialized set of links represents
   two links that share a 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: Linkset with two links with the same context

   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, and 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: Linkset with two links with different contexts

4.2.4.  Link Target Attributes

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

   *  Serialization-defined attributes as described in Section 3.4.1 of
      [RFC8288].

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

   *  Internationalized versions of the "title" attribute as 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, its value MUST be an array (even if there is only 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 -- that follows the same model as the syntax
      discussed in [RFC8288].

   "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 the syntax discussed in [RFC8288].

   "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.

   "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 the syntax discussed in [RFC8288].

   The following example illustrates how the "hreflang" (repeatable)
   target attribute and the "type" (not repeatable) target attribute 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: Linkset with "hreflang" and "type" target attributes

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.

   *  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 objects 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 objects 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
   as 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: Linkset with "title" and "title*" target attributes

   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"
   format and the "application/linkset+json" format.

4.2.4.3.  Extension Target Attributes

   Extension target attributes (e.g., as listed in Section 4.2.4.1) are
   attributes that are not defined by Section 3.4.1 of [RFC8288] 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:

   *  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 is only 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 following example shows a link target object with three extension
   target attributes.  The value for each extension target attribute is
   an array.  The first two 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" } ]
           }
         ]
       }
     ]
   }

             Figure 6: Linkset with extension target attributes

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 unambiguous round trips 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
   discussed in 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.  If 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" as detailed in Sections 4.1 and 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's "Profile URIs" 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.

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 the 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's "Profile URIs" 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.

7.  Examples

   Sections 7.1 and 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 the 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".

   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 that it prefers a response
   in the "application/linkset+json" format.

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

       Figure 9: Client HTTP GET request expressing preference for an
                    "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"
           }
         ]
       },
       { "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 linkset

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

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.

   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 its value the same profile URI
   <https://www.gs1.org/voc/?show=linktypes> as was used in Figure 14.

   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 shown in 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 the anchor and the profile
   URI as the target.  Multiple profile URIs are handled by using
   multiple "href" members.

   { "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 linkset 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 has been registered by IANA in the "Link
   Relation Types" registry as per Section 4.2 of [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:  RFC 9264

8.2.  Media Type: application/linkset

   The Internet media type "application/linkset" for a linkset encoded
   as described in Section 4.1 has been registered by IANA in the "Media
   Types" registry as per [RFC6838].

   Type name:  application

   Subtype name:  linkset

   Required parameters:  N/A

   Optional parameters:  profile

   Encoding considerations:  Linksets are encoded according to the
      definitions provided in [RFC8288].  The encoding discussed in
      [RFC8288] is based on the general encoding rules specified by HTTP
      [RFC9110] and allows specific parameters to be extended by the
      indication of character encoding and language as defined by
      [RFC8187].

   Security considerations:  The security considerations of RFC 9264
      apply.

   Interoperability considerations:  N/A

   Published specification:  RFC 9264

   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 has been registered by IANA in
   the "Media Types" registry as per [RFC6838].

   Type name:  application

   Subtype name:  linkset+json

   Required parameters:  N/A

   Optional parameters:  profile

   Encoding considerations:  The encoding considerations of [RFC8259]
      apply.

   Security considerations:  The security considerations of RFC 9264
      apply.

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

   Published specification:  RFC 9264

   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
   and that 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 standalone resources.

   *  The model introduced in this specification supports "third-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 third party to allow it to provide
      these links.  Applications may choose to treat third-party links
      differently than cases where a resource and the links for that
      resource are provided by the same party.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [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>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [W3C.REC-json-ld]
              Sporny, M., Ed., Kellogg, G., Ed., and M. Lanthaler, Ed.,
              "JSON-LD 1.1: A JSON-based Serialization for Linked Data",
              W3C Recommendation REC-json-ld-20140116, July 2020,
              <https://www.w3.org/TR/json-ld/>.

10.2.  Informative References

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

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

   [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>.

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

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] that maps the JSON keys to corresponding
   Linked Data terms.  And, as per Section 6.1 of [W3C.REC-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 field.

   Figure 19 shows the response to an HTTP GET against the URI of a link
   set resource and illustrates this approach to support the discovery
   of a JSON-LD context.  This 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"
         ; 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": [
           {
             "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 the discovery of a JSON-
                          LD context for a linkset

   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.

   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.

   <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> .
   <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 linkset resulting from
                        applying the JSON-LD context

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.

Authors' Addresses

   Erik Wilde
   Axway
   Email: erik.wilde@dret.net


   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