Internet-Draft Linkset May 2022
Wilde & Van de Sompel Expires 6 November 2022 [Page]
Workgroup:
Network Working Group
Published:
Intended Status:
Standards Track
Expires:
Authors:
E. Wilde
Axway
H. Van de Sompel
Data Archiving and Networked Services

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

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.

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Table of Contents

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.

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.

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.

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.

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":

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:

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 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.

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].

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.

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.

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

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.

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
4.2.4.3. 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 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" } ]
        }
      ]
    }
  ]
}
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.

8. IANA Considerations

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 ]]

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
  • 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

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
  • 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, 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:

10. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc5646>.
[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, , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8259>.
[RFC8288]
Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, , <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, , <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, , <https://www.w3.org/TR/2014/REC-rdf11-concepts-20140225>.
[RFC5988]
Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, , <https://www.rfc-editor.org/info/rfc5988>.
[RFC6690]
Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, , <https://www.rfc-editor.org/info/rfc6690>.
[RFC6906]
Wilde, E., "The 'profile' Link Relation Type", RFC 6906, DOI 10.17487/RFC6906, , <https://www.rfc-editor.org/info/rfc6906>.
[RFC7284]
Lanthaler, M., "The Profile URI Registry", RFC 7284, DOI 10.17487/RFC7284, , <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, , <https://www.rfc-editor.org/info/rfc7942>.
[DCMI-TERMS]
Initiative, D. C. M., "DCMI Metadata Terms", , <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, 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"
      ; 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 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.

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 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".

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.

Authors' Addresses

Erik Wilde
Axway
URI: http://dret.net/netdret/
Herbert Van de Sompel
Data Archiving and Networked Services
URI: https://orcid.org/0000-0002-0715-6126

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