Internet-Draft Linkset January 2021
Wilde & Van de Sompel Expires 18 July 2021 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-ietf-httpapi-linkset-00
Published:
Intended Status:
Informational
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 document formats and respective media types for representing sets of links as stand-alone resources. One format is JSON-based, the other aligned with the format for representing links in the HTTP "Link" header field. This specification also introduces a link relation type to support discovery of sets of links.

Note to Readers

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

Online access to all versions and files is available on GitHub (https://github.com/ietf-wg-httpapi/linkset).

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute 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 18 July 2021.

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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 for documents of any media type. In some cases, however, providing links in this manner is impractical or impossible and delivering a set of links as a stand-alone document is preferable.

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

2. Terminology

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

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

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

3. Scenarios

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

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

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

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.
  • 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 headers. In such cases, providing links by means of a standalone document can be a solution. Making the resource that provides these links discoverable can be achieved by means of a typed link.

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":

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

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

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

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

  • For every link provided in the set of links, explicitly provide the link context using the "anchor" attribute.
  • For link context ("anchor" attribute) and link target ("href" attribute), use absolute URIs (as defined in Section 4.3 of [RFC3986]).

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

4.2. JSON Document Format: application/linkset+json

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

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

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

  • For every link provided in the set of links, explicitly provide the link context using the "anchor" member.
  • For link context ("anchor" member) and link target ("href" member), use absolute URIs (as defined in Section 4.3 of [RFC3986]).

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

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

In the JSON representation of a set of links:

  • A set of links MUST be represented as a JSON object which MUST have "linkset" as its sole member.
  • The "linkset" member is an array in which a distinct JSON object - the "link context object" (see Section 4.2.2) - MUST be used to represent links that have the same link context.
  • If necessary, the "linkset" member MAY contain information in addition to link context objects, in which case that information MUST NOT change the semantics of the links provided by those link context objects.
  • Even if there is only one link context object, it MUST be wrapped in an array. Members other than link context objects MUST NOT be included in this array.

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

  • Attributes defined by the serialization of Web Linking [RFC8288].
  • Extension attributes defined and used by communities as allowed by [RFC8288].
  • Internationalized versions of the "title" attribute defined by [RFC8288] and of extension attributes allowed by [RFC8288].

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

4.2.4.1. Target Attributes Defined by Web Linking

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

  • "hreflang": The optional and repeatable "hreflang" target attribute MUST be represented by an array (even if there only is one value to be represented), and each value in that array MUST be a string - representing one value of the "hreflang" target attribute for a link - which follows the same model as in the [RFC8288] syntax.
  • "media": The optional and not repeatable "media" target attribute MUST be represented by a "media" member in the link target object, and its value MUST be a string that follows the same model as in the [RFC8288] syntax.
  • "type": The optional and not repeatable "type" target attribute MUST be represented by a "type" member in the link target object, and its value MUST be a string that follows the same model as in the [RFC8288] syntax.
  • "title": The optional and not repeatable "title" target attribute MUST be represented by a "title" member in the link target object, and its value MUST be a string that follows the same model as in the [RFC8288] syntax.
  • "title*": The optional and not repeatable "title*" target attribute is motivated by character encoding and language issues and follows the model defined in [RFC8187]. The details of the JSON representation that applies to title* are described in Section 4.2.4.2.

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

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

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

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

  • An extension target attribute is represented as a member of the link context object with the same name of the attribute, including the * if applicable.
  • 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 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": "http://example.net/bar",
        "next": [
              { "href": "http://example.com/foo",
                "type": "text/html",
                "foo":  [ "foovalue" ],
                "bar":  [ "barone", "bartwo" ],
                "baz*": [ { "value": "bazvalue" , "language" : "en" } ]
              }
        ]
      }
    ]
}

7. Implementation Status

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

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

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

7.1. GS1

GS1 is a provider of barcodes (GS1 GTINs and EAN/UPC) for retail products and manages an ecology of services and standards to leverage them at a global scale. GS1 has indicated that it will implement this "linkset" specification as a means to allow requesting and representing links pertaining to products from various retailers. Currently, the GS1 Digital Link specification makes an informative reference to version 03 of the "linkset" I-D. GS1 expresses confidence that this will become a normative reference in the next iteration of that specification, likely to be ratified as a GS1 standard around February 2021.

7.2. Open Journal Systems (OJS)

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

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

8. IANA Considerations

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

  • Relation Name: linkset
  • Description: The Target IRI of a link with the "linkset" relation type provides a set of links, including links in which the Context IRI of the link participates.
  • Reference: [[ This document ]]

8.2.1. IANA Considerations

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

  • Type name: application
  • Subtype name: linkset
  • Required parameters: none
  • Optional parameters: none
  • Encoding considerations: Linksets are encoded according to the definition of [RFC8288]. The encoding of [RFC8288] is based on the general encoding rules of [RFC7230], with the addition of allowing indicating character encoding and language for specific parameters as defined by [RFC8187].
  • Security considerations: The security considerations of [[ This document ]] apply.
  • Interoperability considerations: The interoperability considerations of [RFC7230] apply.
  • Published specification: [[ This document ]]
  • Applications that use this media type: This media type is not specific to any application, as it can be used by any application that wants to interchange web links.

  • Additional information:

    • Magic number(s): N/A
    • File extension(s): This media type does not propose a specific extension.
    • Macintosh file type code(s): TEXT
  • Person & email address to contact for further information: Erik Wilde <erik.wilde@dret.net>
  • Intended usage: COMMON
  • Restrictions on usage: none
  • Author: Erik Wilde <erik.wilde@dret.net>
  • Change controller: IETF

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

  • Type name: application
  • Subtype name: linkset+json
  • Required parameters: none
  • Optional parameters: none
  • 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 Web Linking [RFC8288] apply, as long as they are not specifically discussing the risks of exposing information in HTTP header fields.

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

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

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, , <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>.
[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>.
[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>.
[RFC6982]
Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, DOI 10.17487/RFC6982, , <https://www.rfc-editor.org/info/rfc6982>.
[RFC7230]
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, , <https://www.rfc-editor.org/info/rfc7230>.
[RFC7231]
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, , <https://www.rfc-editor.org/info/rfc7231>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <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>.

10.2. Informative References

[RFC4287]
Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication Format", RFC 4287, DOI 10.17487/RFC4287, , <https://www.rfc-editor.org/info/rfc4287>.
[RFC5988]
Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, , <https://www.rfc-editor.org/info/rfc5988>.
[W3C.REC-json-ld-20140116]
Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0", World Wide Web Consortium Recommendation REC-json-ld-20140116, , <https://www.w3.org/TR/2014/REC-json-ld-20140116>.

Appendix A. Acknowledgements

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

Appendix B. JSON-LD Context

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

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

HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:48:22 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Link: <https://example.org/contexts/linkset.jsonld>
      ; rel="http://www.w3.org/ns/json-ld#context"
      ; type="application/ld+json"
Content-Length: 846

{
  "linkset": [
    {
      "anchor": "https://example.org/article/view/7507",
      "author": [
        {
          "href": "https://orcid.org/0000-0002-1825-0097"
        }
      ],
      "item": [
        {
          "href": "https://example.org/article/7507/item/1",
          "type": "application/pdf"
        },
        {
          "href": "https://example.org/article/7507/item/2",
          "type": "text/csv"
        }
      ],
      "cite-as": [
        {
          "href": "https://doi.org/10.5555/12345680",
          "title": "A Methodology for the Emulation of Architecture"
        }
      ]
    },
    {
      "anchor": "https://example.com/links/article/7507",
      "alternate": [
        {
          "href": "https://mirror.example.com/links/article/7507",
          "type": "application/linkset"
        }
      ]
    }
  ]
}
Figure 7: Using a typed link to support discovery of a JSON-LD Context for a Set of Links

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

HTTP/1.1 200 OK
Content-Type: application/ld+json
Content-Length: 638

{
  "@context": [
    {
      "@vocab":  "http://www.iana.org/assignments/relation/",
      "anchor":  "@id",
      "href":    "@id",
      "linkset": "@graph",
      "_linkset": "@graph",
      "title":   {
        "@id":    "http://purl.org/dc/terms/title"
      },
      "title*":  {
        "@id":    "http://purl.org/dc/terms/title"
      },
      "type":    {
        "@id":    "http://purl.org/dc/terms/format"
      }
    },
    {
      "language": "@language",
      "value":    "@value",
      "hreflang": {
        "@id":        "http://purl.org/dc/terms/language",
        "@container": "@set"
      }
    }
  ]
}
Figure 8: JSON-LD Context mapping to schema.org and IANA assignments

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

<https://example.org/article/view/7507>
    <http://www.iana.org/assignments/relation/author>
    <https://orcid.org/0000-0002-1825-0097> .

<https://example.org/article/view/7507>
    <http://www.iana.org/assignments/relation/item>
    <https://example.org/article/7507/item/1> .

<https://example.org/article/7507/item/1>
    <http://purl.org/dc/terms/format>
    "application/pdf" .

<https://example.org/article/view/7507>
    <http://www.iana.org/assignments/relation/item>
    <https://example.org/article/7507/item/2> .

<https://example.org/article/7507/item/2>
    <http://purl.org/dc/terms/format>
    "text/csv" .

<https://example.org/article/view/7507>
    <http://www.iana.org/assignments/relation/cite-as>
    <https://doi.org/10.5555/12345680> .

<https://doi.org/10.5555/12345680>
    <http://purl.org/dc/terms/title>
    "A Methodology for the Emulation of Architecture" .

<https://example.com/links/article/7507>
    <http://www.iana.org/assignments/relation/alternate>
    <https://mirror.example.com/links/article/7507> .

<https://mirror.example.com/links/article/7507>
    <http://purl.org/dc/terms/format>
    "application/linkset" .
Figure 9: RDF serialization of the link set resulting from applying the JSON-LD context

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

Authors' Addresses

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