Network Working Group E. Wilde Internet-Draft Axway Intended status: Informational H. Van de Sompel Expires: 12 August 2022 Data Archiving and Networked Services 8 February 2022 Linkset: Media Types and a Link Relation Type for Link Sets draft-ietf-httpapi-linkset-07 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 12 August 2022. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Wilde & Van de Sompel Expires 12 August 2022 [Page 1] Internet-Draft Linkset February 2022 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Use Cases and Motivation . . . . . . . . . . . . . . . . . . 3 3.1. Third-Party Links . . . . . . . . . . . . . . . . . . . . 4 3.2. Challenges Writing to HTTP Link Header Field . . . . . . 5 3.3. Large Number of Links . . . . . . . . . . . . . . . . . . 5 4. Document Formats for Sets of Links . . . . . . . . . . . . . 5 4.1. HTTP Link Document Format: application/linkset . . . . . 6 4.2. JSON Document Format: application/linkset+json . . . . . 7 4.2.1. Set of Links . . . . . . . . . . . . . . . . . . . . 7 4.2.2. Link Context Object . . . . . . . . . . . . . . . . . 8 4.2.3. Link Target Object . . . . . . . . . . . . . . . . . 8 4.2.4. Link Target Attributes . . . . . . . . . . . . . . . 10 4.2.5. JSON Extensibility . . . . . . . . . . . . . . . . . 14 5. The "profile" parameter for media types to Represent Sets of Links . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6. The "linkset" Relation Type for Linking to a Set of Links . . 15 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7.1. Set of Links Provided as application/linkset . . . . . . 16 7.2. Set of Links Provided as application/linkset+json . . . . 17 7.3. Discovering a Link Set via the "linkset" Link Relation Type . . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.4. Link Set Profiles . . . . . . . . . . . . . . . . . . . . 20 7.4.1. Using a "profile" Attribute with a "linkset" Link . . 20 7.4.2. Using a "profile" Parameter with a Link Set Media Type . . . . . . . . . . . . . . . . . . . . . . . . 20 7.4.3. Using a Link with a "profile" Link Relation Type . . 21 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 8.1. Link Relation Type: linkset . . . . . . . . . . . . . . . 22 8.2. Media Type: application/linkset . . . . . . . . . . . . . 22 8.3. Media Type: application/linkset+json . . . . . . . . . . 23 9. Security Considerations . . . . . . . . . . . . . . . . . . . 24 10. Normative References . . . . . . . . . . . . . . . . . . . . 25 11. Informative References . . . . . . . . . . . . . . . . . . . 26 Appendix A. JSON-LD Context . . . . . . . . . . . . . . . . . . 27 Appendix B. Implementation Status . . . . . . . . . . . . . . . 32 B.1. GS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Wilde & Van de Sompel Expires 12 August 2022 [Page 2] Internet-Draft Linkset February 2022 B.2. FAIR Signposting Profile . . . . . . . . . . . . . . . . 33 B.3. Open Journal Systems (OJS) . . . . . . . . . . . . . . . 33 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 33 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 1. Introduction Resources on the Web often use typed Web Links [RFC8288], either embedded in resource representations, for example using the 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 HTTP the 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" as defined in [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. Wilde & Van de Sompel Expires 12 August 2022 [Page 3] Internet-Draft Linkset February 2022 For all scenarios, links could be provided by means of a stand-alone document that is formatted according to the JSON-based serialization, the serialization aligned with the HTTP "Link" field format, or both. The former serialization is motivated by the widespread use of JSON and related tools, which suggests that handling sets of links expressed as JSON documents should be attractive to developers. The latter serialization is provided for compatibility with the existing serialization used in the HTTP "Link" field and to allow reuse of tools created to handle it. It is important to keep in mind that when providing links by means of a standalone representation, other links can still be provided using other approaches, i.e. it is possible to combine various mechanisms to convey links. 3.1. Third-Party Links In some cases it is useful that links pertaining to a resource are provided by a server other than the one that hosts the resource. For example, this allows: * Providing links in which the resource is involved not just as link context but also as link target. * 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. Wilde & Van de Sompel Expires 12 August 2022 [Page 4] Internet-Draft Linkset February 2022 3.2. Challenges Writing to HTTP Link Header Field In some cases, it is not straightforward to write links to the HTTP "Link" header field from an application. This can, for example, be the case because not all required link information is available to the application or because the application does not have the capability to directly write HTTP fields. In such cases, providing links by means of a standalone document can be a solution. Making the resource that provides these links discoverable can be achieved by means of a typed link. 3.3. Large Number of Links When conveying links in an HTTP "Link" header field, it is possible for the size of the HTTP response fields to become unpredictable. This can be the case when links are determined dynamically dependent on a range of contextual factors. It is possible to statically configure a web server to correctly handle large HTTP response fields by specifying an upper bound for their size. But when the number of links is unpredictable, estimating a reliable upper bound is challenging. Section 15 of HTTP [I-D.ietf-httpbis-semantics] defines error codes related to excess communication by the user agent ("413 Request Entity Too Large" and "414 Request-URI Too Long"), but no specific error codes are defined to indicate that response field content exceeds the upper bound that can be handled by the server, and thus it has been truncated. As a result, applications take counter measures aimed at controlling the size of the HTTP "Link" header field, for example by limiting the links they provide to those with select relation types, thereby limiting the value of the HTTP "Link" header field to clients. Providing links by means of a standalone document overcomes challenges related to the unpredictable nature of the size of HTTP "Link" header fields. 4. Document Formats for Sets of Links This section specifies two document formats to convey a set of links. Both are based on the abstract model specified in Section 2 of Web Linking [RFC8288] that defines a link as consisting of a "link context", a "link relation type", a "link target", and optional "target attributes": * The format defined in Section 4.1 is near identical to the field value of the HTTP "Link" header field as specified in Web Linking Section 3 of [RFC8288]. * The format defined in Section 4.2 is based on JSON [RFC8259]. Wilde & Van de Sompel Expires 12 August 2022 [Page 5] Internet-Draft Linkset February 2022 Note 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 position of the resources involved in the link. 4.1. HTTP Link Document Format: application/linkset This document format is near 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 only from the format for field values of the HTTP "Link" header 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. 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 SHOULD be removed in order to comply with Section 5.5 of [I-D.ietf-httpbis-semantics]. 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 URI references that are not relative references (as defined in Section 4.1 of [RFC3986]). If these recommendations are not followed, interpretation of links in "application/linkset" documents will depend on which URI is used as context. It should be noted that the "application/linkset" format specified here is different than the "application/link-format" format specified in [RFC6690] in that the former fully matches the 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. Wilde & Van de Sompel Expires 12 August 2022 [Page 6] Internet-Draft Linkset February 2022 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 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 URI references that are not relative references (as defined in Section 4.1 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 allows for OPTIONAL support of a JSON-LD [W3C.REC-json-ld-20140116] serialization. This can be achieved by adding an appropriate context to the "application/ linkset+json" serialization using the approach described in [W3C.REC-json-ld-20140116]. Communities of practice can decide which context best meets their application needs. Appendix A shows an example of a possible context that, when added to a JSON serialization, allows it to be interpreted as Resource Description Framework (RDF) [W3C.REC-rdf11-concepts-20140225] data. 4.2.1. Set of Links In the JSON representation of a set of links: * A set of links is represented in JSON as an object which MUST contain "linkset" as its sole member. * The "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. Wilde & Van de Sompel Expires 12 August 2022 [Page 7] Internet-Draft Linkset February 2022 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 per 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. 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 per 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 ("href": ""). Wilde & Van de Sompel Expires 12 August 2022 [Page 8] Internet-Draft Linkset February 2022 * In many cases, a link target is further qualified by target attributes. Various types of attributes exist and they are conveyed as additional members of the link target object as detailed in Section 4.2.4. The following example of a JSON-serialized set of links represents one link with its core components: link context, link relation type, and link target. { "linkset": [ { "anchor": "http://example.net/bar", "next": [ {"href": "http://example.com/foo"} ] } ] } Figure 1 The following example of a JSON-serialized set of links represents two links that share link context and relation type but have different link targets. { "linkset": [ { "anchor": "http://example.net/bar", "item": [ {"href": "http://example.com/foo1"}, {"href": "http://example.com/foo2"} ] } ] } Figure 2 The following example shows a set of links that represents two links, each with a different link context, link target, and relation type. One relation type is registered, the other is an extension relation type. Wilde & Van de Sompel Expires 12 August 2022 [Page 9] Internet-Draft Linkset February 2022 { "linkset": [ { "anchor": "http://example.net/bar", "next": [ {"href": "http://example.com/foo1"} ] }, { "anchor": "http://example.net/boo", "http://example.com/relations/baz" : [ {"href": "http://example.com/foo2"} ] } ] } Figure 3 4.2.4. Link Target Attributes A link may be further qualified by target attributes as defined by Section 2 of Web Linking [RFC8288]. Three types of attributes exist: * Serialisation-defined attributes described in Section 3.4.1 of Web Linking [RFC8288]. * Extension attributes defined and used by communities as allowed by Section 3.4.2 of [RFC8288]. * Internationalized versions of the "title" attribute defined by [RFC8288] and of extension attributes allowed by Section 3.4 of [RFC8288]. The handling of these different types of attributes is described in the sections below. 4.2.4.1. Target Attributes Defined by Web Linking Section 3.4.1 of [RFC8288] defines the following target attributes that may be used to annotate links: "hreflang", "media", "title", "title*", and "type"; these target attributes follow different occurrence and value patterns. In the JSON representation, these attributes MUST be conveyed as additional members of the link target object as follows: Wilde & Van de Sompel Expires 12 August 2022 [Page 10] Internet-Draft Linkset February 2022 * "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" ] } ] } ] } Figure 4 Wilde & Van de Sompel Expires 12 August 2022 [Page 11] Internet-Draft Linkset February 2022 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 *) of the attribute. * The character encoding information as prescribed by [RFC8187] is not preserved; instead, the content of the internationalized attribute is represented in the character encoding used for the JSON set of links. * The value of the internationalized target attribute is an array that contains one or more JSON objects. The name of one member of such JSON object is "value" and its value is the actual content (in its unescaped version) of the internationalized target attribute, i.e. the value of the attribute from which the encoding and language information are removed. The name of another, optional, member of such JSON object is "language" and its value is the language tag [RFC5646] for the language in which the attribute content is conveyed. The following example illustrates how the "title*" target attribute defined by Section 3.4.1 of [RFC8288] is represented in a link target object. Wilde & Van de Sompel Expires 12 August 2022 [Page 12] Internet-Draft Linkset February 2022 { "linkset": [ { "anchor": "http://example.net/bar", "next": [ { "href": "http://example.com/foo", "type": "text/html", "hreflang": [ "en" , "de" ], "title": "Next chapter", "title*": [ { "value": "nächstes Kapitel" , "language" : "de" } ] } ] } ] } Figure 5 The above example assumes that the German title contains an umlaut character (in the native syntax it would be encoded as title*=UTF- 8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped form in the JSON representation. Implementations MUST properly decode/encode internationalized target attributes that follow the model of [RFC8187] when transcoding between the "application/linkset" and the "application/linkset+json" formats. 4.2.4.3. Extension Target Attributes Extension target attributes are attributes that are not defined by Section 3.4.1 of [RFC8288] (as listed in Section 4.2.4.1), but are nevertheless used to qualify links. They can be defined by communities in any way deemed necessary, and it is up to them to make sure their usage is understood by target applications. However, lacking standardization, there is no interoperable understanding of 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 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. Wilde & Van de Sompel Expires 12 August 2022 [Page 13] Internet-Draft Linkset February 2022 * 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" } ] } ] } ] } 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. No other form of extensions SHOULD be used. This limitation of the JSON format 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 publishers needs to include descriptive or technical metadata for Wilde & Van de Sompel Expires 12 August 2022 [Page 14] Internet-Draft Linkset February 2022 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. 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. 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. A user agent that follows a "linkset" link MUST be aware that the set of links provided by the resource that is the target of the link can contain links in which the resource that is the context of the link does not participate; it MAY decide to ignore those links. Wilde & Van de Sompel Expires 12 August 2022 [Page 15] Internet-Draft Linkset February 2022 A user agent that follows a "linkset" link and obtains links for which anchors and targets are expressed as relative references (as per Section 4.1 of [RFC3986]) MUST determine what the context is for these links; it SHOULD ignore links for which it is unable to unambiguously make that determination. As a means to convey specific constraints or conventions (as per [RFC6906]) that apply to a link set document, the "profile" attribute MAY be used in conjunction with the "linkset" link relation type. For example, the attribute could be used to indicate that a link set uses a specific, limited set of link relation types. The value of the "profile" attribute MUST be a non-empty list of space-separated URIs, each of which identifies specific constraints or conventions that apply to the link set document. Profile URIs MAY be registered in the IANA Profile URI Registry in the manner specified by [RFC7284]. Section 7.4.1 shows an example of using the "profile" attribute on a link with the "linkset" relation type, making both the link set and the profile(s) to which it complies discoverable. 7. Examples Section 7.1 and Section 7.2 show examples whereby a set of links is provided as "application/linkset" and "application/linkset+json" documents, respectively. Section 7.3 illustrates the use of the "linkset" link relation type to support discovery of sets of links and Section 7.4 shows how to convey profile information pertaining to a links set. 7.1. Set of Links Provided as application/linkset Figure 7 shows a client issuing an HTTP GET request against resource . 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 , 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". Wilde & Van de Sompel Expires 12 August 2022 [Page 16] Internet-Draft Linkset February 2022 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: ; rel="alternate" ; type="application/linkset+json" ; rel="author" ; type="application/rdf+xml" ; anchor="https://example.org/resource1", ; rel="latest-version" ; type="text/html" ; anchor="https://example.org/resource1", ; rel="predecessor-version" ; type="text/html" ; anchor="https://example.org/resource1?version=3", ; rel="predecessor-version" ; type="text/html" ; anchor="https://example.org/resource1?version=2", ; rel="memento" ; type="text/html" ; datetime="Thu, 13 Jun 2019 09:34:33 GMT" ; anchor="https://example.org/resource1", ; rel="memento" ; type="text/html" ; datetime="Sun, 21 Jul 2019 12:22:04 GMT" ; anchor="https://example.org/resource1", ; 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 . In the request, the client uses an "Accept" header field to indicate it prefers a response in the "application/linkset+json" format. Wilde & Van de Sompel Expires 12 August 2022 [Page 17] Internet-Draft Linkset February 2022 GET links/resource1 HTTP/1.1 Host: example.org Accept: application/linkset+json Figure 9: Client HTTP GET request expressing preference for "application/ linkset+json" response Figure 10 shows the response to the HTTP GET request of Figure 9. The set of links is serialized according to the media type "application/linkset+json". HTTP/1.1 200 OK Date: Mon, 12 Aug 2019 10:46:22 GMT Server: Apache-Coyote/1.1 Content-Type: application/linkset+json Link: ; rel="alternate" ; type="application/linkset" Content-Length: 1349 { "linkset": [ { "anchor": "https://example.org/resource1", "author": [ { "href": "https://authors.example.net/johndoe", "type": "application/rdf+xml" } ], "memento": [ { "href": "https://example.org/resource1?version=1", "type": "text/html", "datetime": "Thu, 13 Jun 2019 09:34:33 GMT" }, { "href": "https://example.org/resource1?version=2", "type": "text/html", "datetime": "Sun, 21 Jul 2019 12:22:04 GMT" } ], "latest-version": [ { "href": "https://example.org/resource1?version=3", "type": "text/html" } ] }, { "anchor": "https://example.org/resource1?version=3", "predecessor-version": [ { "href": "https://example.org/resource1?version=2", "type": "text/html" Wilde & Van de Sompel Expires 12 August 2022 [Page 18] Internet-Draft Linkset February 2022 } ] }, { "anchor": "https://example.org/resource1?version=2", "predecessor-version": [ { "href": "https://example.org/resource1?version=1", "type": "text/html" } ] }, { "anchor": "https://example.org/resource1#comment=1", "author": [ { "href": "https://authors.example.net/alice"} ] } ] } Figure 10: Response to the client's request for the set of links 7.3. Discovering a Link Set via the "linkset" Link Relation Type Figure 11 shows a client issuing an HTTP HEAD request against resource . 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 , 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: ; rel="linkset" ; type="application/linkset+json" Content-Length: 236 Content-Type: text/html;charset=utf-8 Figure 12: Response to HTTP HEAD request Wilde & Van de Sompel Expires 12 August 2022 [Page 19] Internet-Draft Linkset February 2022 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 . 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 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. For posterity that profile document was saved in the Internet Archive at 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://dalgiardino.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 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 Wilde & Van de Sompel Expires 12 August 2022 [Page 20] Internet-Draft Linkset February 2022 Figure 15: Client HTTP HEAD request Figure 16 shows the server's response to the request of Figure 15. Note the "profile" parameter for the application/linkset+json media type, which has as value the same Profile URI as was used in . HTTP/1.1 200 OK Date: Mon, 27 Sep 2021 16:03:33 GMT Server: nginx Content-Type: application/linkset+json; profile="https://www.gs1.org/voc/?show=linktypes" Content-Length: 396 Figure 16: Response to the client's HEAD request including a "profile" parameter for the "application/linkset+json" media type 7.4.3. Using a Link with a "profile" Link Relation Type Note that the response Figure 16 from the link set resource is equivalent to the response shown in Figure 17, which leverages the "profile" link relation type defined in [RFC6906]. HTTP/1.1 200 OK Date: Mon, 27 Sep 2021 16:03:33 GMT Server: nginx Content-Type: application/linkset+json Link: https://www.gs1.org/voc/?show=linktypes; rel="profile" Content-Length: 396 Figure 17: Response to the client's HEAD request including a "profile" link A link with a "profile" link relation type as shown in Figure 17 can also be conveyed in the link set document itself. This is illustrated by Figure 18. Following the recommendation that all links in a link set document should have an explicit anchor, such a link has the URI of the link set itself as anchor and the Profile URI as target. Multiple Profile URIs are handled by using multiple "href" members. Wilde & Van de Sompel Expires 12 August 2022 [Page 21] Internet-Draft Linkset February 2022 { "linkset": [ { "anchor": "https://id.gs1.org/01/9506000134352?linkType=all", "profile": [ {"href": "https://www.gs1.org/voc/?show=linktypes"} ] }, { "anchor": "https://id.gs1.org/01/9506000134352", "https://gs1.org/voc/whatsInTheBox": [ {"href": "https://example.com/en/packContents/GB"} ] } ] } Figure 18: A Link Set that declares the profile it complies with using a "profile" link 8. IANA Considerations 8.1. Link Relation Type: linkset The link relation type below should be registered by IANA per Section 4.2 of Web Linking [RFC8288]: Relation Name: linkset Description: The link target of a link with the "linkset" relation type provides a set of links, including links in which the link context of the link participates. Reference: [[ This document ]] 8.2. Media Type: application/linkset The Internet media type [RFC6838] for a natively encoded linkset is application/linkset. Type name: application Subtype name: linkset Required parameters: none Optional parameters: profile Wilde & Van de Sompel Expires 12 August 2022 [Page 22] Internet-Draft Linkset February 2022 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 Intended usage: COMMON Restrictions on usage: none Author: Erik Wilde Change controller: IETF 8.3. Media Type: application/linkset+json The Internet media type [RFC6838] for a JSON-encoded linkset is application/linkset+json. Type name: application Subtype name: linkset+json Required parameters: none Wilde & Van de Sompel Expires 12 August 2022 [Page 23] Internet-Draft Linkset February 2022 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 Intended usage: COMMON Restrictions on usage: none Author: Erik Wilde 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 Wilde & Van de Sompel Expires 12 August 2022 [Page 24] Internet-Draft Linkset February 2022 exposed in links should be supposed to be "hidden"; instead, if these URIs are supposed to be limited to certain users, then technical measures should be put in place so that accidentally exposing them does not cause any harm. For the specific mechanisms defined in this specification, two security considerations should be taken into account: * The Web Linking model always has an "implicit context", which is the resource of the HTTP interaction. This original context can be lost or can change when self-contained link representations are moved. Changing the context can change the interpretation of links when they have no explicit anchor, or when they use relative URIs. Applications may choose to ignore links that have no explicit anchor or that use relative URIs when these are exchanged in stand-alone resources. * The model introduced in this specification supports "3rd party links", where one party can provide links that have another party's resource as an anchor. Depending on the link semantics and the application context, it is important to verify that there is sufficient trust in that 3rd party to allow it to provide these links. Applications may choose to treat 3rd party links differently than cases where a resource and the links for that resource are provided by the same party. 10. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009, . Wilde & Van de Sompel Expires 12 August 2022 [Page 25] Internet-Draft Linkset February 2022 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, . [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, . [RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906, DOI 10.17487/RFC6906, March 2013, . [RFC7284] Lanthaler, M., "The Profile URI Registry", RFC 7284, DOI 10.17487/RFC7284, June 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8187] Reschke, J., "Indicating Character Encoding and Language for HTTP Header Field Parameters", RFC 8187, DOI 10.17487/RFC8187, September 2017, . [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, . [RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017, . [I-D.ietf-httpbis-semantics] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf- httpbis-semantics-19, 12 September 2021, . 11. Informative References Wilde & Van de Sompel Expires 12 August 2022 [Page 26] Internet-Draft Linkset February 2022 [W3C.REC-rdf11-concepts-20140225] Cyganiak, R., Wood, D., and M. Lanthaler, "RDF 1.1 Concepts and Abstract Syntax", World Wide Web Consortium Recommendation REC-rdf11-concepts-20140225, 25 February 2014, . [W3C.REC-json-ld-20140116] Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0", World Wide Web Consortium Recommendation REC-json-ld- 20140116, 16 January 2014, . [RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010, . [RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, July 2016, . [DCMI-TERMS] Initiative, D. C. M., "DCMI Metadata Terms", 2020, . Appendix A. JSON-LD Context A set of links rendered according to the JSON serialization defined in Section 4.2 can be interpreted as RDF triples by adding a JSON-LD context [W3C.REC-json-ld-20140116] that maps the JSON keys to corresponding Linked Data terms. And, as per [W3C.REC-json-ld-20140116] section 6.8 (https://www.w3.org/TR/2014/ REC-json-ld-20140116/#interpreting-json-as-json-ld), when delivering a link set that is rendered according to the "application/ linkset+json" media type to a user agent, a server can convey the availability of such a JSON-LD context by using a link with the relation type "http://www.w3.org/ns/json-ld#context" in the HTTP "Link" header. Figure 19 shows the response of 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 that are expressed as HTTP URIs. Wilde & Van de Sompel Expires 12 August 2022 [Page 27] Internet-Draft Linkset February 2022 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: ; 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": [ Wilde & Van de Sompel Expires 12 August 2022 [Page 28] Internet-Draft Linkset February 2022 "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. Wilde & Van de Sompel Expires 12 August 2022 [Page 29] Internet-Draft Linkset February 2022 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. Wilde & Van de Sompel Expires 12 August 2022 [Page 30] Internet-Draft Linkset February 2022 "text/html" . "en" . "Product information" . "en" . "What's in the box?" . "fr" . "Information produit" . "fr" . "Qu'y a-t-il dans la boite?" . "fr" . "Qu'y a-t-il dans la boite?" . . . . . . Wilde & Van de Sompel Expires 12 August 2022 [Page 31] Internet-Draft Linkset February 2022 . "en" . "fr" . "See it in action!"@en . "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". Wilde & Van de Sompel Expires 12 August 2022 [Page 32] Internet-Draft Linkset February 2022 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, and Sarven Capadisli. Wilde & Van de Sompel Expires 12 August 2022 [Page 33] Internet-Draft Linkset February 2022 Authors' Addresses Erik Wilde Axway Email: erik.wilde@dret.net URI: http://dret.net/netdret/ Herbert Van de Sompel Data Archiving and Networked Services Email: herbert.van.de.sompel@dans.knaw.nl URI: https://orcid.org/0000-0002-0715-6126 Wilde & Van de Sompel Expires 12 August 2022 [Page 34]