| Network Working Group | E. Wilde |
| Internet-Draft | |
| Intended status: Informational | H. Van de Sompel |
| Expires: July 3, 2020 | Data Archiving and Networked Services |
| December 31, 2019 |
Linkset: Media Types and a Link Relation Type for Link Sets
draft-wilde-linkset-05
This specification defines two media types for representing sets of links and a link relation type for referring to sets of links. The media types can be used to represents links in a standalone fashion, in one case in the native format as used in the HTTP "Link" header field, and in the other case in a JSON-based format. The link relation type can be used to point at a resource that provides a set of links.
Please discuss this draft on the ART mailing list (<https://www.ietf.org/mailman/listinfo/art>).
Online access to all versions and files is available on GitHub (<https://github.com/dret/I-D/tree/master/linkset>).
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Resources on the Web often use typed Web Links [RFC8288], either as an embedded part of resource representations, for example using the <link> element for HTML representations, or by using the HTTP "Link" header field for representations using any media type. In some cases, however, providing links in this manner is impractical or impossible.
To that end, this specification defines two document formats and associated media types to represent sets of links. It also defines the "linkset" relation type that can be used to support discovery of a resource that conveys links.
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 RFC 5988 [RFC5988]. Although defined as IRIs, in common scenarios they are also URIs.
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. It is important to keep in mind that when providing links by means of a standalone representation and possibly linking to it, other links can still be provided by value (i.e., it is possible combine various mechanisms of how links are conveyed).
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:
In such cases, a specific resource can provide a set of links pertaining to a resource. This specific resource may be managed by the same or by another custodian as the linked resource. For clients intent on consuming links pertaining to the resource, it would be beneficial if the following conditions were met:
These two needs (linking to a resource that represents a set of links, and providing media types for representing sets of links) are addressed in this specification through the link relation type and the two defined media types.
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.
When conveying links in an HTTP "Link" header field, it is possible for the size of the HTTP response header 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 headers by specifying an upper bound for their size. But when the number of links is unpredictable, estimating a reliable upper bound is challenging.
HTTP [RFC7231] 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 header 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.
In more extreme scenarios it is conceivable that the number of links to be conveyed becomes so large that even a standalone document would become too large. For example, this could be the case for highly interlinked resources and when links are provided in which such resources participates as both link context and link target. In such cases, the links could be delivered incrementally, for example, by means of a paged resource model.
A set of links can be represented by serializations that support conveying both link contexts and link targets. This section specifies two document formats 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 identical to the payload of the HTTP "Link" header field as specified in Web Linking [RFC8288].
The format defined in Section 4.2 is based on JSON but encodes the same abstract model as defined in Web Linking [RFC8288].
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:
If these recommendations are not followed, interpretation of the "application/linkset" will depend on the which URI is used as context URI.
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:
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 the a JSON serialization, allows it to be interpreted as RDF.
In the JSON representation of a set of links:
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:
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:
Note that the JSON representation does not use the "rel" or "rev" constructs used by Web Linking [RFC8288] and the above specified HTTP "Link" document format. Rather:
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"}
]
}
]
}
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"}
]
}
]
}
The following example shows a set of links that represents two links, each with a different link context, link target, and relation type. One relation type is registered, the other is an extension relation type.
{
"linkset":
[
{ "anchor": "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"}
]
}
]
}
A link may be further qualified by target attributes, either defined by the serialization of Web Linking [RFC8288] or extension attributes defined and used by communities.
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:
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" ]
}
]
}
]
}
In addition to the target attributes described in Section 4.2.4.1, [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:
The following example illustrates how the "title*" target attribute defined by [RFC8288] is represented in a link target object.
{
"linkset":
[
{ "anchor": "http://example.net/bar",
"next": [
{"href": "http://example.com/foo",
"type": "text/html",
"hreflang": [ "en" , "de" ],
"title": "Next chapter",
"title*": [ { "value": "nachstes Kapitel" , "language" : "de" } ]
}
]
}
]
}
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. This is not shown in the above example due to the limitations of RFC publication. 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.
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:
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" } ]
}
]
}
]
}
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 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.
A user agent that follows a "linkset" link and obtains links for which anchors and targets are not expressed as absolute URIs MUST properly determine what the context is for these links; it SHOULD ignore links for which it is unable to unambiguously make that determination.
Section 6.1 and Section 6.2 show examples whereby the set of links are provided as "application/linkset" and "application/linkset+json" documents, respectively.
Figure 1 shows a client issuing an HTTP GET request against resource http://example.org/resource1.
GET /resource1 HTTP/1.1
Host: example.org
Connection: close
Figure 1: Client HTTP GET request
Figure 2 shows the response to the GET request of Figure 1. The response contains a Content-Type header specifying application/linkset. A set of links, including links that pertain to the responding resource, is provided in the body of the response.
HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:35:51 GMT
Server: Apache-Coyote/1.1
Content-Length: 729
Content-Type: application/linkset
Connection: close
<http://authors.example.net/johndoe>
; rel="author"
; type="application/rdf+xml"
; anchor="http://example.org/resource1",
<http://authors.example.net/janedoe>
; rel="author"
; type="application/rdf+xml"
; anchor="http://example.org/resource1",
<http://example.org/resource1/items/AF48EF.pdf>
; rel="item"
; type="application/pdf"
; anchor="http://example.org/resource1",
<http://example.org/resource1/items/CB63DA.html>
; rel="item"
; type="text/html"
; anchor="http://example.org/resource1",
<http://example.org/resource1>
; rel="latest-version"
; anchor="http://example.org/resource41/",
<http://example.org/resource40>
; rel="prev"
; anchor="http://example.org/resource41/"
Figure 2: Response to HTTP GET includes a set of links
Figure 3 shows a client issuing an HTTP HEAD request against resource http://example.org/article/view/7507
HEAD article/view/7507 HTTP/1.1
Host: example.org
Connection: close
Figure 3: Client HTTP HEAD request
Figure 4 shows the response to the HEAD request of Figure 3. The response contains a Link header with a link that has the "linkset" relation type. It indicates that a set of links is provided by resource http://example.com/links/article/7507, 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: <http://example.com/links/article/7507>
; rel="linkset"
; type="application/linkset+json"
Content-Length: 236
Content-Type: text/html;charset=utf-8
Connection: close
Figure 4: Response to HTTP HEAD request
Figure 5 shows the client issuing an HTTP GET request against the target IRI of the "linkset" link provided in Figure 4. In the request, the client uses an Accept header to indicate it requests a response in "application/linkset+json" format.
GET links/article/7507 HTTP/1.1 Host: example.com Accept: application/linkset+json Connection: close
Figure 5: Client issues HTTP GET to obtain set of links
Figure 6 shows the response to the HTTP GET request of Figure 5. The set of links is serialized according to the media type "application/linkset+json" and includes links in which the Context IRI of the "linkset" link of Figure 4 participates. It also includes a link in which it does not.
HTTP/1.1 200 OK
Date: Mon, 12 Aug 2019 10:46:22 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Content-Length: 802
{
"linkset": [
{
"anchor": "https://example.org/article/view/7507",
"author": [
{
"href": "https://orcid.org/0000-0002-4311-0897",
"type": "rdf/xml"
}
],
"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/10.841/zk2557"
}
]
},
{
"anchor": "https://example.com/links/article/7507",
"alternate": [
{
"href": "https://mirror.example.com/links/article/7507",
"type": "application/linkset"
}
]
}
]
}
Figure 6: Response to the client's request for the set of links
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".
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).
The link relation type below has been registered by IANA per Section 6.2.1 of Web Linking [RFC8288]:
The Internet media type [RFC6838] for a natively encoded link set is application/linkset.
The Internet media type [RFC6838] for a JSON-encoded link set is application/linkset+json.
...
| [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. |
| [RFC5646] | Phillips, A. and M. Davis, "Tags for Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009. |
| [RFC5988] | Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010. |
| [RFC6838] | Freed, N., Klensin, J. and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013. |
| [RFC6982] | Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, DOI 10.17487/RFC6982, July 2013. |
| [RFC7231] | Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, 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., "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. |
Thanks for comments and suggestions provided by Mark Nottingham, Stian Soiland-Reyes, Sarven Capadisli, ...
When adding the below context to the representation of a link set rendered according to the JSON serialization defined in Section 4.2, it can be interpreted as RDF.
{
"@context": [
{
"@vocab": "https://www.iana.org/assignments/link-relations/",
"anchor": "@id",
"href": "@id",
"dct": "http://purl.org/dc/terms/",
"link": "https://www.iana.org/assignments/link-relations#",
"title": {
"@id": "http://purl.org/dc/terms/title"
},
"title*": {
"@id": "http://purl.org/dc/terms/title"
},
"type": {
"@id": "dct:format",
"@type": "@vocab"
}
},
{
"language": "@language",
"value": "@value",
"hreflang": {
"@id": "https://www.w3.org/ns/activitystreams#hreflang",
"@container": "@set"
}
}
]
}
Figure 7: Context for JSON-LD that could be used with the JSON serialization