| Network Working Group | E. Wilde |
| Internet-Draft | CA Technologies |
| Intended status: Informational | H. Van de Sompel |
| Expires: March 10, 2018 | Los Alamos National Laboratory |
| September 6, 2017 |
Linkset: Media Types and a Link Relation Type for Link Sets
draft-wilde-linkset-01
This specification defines two media types and a link relation type for sets of links. The media types can be used to represents links in a standalone fashion, in one case in the native format used in the HTTP Link header, and in the other case in a JSON-based format. The link relation can be used to reference these kind of standalone link sets, so that a resource can indicate that additional links putting it into context are available someplace else. One typical scenario is when the number of links to put in an HTTP Link header field becomes too big, and thus these links should be provided by a distinct resource.
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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 convey typed Web Links [I-D.nottingham-rfc5988bis] as a part of resource representations, for example, using the <link> element for HTML representations, or the "Link" header field in HTTP response headers for representations of any media type. In some cases, however, providing links by value is impractical or impossible. In these cases, an approach to provide links by reference (instead of by value) can solve the problem.
To that end, this specification defines two document formats and associated media types to represent a set of links. It also defines the "linkset" relation type that a resource can use to support discovery of another resource that conveys links in which the former resource participates.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
This specification uses the terms "link context" and "link target" as defined in [I-D.nottingham-rfc5988bis]. (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.
Additionally, this specification uses the following terms for types of resources involved in providing links by reference:
The following sections outline some scenarios in which it is useful to have the ability to separate resources from the links that pertain to them.
These are all scenarios in which providing links by reference is advantageous or necessary. It is important to keep in mind that, when providing some links by reference, some/other links can still be provided by value.
In some cases, it is useful that links pertaining to an origin resource are provided by a server other than the one that hosts the origin resource. For example, this allows:
In such cases, a third-party link set resource provides links pertaining to the origin resource. This link set resource may be managed by the same custodian as the origin resource, or by a third party.
In order for the server hosting the origin resource to provide an up-to-date and complete set of links in which the origin resource participates, the server would need to obtain the links from the link set resource, and embed them in the origin resource's representations prior to responding to a client. Doing so would increase latency and load, which may be unnecessary if a client is not intent on consuming these links. Providing links by reference, instead of by value, removes the server-to-server communication and resulting overhead required to obtain links. Instead, the consumer of the origin resource can decide if they need the additional links as context for the resource.
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 reference can be a solution because a link to a link set resource associated with an origin resource can typically be added by means of a web server rewrite rule that leverages the resource's URI.
When conveying links in the HTTP "Link" header, 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 boundary for their size. But when the number of links is unpredictable, estimating a reliable upper boundary 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 boundary 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, for example by limiting the links they provide to those with select relation types, thereby limiting the value of the HTTP "Link" header to clients. Providing links by reference, instead of by value, overcomes challenges related to the unpredictable nature of the extent of HTTP "Link" headers.
In more extreme scenarios it is conceivable that the number of links pertaining to the origin resource becomes so large that the response from the associated link set resource becomes too large. This could be the case for highly popular origin resources, when the link set includes links in which the origin resource participates as both link context and link target. In such cases, the link set resource could deliver responses incrementally, for example, using a paged resource model that clients can consume as required.
Link set resources 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 [I-D.nottingham-rfc5988bis] that defines a link as consisting of a "link context", a "link relation type", a "link target", and optional "target attributes". In those terms, a link set resource is a resource that conveys a collection of one or more such links.
The format defined in Section 4.1 is identical to the payload of the HTTP Link header as specified in [I-D.nottingham-rfc5988bis]. The format defined in Section 4.2 is based on JSON and thus does not have the character encoding limitations of HTTP header fields.
This document format is identical to the payload of the HTTP Link header. It is defined in Section 3 of [I-D.nottingham-rfc5988bis], more specifically by its ABNF production rule for "Link" and subsequent ones. The assigned media type for this format is "application/linkset".
This document format is based on the native Link header field representation defined by [I-D.nottingham-rfc5988bis], but uses JSON [RFC7159] as the syntax to represent the information contained in the native format. It is described in this section. The assigned media type for this format is "application/linkset+json".
In the JSON representation of a link set:
In the JSON representation, individual links (as well as multiple links that have the same link context and link target) are represented by a JSON object, the link object. A link object adheres to the following rules:
The following example of a JSON-serialized link set represents one link with the core components of a link: link context, link relation type, and link target.
[ { "href" : "http://example.com/foo",
"anchor" : "http://example.net/bar",
"rel" : [ "next" ] } ]
The following example of a JSON-serialized link set represents two links with the core components of a link: link context, link relation type, and link target. The syntax of this example takes advantage of the fact that multiple relation types can be used in one link object to represent multiple links that have the same link context and link target.
[ { "href" : "http://example.com/foo",
"anchor" : "http://example.net/bar",
"rel " : [ "next" , "http://example.net/linkrel" ] } ]
The previous example is equivalent to the following one, which does not use the syntax shortcut and therefore represents the two links individually.
[ { "href" : "http://example.com/foo",
"anchor" : "http://example.net/bar",
"rel" : [ "next" ] },
{ "href" : "http://example.com/foo",
"anchor" : "http://example.net/bar",
"rel" : [ "http://example.net/linkrel" ] } ]
In many cases, a link is further qualified by target attributes, either defined by the serialization of [I-D.nottingham-rfc5988bis] or extension attributes defined and used by communities.
RFC 5988bis 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 and MUST be handled in the JSON representation as follows:
The following example illustrates how the repeatable "hreflang" and the not repeatable "type" target attributes are represented in a link object.
[ { "href" : "http://example.com/foo",
"anchor" : "http://example.net/bar",
"rel" : [ "next" ] ,
"hreflang" : [ "en" , "de" ] ,
"type" : "text/html" } ]
In addition to the target attributes described in Section 4.2.3, [I-D.nottingham-rfc5988bis] also supports attributes that follow the content model of [RFC5987]. In [I-D.nottingham-rfc5988bis], these target attributes are recognizable by the use of a trailing asterisk in the attribute name, such as "title*". The content model of [RFC5987] 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 attributes is as follows:
The following example illustrates how the "title*" target attribute defined by [I-D.nottingham-rfc5988bis] is represented in a link object.
[ { "href" : "http://example.com/foo",
"anchor" : "http://example.net/bar",
"rel" : [ "next" ] ,
"title*" : [ [ "nachstes Kapitel", "de" ] ] } ]
The above example assumes that the german title contains an umlaut character (which in the native syntax would be encoded as title*=UTF-8'de'n%c3%a4chstes%20Kapitel), which gets encoded in its unescaped form (but not shown here due to the limitations of RFC publication) in the JSON representation. Implementations MUST properly decode/encode RFC 5987 style target attributes when transcoding between the native and the JSON syntax.
Extension target attributes are attributes that are not defined by RFC 5988bis (as listed in Section 4.2.3.1), 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 target of a link with the "linkset" relation type - the link set resource - provides a set of links, including links in which the link context - the origin resource - participates.
A link with the "linkset" relation type MAY be provided in the header and/or the body of the origin resource's representation. It may also be discovered by other means, such as through client-side information.
More than one link with a "linkset" relation type MAY be provided. Multiple such links can refer to the same set of links expressed using different representations, or to different sets of links (potentially provided by different services).
The use of a link with the "linkset" relation type does not preclude the provision of links with other relation types, i.e. the origin resource can provide typed links other than a "linkset" link. Therefore, the effective set of links pertaining to the origin resource is the union of the links that the resource itself provides, and of all links - provided by the link set resource - in which it participates.
The link set resource MAY provide the links that pertain to the origin resource in its HTTP response header and/or body.
In common scenarios (the origin resource is distinct from the link set resource), it is essential for link set representations to make the URI of the origin resource explicit for those links in which the origin resource acts as link context.
A user agent that follows a "linkset" link from an origin resource MUST be aware that the link set resource can provide links in which the origin resource does not participate and MAY decide to ignore those links.
There is no constraint on the target URI of a link with the "linkset" relation type; designing and using these URIs is left to the discretion of implementers.
If an origin resource provides a "linkset" link pointing at a link set resource, and that link set resource provides a "linkset" link in turn, then this latter link points at links pertaining to the link set resource. This means that in the context of the latter link, the link set resource is an origin resource. This means that linkset relations are not transitive, and it is up to a client to decide whether they follow "nested chains" of "linkset" links or not.
Sections Section 6.1 and Section 6.2 show examples whereby the link set resource provides links pertaining to the origin resource, in its response header and body, respectively.
Figure 1 shows a client issuing an HTTP head request against origin resource http://example.org/resource1.
HEAD /resource1 HTTP/1.1
Host: example.org
Connection: close
Figure 1: Client HTTP HEAD Request
Figure 2 shows the response to the HEAD request of Figure 1. The response contains a Link header with a link that uses the "linkset" relation type. It indicates that links pertaining to the origin resource are provided by link set resource http://example.com/links?uri=http%3A%2F%2Fexample.org%2Fresource.
HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:37:51 GMT
Server: Apache-Coyote/1.1
Link: <http://example.com/links?uri=http%3A%2F%2Fexample.org%2Fresource>
; rel="linkset"
; type="text/html"
Content-Length: 5214
Content-Type: text/html;charset=utf-8
Connection: close
Figure 2: Response to HTTP HEAD on Origin Resource
While in this example the URI of the link set resource uses a pattern that represents the URI of the origin resource, this is opaque to the client, which simply follows target URI to retrieving the link set resource.
Figure 3 shows the client issuing an HTTP GET request against the link set resource provided in Figure 2.
GET /links?uri=http%3A%2F%2Fexample.org%2Fresource HTTP/1.1
Host: example.com
Connection: close
Figure 3: Client HTTP GET against the Link Set Resource
Figure 4 shows the response headers to the HTTP GET request of Figure 3. The links pertaining to the origin resource are provided in the Link response header of the link set resource. As can be seen, in order to support an unambiguous determination of the link context of each link, the "anchor" attribute is always provided. Note that most, but not all, links have the origin resource as link context (anchor).
HTTP/1.1 200 OK Date: Mon, 28 Nov 2016 14:40:02 GMT Server: Apache-Coyote/1.1 Link: <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.net/resource41/> ; rel="related" ; type="application/pdf" ; anchor="http://example.org/resource1/items/AF48EF.pdf" Content-Type: text/html Content-Length: 3018
Figure 4: Response to HTTP GET against the Link Set Resource
Figure 5 is an example of a client issuing an HTTP head request against origin resource http://example.org/article?id=10.1371/journal.pone.0167475
HEAD article?id=10.1371/journal.pone.0167475 HTTP/1.1
Host: example.org
Connection: close
Figure 5: Client HTTP HEAD Request
Figure 6 shows the response to the HEAD request of Figure 5. The response contains a Link header with a link that has the "linkset" relation type. It indicates that links pertaining to the origin resource are provided by link set resource http://example.com/links/10.1371/journal.pone.0167475, which provides a representation with media type application/linkset+json.
HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:37:51 GMT
Server: Apache-Coyote/1.1
Link: <http://example.com/links/10.1371/journal.pone.0167475>
; rel="linkset"
; type="application/linkset+json"
Content-Length: 236
Content-Type: text/html;charset=utf-8
Connection: close
Figure 6: Response to HTTP HEAD on Origin Resource
In this example, the URI of the linkset resource does not directly represent the URI of the origin resource anymore. There still is an association possible through the use of a URI pattern that is includes identifying information. But as in the example above, the URI of the link set resource is opaque to the client that simply accesses the URI to retrieve the link set resource.
Figure 7 shows the client issuing an HTTP GET request against the link set resource provided in Figure 6.
GET /links/10.1371/journal.pone.0167475 HTTP/1.1 Host: example.com Accept: application/linkset+json Connection: close
Figure 7: Client HTTP GET against the Link Set Resource
Figure 8 shows the response headers to the HTTP GET request of Figure 7. The links pertaining to the origin resource are provided in the response body of the link set resource and are serialized according to the media type application/linkset+json.
HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:40:02 GMT
Server: Apache-Coyote/1.1
Content-Type: application/linkset+json
Content-Length: 794
[{"href":"http://authors.example.net/johndoe",
"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
"rel":["author"],"type":"application/rdf+xml"},
{"href":"http://authors.example.net/janedoe",
"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
"rel":["author"],"type":"application/rdf+xml"},
{"href":"http://example.org/resource1/items/AF48EF.pdf",
"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
"rel":["item"],"type":"text/html"},
{"href":"http://example.org/resource1/items/CB63DA.html",
"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475",
"rel":"[item"],"type":"application/pdf"},
{"href":"http://example.net/resource41/",
"anchor":"http://example.org/resource1/items/AF48EF.pdf",
"rel":["related"],"type":"application/pdf"}]
Figure 8: Response to HTTP GET against the Link Set Resource
If Figure 6 would have provided a link to a link set with media type application/linkset, and if the client would have requested that link set, then the body of the response would have been similar to Figure 8. But it would have had application/linkset as Content-Type, the payload of the HTTP Link header of Figure 4 as body, and an accordingly adjusted value for Content-Length.
The link relation type below has been registered by IANA per Section 6.2.1 of Web Linking [I-D.nottingham-rfc5988bis]:
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.
...
| [I-D.nottingham-rfc5988bis] | Nottingham, M., "Web Linking", Internet-Draft draft-nottingham-rfc5988bis-08, August 2017. |
| [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. |
| [RFC5987] | Reschke, J., "Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010. |
| [RFC5988] | Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010. |
| [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. |
| [RFC7159] | Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014. |
| [RFC7231] | Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014. |
| [W3C.REC-html401-19991224] | Rivoal, F., "Media Queries", World Wide Web Consortium Recommendation REC-css3-mediaqueries-20120619, June 2012. |