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Applications and Real-Time Building Blocks for HTTP APIs link relation This specification defines the Link-Template HTTP header field, providing a means for describing the structure of a link between two resources, so that new links can be generated. The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Building Blocks for HTTP APIs Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction defines a syntax for templates that, when expanded using a set of variables, results in a URI . This specification defines a HTTP header field for conveying templates for links in the headers of a HTTP message. It is complimentary to the Link header field defined in , which carries links directly.

Notational Conventions 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 when, and only when, they appear in all capitals, as shown here. This specification uses the following terms from : List, String, Display String, Parameter.

The Link-Template Header Field The Link-Template header field is a Structured Field that serializes one or more links into HTTP message metadata. It is semantically equivalent to the Link header field defined in , except that the link target and link anchor can contain URI Templates . Its value is a List of Strings. Each String is a URI Template, and Parameters on it carry associated metadata. For example:

Link-Template: "/{username}"; rel="item"

indicates that a resource with the relation type "item" for a given "username" can be found by expanding the "username" variable into the template given. The link target (as defined in ) is the result of expanding the URI Template (being converted to an absolute URI after expansion, if necessary). The link context and link relation type for the link (as defined in ) are conveyed using the "anchor" and "rel" Parameters, as they are for the Link header field in . Their values MUST be Strings. However, the "anchor" Parameter MAY contain a URI Template. For example:

Link-Template: "/books/{book_id}/author"; rel="author"; anchor="#{book_id}"

Here, the link to the author for a particular book in a list of books can be found by following the link template. This specification defines additional semantics for the "var-base" Parameter on templated links; see below. The link's target attributes (as defined in ) are conveyed using other Parameters, in a manner similar to the Link header field. These Parameter values MUST be Strings, unless they contain non-ASCII characters, in which case they MUST be Display Strings. Note that some target attribute names will not serialise as Structure Field Parameter keys (see ). For example:

Link-Template: "/author"; rel="author"; title=%"Bj%c3%b6rn J%c3%a4rnsida"

Implementations MUST support all levels of template defined by in the link String and the anchor Parameter.

The 'var-base' parameter When a templated link has a 'var-base' parameter, its value conveys a URI-reference that is used as a base URI for the variable names in the URI template. This allows template variables to be globally identified, rather than specific to the context of use. Dereferencing the URI for a particular variable might lead to more information about the syntax or semantics of that variable; specification of particular formats for this information is out of scope for this document. To determine the URI for a given variable, the value given is used as a base URI in reference resolution (as specified in ). If the resulting URI is still relative, the context of the link is used as the base URI in a further resolution; see . For example:

Link-Template: "/widgets/{widget_id}"; rel="https://un5mkq9urycymemmv4.julianrbryant.com/rel/widget"; var-base="https://un5mkq9urycymemmv4.julianrbryant.com/vars/"

indicates that a resource with the relation type "https://un5mkq9urycymemmv4.julianrbryant.com/rel/widget" can be found by expanding the "https://un5mkq9urycymemmv4.julianrbryant.com/vars/widget_id" variable into the template given. If the current context of the message that the header appears within is "https://un5mkq9urycymemmv4.julianrbryant.com/", the same information could be conveyed by this header field:

Link-Template: "/widgets/{widget_id}"; rel="https://un5mkq9urycymemmv4.julianrbryant.com/rel/widget"; var-base="/vars/"

Security Considerations The security consideration for the Link header field in and those for URI Templates both apply.

IANA Considerations This specification enters the "Link-Template" into the Hypertext Transfer Protocol (HTTP) Field Name Registry. Field Name: Link-Template Status: permanent Specification document: [this document]

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