The Link-Template HTTP Header Field

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Table of Contents

1. Introduction

[URI-TEMPLATE] defines a syntax for templates that, when expanded using a set of variables, results in a URI [URI].¶

This specification defines a HTTP header field [HTTP] for conveying templates for links in the headers of a HTTP message. It is complimentary to the Link header field defined in Section 3 of [WEB-LINKING], which carries links directly.¶

2. The Link-Template Header Field

The Link-Template header field is a Structured Field [STRUCTURED-FIELDS] that serializes one or more links into HTTP message metadata. It is semantically equivalent to the Link header field defined in Section 3 of [WEB-LINKING], except that it uses URI Templates [URI-TEMPLATE] to convey the structure of links.¶

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="https://example.org/rel/user"

indicates that a resource with the relation type "https://example.org/rel/user" can be found by expanding the "username" variable into the template given.¶

The target for the link (as defined in Section 2 of [WEB-LINKING]) is the result of expanding the URI Template [URI-TEMPLATE] (being converted to an absolute URI after expansion, if necessary).¶

The context, relation type and target attributes for the link are determined as defined for the Link header field in Section 3 of [WEB-LINKING].¶

Parameters on a templated link have identical semantics to those of a Link header field. This includes (but is not limited to) the use of the "rel" parameter to convey the relation type, the "anchor" parameter to modify the context IRI, and so on. Parameter values MUST be Strings.¶

Likewise, the requirements for parameters on templated links are the same as those for a Link header field.¶

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.¶

Implementations MUST support all levels of template defined by [URI-TEMPLATE] in both the rel and anchor parameters.¶

This specification defines additional semantics for the "var-base" parameter on templated links; see below.¶

Link-Template: "/widgets/{widget_id}"
               rel="https://example.org/rel/widget";
               var-base="https://example.org/vars/"

Link-Template: "/widgets/{widget_id}";
               rel="https://example.org/rel/widget";
               var-base="/vars/"

3. Security Considerations

The security consideration for the Link header field in [WEB-LINKING] and those for URI Templates [URI-TEMPLATE] both apply.¶

4. 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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