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 the link target and link anchor can contain URI Templates [URI-TEMPLATE].¶

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 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 link context and link relation type for the link (as defined in Section 2 of [WEB-LINKING]) are conveyed using the "anchor" and "rel" Parameters, as they are for the Link header field in Section 3 of [WEB-LINKING]. 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 Section 2.2 of [WEB-LINKING]) 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 Structured Field Parameter keys (see Section 3.1.2 of [STRUCTURED-FIELDS]).¶

For example:¶

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

Implementations MUST support all levels of template defined by [URI-TEMPLATE] in the link String and the anchor Parameter.¶

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.¶

Target attributes that are conveyed via Display Strings can be vulnerable to a wide variety of attacks. See [UNICODE-SECURITY] for advice regarding their handling. Specific advice is not given by this specification, since there are a variety of potential use cases for such attributes.¶

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]¶

5. Normative References

[HTTP]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/rfc/rfc9110>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[STRUCTURED-FIELDS]

Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", Work in Progress, Internet-Draft, draft-ietf-httpbis-sfbis-05, 23 January 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-sfbis-05>.

[UNICODE-SECURITY]

Davis, M. and M. Suignard, "Unicode Security Considerations", Unicode Technical Report #16, 19 September 2014, <http://www.unicode.org/reports/tr36/>.

[URI]

Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <https://www.rfc-editor.org/rfc/rfc3986>.

[URI-TEMPLATE]

Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, March 2012, <https://www.rfc-editor.org/rfc/rfc6570>.

[WEB-LINKING]

Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017, <https://www.rfc-editor.org/rfc/rfc8288>.

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