The Link-Template HTTP Header Field

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 3 October 2024.¶

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

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>.

Author's Address