The Link-Template HTTP Header Field
draft-ietf-httpapi-link-template-02

Building Blocks for HTTP APIs                              M. Nottingham
Internet-Draft                                           1 February 2023
Intended status: Standards Track                                        
Expires: 5 August 2023

                  The Link-Template HTTP Header Field
                  draft-ietf-httpapi-link-template-02

Abstract

   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.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   httpapi.github.io/link-template/draft-ietf-httpapi-link-
   template.html.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-httpapi-link-template/.

   Discussion of this document takes place on the Building Blocks for
   HTTP APIs Working Group mailing list (mailto:httpapi@ietf.org), which
   is archived at https://mailarchive.ietf.org/arch/browse/httpapi/.
   Subscribe at https://www.ietf.org/mailman/listinfo/httpapi/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-httpapi/link-template.

Status of This Memo

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

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   This Internet-Draft will expire on 5 August 2023.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   2
   2.  The Link-Template Header Field  . . . . . . . . . . . . . . .   3
     2.1.  The 'var-base' parameter  . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

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.

1.1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification uses the following terms from [STRUCTURED-FIELDS]:
   List, String, Parameter.

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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.

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

2.1.  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 [URI]).  If the
   resulting URI is still relative, the context of the link is used as
   the base URI in a further resolution; see [WEB-LINKING].

   For example:

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

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

   If the current context of the message that the header appears within
   is "https://example.org/", the same information could be conveyed by
   this header field:

   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.

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   Field Name: Link-Template
   Status: permanent
   Specification document: [this document]

5.  Normative References

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [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-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8941>.

   [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

   Mark Nottingham
   Prahran
   Australia
   Email: mnot@mnot.net

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   URI:   https://www.mnot.net/

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