



Internet Engineering Task Force (IETF)                     M. Nottingham
Request for Comments: 9652                                September 2024
Category: Standards Track                                               
ISSN: 2070-1721


                  The Link-Template HTTP Header Field

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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9652.

Copyright Notice

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

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   in the Revised BSD License.

Table of Contents

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

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, Display String, and Parameter.

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].

   The Link-template header field's value is a List of Strings (see
   [STRUCTURED-FIELDS]).  Each String contains a URI Template and can
   have Parameters that carry metadata associated with that template.

   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 (see Section 2 of [WEB-LINKING]) is determined by
   expanding the template and converting it to an absolute URI (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 Section 2.1.

   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 serialize 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.

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 considerations for the Link header field in
   [WEB-LINKING] and those for URI Templates [URI-TEMPLATE] 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" field name into the
   "Hypertext Transfer Protocol (HTTP) Field Name Registry".

               +===============+===========+===============+
               | Field Name    | Status    | Reference     |
               +===============+===========+===============+
               | Link-Template | Permanent | This document |
               +---------------+-----------+---------------+

                                  Table 1

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/info/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/info/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/info/rfc8174>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,
              <https://www.rfc-editor.org/info/rfc9651>.

   [UNICODE-SECURITY]
              Davis, M. and M. Suignard, "Unicode Security
              Considerations", Unicode Technical Report #16, 19
              September 2014, <https://www.unicode.org/reports/tr36/>.
              Latest version available at
              <https://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/info/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/info/rfc6570>.

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

Author's Address

   Mark Nottingham
   Prahran
   Australia
   Email: mnot@mnot.net
   URI:   https://www.mnot.net/
