Building Blocks for HTTP APIs                              M. Nottingham
Internet-Draft                                             28 April 2022
Intended status: Standards Track
Expires: 30 October 2022

                  The Link-Template HTTP Header Field


   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-
   template.html.  Status information for this document may be found at

   Discussion of this document takes place on the Building Blocks for
   HTTP APIs Working Group mailing list (, which
   is archived at

   Source for this draft and an issue tracker can be found at

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   This Internet-Draft will expire on 30 October 2022.

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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  . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   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",
   "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 document uses the Augmented BNF defined in [HTTP] to specify
   valid protocol elements.  Additionally, it uses the modified
   "parameter" rule from [RFC5987] and the "URI-Template" rule from

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2.  The Link-Template Header Field

   The Link-Template header field provides a means for serialising 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.

   Link-Template  = "Link-Template" ":" #templated-link
   templated-link = "<" URI-Template ">" *( ";" parameter )

   For example:

   Link-Template: </{username}>; rel=""

   indicates that a resource with the relation type
   "" can be found by interpolating 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

   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.

   Likewise, the requirements for parameters on templated-links are the
   same as those for a Link header field; in particular, the "rel"
   parameter MUST NOT appear more than once, and if it does, the
   templated-link MUST be ignored by parsers.

   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.

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   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}>;

   indicates that a resource with the relation type
   "" can be found by interpolating the
   "" variable into the template

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

   Link-Template: </widgets/{widget_id}>;

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]

5.  Normative References

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   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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,

              Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,

              Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

Author's Address

   Mark Nottingham

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