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Versions: 00 01 02 03                                                   
Network Working Group                                      M. Nottingham
Internet-Draft                                          1 September 2021
Intended status: Standards Track
Expires: 5 March 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.

Note to Readers

   The issues list can be found at https://github.com/mnot/I-D/labels/
   link-template (https://github.com/mnot/I-D/labels/link-template).

   The most recent (often, unpublished) draft is at
   https://mnot.github.io/I-D/link-template/ (https://mnot.github.io/I-

   Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
   pages/link-template (https://github.com/mnot/I-D/commits/gh-pages/

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 5 March 2022.

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Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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="https://example.org/rel/user"

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

   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}>;

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-18, 18 August 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, <https://www.rfc-editor.org/info/rfc8174>.

   [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

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

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