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Network Working Group                                      M. Nottingham
Internet-Draft                                             June 19, 2017
Intended status: Standards Track
Expires: December 21, 2017

                  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 .

   The most recent (often, unpublished) draft is at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

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

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
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   This Internet-Draft will expire on December 21, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   2
   2.  The Link-Template Header Field  . . . . . . . . . . . . . . .   2
     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

   [RFC6570] defines a syntax for templates that, when expanded using a
   set of variables, results in a URI [RFC3986].

   This specification defines a HTTP header field for conveying
   templates for links in the headers of a HTTP message.  It is
   complimentary to the Link header field [RFC5988], which carries links

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document uses the Augmented BNF defined in [RFC7230] to specify
   valid protocol elements.  Additionally, it uses the modified
   "parameter" rule from [RFC5987], and the "URI-Template" rule from

2.  The Link-Template Header Field

   The Link-Template entity-header field provides a means for
   serialising one or more links into HTTP headers.  It is semantically
   equivalent to the Link header field [RFC5988], except that it uses
   URI Templates [RFC6570] to convey the structure of links.

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     Link-Template  = "Link-Template" ":" #linkt-value
     linkt-value    = "<" URI-Template ">" *( ";" parameter )

   For example:

   Link-Template: </{username}>; rel="http://example.org/rel/user"

   indicates that a resource with the relation type
   "http://example.org/rel/user" can be found by interpolating the
   "username" variable into the template given.

   The target for the link (as defined by [RFC5988]) is the result of
   expanding the URI Template [RFC6570] (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 [RFC5988].

   The parameters on a linkt-value have identical semantics to those of
   a Link header field [RFC5988].  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 linkt-values 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 linkt-
   value MUST be ignored by parsers.

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

2.1.  The 'var-base' parameter

   When a linkt-value 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 mechanism 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 [RFC3986]).  If
   the resulting URI is still relative, the context of the link is used
   as the base URI in a further resolution; see [RFC5988].

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   For example:

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

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

   If the current context of the message that the header appears within
   is "http://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 [RFC5988] and
   those for URI Templates [RFC6570] both apply.

4.  IANA Considerations

   This specification enters the "Link-Template" into the registry of
   Permanent Message Header Field Names.

   Header Field Name: Link-Template
   Protocol: http
   Reference: [this document]

5.  Normative References

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

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

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,

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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

Author's Address

   Mark Nottingham

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

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