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Versions: 00 01

Network Working Group                                      M. Nottingham
Internet-Draft                                                 Rackspace
Intended status: Informational                              July 3, 2012
Expires: January 4, 2013


                  The Link-Template HTTP Header Field
                   draft-nottingham-link-template-00

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 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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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on January 4, 2013.

Copyright Notice

   Copyright (c) 2012 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
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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 3
   4.  The Link-Template Header Field  . . . . . . . . . . . . . . . . 3
   5.  The 'var-base' parameter  . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6






































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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
   templated links in the headers of a HTTP message.  It is
   complimentary to the Link header field [RFC5988], which carries links
   directly.


2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  Notational Conventions

   This document uses the Augmented BNF defined in [RFC2616] to specify
   valid protocol elements.

   Additionally, it uses the modified "parameter" rule from [RFC5987],
   and the "URI-Template" rule from [RFC6570].


4.  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.

     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).



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   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.


5.  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].

   For example:

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

   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 link is "http://example.org/", the same
   information could be conveyed by this header field:





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


6.  Security Considerations

   The security consideration for the Link header field in [RFC5988] and
   those for URI Templates [RFC6570] both apply.


7.  IANA Considerations

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

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


8.  References

8.1.  Normative References

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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

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

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

8.2.  Informative References

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




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Author's Address

   Mark Nottingham
   Rackspace

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












































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