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.

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This Internet-Draft will expire on January 02, 2013.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

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

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).

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:

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.

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

8. References

8.1. Normative References

8.2. Informative References

Author's Address