Network Working Group M. Nottingham
Internet-Draft June 19, 2017
Intended status: Standards Track
Expires: December 21, 2017
The Link-Template HTTP Header Field
draft-nottingham-link-template-02
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.
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The most recent (often, unpublished) draft is at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
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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
directly.
1.1. Notational Conventions
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].
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
[RFC6570].
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}>;
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 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}>;
rel="http://example.org/rel/widget";
var-base="/vars/"
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
Status:
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,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
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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,
<http://www.rfc-editor.org/info/rfc5987>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<http://www.rfc-editor.org/info/rfc6570>.
[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,
<http://www.rfc-editor.org/info/rfc7230>.
Author's Address
Mark Nottingham
EMail: mnot@mnot.net
URI: https://www.mnot.net/
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