Internet DRAFT - draft-ietf-httpapi-link-template
draft-ietf-httpapi-link-template
Building Blocks for HTTP APIs M. Nottingham
Internet-Draft 31 January 2024
Intended status: Standards Track
Expires: 3 August 2024
The Link-Template HTTP Header Field
draft-ietf-httpapi-link-template-03
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.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://ietf-wg-
httpapi.github.io/link-template/draft-ietf-httpapi-link-
template.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-httpapi-link-template/.
Discussion of this document takes place on the Building Blocks for
HTTP APIs Working Group mailing list (mailto:httpapi@ietf.org), which
is archived at https://mailarchive.ietf.org/arch/browse/httpapi/.
Subscribe at https://www.ietf.org/mailman/listinfo/httpapi/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-httpapi/link-template.
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 3 August 2024.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 2
2. The Link-Template Header Field . . . . . . . . . . . . . . . 3
2.1. The 'var-base' parameter . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Normative References . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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 specification uses the following terms from [STRUCTURED-FIELDS]:
List, String, Display String, Parameter.
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2. The Link-Template Header Field
The Link-Template header field is a Structured Field
[STRUCTURED-FIELDS] that serializes 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 the link
target and link anchor can contain URI Templates [URI-TEMPLATE].
Its value is a List of Strings. Each String is a URI Template, and
Parameters on it carry associated metadata.
For example:
Link-Template: "/{username}"; rel="item"
indicates that a resource with the relation type "item" for a given
"username" can be found by expanding the "username" variable into the
template given.
The link target (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 link context and link relation type for the link (as defined in
Section 2 of [WEB-LINKING]) are conveyed using the "anchor" and "rel"
Parameters, as they are for the Link header field in Section 3 of
[WEB-LINKING]. Their values MUST be Strings.
However, the "anchor" Parameter MAY contain a URI Template. For
example:
Link-Template: "/books/{book_id}/author";
rel="author"; anchor="#{book_id}"
Here, the link to the author for a particular book in a list of books
can be found by following the link template.
This specification defines additional semantics for the "var-base"
Parameter on templated links; see below.
The link's target attributes (as defined in Section 2.2 of
[WEB-LINKING]) are conveyed using other Parameters, in a manner
similar to the Link header field. These Parameter values MUST be
Strings, unless they contain non-ASCII characters, in which case they
MUST be Display Strings. Note that some target attribute names will
not serialise as Structure Field Parameter keys (see Section 3.1.2 of
[STRUCTURED-FIELDS]).
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For example:
Link-Template: "/author"; rel="author";
title=%"Bj%c3%b6rn J%c3%a4rnsida"
Implementations MUST support all levels of template defined by
[URI-TEMPLATE] in the link String and the anchor Parameter.
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.
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}";
rel="https://example.org/rel/widget";
var-base="https://example.org/vars/"
indicates that a resource with the relation type
"https://example.org/rel/widget" can be found by expanding the
"https://example.org/vars/widget_id" variable into the template
given.
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}";
rel="https://example.org/rel/widget";
var-base="/vars/"
3. Security Considerations
The security consideration for the Link header field in [WEB-LINKING]
and those for URI Templates [URI-TEMPLATE] both apply.
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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
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[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/rfc/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-sfbis-05, 23 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
sfbis-05>.
[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,
<https://www.rfc-editor.org/rfc/rfc3986>.
[URI-TEMPLATE]
Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/rfc/rfc6570>.
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[WEB-LINKING]
Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/rfc/rfc8288>.
Author's Address
Mark Nottingham
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
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