Building Blocks for HTTP APIs
Internet Engineering Task Force (IETF) M. Nottingham
Intended status:
Request for Comments: 9652 September 2024
Category: Standards Track
Expires: 3 October 2024
ISSN: 2070-1721
The Link-Template HTTP Header Field
draft-ietf-httpapi-link-template-04
Abstract
This specification defines the Link-Template HTTP header field,
providing a means for describing the structure of a link between two
resources,
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.
Internet-Drafts are working documents an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 October 2024.
https://www.rfc-editor.org/info/rfc9652.
Copyright Notice
Copyright (c) 2024 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info)
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
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 Parameter
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, and Parameter.
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. Section 2.1.
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 serialize as Structured Field Parameter keys (see Section 3.1.2
of [STRUCTURED-FIELDS]).
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 Parameter
When a templated link has a 'var-base' parameter, Parameter, its value conveys a
URI-reference that is used as a base URI for the variable names in
the URI template. 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 considerations for the Link header field in
[WEB-LINKING] and those for URI Templates [URI-TEMPLATE] both apply.
Target attributes that are conveyed via Display Strings can be
vulnerable to a wide variety of attacks. See [UNICODE-SECURITY] for
advice regarding their handling. Specific advice is not given by
this specification, specification since there are a variety of potential use cases
for such attributes.
4. IANA Considerations
This specification enters the "Link-Template" field name into the Hypertext
"Hypertext Transfer Protocol (HTTP) Field Name Registry.
* Registry".
+===============+===========+===============+
| Field Name: Name | Status | Reference |
+===============+===========+===============+
| Link-Template
* Status: permanent
* Specification document: [this document] | Permanent | This document |
+---------------+-----------+---------------+
Table 1
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>.
<https://www.rfc-editor.org/info/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>.
<https://www.rfc-editor.org/info/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>. <https://www.rfc-editor.org/info/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. P.-H. Kamp, "Structured Field Values
for HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-sfbis-05, 23 January RFC 9651, DOI 10.17487/RFC9651, September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
sfbis-05>.
<https://www.rfc-editor.org/info/rfc9651>.
[UNICODE-SECURITY]
Davis, M. and M. Suignard, "Unicode Security
Considerations", Unicode Technical Report #16, 19
September 2014, <http://www.unicode.org/reports/tr36/>. <https://www.unicode.org/reports/tr36/>.
Latest version available at
<https://www.unicode.org/reports/tr36/>.
[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>.
<https://www.rfc-editor.org/info/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>.
<https://www.rfc-editor.org/info/rfc6570>.
[WEB-LINKING]
Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/rfc/rfc8288>.
<https://www.rfc-editor.org/info/rfc8288>.
Author's Address
Mark Nottingham
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/