draft-ietf-httpapi-link-template-02.unpg.txt   draft-ietf-httpapi-link-template-latest.txt 
Building Blocks for HTTP APIs M. Nottingham Building Blocks for HTTP APIs M. Nottingham
Internet-Draft February 1, 2023 Internet-Draft May 26, 2023
Intended status: Standards Track Intended status: Standards Track
Expires: August 5, 2023 Expires: November 27, 2023
The Link-Template HTTP Header Field The Link-Template HTTP Header Field
draft-ietf-httpapi-link-template-02 draft-ietf-httpapi-link-template-latest
Abstract Abstract
This specification defines the Link-Template HTTP header field, This specification defines the Link-Template HTTP header field,
providing a means for describing the structure of a link between two providing a means for describing the structure of a link between two
resources, so that new links can be generated. resources, so that new links can be generated.
About This Document About This Document
This note is to be removed before publishing as an RFC. This note is to be removed before publishing as an RFC.
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This Internet-Draft will expire on November 27, 2023.
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Copyright Notice Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 2
2. The Link-Template Header Field 2. The Link-Template Header Field . . . . . . . . . . . . . . . 3
2.1. The 'var-base' parameter 2.1. The 'var-base' parameter . . . . . . . . . . . . . . . . 4
3. Security Considerations 3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Normative References 5. Normative References . . . . . . . . . . . . . . . . . . . . 5
Author's Address Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction 1. Introduction
[URI-TEMPLATE] defines a syntax for templates that, when expanded [URI-TEMPLATE] defines a syntax for templates that, when expanded
using a set of variables, results in a URI [URI]. using a set of variables, results in a URI [URI].
This specification defines a HTTP header field [HTTP] for conveying This specification defines a HTTP header field [HTTP] for conveying
templates for links in the headers of a HTTP message. It is templates for links in the headers of a HTTP message. It is
complimentary to the Link header field defined in Section 3 of complimentary to the Link header field defined in Section 3 of
[WEB-LINKING], which carries links directly. [WEB-LINKING], which carries links directly.
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[STRUCTURED-FIELDS] that serializes one or more links into HTTP [STRUCTURED-FIELDS] that serializes one or more links into HTTP
message metadata. It is semantically equivalent to the Link header message metadata. It is semantically equivalent to the Link header
field defined in Section 3 of [WEB-LINKING], except that it uses URI field defined in Section 3 of [WEB-LINKING], except that it uses URI
Templates [URI-TEMPLATE] to convey the structure of links. Templates [URI-TEMPLATE] to convey the structure of links.
Its value is a List of Strings. Each String is a URI Template, and Its value is a List of Strings. Each String is a URI Template, and
Parameters on it carry associated metadata. Parameters on it carry associated metadata.
For example: For example:
Link-Template: "/{username}"; rel="https://example.org/rel/user" Link-Template: "/{username}"; rel="item"
indicates that a resource with the relation type
"https://example.org/rel/user" can be found by expanding the
"username" variable into the template given.
The target for the link (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 context, relation type and target attributes for the link are indicates that a resource with the relation type "item" can be found
determined as defined for the Link header field in Section 3 of by expanding the "username" variable into the template given.
[WEB-LINKING].
Parameters on a templated link have identical semantics to those of a The link target (as defined in Section 2 of [WEB-LINKING]) is the
Link header field. This includes (but is not limited to) the use of result of expanding the URI Template [URI-TEMPLATE] (being converted
the "rel" parameter to convey the relation type, the "anchor" to an absolute URI after expansion, if necessary).
parameter to modify the context IRI, and so on. Parameter values
MUST be Strings.
Likewise, the requirements for parameters on templated links are the The link context and link relation type for the link (as defined in
same as those for a Link header field. 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 However, the "anchor" Parameter MAY contain a URI Template. For
example: example:
Link-Template: </books/{book_id}/author>; Link-Template: "/books/{book_id}/author";
rel="author" anchor="#{book_id}" rel="author"; anchor="#{book_id}"
Here, the link to the author for a particular book in a list of books Here, the link to the author for a particular book in a list of books
can be found by following the link template. can be found by following the link template.
Implementations MUST support all levels of template defined by
[URI-TEMPLATE] in both the rel and anchor parameters.
This specification defines additional semantics for the "var-base" This specification defines additional semantics for the "var-base"
parameter on templated links; see below. 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. Note that some target attribute names will not serialise as
Structure Field Parameter keys (see Section 3.1.2 of
[STRUCTURED-FIELDS]).
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 2.1. The 'var-base' parameter
When a templated link has a 'var-base' parameter, its value conveys a 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 URI-reference that is used as a base URI for the variable names in
the URI template. This allows template variables to be globally the URI template. This allows template variables to be globally
identified, rather than specific to the context of use. identified, rather than specific to the context of use.
Dereferencing the URI for a particular variable might lead to more Dereferencing the URI for a particular variable might lead to more
information about the syntax or semantics of that variable; information about the syntax or semantics of that variable;
specification of particular formats for this information is out of specification of particular formats for this information is out of
scope for this document. scope for this document.
To determine the URI for a given variable, the value given is used as 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 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 resulting URI is still relative, the context of the link is used as
the base URI in a further resolution; see [WEB-LINKING]. the base URI in a further resolution; see [WEB-LINKING].
For example: For example:
Link-Template: "/widgets/{widget_id}" Link-Template: "/widgets/{widget_id}";
rel="https://example.org/rel/widget"; rel="https://example.org/rel/widget";
var-base="https://example.org/vars/" var-base="https://example.org/vars/"
indicates that a resource with the relation type indicates that a resource with the relation type
"https://example.org/rel/widget" can be found by expanding the "https://example.org/rel/widget" can be found by expanding the
"https://example.org/vars/widget_id" variable into the template "https://example.org/vars/widget_id" variable into the template
given. given.
If the current context of the message that the header appears within If the current context of the message that the header appears within
is "https://example.org/", the same information could be conveyed by is "https://example.org/", the same information could be conveyed by
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3. Security Considerations 3. Security Considerations
The security consideration for the Link header field in [WEB-LINKING] The security consideration for the Link header field in [WEB-LINKING]
and those for URI Templates [URI-TEMPLATE] both apply. and those for URI Templates [URI-TEMPLATE] both apply.
4. IANA Considerations 4. IANA Considerations
This specification enters the "Link-Template" into the Hypertext This specification enters the "Link-Template" into the Hypertext
Transfer Protocol (HTTP) Field Name Registry. Transfer Protocol (HTTP) Field Name Registry.
Field Name: Link-Template o Field Name: Link-Template
Status: permanent o Status: permanent
Specification document: [this document]
o Specification document: [this document]
5. Normative References 5. Normative References
[HTTP] Fielding, R., Nottingham, M., and J. Reschke, "HTTP [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Semantics", draft-ietf-httpbis-semantics-19 (work in Ed., "HTTP Semantics", STD 97, RFC 9110,
progress), September 2021. DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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