Prahran Australia mnot@mnot.net https://www.mnot.net/

WIT httpapi link relation 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 is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
  • .  Notational Conventions
• .  The Link-Template Header Field
  • .  The 'var-base' Parameter
• .  Security Considerations
• .  IANA Considerations
• .  Normative References
• Author's Address

Introduction defines a syntax for templates that, when expanded using a set of variables, results in a URI . 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 defined in , which carries links directly.

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 when, and only when, they appear in all capitals, as shown here. This specification uses the following terms from : List, String, Display String, and Parameter.

The Link-Template Header Field The Link-Template header field is a Structured Field that serializes one or more links into HTTP message metadata. It is semantically equivalent to the Link header field defined in , except that the link target and link anchor can contain URI Templates . The Link-template header field's value is a List of Strings (see ). Each String contains a URI Template and can have Parameters that carry metadata associated with that template. 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 (see ) is determined by expanding the template and converting it to an absolute URI (if necessary). The link context and link relation type for the link (as defined in ) are conveyed using the 'anchor' and 'rel' Parameters, as they are for the Link header field in . 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 . The link's target attributes (as defined in ) 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 serialize as Structured Field Parameter keys (see ). For example: Link-Template: "/author"; rel="author"; title=%"Bj%c3%b6rn J%c3%a4rnsida" Implementations MUST support all levels of template defined by in the link String and the 'anchor' Parameter.

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 ). If the resulting URI is still relative, the context of the link is used as the base URI in a further resolution; see . 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/"

Security Considerations The security considerations for the Link header field in and those for URI Templates apply. Target attributes that are conveyed via Display Strings can be vulnerable to a wide variety of attacks. See for advice regarding their handling. Specific advice is not given by this specification since there are a variety of potential use cases for such attributes.

IANA Considerations This specification enters the "Link-Template" field name into the "Hypertext Transfer Protocol (HTTP) Field Name Registry".

Field Name	Status	Reference
Link-Template	Permanent	This document

Normative References The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Cloudflare The Varnish Cache Project Latest version available at . A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] A URI Template is a compact sequence of characters for describing a range of Uniform Resource Identifiers through variable expansion. This specification defines the URI Template syntax and the process for expanding a URI Template into a URI reference, along with guidelines for the use of URI Templates on the Internet. [STANDARDS-TRACK] This specification defines a model for the relationships between resources on the Web ("links") and the type of those relationships ("link relation types"). It also defines the serialisation of such links in HTTP headers with the Link header field.

Author's Address

Prahran Australia mnot@mnot.net https://www.mnot.net/