HTTP Header Linking

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This specification clarifies the status of the Link HTTP header and introduces the complimentary Profile and Link-Template HTTP headers.¶

A means of indicating the relationships between documents on the Web has been available for some time in HTML, and was considered as a HTTP header in [RFC2068], but removed from [RFC2616], due to a lack of implementation experience.¶

There have since surfaced many cases where a means of including this information in HTTP headers has proved useful. However, because it was removed, the status of the Link header is unclear, leading some to consider minting new application-specific HTTP headers instead of reusing it.¶

Additionally, the complementary "profile" mechanism -- which is often used to disambiguate link relationship types -- is not available as a HTTP header.¶

This specification seeks to address these shortcomings. It also introduces a new header, Link-Template, that allows the structure of links to be described.¶

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 BCP 14, [RFC2119], as scoped to those conformance targets.¶

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC2616], and explicitly includes the following rules from it: quoted-string, token, SP (space), ALPHA (letters), DIGIT (decimal digit). Additionally, the following rules are included from [RFC3986]: URI-Reference, reserved, unreserved.¶

The Link entity-header field provides a means for describing a relationship between two resources, generally between the requested resource and some other resource. An entity MAY include multiple Link values. Links at the metainformation level typically indicate relationships like hierarchical structure and navigation paths. The Link field is semantically equivalent to the <LINK> element in HTML.¶

Relationship values are case-insensitive and MAY be extended within the constraints of the sgml-name syntax. The title parameter MAY be used to label the destination of a link such that it can be used as identification within a human-readable menu. The anchor parameter MAY be used to indicate a source anchor other than the entire current resource, such as a fragment of this resource or a third resource.¶

The first example indicates that chapter2 is previous to this resource in a logical navigation path. The second indicates that the person responsible for making the resource available is identified by the given e-mail address.¶

The Profile entity-header field provides a means to indicate the meta data profile of the entity. Commonly, it is used to disambiguate the meaning of relationships in links.¶

Note that this URI MAY be used as either an identifier (e.g., to uniquely identify links, without dereferencing the URI), or as a link that is intended to be dereferenced.¶

The Profile field is semantically equivalent to the profile attribute of the HEAD element in HTML [W3C.REC-html401-19991224]. Note, however, that its use is not limited to HTML entities.¶

The Link-Template entity-header field provides a means for describing the structure of a link between two resources, so that new links can be generated.¶

It does so through by allowing brace ("{}") -delimited strings to be interposed throughout a URI reference. These correspond to variables which, after being replaced with content in a relation-specified manner, are semantically equivalent to the corresponding Link header.¶

For example,¶

This link indicates that the "home" link relation can be constructed if the userid variable is known; if it were known to be "bob", this header would be considered equivalent to¶

This specification does not define when or how template variables are interposed into link templates. Link relations that wish to allow templating SHOULD specify such details.¶

This specification does not define the correct behaviour in the face of a conflict between a Link-Template header and a Link header with the same relation. Link relations allowing templating SHOULD specify their relative precedence.¶

Applications SHOULD NOT use link relations that do not explicitly allow such templating in the Link-Template header.¶

This specification requires registration of two Message Header Fields for HTTP [RFC3864]. Note that "Link" is already present in the registry; this registration only updates its specification document.¶

The content of both the Link and Profile headers are not secure, private or integrity-guaranteed, and due caution should be excercised when using them.¶

Applications that take advantage of these mechanisms should consider the attack vectors opened by automatically following, trusting, or otherwise using links gathered from HTTP headers.¶

This specification lifts the definition of the Link header from RFC2068; credit for it belongs entirely to the authors of and contributors to that document.¶

The semantics and much of the syntax of the Profile header was defined by HTML 4.01; credit for them belongs to the authors of and contributors to that document.¶

Joe Gregorio, Marc Hadley and David Orchard contributed to the design of the Link-Template mechanism.¶

Copyright © The Internet Society (2006).¶

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