HTTP Header Linking

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This document clarifies the status of the Link HTTP header and attempts to consolidate link relations in a single registry.¶

A means of indicating the relationships between documents on the Web has been available for some time in HTML [W3C.REC-html401-19991224], 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.¶

This document seeks to address these shortcomings.¶

Additionally, formats other than HTML -- namely, Atom [RFC4287] -- have also defined generic linking mechanisms that are similar to those in HTML, but not identical. This document aims to reconcile these differences when such links are expressed as headers.¶

[[ NOTE: This is a straw-man draft that is intended to give a ROUGH idea of what it would take to align and consolidate the HTML and Atom link relations into a single registry with reasonable extensibility rules. In particular; a) it changes the registry for Atom link relations, and the process for registration; b) it assigns more generic semantics to several existing link relations, both Atom and HTML; c) it changes the syntax of the Link header (in the case where extensions are present). Feedback is welcome on the ietf-http-wg@w3.org mailing list, although this is NOT a work item of the HTTPBIS WG. ]]¶

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 document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC2616], and explicitly includes the following rules from it: quoted-string, token, SP (space). Additionally, the following rules are included from [RFC3986]: URI-Reference, and from [RFC4288]: type-name.¶

The Link entity-header field provides a means for describing a relationship between two resources, generally between that of the entity associated with the header and some other resource. An entity MAY include multiple Link values.¶

The Link header field is semantically equivalent to the <LINK> element in HTML, as well as the atom:link element in Atom [RFC4287].¶

Relationship values that include a semicolon (";") or comma (",") MUST be quoted.¶

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.¶

This indicates that chapter2 is previous to this resource in a logical navigation path.¶

The content the Link headers is not secure, private or integrity-guaranteed, and due caution should be excercised when using it.¶

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.¶

HTML motivated the original syntax of the Link header, and many of the design decisions in this document are driven by a desire to stay compatible with these uses.¶

All of the link relations defined by HTML4 have been included in the link relation registry, so they can be used without modification. However, extension link relations work differently in HTML4 and the Link header; the former uses a document-wide "profile" URI to scope the relations, while the latter allows the use of full URIs on individual relations.¶

could be represented as a header like this;¶

Profile authors should note this when creating profile URIs; it may be desirable to use URIs that end in a delimiter (e.g., "/" or "#"), to make extracting the specific relation in use easier.¶

HTML defines link relation values as case-insensitive, while the Link header's syntax does not. Therefore, it is important to case-normalise relation values in HTML before comparing or converting them to Link headers.¶

HTML also defines several attributes on links that are not explicitly defined by the Link header. Although most of these are believed to be defunct, they can be used as link-extensions.¶

Atom conveys links in the atom:link element. When serialising an atom:link into a Link header, it is necessary to convert IRIs (if used) to URIs.¶

Additionally, since the base URI for link relations in Link headers is fixed, extension links (i.e,. those not in the registry) MUST be represented as absolute URIs.¶

Note also that while the Link header allows multiple relations to be associated with a single link, atom:link does not. In this case, a single Link header may map to several atom:link elements.¶

As with HTML, atom:link defines some attributes that are not explicitly mirrored in the Link header syntax, but they may also be used as link-extensions.¶

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 link relation registrations themselves are sourced from several documents; see the applicable references.¶

The author would like to thank the many people who commented upon, encouraged and gave feedback to this draft, especially including Frank Ellermann and Julian Reschke.¶

-02¶

• Dropped XLink language.
• Removed 'made' example.
• Removed 'rev'. Can still be used as an extension.
• Added HTML reference to introduction.
• Required relationship values that have a ; or , to be quoted.
• Changed base URI for relation values.
• Noted registry location.
• Added advisory text about HTML profile URIs.
• Disallowed registration of relations that only differ in case.
• Clarified language about IRIs in Atom.
• Added descriptions for 'first', 'last', and 'payment', referring to current IANA registry entries, as these were sourced from e-mail. Will this cause self-referential implosion?
• Explicitly updates RFC4287.
• Added 'type' parameter.
• Removed unnecessary advice about non-HTML relations in HTML section.

-01¶

• Changed syntax of link-relation to one or more URI; dropped Profile.
• Dropped anchor parameter; can still be an extension.
• Removed Link-Template header; can be specified by templates spec or elsewhere.
• Straw-man for link relation registry.

-00¶

• Initial draft; normative text lifted from RFC2068.

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