Link Relations and HTTP Header Linking

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This document specifies relation types for Web links, and defines a registry for them. It also defines how to send such links in HTTP headers with the Link header-field.¶

A means of indicating the relationships between resources on the Web, as well as indicating the type of those relationships, has been available for some time in HTML [W3C.REC-html401-19991224], and more recently in Atom [RFC4287]. These mechanisms, although conceptually similar, are separately specified. However, links between resources need not be format-specific; it can be useful to have typed links that are independent of the format, especially when a resource has representations in multiple formats.¶

To this end, this document defines a framework for typed links that isn't specific to a particular serialisation or context of use. It does so by re-defining the link relation registry established by Atom to have a broader scope, and adding to it the relations that are defined by HTML.¶

Furthermore, an HTTP header-field for conveying typed links was defined in [RFC2068], but removed from [RFC2616], due to a lack of implementation experience. Since then, it has been implemented in some User-Agents (e.g., for stylesheets), and several additional use cases have surfaced. 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 addresses this by re-specifying the Link header with updated but backwards-compatible syntax.¶

[[ 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 and URI-Reference, and from [RFC4288]: type-name.¶

In this specification, a link is a typed connection between two resources that are identified by IRIs [RFC3987], and is comprised of: ¶

• A context IRI, and
• A link relation type (Section 4), and
• A target IRI.

A link can be viewed as a statement of the form "(context IRI) has a (relation type) resource at (target IRI)."¶

Note that in the common case, the context IRI will also be a URI [RFC3986], because common protocols (such as HTTP) do not support dereferencing IRIs. Likewise, the target IRI will be converted to a URI in serialisations that do not support IRIs (e.g., the Link header).¶

This specification does not place restrictions on the cardinality of links; there can be multiple links from and to a particular IRI, and multiple links of different types between two given IRIs.¶

Additionally, this specification does not define a general syntax for expressing links, nor mandate a specific context for any given link; it is expected that applications of links will specify both aspects. One such application is communication of links through HTTP headers, specified in Section 5.¶

Such applications may further constrain or extend links (e.g., associating a media type hint).¶

A link relation type identifies the semantics of a link. For example, a link with the relation type "copyright" indicates that the resource identified by the target IRI is a statement of the copyright terms applying to the current context IRI.¶

Relation types are not to be confused with media types [RFC4288]; they do not identify the format of the representation that results when the link is dereferenced. Rather, they only describe how the current context is related to another resource.¶

As such, relation types are not format-specific, and MUST NOT specify a particular format or media type that they are to be used with. Likewise, the context IRI for a given link is usually determined by the serialisation of the link (e.g., the Link header, a HTML document, etc.); a relation type SHOULD NOT specify the context IRI.¶

Consuming implementations SHOULD ignore relation types that they do not understand or have no need to process.¶

There are two kinds of relation types; registered and extension.¶

The Link entity-header field provides a means for conveying one or more links in HTTP headers. It is semantically equivalent to the <LINK> element in HTML, as well as the atom:link feed-level element in Atom [RFC4287].¶

For example:¶

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

Each link-value conveys one target IRI as a URI-Reference (after conversion, if necessary) inside angle brackets ("<>"). If the URI-Reference is relative, it MUST be resolved as per [RFC3986]. Note that any base IRI from the body's content is not applied.¶

By default, the context of a link conveyed in the Link header field is the IRI associated with the representation it occurs in. When present, the anchor parameter overrides this with another URI, such as a fragment of this resource, or a third resource (i.e., when the anchor value is an absolute URI).¶

Normally, the relation type of a link is conveyed in the "rel" parameter's value. The "rev" parameter has also been used for this purpose historically by some formats, and is included here for compatibility with those uses, but its use is not encouraged nor defined by this specification.¶

Note that extension relation types are REQUIRED to be absolute URIs in Link headers, and MUST be quoted if they contain a semicolon (";") or comma (",").¶

The title parameter is used to label the destination of a link such that it can be used as a human-readable identifier (e.g. a menu entry). The title* parameter MAY be used to instead to encode this label in an alternate character set, and/or contain language information as per [RFC2231]. When using the enc2231-string syntax, producers MUST NOT use a charset value other than 'ISO-8859-1' or 'UTF-8'.¶

Note that link-values may convey multiple links between the same target and context IRIs; for example¶

Here, the link to "http://example.org/" has the registered relation types "index" and "start", and the extension relation type "http://example.net/relation/other".¶

The content of the Link header-field is not secure, private or integrity-guaranteed, and due caution should be exercised when using it.¶

Applications that take advantage of typed links should consider the attack vectors opened by automatically following, trusting, or otherwise using links gathered from HTTP headers. In particular, Link headers that use the "anchor" parameter to associate a link's context with another resource should be treated with due caution.¶

Target IRIs may need to be converted to URIs in order to serialise them in applications that do not support IRIs. This includes the Link HTTP header.¶

Similarly, the anchor parameter of the Link header does not support IRIs, and therefore IRIs must be converted to URIs before inclusion there.¶

Relation types are defined as URIs, not IRIs, to aid in their comparison. It is not expected that they will be displayed to end users.¶

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.¶

In HTML4, the link element can be mapped to links as specified here by using the "href" attribute for the target URI, and "rel" to convey both the relation type, as in the Link header. The context of the link is the URI associated with the entire HTML document.¶

HTML4 also has a "rev" parameter for links that allows a link's relation to be reversed. The Link header has a "rev" parameter to allow the expression of these links in HTTP headers, but its use is not encouraged, due to the confusion this mechanism causes as well as conflicting interpretations among HTML versions.¶

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.¶

Therefore, when using the profile mechanism in HTML4, it is necessary to map the profiled link relations to URIs when expressed in Link headers. For example, in HTML:¶

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.¶

Surveys of existing HTML content have shown that unregistered link relation types that are not URIs are (perhaps inevitably) common. Consuming HTML implementations should not consider such unregistered short links to be errors, but rather relation types with a local scope (i.e., their meaning is specific and perhaps private to that document).¶

HTML4 also defines several attributes on links that are not explicitly defined by the Link header. These attributes can be serialised as link-extensions to maintain fidelity.¶

Atom conveys links in the atom:link element, with the "href" attribute indicating the target IRI and the "rel" attribute containing the relation type. The context of the link is either a feed IRI or an entry ID, depending on where it appears; generally, feed-level links are candidates for transmission as a Link header.¶

When serialising an atom:link into a Link header, it is necessary to convert target IRIs (if used) to URIs.¶

Atom defines extension relation types in terms of IRIs. This specification defines them as URIs, to aid in their comparison.¶

Atom allows registered link relation types to be serialised as absolute URIs, because a base URI is defined for the registry. Such relation types SHOULD be converted to the appropriate registered form (e.g., "http://www.iana.org/assignments/relation/self" to "self") so that they are not mistaken for extension relation types.¶

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-value 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 idea and definition for 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, Roy Fielding and Julian Reschke.¶

[[ to be removed by the RFC editor before publication as an RFC. ]]¶

-04¶

• Defined context as a resource, rather than a representation.
• Removed concept of link directionality; relegated to a deprecated Link header extension.
• Relation types split into registered (non-URI) and extension (URI).
• Changed wording around finding URIs for registered relation types.
• Changed target and context URIs to IRIs (but not extension relation types).
• Add RFC2231 encoding for title parameter, explicit BNF for title*.
• Add i18n considerations.
• Specify how to compare relation types.
• Changed registration procedure to Designated Expert.
• Softened language around presence of relations in the registry.
• Added describedby relation.
• Re-added 'anchor' parameter, along with security consideration for third-party anchors.
• Softened language around HTML4 attributes that aren't directly accommodated.
• Various tweaks to abstract, introduction and examples.

-03¶

• Inverted focus from Link headers to link relations.
• Specified was a link relation type is.
• Based on discussion, re-added 'rev'.
• Changed IESG Approval to IETF Consensus for relation registrations (i.e., require a document).
• Updated RFC2434 reference to RFC5226.
• Registered relations SHOULD conform to sgml-name.
• Cautioned against confusing relation types with media types.

-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.