| Network Working Group | M. Nottingham |
| Internet-Draft | January 19, 2010 |
| Updates: 4287 (if approved) | |
| Intended status: Standards Track | |
| Expires: July 23, 2010 |
This document specifies relation types for Web links, and defines a registry for them. It also defines the use of such links in HTTP headers with the Link header-field.¶
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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 their serialisation, 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 application. It does so by re-defining the link relation registry established by Atom to have a broader domain, 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 as one such serialisation, 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), LOALPHA, DIGIT.¶
Additionally, the following rules are included from [RFC3986]: URI and URI-Reference; from [RFC4288]: type-name and subtype-name; from [W3C.REC-html401-19991224]: MediaDesc, and from [RFC4646]: Language-Tag.¶
In this specification, a link is a typed connection between two resources that are identified by IRIs [RFC3987], and is comprised of: ¶
A link can be viewed as a statement of the form "{context IRI} has a {relation type} resource at {target IRI}, which has {target attributes}."¶
Note that in the common case, the context IRI will also be a URI [RFC3986], because many protocols (such as HTTP) do not support dereferencing IRIs. Likewise, the target IRI will be converted to a URI (see [RFC3987], Section 3.1) 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. Likewise, the relative ordering of links in any particular serialisation, or between serialisations (e.g., the Link header and in-content links) is not specified or significant in this specification; applications that wish to consider ordering significant can do so.¶
Target attributes are a set of key/value pairs that describe the link or its target; for example, a media type hint. This specification does not attempt to coordinate their names or use, but does provide common target attributes for use in the Link HTTP header.¶
Finally, this specification does not define a general syntax for expressing links, nor mandate a specific context for any given link; it is expected that serialisations of links will specify both aspects. One such serialisation is communication of links through HTTP headers, specified in Section 5.¶
In the simplest case, 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.¶
Link relation types can also be used to indicate that the target resource has particular attributes, or exhibits particular behaviours; for example, a "service" link implies that the identified resource is part of a defined protocol (in this case, a service description).¶
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.¶
Relation types SHOULD NOT infer any additional semantics based upon the presence or absence of another link relation type, or its own cardinality of occurrence. An exception to this is the combination of the "alternate" and "stylesheet" registered relation types, which has special meaning in HTML4 for historical reasons.¶
There are two kinds of relation types: registered and extension.¶
Well-defined relation types can be registered as tokens for convenience and/or to promote reuse by other applications. This specification establishes an IANA registry of such relation types; see Section 6.2.¶
Registered relation type names MUST conform to the reg-relation-type rule, and MUST be compared character-by-character in a case-insensitive fashion. They SHOULD be appropriate to the specificity of the relation type; i.e., if the semantics are highly specific to a particular application, the name should reflect that, so that more general names are available for less specific use.¶
Registered relation types MUST NOT constrain the media type of the context IRI, and MUST NOT constrain the available representation media types of the target IRI. However, they MAY specify the behaviours and properties of the target resource (e.g., allowable methods, request and response media types which must be supported).¶
Additionally, specific applications of linking may have additional per-relation type attributes which are advantageous to register. For example, some link relations might not be appropriate to use in particular contexts, or might have common behaviour such as whether their content should be archived with the page. To accommodate this, new per-entry fields MAY be added to the registry, by registering them in the Link Relation Field Registry Section 6.3.¶
Applications that don't wish to register a relation type may use an extension relation type, which is a URI [RFC3986] that uniquely identifies the relation type. Although the URI can point to a resource that contains a definition of the semantics of the relation type, clients SHOULD NOT automatically access that resource to avoid overburdening its server.¶
When extension relation types are compared, they MUST be compared as URIs in a case-insensitive fashion, character-by-character. Because of this, all-lowercase URIs SHOULD be used for extension relations.¶
Note that while extension relation types are required to be URIs, a serialisation of links MAY specify that they are expressed in another form, as long as they can be converted to URIs.¶
The Link entity-header field provides a means for serialising 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].¶
Link = "Link" ":" #link-value
link-value = "<" URI-Reference ">" *( ";" link-param )
link-param = ( ( "rel" "=" relation-types )
| ( "anchor" "=" <"> URI-Reference <"> )
| ( "rev" "=" relation-types )
| ( "hreflang" "=" Language-Tag )
| ( "media" "=" ( MediaDesc | <"> MediaDesc <"> ) )
| ( "title" "=" quoted-string )
| ( "title*" "=" enc2231-string )
| ( "type" "=" type-name "/" subtype-name )
| ( link-extension ) )
link-extension = token [ "=" ( token | quoted-string ) ]
enc2231-string = <extended-initial-value [RFC2231] Section 7>
relation-types = relation-type |
<"> relation-type *( 1*SP relation-type ) <">
relation-type = reg-relation-type | ext-relation-type
reg-relation-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
ext-relation-type = URI
Each link-value conveys one target IRI as a URI-Reference (after conversion to one, if necessary; see [RFC3987], Section 3.1) inside angle brackets ("<>"). If the URI-Reference is relative, parsers MUST resolve it as per [RFC3986], Section 5. Note that any base IRI from the message's content is not applied.¶
By default, the context of a link conveyed in the Link header field is the IRI of the requested resource.¶
When present and explicitly specified by use by an application, 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). If the anchor parameter's value is a relative URI, parsers MUST resolve it as per [RFC3986], Section 5. Note that any base URI from the body's content is not applied.¶
The anchor parameter MUST be ignored by consuming implementations, unless its use is specified by the application in use.¶
The relation type of a link is conveyed in the "rel" parameter's value. Note that the "rev" parameter has also been used by some formats, and MAY be accommodated as a link-extension, but its use is neither encouraged nor defined by this specification.¶
The "rel" parameter MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by parsers.¶
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 "hreflang", "media", "title", "title*", "type" and any link-extension link-params are considered to be target attributes for the link.¶
The "hreflang" parameter, when present, is a hint indicating what the language of the result of dereferencing the link should be. Note that this is only a hint; for example, it does not override the Content-Language header of a HTTP response obtained by actually following the link. Multiple hreflang parameters on a single link-value indicate that multiple languages are available from the indicated resource.¶
The "media" parameter, when present, is used to indicate intended destination medium or media for style information (see [W3C.REC-html401-19991224], Section 6.13. Note that this may be updated by [W3C.CR-css3-mediaqueries-20090915]). Its value MUST be quoted if it contains a semicolon (";") or comma (","), and there MUST NOT be more than one media parameter in a link-value.¶
The "title" parameter, when present, 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 MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by parsers.¶
The "title*" parameter MAY be used encode this label in a different 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'. The "title*" parameter MAY appear more than once in a given link-value, but each occurrence MUST indicate a different language; occurrences after the first for a given language MUST be ignored by parsers.¶
When presenting links to users, agents SHOULD use the most appropriate "title*" value, according to user preferences. If an appropriate "title*" value cannot be found, the "title" parameter's value, if available, can be used.¶
The "type" parameter, when present, is a hint indicating what the media type of the result of dereferencing the link should be. Note that this is only a hint; for example, it does not override the Content-Type header of a HTTP response obtained by actually following the link. There MUST NOT be more than one type parameter in a link-value.¶
For example:¶
Link: <http://example.com/TheBook/chapter2>; rel="previous";
title="previous chapter"
indicates that "chapter2" is previous to this resource in a logical navigation path.¶
Similarly,¶
Link: </>; rel="http://example.net/foo"
indicates that the root resource ("/") is related to this resource with the extension relation type "http://example.net/foo".¶
The example below shows an instance of the Link header encoding multiple links, and also the use of RFC 2231 encoding to encode both non-ASCII characters and language information.¶
Link: </TheBook/chapter2>;
rel="previous"; title*=UTF-8'de'letztes%20Kapitel,
</TheBook/chapter4>;
rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel
␉␉␉␉Here, both links have titles encoded in UTF-8, use the German language ("de"), and the second link contains the Unicode code point U+00E4 ("LATIN SMALL LETTER A WITH DIAERESIS").¶
Note that link-values may convey multiple links between the same target and context IRIs; for example:¶
Link: <http://example.org/>;
rel="start http://example.net/relation/other"
Here, the link to "http://example.org/" has the registered relation type "start" and the extension relation type "http://example.net/relation/other".¶
This specification updates the Message Header Registry entry for "Link" in HTTP [RFC3864] to refer to this document.¶
Header field: Link
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[ this document ]
This specification establishes the Link Relation Type Registry, and updates Atom [RFC4287] to refer to it in place of the "Registry of Link Relations".¶
[[ Note to IESG: Entries in the Atom registry that are not listed below at the time that IANA implements this change (i.e., those that are registered before this document comes into effect) should be referred to the Designated Expert. ]]¶
Relation types are registered on the advice of a Designated Expert (appointed by the IESG or their delegate), with a Specification Required (using terminology from [RFC5226]).¶
The requirements for registered relation types are described in Section 4.1.¶
Registration requests consist of the completed registration template below, typically published in an RFC or Open Standard (in the sense described by [RFC2026], Section 7). However, to allow for the allocation of values prior to publication, the Designated Expert may approve registration once they are satisfied that a specification will be published.¶
The registration template is:¶
Registration requests should be sent to the [TBD]@ietf.org mailing list, marked clearly in the subject line (e.g,. "NEW RELATION REQUEST").¶
Within at most 14 days of the request, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. Registration requests that are undetermined for a period longer than 21 days can be brought to the IESG's attention (using the iesg@iesg.org mailing list) for resolution.¶
When a registration request is successful, the Designated Expert(s) will update the registry XML file (using the format described in Appendix A and send it to the [TBD-2]@ietf.org mailing list (which SHOULD NOT be centrally archived, and only accept posts from the Designated Expert(s)), so that implementers interested in receiving a machine-readable registry can do so. Simultaneously, they will send a text (not XML) version of the registry to IANA for publication.¶
IANA should only accept registry updates from the Designated Expert(s), and should direct all requests for registration to the review mailing list.¶
The Link Relation Type registry's initial contents are:¶
This specification also establishes the Link Relation Field Registry, to allow entries in the Link Relation Type Registry to be extended with application-specific data (hereafter, "fields").¶
Fields are registered on the advice of a Designated Expert (appointed by the IESG or their delegate), with a Specification Required (using terminology from [RFC5226]).¶
Registration requests consist of the completed registration template below;¶
The Description SHOULD identify the value space of the field. The Default Value MUST be appropriate to entries which the field does not apply to.¶
Entries that pre-date the addition of a field will automatically be considered to have the default value for that field; if there are exceptions, the modification of such entries should be coordinated by the Designated Expert(s), in consultation with the author of the proposed field as well as the registrant of the existing entry (if possible).¶
Registration requests should be sent to the [TBD]@ietf.org mailing list, marked clearly in the subject line (e.g,. "NEW EXTENSION FIELD").¶
Within at most 14 days of the request, the Designated Expert will either approve or deny the registration request, communicating this decision to the review list. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. Registration requests that are undetermined for a period longer than 21 days MAY be brought to the IESG's attention (using the iesg@iesg.org mailing list) for resolution.¶
When a registration request is successful, the Designated Expert will forward it to IANA for publication. IANA should only accept registry updates from the Designated Expert(s), and should direct all requests for registration to the review mailing list.¶
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 express them in serialisations 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.¶
To facilitate applications that wish to use registry data, this specification defines an XML-based format for the registry entries.¶
Each registered relation type is represented by a RelationType element, and if any of the Field values are other than the default value identified in the Field Registry, they will be represented by field elements.¶
Note that this format is NOT that which IANA publishes the registry in, because doing so would subject IANA's servers to, potentially, very high load (e.g., if Web browsers were to automatically update their copies of the registry). Instead, this format is published to the [TBD-2]@ietf.org mailing list, so that interested implementors can subscribe and distribute the machine-readable document using their own infrastructure.¶
element RelationTypes {
element RelationType {
attribute name { text },
attribute reference { text },
element description { text },
element notes { text }?,
element field {
attribute name { text },
text
}*
}+
}
<RelationTypes>
<RelationType name="example"
reference="http://www.example.org/example_spec">
<description>This is an example relation type.</description>
<field name="foo">This is the value of the Foo field.</field>
</RelationType>
<!-- ... -->
</RelationTypes>
␉␉␉␉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 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 does not define a corresponding "rev" parameter to allow the expression of these links in HTTP headers, due to the confusion this mechanism causes as well as conflicting interpretations (briefly, some hold that rev reverses the direction of the link, while others that it reverses the semantics of the relation itself).¶
All of the link relation types defined by HTML4 have been included in the link relation type registry, so they can be used without modification. However, there are several potential ways to serialise extension relation types into HTML4, including¶
Individual applications of linking will therefore need to define how their extension links should be serialised into HTML4.¶
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.¶
Finally, the HTML4 specification gives a special meaning when the "alternate" and "stylesheet" relation types coincide in the same link. Such links should be serialised in the Link header using a single list of relation-types (e.g., rel="alternate stylesheet") to preserve this relationship.¶
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 obvious 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 re-defines them as URIs, to simplify and reduce errors in their comparison.¶
Atom allows registered link relation types to be serialised as absolute URIs. 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.¶
Furthermore, Atom link relation types are always compared in a case-sensitive fashion; therefore, registered link relation types SHOULD be converted to their registered form (usually, lower case) when serialised in an Atom document.¶
Note also that while the Link header allows multiple relations to be serialised in 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 to maintain fidelity.¶
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 type 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 specification, especially including Frank Ellermann, Roy Fielding, Eran Hammer-Lahav, and Julian Reschke.¶
[[ to be removed by the RFC editor before publication as an RFC. ]]¶
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