One document matched: draft-nottingham-http-link-header-07.xml
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<rfc category="std" docName="draft-nottingham-http-link-header-07" ipr="pre5378Trust200902" updates="4287">
<front>
<title>Web Linking</title>
<author fullname="Mark Nottingham" initials="M." surname="Nottingham">
<organization />
<address>
<email>mnot@mnot.net</email>
<uri>http://www.mnot.net/</uri>
</address>
</author>
<date year="2010" />
<abstract>
<t>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. </t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>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 <xref
target="W3C.REC-html401-19991224" />, and more recently in Atom <xref target="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.</t>
<t>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.</t>
<t>Furthermore, an HTTP header-field for conveying typed links was defined in <xref
target="RFC2068" />, but removed from <xref target="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.</t>
<t>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.</t>
<t>[[ Feedback is welcome on the ietf-http-wg@w3.org mailing list, although this is NOT a work
item of the HTTPBIS WG. ]]</t>
</section>
<section title="Notational Conventions">
<t>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, <xref target="RFC2119" />, as scoped to those conformance targets.</t>
<t>This document uses the Augmented Backus-Naur Form (ABNF) notation of <xref target="RFC2616"
/>, and explicitly includes the following rules from it: quoted-string, token, SP (space),
LOALPHA, DIGIT.</t>
<t>Additionally, the following rules are included from <xref target="RFC3986" />: URI and
URI-Reference; from <xref target="RFC4288" />: type-name and subtype-name; from <xref
target="W3C.REC-html401-19991224" />: MediaDesc, and from <xref target="RFC4646"/>: Language-Tag.</t>
</section>
<section title="Links" anchor="links">
<t>In this specification, a link is a typed connection between two resources that are identified by IRIs <xref target="RFC3987"/>, and is comprised of: <list style="symbols">
<t>A context IRI, and</t>
<t>a link relation type (<xref target="link-relationship-types" />), and</t>
<t>a target IRI, and</t>
<t>optionally, target attributes.</t>
</list>
</t>
<t>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}."</t>
<t>Note that in the common case, the context IRI will also be a URI <xref target="RFC3986"/>, because many protocols
(such as HTTP) do not support dereferencing IRIs. Likewise, the target IRI will be converted to a URI (see
<xref target="RFC3987"/>, Section 3.1) in serialisations that do not support IRIs (e.g., the Link header).</t>
<t>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.</t>
<t>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.</t>
<t>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 <xref target="header.link" />.</t>
</section>
<section anchor="link-relationship-types" title="Link Relation Types">
<t>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.</t>
<t>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).</t>
<t>Relation types are not to be confused with media types <xref target="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.</t>
<t>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.</t>
<t>There are two kinds of relation types: registered and extension.</t>
<section title="Registered Relation Types" anchor="registered-relation-types">
<t>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 <xref target="link-relation-registry"/>.</t>
<t>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.</t>
<t>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).</t>
<t>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 <xref target="field_registry"/>.</t>
</section>
<section title="Extension Relation Types">
<t>Applications that don't wish to register a relation type may use an extension relation type, which is
a URI <xref target="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.</t>
<t>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.</t>
<t>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.</t>
</section>
</section>
<section anchor="header.link" title="The Link Header Field">
<t>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 <xref target="RFC4287" />. </t>
<figure>
<artwork xml:space="preserve"><![CDATA[
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
]]></artwork>
</figure>
<section title="Target IRI">
<t>Each link-value conveys one target IRI as a URI-Reference (after conversion to one, if necessary; see <xref target="RFC3987"/>, Section 3.1) inside angle
brackets ("<>"). If the URI-Reference is relative, parsers MUST resolve it as per <xref target="RFC3986"/>, Section 5.
Note that any base IRI from the message's content is not applied.</t>
</section>
<section title="Context IRI">
<t>By default, the context of a link conveyed in the Link header field is the IRI of the requested resource.</t>
<t>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 <xref target="RFC3986"/>, Section 5. Note that any
base URI from the body's content is not applied.</t>
<t>The anchor parameter MUST be ignored by consuming implementations, unless its use is specified by the
application in use.</t>
</section>
<section title="Relation Type">
<t>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.</t>
<t>The "rel" parameter MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored
by parsers.</t>
<t>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 (",").</t>
</section>
<section title="Target Attributes">
<t>The "hreflang", "media", "title", "title*", "type" and any link-extension link-params are considered to be target
attributes for the link.</t>
<t>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.</t>
<t>The "media" parameter, when present, is used to indicate intended destination medium or media for style information (see
<xref target="W3C.REC-html401-19991224" />, Section 6.13. Note that this may be updated by <xref target="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.</t>
<t>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.</t>
<t>The "title*" parameter MAY be used encode this label in a different character set, and/or contain language information as per
<xref target="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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Examples">
<t>For example:</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
Link: <http://example.com/TheBook/chapter2>; rel="previous";
title="previous chapter"
]]></artwork>
</figure>
<t>indicates that "chapter2" is previous to this resource in a logical navigation path.</t>
<t>Similarly,</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
Link: </>; rel="http://example.net/foo"
]]></artwork>
</figure>
<t>indicates that the root resource ("/") is related to this resource with the extension relation type "http://example.net/foo".</t>
<t>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.</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
Link: </TheBook/chapter2>;
rel="previous"; title*=UTF-8'de'letztes%20Kapitel,
</TheBook/chapter4>;
rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel
]]></artwork>
</figure>
<t>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").</t>
<t>Note that link-values may convey multiple links between the same target and context IRIs; for example:</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
Link: <http://example.org/>;
rel="start http://example.net/relation/other"
]]></artwork>
</figure>
<t>Here, the link to "http://example.org/" has the registered relation type
"start" and the extension relation type "http://example.net/relation/other".</t>
</section>
</section>
<section title="IANA Considerations">
<section title="Link HTTP Header Registration">
<t>This specification updates the Message Header Registry entry for "Link" in HTTP
<xref target="RFC3864" /> to refer to this document.</t>
<figure>
<artwork xml:space="preserve">
Header field: Link
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[ this document ]
</artwork>
</figure>
</section>
<section title="Link Relation Type Registry" anchor="link-relation-registry">
<t>This specification establishes the Link Relation Type Registry, and updates Atom
<xref target="RFC4287" /> to refer to it in place of the "Registry of Link Relations".</t>
<t>[[ 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. ]]</t>
<section title="Registering new Link Relation Types">
<t>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 <xref target="RFC5226"/>).</t>
<t>The requirements for registered relation types are described in <xref target="registered-relation-types"/>.</t>
<t>Registration requests consist of the completed registration template
below, typically published in an RFC or Open Standard (in the sense described
by <xref target="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.</t>
<t>The registration template is:</t>
<t>
<list style="symbols">
<t>Relation Name: </t>
<t>Description:</t>
<t>Reference: </t>
<t>Notes: [optional]</t>
<t>Fields: [optional]</t>
</list>
</t>
<t>Registration requests should be sent to the [TBD]@ietf.org mailing list, marked clearly in the subject line
(e.g,. "NEW RELATION REQUEST").</t>
<t>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.</t>
<t>When a registration request is successful, the Designated Expert(s) will update the registry XML file (using the format
described in <xref target="registry_format"/> 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.</t>
<t>IANA should only accept registry updates from the Designated Expert(s), and should direct all requests for registration
to the review mailing list.</t>
</section>
<section title="Initial Registry Contents">
<t>The Link Relation Type registry's initial contents are:</t>
<t>
<list style="symbols">
<t>Relation Name: alternate</t>
<t>Description: Designates a substitute for the link's context.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: appendix</t>
<t>Description: Refers to an appendix.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: bookmark</t>
<t>Description: Refers to a bookmark or entry point. </t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: chapter</t>
<t>Description: Refers to a chapter in a collection of resources.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: contents</t>
<t>Description: Refers to a table of contents.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: copyright</t>
<t>Description: Refers to a copyright statement that applies to the link's context.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: current</t>
<t>Description: Refers to a resource containing the most recent item(s) in a collection
of resources.</t>
<t>Reference: <xref target="RFC5005" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: describedby</t>
<t>Description: Refers to a resource providing information about the link's context.</t>
<t>Documentation: <eref target="http://www.w3.org/TR/powder-dr/#assoc-linking"/></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: edit</t>
<t>Description: Refers to a resource that can be used to edit the link's context.</t>
<t>Reference: <xref target="RFC5023" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: edit-media</t>
<t>Description: Refers to a resource that can be used to edit media associated with the
link's context.</t>
<t>Reference: <xref target="RFC5023" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: enclosure</t>
<t>Description: Identifies a related resource that is potentially large and might
require special handling.</t>
<t>Reference: <xref target="RFC4287" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: first</t>
<t>Description: An IRI that refers to the furthest preceding resource in a series of
resources.</t>
<t>Reference: [this document]</t>
<t>Notes: this relation type pre-exists this specification, and did not indicate a reference. Originally
requested by Mark Nottingham in December 2004.</t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: glossary</t>
<t>Description: Refers to a glossary of terms.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: help</t>
<t>Description: Refers to a resource offering help (more information, links to other
sources information, etc.)</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: index</t>
<t>Description: Refers to an index.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: last</t>
<t>Description: An IRI that refers to the furthest following resource in a series of
resources.</t>
<t>Reference: [this document]</t>
<t>Notes: this relation type pre-exists this specification, and did not indicate a reference. Originally
requested by Mark Nottingham in December 2004.</t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: license</t>
<t>Description: Refers to a license associated with the link's context.</t>
<t>Reference: <xref target="RFC4946" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: next</t>
<t>Description: Refers to the next resource in a ordered series of resources.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: next-archive</t>
<t>Description: Refers to the immediately following archive resource.</t>
<t>Reference: <xref target="RFC5005" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: payment</t>
<t>Description: indicates a resource where payment is accepted.</t>
<t>Reference: [this document]</t>
<t>Notes: this relation type pre-exists this specification, and did not indicate a reference. Requested
by Joshua Kinberg and Robert Sayre.</t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: prev</t>
<t>Description: Refers to the previous resource in an ordered series of resources.
Synonym for "previous".</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: previous</t>
<t>Description: Refers to the previous resource in an ordered series of resources.
Synonym for "prev".</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: prev-archive</t>
<t>Description: Refers to the immediately preceding archive resource.</t>
<t>Reference: <xref target="RFC5005" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: related</t>
<t>Description: Identifies a related resource.</t>
<t>Reference: <xref target="RFC4287" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: replies</t>
<t>Description: Identifies a resource that is a reply to the context of the link.</t>
<t>Reference: <xref target="RFC4685" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: section</t>
<t>Description: Refers to a section in a collection of resources.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: self</t>
<t>Description: Conveys an identifier for the link's context.</t>
<t>Reference: <xref target="RFC4287" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: service</t>
<t>Description: Indicates a URI that can be used to retrieve a service document. </t>
<t>Reference: <xref target="RFC5023" /></t>
<t>Notes: When used in an Atom document, this relation type specifies Atom Publishing Protocol service
documents by default. Requested by James Snell.</t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: start</t>
<t>Description: Refers to the first resource in a collection of resources.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: stylesheet</t>
<t>Description: Refers to an external style sheet.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: subsection</t>
<t>Description: Refers to a resource serving as a subsection in a collection of
resources.</t>
<t>Reference: <xref target="W3C.REC-html401-19991224" /></t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: up</t>
<t>Description: Refers to a parent document in a hierarchy of documents.</t>
<t>Reference: [this document]</t>
<t>Notes: this relation type pre-exists this specification, and did not indicate a reference. Requested
by Noah Slater.</t>
</list>
</t>
<t>
<list style="symbols">
<t>Relation Name: via</t>
<t>Description: Identifies a resource that is the source of the information in the
link's context.</t>
<t>Reference: <xref target="RFC4287" /></t>
</list>
</t>
</section>
</section>
<section title="Link Relation Field Registry" anchor="field_registry">
<t>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").</t>
<t>Fields are registered on the advice of a Designated Expert (appointed by the
IESG or their delegate), with a Specification Required (using terminology from <xref target="RFC5226"/>).</t>
<t>Registration requests consist of the completed registration template below;</t>
<t>
<list style="symbols">
<t>Field Name: </t>
<t>Description: </t>
<t>Default Value: </t>
<t>Notes: [optional]</t>
</list>
</t>
<t>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.</t>
<t>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).</t>
<t>Registration requests should be sent to the [TBD]@ietf.org mailing list, marked clearly in the subject line
(e.g,. "NEW EXTENSION FIELD").</t>
<t>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.</t>
<t>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.</t>
</section>
</section>
<section title="Security Considerations">
<t>The content of the Link header-field is not secure, private or integrity-guaranteed, and
due caution should be exercised when using it.</t>
<t>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.</t>
</section>
<section title="Internationalisation Considerations">
<t>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.</t>
<t>Similarly, the anchor parameter of the Link header does not support IRIs, and therefore
IRIs must be converted to URIs before inclusion there.</t>
<t>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.</t>
</section>
</middle>
<back>
<references title="Normative References"> &rfc2026; &rfc2119; &rfc2231; &rfc3986; &rfc3987;
&rfc2616; &rfc3864; &rfc4288; &rfc4646; &rfc5226;</references>
<references title="Informative References"> &rfc2068; &rfc4287; &rfc4685;
&rfc4946; &rfc5005; &rfc5023; &html; &xhtml; &rdfa; &mediaq;</references>
<section title="Link Relation Registry Format" anchor="registry_format">
<t>To facilitate applications that wish to use registry data, this specification defines an XML-based
format for the registry entries.</t>
<t>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.</t>
<t>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.</t>
<section title="Relax NG Grammar">
<figure>
<artwork xml:space="preserve"><![CDATA[
element RelationTypes {
element RelationType {
attribute name { text },
attribute reference { text },
element description { text },
element notes { text }?,
element field {
attribute name { text },
text
}*
}+
}
]]></artwork>
</figure>
</section>
<section title="Example">
<figure>
<artwork xml:space="preserve"><![CDATA[
<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>
]]></artwork>
</figure>
</section>
</section>
<section title="Notes on Using the Link Header with the HTML4 Format">
<t>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.</t>
<t>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.</t>
<t>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).</t>
<t>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</t>
<t><list style="symbols">
<t>As absolute URIs, or</t>
<t>using the document-wide "profile" attribute's URI as a prefix for relation types, or</t>
<t>using the RDFa <xref target="W3C.REC-rdfa-syntax-20081014"/> convention of mapping token prefixes to
URIs (in a manner similar to XML name spaces) (note that RDFa is only defined to work in XHTML <xref
target="W3C.REC-xhtml-basic-20080729"/>, but is sometimes used in HTML4).</t>
</list></t>
<t>Individual applications of linking will therefore need to define how their extension links should be
serialised into HTML4.</t>
<t>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).</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Notes on Using the Link Header with the Atom Format">
<t>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.</t>
<t>When serialising an atom:link into a Link header, it is necessary to convert target IRIs (if used)
to URIs.</t>
<t>Atom defines extension relation types in terms of IRIs. This specification re-defines
them as URIs, to simplify and reduce errors in their comparison.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Acknowledgements">
<t>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.</t>
<t>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.</t>
</section>
<section title="Document history">
<t>[[ to be removed by the RFC editor before publication as an RFC. ]]</t>
<t>-07</t>
<t>
<list style="symbols">
<t>Allowed multiple spaces between relation types.</t>
<t>Relaxed requirements for registered relations.</t>
<t>Removed Defining New Link Serialisations appendix.</t>
<t>Added Field registry.</t>
<t>Added registry XML format.</t>
<t>Changed registration procedure to use mailing list(s), giving the Designated Experts more responsibility for
the smooth running of the registry.</t>
<t>Loosened prohibition against media-specific relation types to SHOULD NOT.</t>
<t>Disallowed registration of media-specific relation types (can still be used as extension types).</t>
<t>Clarified that parsers are responsible for resolving relative URIs.</t>
<t>Fixed ABNF for extended-initial-value.</t>
<t>Fixed title* parameter quoting in example.</t>
<t>Added notes for registered relations that lack a reference.</t>
<t>Added hreflang parameter.</t>
<t>Clarified status of 'rev'.</t>
<t>Removed advice to use @profile in HTML4.</t>
<t>Clarified what multiple *title and hreflang attributes mean.</t>
<t>Disallowed multiple type, rel and title attributes.</t>
<t>Removed text about absolute URI form of registered relations.</t>
<t>Required registered relations to conform to sgml-name (now just rel-relation-type).</t>
<t>Required registered relations to be lowercase.</t>
<t>Made comparison of extension relations case insensitive.</t>
<t>Clarified requirements on registered relation types regarding media types, etc.</t>
<t>Allowed applications to ignore links with anchor parameters if they're concerned.</t>
<t>Made 'rev' text a bit less confusing.</t>
<t>Extension relation URIs SHOULD be all-lowercase.</t>
<t>Added media parameter.</t>
<t>Required applications to specifically call out use of anchor parameter.</t>
</list>
</t>
<t>-06</t>
<t>
<list style="symbols">
<t>Added "up" and "service" relation types.</t>
<t>Fixed "type" attribute syntax and added prose.</t>
<t>Added note about RDFa and XHTML to HTML4 notes.</t>
<t>Removed specific location for the registry, since IANA seems to have its own ideas about that.</t>
</list>
</t>
<t>-05</t>
<t>
<list style="symbols">
<t>Clarified how to resolve relative URIs in the 'anchor' parameter.</t>
<t>Tweaked language about dereferencing relation type URIs.</t>
<t>Separated out examples.</t>
<t>Made target-parameters more explicit in the model.</t>
<t>Discourage special semantics between different relations, or based upon cardinality.</t>
<t>Grandfathered in special semantics of 'alternate stylesheet' for HTML4.</t>
<t>Note that extension types can be serialised in ways other than as URIs, as long as they can be
converted to URIs.</t>
<t>Change default context of a link header to that of the requested resource.</t>
<t>Use this document as reference for relations that don't have a formal definition other than the
registry entries; avoids circular references.</t>
<t>Noted that ordering of links is not significant or defined in this spec, but may be in specific applications.</t>
<t>Adjusted uses of 'application' to 'serialisation' where appropriate.</t>
<t>Added 'Defining New Link Serialisations' section.</t>
<t>Added note about case sensitivity when comparing registered relation types in Atom.</t>
</list>
</t>
<t>-04</t>
<t>
<list style="symbols">
<t>Defined context as a resource, rather than a representation.</t>
<t>Removed concept of link directionality; relegated to a deprecated Link header extension.</t>
<t>Relation types split into registered (non-URI) and extension (URI).</t>
<t>Changed wording around finding URIs for registered relation types.</t>
<t>Changed target and context URIs to IRIs (but not extension relation types).</t>
<t>Add RFC2231 encoding for title parameter, explicit BNF for title*.</t>
<t>Add i18n considerations.</t>
<t>Specify how to compare relation types.</t>
<t>Changed registration procedure to Designated Expert.</t>
<t>Softened language around presence of relations in the registry.</t>
<t>Added describedby relation.</t>
<t>Re-added 'anchor' parameter, along with security consideration for third-party anchors.</t>
<t>Softened language around HTML4 attributes that aren't directly accommodated.</t>
<t>Various tweaks to abstract, introduction and examples.</t>
</list>
</t>
<t>-03</t>
<t>
<list style="symbols">
<t>Inverted focus from Link headers to link relations.</t>
<t>Specified was a link relation type is.</t>
<t>Based on discussion, re-added 'rev'.</t>
<t>Changed IESG Approval to IETF Consensus for relation registrations (i.e., require a document).</t>
<t>Updated RFC2434 reference to RFC5226.</t>
<t>Registered relations SHOULD conform to sgml-name.</t>
<t>Cautioned against confusing relation types with media types.</t>
</list>
</t>
<t>-02</t>
<t>
<list style="symbols">
<t>Dropped XLink language.</t>
<t>Removed 'made' example.</t>
<t>Removed 'rev'. Can still be used as an extension.</t>
<t>Added HTML reference to introduction.</t>
<t>Required relationship values that have a ; or , to be quoted.</t>
<t>Changed base URI for relation values.</t>
<t>Noted registry location.</t>
<t>Added advisory text about HTML profile URIs.</t>
<t>Disallowed registration of relations that only differ in case.</t>
<t>Clarified language about IRIs in Atom.</t>
<t>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?</t>
<t>Explicitly updates RFC4287.</t>
<t>Added 'type' parameter.</t>
<t>Removed unnecessary advice about non-HTML relations in HTML section.</t>
</list>
</t>
<t>-01</t>
<t>
<list style="symbols">
<t>Changed syntax of link-relation to one or more URI; dropped Profile.</t>
<t>Dropped anchor parameter; can still be an extension.</t>
<t>Removed Link-Template header; can be specified by templates spec or elsewhere.</t>
<t>Straw-man for link relation registry.</t>
</list>
</t>
<t>-00</t>
<t>
<list style="symbols">
<t>Initial draft; normative text lifted from RFC2068.</t>
</list>
</t>
</section>
</back>
</rfc>
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