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rfc:rfc5988

Internet Engineering Task Force (IETF) M. Nottingham Request for Comments: 5988 October 2010 Updates: 4287 Category: Standards Track ISSN: 2070-1721

                            Web Linking

Abstract

 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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5988.

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Nottingham Standards Track [Page 1] RFC 5988 Web Linking October 2010

 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................3
 2. Notational Conventions ..........................................3
 3. Links ...........................................................4
 4. Link Relation Types .............................................5
    4.1. Registered Relation Types ..................................5
    4.2. Extension Relation Types ...................................6
 5. The Link Header Field ...........................................6
    5.1. Target IRI .................................................7
    5.2. Context IRI ................................................7
    5.3. Relation Type ..............................................8
    5.4. Target Attributes ..........................................8
    5.5. Examples ...................................................9
 6. IANA Considerations ............................................10
    6.1. Link HTTP Header Registration .............................10
    6.2. Link Relation Type Registry ...............................10
         6.2.1. Registering New Link Relation Types ................11
         6.2.2. Initial Registry Contents ..........................12
    6.3. Link Relation Application Data Registry ...................16
 7. Security Considerations ........................................17
 8. Internationalisation Considerations ............................18
 9. References .....................................................18
    9.1. Normative References ......................................18
    9.2. Informative References ....................................19
 Appendix A.  Notes on Using the Link Header with the HTML4
              Format ...............................................21
 Appendix B.  Notes on Using the Link Header with the Atom
              Format ...............................................22
 Appendix C.  Acknowledgements .....................................23

Nottingham Standards Track [Page 2] RFC 5988 Web Linking October 2010

1. Introduction

 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 redefining 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 Section 19.6.2.4 of [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.

2. Notational Conventions

 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; from [RFC5646]: Language-Tag;
 and from [RFC5987], ext-value and parmname.

Nottingham Standards Track [Page 3] RFC 5988 Web Linking October 2010

3. Links

 In this specification, a link is a typed connection between two
 resources that are identified by Internationalised Resource
 Identifiers (IRIs) [RFC3987], and is comprised of:
 o  A context IRI,
 o  a link relation type (Section 4),
 o  a target IRI, and
 o  optionally, target attributes.
 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 to and from 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 does it 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.

Nottingham Standards Track [Page 4] RFC 5988 Web Linking October 2010

4. Link Relation Types

 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.

4.1. Registered Relation Types

 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-rel-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 can specify the
 behaviours and properties of the target resource (e.g., allowable
 HTTP methods, request and response media types that must be
 supported).

Nottingham Standards Track [Page 5] RFC 5988 Web Linking October 2010

 Additionally, specific applications of linking may require additional
 data to be included in the registry.  For example, Web browsers might
 want to know what kinds of links should be downloaded when they
 archive a Web page; if this application-specific information is in
 the registry, new link relation types can control this behaviour
 without unnecessary coordination.
 To accommodate this, per-entry application data can be added to the
 Link Relation Type registry, by registering it in the Link Relation
 Application Data registry (Section 6.3).

4.2. Extension Relation Types

 Applications that don't wish to register a relation type can 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
 strings (after converting to URIs if serialised in a different
 format, such as a Curie [W3C.CR-curie-20090116]) 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 can specify that they are expressed in another
 form, as long as they can be converted to URIs.

5. The Link Header Field

 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].

Nottingham Standards Track [Page 6] RFC 5988 Web Linking October 2010

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*" "=" ext-value )
               | ( "type" "=" ( media-type | quoted-mt ) )
               | ( link-extension ) )
link-extension = ( parmname [ "=" ( ptoken | quoted-string ) ] )
               | ( ext-name-star "=" ext-value )
ext-name-star  = parmname "*" ; reserved for RFC2231-profiled
                              ; extensions.  Whitespace NOT
                              ; allowed in between.
ptoken         = 1*ptokenchar
ptokenchar     = "!" | "#" | "$" | "%" | "&" | "'" | "("
               | ")" | "*" | "+" | "-" | "." | "/" | DIGIT
               | ":" | "<" | "=" | ">" | "?" | "@" | ALPHA
               | "[" | "]" | "^" | "_" | "`" | "{" | "|"
               | "}" | "~"
media-type     = type-name "/" subtype-name
quoted-mt      = <"> media-type <">
relation-types = relation-type
               | <"> relation-type *( 1*SP relation-type ) <">
relation-type  = reg-rel-type | ext-rel-type
reg-rel-type   = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
ext-rel-type   = URI

5.1. Target IRI

 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.

5.2. Context IRI

 By default, the context of a link conveyed in the Link header field
 is the IRI of the requested resource.
 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).  If the anchor parameter's

Nottingham Standards Track [Page 7] RFC 5988 Web Linking October 2010

 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.
 Consuming implementations can choose to ignore links with an anchor
 parameter.  For example, the application in use may not allow the
 context IRI to be assigned to a different resource.  In such cases,
 the entire link is to be ignored; consuming implementations MUST NOT
 process the link without applying the anchor.
 Note that depending on HTTP status code and response headers, the
 context IRI might be "anonymous" (i.e., no context IRI is available).
 For instance, this is the case on a 404 response to a GET request.

5.3. Relation Type

 The relation type of a link is conveyed in the "rel" parameter's
 value.  The "rel" parameter MUST NOT appear more than once in a given
 link-value; occurrences after the first MUST be ignored by parsers.
 The "rev" parameter has been used in the past to indicate that the
 semantics of the relationship are in the reverse direction.  That is,
 a link from A to B with REL="X" expresses the same relationship as a
 link from B to A with REV="X". "rev" is deprecated by this
 specification because it often confuses authors and readers; in most
 cases, using a separate relation type is preferable.
 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 (",") (as these characters are used as delimiters in the
 header itself).

5.4. Target Attributes

 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.

Nottingham Standards Track [Page 8] RFC 5988 Web Linking October 2010

 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) in the language indicated by the Content-
 Language header (if present).  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 can be used to encode this label in a
 different character set, and/or contain language information as per
 [RFC5987].  The "title*" parameter MUST NOT appear more than once in
 a given link-value; occurrences after the first MUST be ignored by
 parsers.  If the parameter does not contain language information, its
 language is indicated by the Content-Language header (when present).
 If both the "title" and "title*" parameters appear in a link-value,
 processors SHOULD use the "title*" parameter's value.
 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.

5.5. Examples

 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".

Nottingham Standards Track [Page 9] RFC 5988 Web Linking October 2010

 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 can 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".

6. IANA Considerations

6.1. Link HTTP Header Registration

 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):
     [RFC5988]

6.2. Link Relation Type Registry

 This specification establishes the Link Relation Type registry, and
 updates Atom [RFC4287] to refer to it in place of the "Registry of
 Link Relations".
 The underlying registry data (e.g., the XML file) must include
 Simplified BSD License text as described in Section 4.e of the Trust
 Legal Provisions (<http://trustee.ietf.org/license-info>).

Nottingham Standards Track [Page 10] RFC 5988 Web Linking October 2010

6.2.1. Registering New Link Relation Types

 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.
 Note that relation types can be registered by third parties, if the
 Designated Expert determines that an unregistered relation type is
 widely deployed and not likely to be registered in a timely manner.
 The registration template is:
 o  Relation Name:
 o  Description:
 o  Reference:
 o  Notes: [optional]
 o  Application Data: [optional]
 Registration requests should be sent to the link-relations@ietf.org
 mailing list, marked clearly in the subject line (e.g., "NEW RELATION
 - example" to register an "example" relation type).
 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 and IANA.  Denials should include an
 explanation and, if applicable, suggestions as to how to make the
 request successful.
 Decisions (or lack thereof) made by the Designated Expert can be
 first appealed to Application Area Directors (contactable using
 app-ads@tools.ietf.org email address or directly by looking up their
 email addresses on http://www.iesg.org/ website) and, if the
 appellant is not satisfied with the response, to the full IESG (using
 the iesg@iesg.org mailing list).

Nottingham Standards Track [Page 11] RFC 5988 Web Linking October 2010

 IANA should only accept registry updates from the Designated
 Expert(s), and should direct all requests for registration to the
 review mailing list.

6.2.2. Initial Registry Contents

 The Link Relation Type registry's initial contents are:
 o  Relation Name: alternate
 o  Description: Designates a substitute for the link's context.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: appendix
 o  Description: Refers to an appendix.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: bookmark
 o  Description: Refers to a bookmark or entry point.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: chapter
 o  Description: Refers to a chapter in a collection of resources.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: contents
 o  Description: Refers to a table of contents.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: copyright
 o  Description: Refers to a copyright statement that applies to the
    link's context.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: current
 o  Description: Refers to a resource containing the most recent
    item(s) in a collection of resources.
 o  Reference: [RFC5005]
 o  Relation Name: describedby
 o  Description: Refers to a resource providing information about the
    link's context.
 o  Documentation: <http://www.w3.org/TR/powder-dr/#assoc-linking>
 o  Relation Name: edit
 o  Description: Refers to a resource that can be used to edit the
    link's context.
 o  Reference: [RFC5023]

Nottingham Standards Track [Page 12] RFC 5988 Web Linking October 2010

 o  Relation Name: edit-media
 o  Description: Refers to a resource that can be used to edit media
    associated with the link's context.
 o  Reference: [RFC5023]
 o  Relation Name: enclosure
 o  Description: Identifies a related resource that is potentially
    large and might require special handling.
 o  Reference: [RFC4287]
 o  Relation Name: first
 o  Description: An IRI that refers to the furthest preceding resource
    in a series of resources.
 o  Reference: [RFC5988]
 o  Notes: this relation type registration did not indicate a
    reference.  Originally requested by Mark Nottingham in December
    2004.
 o  Relation Name: glossary
 o  Description: Refers to a glossary of terms.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: help
 o  Description: Refers to a resource offering help (more information,
    links to other sources information, etc.)
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: hub
 o  Description: Refers to a hub that enables registration for
    notification of updates to the context.
 o  Reference: <http://pubsubhubbub.googlecode.com/> <http://
    pubsubhubbub.googlecode.com/svn/trunk/pubsubhubbub-core-0.3.html>
 o  Notes: this relation type was requested by Brett Slatkin.
 o  Relation Name: index
 o  Description: Refers to an index.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: last
 o  Description: An IRI that refers to the furthest following resource
    in a series of resources.
 o  Reference: [RFC5988]
 o  Notes: this relation type registration did not indicate a
    reference.  Originally requested by Mark Nottingham in December
    2004.

Nottingham Standards Track [Page 13] RFC 5988 Web Linking October 2010

 o  Relation Name: latest-version
 o  Description: Points to a resource containing the latest (e.g.,
    current) version of the context.
 o  Reference: [RFC5829]
 o  Relation Name: license
 o  Description: Refers to a license associated with the link's
    context.
 o  Reference: [RFC4946]
 o  Relation Name: next
 o  Description: Refers to the next resource in a ordered series of
    resources.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: next-archive
 o  Description: Refers to the immediately following archive resource.
 o  Reference: [RFC5005]
 o  Relation Name: payment
 o  Description: indicates a resource where payment is accepted.
 o  Reference: [RFC5988]
 o  Notes: this relation type registration did not indicate a
    reference.  Requested by Joshua Kinberg and Robert Sayre.  It is
    meant as a general way to facilitate acts of payment, and thus
    this specification makes no assumptions on the type of payment or
    transaction protocol.  Examples may include a Web page where
    donations are accepted or where goods and services are available
    for purchase. rel="payment" is not intended to initiate an
    automated transaction.  In Atom documents, a link element with a
    rel="payment" attribute may exist at the feed/channel level and/or
    the entry/item level.  For example, a rel="payment" link at the
    feed/channel level may point to a "tip jar" URI, whereas an entry/
    item containing a book review may include a rel="payment" link
    that points to the location where the book may be purchased
    through an online retailer.
 o  Relation Name: prev
 o  Description: Refers to the previous resource in an ordered series
    of resources.  Synonym for "previous".
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: predecessor-version
 o  Description: Points to a resource containing the predecessor
    version in the version history.
 o  Reference: [RFC5829]

Nottingham Standards Track [Page 14] RFC 5988 Web Linking October 2010

 o  Relation Name: previous
 o  Description: Refers to the previous resource in an ordered series
    of resources.  Synonym for "prev".
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: prev-archive
 o  Description: Refers to the immediately preceding archive resource.
 o  Reference: [RFC5005]
 o  Relation Name: related
 o  Description: Identifies a related resource.
 o  Reference: [RFC4287]
 o  Relation Name: replies
 o  Description: Identifies a resource that is a reply to the context
    of the link.
 o  Reference: [RFC4685]
 o  Relation Name: section
 o  Description: Refers to a section in a collection of resources.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: self
 o  Description: Conveys an identifier for the link's context.
 o  Reference: [RFC4287]
 o  Relation Name: service
 o  Description: Indicates a URI that can be used to retrieve a
    service document.
 o  Reference: [RFC5023]
 o  Notes: When used in an Atom document, this relation type specifies
    Atom Publishing Protocol service documents by default.  Requested
    by James Snell.
 o  Relation Name: start
 o  Description: Refers to the first resource in a collection of
    resources.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: stylesheet
 o  Description: Refers to an external style sheet.
 o  Reference: [W3C.REC-html401-19991224]
 o  Relation Name: subsection
 o  Description: Refers to a resource serving as a subsection in a
    collection of resources.
 o  Reference: [W3C.REC-html401-19991224]

Nottingham Standards Track [Page 15] RFC 5988 Web Linking October 2010

 o  Relation Name: successor-version
 o  Description: Points to a resource containing the successor version
    in the version history.
 o  Reference: [RFC5829]
 o  Relation Name: up
 o  Description: Refers to a parent document in a hierarchy of
    documents.
 o  Reference: [RFC5988]
 o  Notes: this relation type registration did not indicate a
    reference.  Requested by Noah Slater.
 o  Relation Name: version-history
 o  Description: points to a resource containing the version history
    for the context.
 o  Reference: [RFC5829]
 o  Relation Name: via
 o  Description: Identifies a resource that is the source of the
    information in the link's context.
 o  Reference: [RFC4287]
 o  Relation Name: working-copy
 o  Description: Points to a working copy for this resource.
 o  Reference: [RFC5829]
 o  Relation Name: working-copy-of
 o  Description: Points to the versioned resource from which this
    working copy was obtained.
 o  Reference: [RFC5829]

6.3. Link Relation Application Data Registry

 This specification also establishes the Link Relation Application
 Field registry, to allow entries in the Link Relation Type registry
 to be extended with application-specific data (hereafter, "app data")
 specific to all instances of a given link relation type.
 Application data is registered on the advice of a Designated Expert
 (appointed by the IESG or their delegate), with a Specification
 Required (using terminology from [RFC5226]).

Nottingham Standards Track [Page 16] RFC 5988 Web Linking October 2010

 Registration requests consist of the completed registration template
 below:
 o  Application Name:
 o  Description:
 o  Default Value:
 o  Notes: [optional]
 The Description SHOULD identify the value space of the app data.  The
 Default Value MUST be appropriate to entries to which the app data
 does not apply.
 Entries that pre-date the addition of app data will automatically be
 considered to have the default value for that app data; 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 app data as well as the registrant of the existing entry (if
 possible).
 Registration requests should be sent to the link-relations@ietf.org
 mailing list, marked clearly in the subject line (e.g., "NEW APP DATA
 - example" to register "example" app data).
 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 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 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.

7. Security Considerations

 The content of the Link header field is not secure, private or
 integrity-guaranteed, and due caution should be exercised when using
 it.  Use of Transport Layer Security (TLS) with HTTP ([RFC2818] and
 [RFC2817]) is currently the only end-to-end way to provide such
 protection.

Nottingham Standards Track [Page 17] RFC 5988 Web Linking October 2010

 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.
 The Link entity-header field makes extensive use of IRIs and URIs.
 See [RFC3987] for security considerations relating to IRIs.  See
 [RFC3986] for security considerations relating to URIs.  See
 [RFC2616] for security considerations relating to HTTP headers.

8. Internationalisation Considerations

 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.

9. References

9.1. Normative References

 [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
            3", BCP 9, RFC 2026, October 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
            Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
            Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
            Procedures for Message Header Fields", BCP 90, RFC 3864,
            September 2004.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, January 2005.

Nottingham Standards Track [Page 18] RFC 5988 Web Linking October 2010

 [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
            Identifiers (IRIs)", RFC 3987, January 2005.
 [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
            Registration Procedures", BCP 13, RFC 4288, December 2005.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5646]  Phillips, A. and M. Davis, "Tags for Identifying
            Languages", BCP 47, RFC 5646, September 2009.
 [RFC5987]  Reschke, J., "Character Set and Language Encoding for
            Hypertext Transfer Protocol (HTTP) Header Field
            Parameters", RFC 5987, August 2010.

9.2. Informative References

 [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
            Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
            RFC 2068, January 1997.
 [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
            HTTP/1.1", RFC 2817, May 2000.
 [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
            Syndication Format", RFC 4287, December 2005.
 [RFC4685]  Snell, J., "Atom Threading Extensions", RFC 4685,
            September 2006.
 [RFC4946]  Snell, J., "Atom License Extension", RFC 4946, July 2007.
 [RFC5005]  Nottingham, M., "Feed Paging and Archiving", RFC 5005,
            September 2007.
 [RFC5023]  Gregorio, J. and B. de hOra, "The Atom Publishing
            Protocol", RFC 5023, October 2007.
 [RFC5829]  Brown, A., Clemm, G., and J. Reschke, "Link Relation Types
            for Simple Version Navigation between Web Resources",
            RFC 5829, April 2010.

Nottingham Standards Track [Page 19] RFC 5988 Web Linking October 2010

 [W3C.CR-css3-mediaqueries-20090915]
            van Kesteren, A., Glazman, D., Lie, H., and T. Celik,
            "Media Queries", W3C Candidate Recommendation CR-css3-
            mediaqueries-20090915, September 2009,
            <http://www.w3.org/TR/2009/
            CR-css3-mediaqueries-20090915/>.
            Latest version available at
            <http://www.w3.org/TR/css3-mediaqueries/>.
 [W3C.CR-curie-20090116]
            Birbeck, M. and S. McCarron, "CURIE Syntax 1.0", W3C
            Candidate Recommendation CR-curie-20090116, January 2009,
            <http://www.w3.org/TR/2009/CR-curie-20090116>.
            Latest version available at <http://www.w3.org/TR/curie>.
 [W3C.REC-html401-19991224]
            Le Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
            Specification", W3C Recommendation REC-html401-19991224,
            December 1999,
            <http://www.w3.org/TR/1999/REC-html401-19991224>.
            Latest version available at
            <http://www.w3.org/TR/html401>.
 [W3C.REC-rdfa-syntax-20081014]
            Adida, B., Birbeck, M., McCarron, S., and S. Pemberton,
            "RDFa in XHTML: Syntax and Processing", W3C
            Recommendation REC-rdfa-syntax-20081014, October 2008,
            <http://www.w3.org/TR/2008/REC-rdfa-syntax-20081014>.
            Latest version available at
            <http://www.w3.org/TR/rdfa-syntax>.
 [W3C.REC-xhtml-basic-20080729]
            Baker, M., Ishikawa, M., Stark, P., Matsui, S., Wugofski,
            T., and T. Yamakami, "XHTML[TM] Basic 1.1", W3C
            Recommendation REC-xhtml-basic-20080729, July 2008,
            <http://www.w3.org/TR/2008/REC-xhtml-basic-20080729>.
            Latest version available at
            <http://www.w3.org/TR/xhtml-basic>.

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Appendix A. Notes on Using the Link Header with the HTML4 Format

 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.
 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
 o  As absolute URIs,
 o  using the document-wide "profile" attribute's URI as a prefix for
    relation types, or
 o  using the RDFa [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
    [W3C.REC-xhtml-basic-20080729], but is sometimes used in HTML4).
 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.

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Appendix B. Notes on Using the Link Header with the Atom Format

 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, lowercase) 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 can also be
 used as link-extensions to maintain fidelity.

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Appendix C. Acknowledgements

 This specification lifts the idea and definition for the Link header
 from RFC 2068; 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.

Author's Address

 Mark Nottingham
 EMail: mnot@mnot.net
 URI:   http://www.mnot.net/

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