


Network Working Group                                      M. Nottingham
Internet-Draft                                          December 1, 2008
Updates: 4287 (if approved)
Intended status: Standards Track
Expires: June 4, 2009


                 Link Relations and HTTP Header Linking
                  draft-nottingham-http-link-header-03

Status of this Memo

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   This Internet-Draft will expire on June 4, 2009.

Abstract

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











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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Links  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Link Relation Types  . . . . . . . . . . . . . . . . . . . . .  4
   5.  The Link Header Field  . . . . . . . . . . . . . . . . . . . .  5
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Notes on Using the Link Header with HTML  . . . . . . 11
   Appendix B.  Notes on Using the Link Header with Atom  . . . . . . 12
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 13
   Appendix D.  Document history  . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 15

































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1.  Introduction

   A means of indicating the relationships between documents 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 separate.  However, links between resources need not be
   format-specific; it can be useful to have typed links that are
   independent of the format, especially when a resource has
   representations in multiple formats.

   This document defines typed link relations, independent of the
   context they occur in.  It does so by clarifying the status of the
   link relation registry established by Atom, and registering in 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, several use cases for doing
   so have surfaced.  However, because it was removed, the status of the
   Link header is unclear, leading some to consider minting new
   application-specific HTTP headers instead of reusing it.  This
   document addresses this by re-specifying the Link header with updated
   but backwards-compatible syntax.

   [[ Feedback is welcome on the ietf-http-wg@w3.org mailing list,
   although this is NOT a work item of the HTTPBIS WG. ]]


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).  Additionally, the following rules
   are included from [RFC3986]: URI and URI-Reference, and from
   [RFC4288]: type-name.


3.  Links

   In the context of this specification, a link is comprised of:





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   o  A target URI ([RFC3986]), and
   o  A context of use, and
   o  A link relation type (Section 4), and
   o  A link direction (outbound or inbound).

   A link can be viewed as a statement of the form "(subject) has a
   (relation type) at (object)", where for an outbound link the subject
   is the context of use and the object is the resource identified by
   the target URI, and for an inbound link the subject is the resource
   identified by the target URI and the object is the context of use.

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

   Such applications may further constrain or extend links (e.g.,
   associating a media type hint, only allowing links in one direction).


4.  Link Relation Types

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

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

   As such, relation types are not format-specific, and MUST NOT specify
   a particular format or media type that they are to be used with.
   Likewise, a relation type SHOULD NOT specify what its context of its
   use is.

   Relation types are URIs.  Although specific applications of links may
   specify the use of URI-References, they must also indicate how to
   resolve them to absolute URIs.

   Although anyone may mint a URI to be used as a relation type, it is
   expected that a few standard types will predominate.  To facilitate
   this, Section 6.2 establishes an IANA registry of relation types that
   share a common base URI.





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5.  The Link Header Field

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

       Link           = "Link" ":" #link-value
       link-value     = "<" URI-Reference ">" *( ";" link-param ) )
       link-param     = ( ( "rel" "=" relation-type )
                      | ( "rev" "=" relation-type )
                      | ( "type" "=" type-name )
                      | ( "title" "=" quoted-string )
                      | ( link-extension ) )
       link-extension = token [ "=" ( token | quoted-string ) ]
       relation-type  = URI-Reference |
                      <"> URI-Reference *( SP URI-Reference) <">

   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.

   Each link-value conveys one target URI inside angle brackets ("<>").
   If it is relative, it MUST be resolved as per [RFC3986].  Note that
   because it is conveyed in a header, base URIs from content are not
   applied to it.

   The context of links conveyed in the Link header field is the
   representation that the header is part of.

   Each link-value MUST have at least one "rel" or "rev" parameter whose
   value indicates the relation type.  If the "rel" parameter is used,
   it indicates that the link's direction for that relation type is
   outbound; if the "rev" parameter is used, the given relation type's
   direction is inbound.

   If the relation-type is a relative URI, its base URI MUST be
   considered to be "http://www.iana.org/assignments/relation/", and the
   corresponding value MUST be present in the link relation registry.

   Relation-types that include a semicolon (";") or comma (",") MUST be
   quoted.

   The title parameter MAY be used to label the destination of a link



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   such that it can be used as identification within a human-readable
   menu.

   Note that link-values may contain multiple relations; for example

       Link: <http://example.org/>; rel="index start";
             rel="http://example.net/relation/other";
             rev=copyright

   Here, the link "http://example.org/" has outbound links of the types
   "http://www.iana.org/assignments/relation/index",
   "http://www.iana.org/assignments/relation/start", and
   "http://example.net/relation/other", as well as an inbound link of
   type "http://www.iana.org/assignments/relation/copyright".


6.  IANA Considerations

6.1.  Link 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):
      [ this document ]

6.2.  Link Relation Type Registry

   This specification establishes the Link Relation Type Registry,
   located at <http://www.iana.org/assignments/relation/>, and updates
   Atom [RFC4287] to refer to it in place of the "Registry of Link
   Relations".

   The semantics of relation types is described in Section 4.  This
   registry is intended to contain widely-used, standard relation types
   so that they may be used in "short form" (i.e., as a relative URI) in
   applications that allow this.

   Registered relation types have an implicit base URI of
   <http://www.iana.org/assignments/relation/>, and their name SHOULD be
   limited to the sgml-name rule for simplicity and backwards-
   compatibility.



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       sgml-name      = ALPHA *( ALPHA | DIGIT | "." | "-" )

   Names that differ only in case (e.g., "Foo" and "foo") MUST NOT be
   registered.

   New relation types can be registered by IETF Review, as outlined in
   [RFC5226].  Specifications of new values should use the following
   template:

   o  Relation Name:
   o  Description:
   o  Reference:

   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: edit





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   o  Description: Refers to a resource that can be used to edit the
      link's context.
   o  Reference: [RFC5023]

   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: A URI that refers to the furthest preceding resource
      in a series of resources.
   o  Reference: <http://www.iana.org/assignments/link-relations/first>

   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: index
   o  Description: Refers to an index.
   o  Reference: [W3C.REC-html401-19991224]

   o  Relation Name: last
   o  Description: A URI that refers to the furthest following resource
      in a series of resources.
   o  Reference: <http://www.iana.org/assignments/link-relations/last>

   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]





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   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:
      <http://www.iana.org/assignments/link-relations/payment>

   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: 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: start
   o  Description: Refers to the first resource in a collection of
      resources.
   o  Reference: [W3C.REC-html401-19991224]

   o  Relation Name: stylesheet





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   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]

   o  Relation Name: via
   o  Description: Identifies a resource that is the source of the
      information in the link's context.
   o  Reference: [RFC4287]


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.

   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.


8.  References

8.1.  Normative References

   [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.

   [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



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              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

8.2.  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.

   [RFC4287]  Nottingham, M. and R. Sayre, "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.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", W3C REC REC-html401-19991224,
              December 1999.


Appendix A.  Notes on Using the Link Header with HTML

   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" and rev"
   to convey both the relation type and its direction, as in the Link
   header.  The context of the link is generally the entire HTML
   document.

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




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


   <html>
     <head profile="http://example.com/profile1/">
       <link rel="foo" href="/foo">
     </head>
     [...]


   could be represented as a header like this;

   Link: </foo>; rel="http://example.com/profile1/foo"

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

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

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


Appendix B.  Notes on Using the Link Header with Atom

   Atom conveys links in the atom:link element, with the "href"
   attribute indicating the target URI and the "rel" attribute
   containing the relation type.  The context of the link is either a
   feed or an entry, depending on where it appears; generally, feed-
   level links are candidates for transmission as a Link header.  Since
   atom:link only specifies "rel", only outbound links are allowed by
   non-extended Atom syntax.

   When serialising an atom:link into a Link header, it is necessary to
   convert IRIs (if used) to URIs.  Additionally, since the base URI for
   link relations in Link headers is fixed, extension relation types
   (i.e,. those not in the registry) must be represented as absolute
   URIs.

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



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   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.


Appendix C.  Acknowledgements

   This specification lifts the idea and definition for the Link header
   from RFC2068; credit for it belongs entirely to the authors of and
   contributors to that document.  The link relation registrations
   themselves are sourced from several documents; see the applicable
   references.

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


Appendix D.  Document history

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

   -03

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

   -02

   o  Dropped XLink language.
   o  Removed 'made' example.
   o  Removed 'rev'.  Can still be used as an extension.
   o  Added HTML reference to introduction.
   o  Required relationship values that have a ; or , to be quoted.
   o  Changed base URI for relation values.
   o  Noted registry location.
   o  Added advisory text about HTML profile URIs.
   o  Disallowed registration of relations that only differ in case.





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   o  Clarified language about IRIs in Atom.
   o  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?
   o  Explicitly updates RFC4287.
   o  Added 'type' parameter.
   o  Removed unnecessary advice about non-HTML relations in HTML
      section.

   -01

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

   -00

   o  Initial draft; normative text lifted from RFC2068.


Author's Address

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   http://www.mnot.net/






















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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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