


Network Working Group                                      M. Nottingham
Internet-Draft                                              July 3, 2008
Updates: 4287 (if approved)
Intended status: Standards Track
Expires: January 4, 2009


                          HTTP Header Linking
                  draft-nottingham-http-link-header-02

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Abstract

   This document clarifies the status of the Link HTTP header and
   attempts to consolidate link relations in a single registry.












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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  The Link Header Field  . . . . . . . . . . . . . . . . . . . .  4
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Notes on HTML Links . . . . . . . . . . . . . . . . . 10
   Appendix B.  Notes on Atom Links . . . . . . . . . . . . . . . . . 11
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Appendix D.  Document history  . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13



































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1.  Introduction

   A means of indicating the relationships between documents on the Web
   has been available for some time in HTML [W3C.REC-html401-19991224],
   and was considered as a HTTP header in [RFC2068], but removed from
   [RFC2616], due to a lack of implementation experience.

   There have since surfaced many cases where a means of including this
   information in HTTP headers has proved useful.  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 seeks to address these shortcomings.

   Additionally, formats other than HTML -- namely, Atom [RFC4287] --
   have also defined generic linking mechanisms that are similar to
   those in HTML, but not identical.  This document aims to reconcile
   these differences when such links are expressed as headers.

   [[ NOTE: This is a straw-man draft that is intended to give a ROUGH
   idea of what it would take to align and consolidate the HTML and Atom
   link relations into a single registry with reasonable extensibility
   rules.  In particular; a) it changes the registry for Atom link
   relations, and the process for registration; b) it assigns more
   generic semantics to several existing link relations, both Atom and
   HTML; c) it changes the syntax of the Link header (in the case where
   extensions are present).  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-Reference, and from [RFC4288]: type-
   name.







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3.  The Link Header Field

   The Link entity-header field provides a means for describing a
   relationship between two resources, generally between that of the
   entity associated with the header and some other resource.  An entity
   MAY include multiple Link values.

   The Link header field is semantically equivalent to the <LINK>
   element in HTML, as well as the atom:link element in Atom [RFC4287].

       Link           = "Link" ":" #("<" URI-Reference ">"
                      *( ";" link-param ) )

       link-param     = ( ( "rel" "=" relationship )
                      | ( "type" "=" type-name )
                      | ( "title" "=" quoted-string )
                      | ( link-extension ) )

       link-extension = token [ "=" ( token | quoted-string ) ]

       relationship   = URI-Reference |
                      <"> URI-Reference *( SP URI-Reference) <"> )

   Relationship values that include a semicolon (";") or comma (",")
   MUST be quoted.

   The title parameter MAY be used to label the destination of a link
   such that it can be used as identification within a human-readable
   menu.

   For example:

       Link: <http://www.cern.ch/TheBook/chapter2>; rel="previous";
             title="previous chapter"

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

3.1.  Link Relationships

   Relationship values are URIs that identify the type of link.  If the
   relationship is a relative URI, its base URI MUST be considered to be
   "http://www.iana.org/assignments/relation/", and the value MUST be
   present in the link relation registry.







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4.  IANA Considerations

4.1.  Link Header Registration

   This specification requires that the Message Header Registry entry
   for "Link" in HTTP [RFC3864] be updated to refer to this document,
   and located at "http://www.iana.org/assignments/relation/".

   Header field: Link
   Applicable protocol: http
   Status: standard
   Author/change controller:
       IETF  (iesg@ietf.org)
       Internet Engineering Task Force
   Specification document(s):
      [ this document ]

4.2.  Link Relation Registry

   This specification is intended to update Atom to become the reference
   for the Link Relation registry, and clarifies its nature and use.

   A Link relation is a way of indicating the semantics of a link.  Link
   relations are not format-specific, and MUST NOT specify a particular
   format or media type that they are to be used with.

   The security considerations of following a particular link are not
   determined by the link's relation type; they are determined by the
   specific context of the use and the media type of the response.

   Likewise, a link relation SHOULD NOT specify what the context of its
   use is, although the media type of the dereferenced link may
   constrain how it is applied.

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

   New relations MAY be registered, subject to IESG Approval, as
   outlined in [RFC2434].  Requests should be made by email to IANA,
   which will then forward the request to the IESG, requesting approval.
   The request should use the following template:

   o  Relation Name:
   o  Description:
   o  Reference:

   The Link Relation registry replaces the Atom Link Relation registry,
   using the same address with the following contents:



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   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.
   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
   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.




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   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]

   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".




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   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
   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]


5.  Security Considerations

   The content the Link headers is not secure, private or integrity-
   guaranteed, and due caution should be excercised when using it.




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   Applications that take advantage of these mechanisms should consider
   the attack vectors opened by automatically following, trusting, or
   otherwise using links gathered from HTTP headers.


6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [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.

6.2.  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.




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   [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 HTML Links

   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.

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

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


   <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



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   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 Atom Links

   Atom conveys links in the atom:link element.  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 links (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
   single Link header 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 definition of 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

   -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.





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   o  Noted registry location.
   o  Added advisory text about HTML profile URIs.
   o  Disallowed registration of relations that only differ in case.
   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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