


Network Working Group                                      M. Nottingham
Internet-Draft                                             June 16, 2006
Expires: December 18, 2006


                          HTTP Header Linking
                  draft-nottingham-http-link-header-00

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification clarifies the status of the Link HTTP header and
   introduces the complimentary Profile and Link-Template HTTP headers.










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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 3
   3.  The Link Header Field . . . . . . . . . . . . . . . . . . . . . 3
   4.  The Profile Header Field  . . . . . . . . . . . . . . . . . . . 4
   5.  The Link-Template Header Field  . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9




































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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, 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.

   Additionally, the complementary "profile" mechanism -- which is often
   used to disambiguate link relationship types -- is not available as a
   HTTP header.

   This specification seeks to address these shortcomings.  It also
   introduces a new header, Link-Template, that allows the structure of
   links to be described.


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 specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC2616], and explicitly includes the following rules
   from it: quoted-string, token, SP (space), ALPHA (letters), DIGIT
   (decimal digit).  Additionally, the following rules are included from
   [RFC3986]: URI-Reference, reserved, unreserved.


3.  The Link Header Field

   The Link entity-header field provides a means for describing a
   relationship between two resources, generally between the requested
   resource and some other resource.  An entity MAY include multiple
   Link values.  Links at the metainformation level typically indicate
   relationships like hierarchical structure and navigation paths.  The
   Link field is semantically equivalent to the <LINK> element in HTML.







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

       link-param     = ( ( "rel" "=" relationship )
                      | ( "rev" "=" relationship )
                      | ( "title" "=" quoted-string )
                      | ( "anchor" "=" <"> URI-Reference <"> )
                      | ( link-extension ) )

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

       relationship   = sgml-name
                      | ( <"> sgml-name *( SP sgml-name) <"> )

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

   Relationship values are case-insensitive and MAY be extended within
   the constraints of the sgml-name syntax.  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.  The anchor parameter
   MAY be used to indicate a source anchor other than the entire current
   resource, such as a fragment of this resource or a third resource.

   Examples of usage include:

       Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
       Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"

   The first example indicates that chapter2 is previous to this
   resource in a logical navigation path.  The second indicates that the
   person responsible for making the resource available is identified by
   the given e-mail address.


4.  The Profile Header Field

   The Profile entity-header field provides a means to indicate the meta
   data profile of the entity.  Commonly, it is used to disambiguate the
   meaning of relationships in links.

   Note that this URI MAY be used as either an identifier (e.g., to
   uniquely identify links, without dereferencing the URI), or as a link
   that is intended to be dereferenced.

   The Profile field is semantically equivalent to the profile attribute
   of the HEAD element in HTML [W3C.REC-html401-19991224].  Note,
   however, that its use is not limited to HTML entities.




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       Profile       = "Profile" ":" #("<" URI-Reference ">")

   For example:

       Profile: <http://www.acme.com/profiles/core>
       Profile: <http://example.com/p1>, </profiles/other-profile>


5.  The Link-Template Header Field

   The Link-Template entity-header field provides a means for describing
   the structure of a link between two resources, so that new links can
   be generated.

   It does so through by allowing brace ("{}") -delimited strings to be
   interposed throughout a URI reference.  These correspond to variables
   which, after being replaced with content in a relation-specified
   manner, are semantically equivalent to the corresponding Link header.

   For example,

       Link-Template: <http://example.com/home/{userid}>; rel="home"

   This link indicates that the "home" link relation can be constructed
   if the userid variable is known; if it were known to be "bob", this
   header would be considered equivalent to

       Link: <http://example.com/home/bob>; rel="home"

   This specification does not define when or how template variables are
   interposed into link templates.  Link relations that wish to allow
   templating SHOULD specify such details.

   This specification does not define the correct behaviour in the face
   of a conflict between a Link-Template header and a Link header with
   the same relation.  Link relations allowing templating SHOULD specify
   their relative precedence.

   Applications SHOULD NOT use link relations that do not explicitly
   allow such templating in the Link-Template header.

       Link-Template  = "Link-Template" ":" #("<" template ">"
                      *( ";" link-param ) )

       template       = *( uri-char | template-var )

       template-var   = "{" 1*( uri-char ) "}"




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       uri-char       = ( reserved | unreserved )


6.  IANA Considerations

   This specification requires registration of two Message Header Fields
   for HTTP [RFC3864].  Note that "Link" is already present in the
   registry; this registration only updates its specification 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 ]


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


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


7.  Security Considerations

   The content of both the Link and Profile headers are not secure,
   private or integrity-guaranteed, and due caution should be excercised
   when using them.

   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.



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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.

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

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.


Appendix A.  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 semantics and much of the syntax of the Profile header was
   defined by HTML 4.01; credit for them belongs to the authors of and
   contributors to that document.

   Joe Gregorio, Marc Hadley and David Orchard contributed to the design
   of the Link-Template mechanism.








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Author's Address

   Mark Nottingham

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













































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