Network Working Group                                    E. Hammer-Lahav
Internet-Draft                                                    Yahoo!
Intended status: Informational                          October 26, 2009
Expires: April 29, 2010

                      Host-Meta: Web Host Metadata

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   This memo describes a method for locating host metadata for Web-based

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Namespace and Version  . . . . . . . . . . . . . . . . . .  4
     1.3.  Notational Conventions . . . . . . . . . . . . . . . . . .  4
   2.  Metadata Scope . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  The host-meta Document Format  . . . . . . . . . . . . . . . .  5
     3.1.  The 'host-meta:Scope' Element  . . . . . . . . . . . . . .  6
     3.2.  The 'Link' Element . . . . . . . . . . . . . . . . . . . .  6
   4.  Obtaining host-meta Documents  . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  The host-meta Well-Known URI . . . . . . . . . . . . . . . 10
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 10
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 10
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   Web-based protocols often require the discovery of host policy or
   metadata, where host is not a single resource but the collection of
   resources identified by URIs with a common scheme and authority as
   defined by [RFC3986].  While these protocols have a wide range of
   metadata needs, they often define metadata that is concise, has
   simple syntax requirements, and can benefit from sharing its metadata
   store with other related protocols.

   Because there is no URI or a resource available to describe a host,
   many of the methods used for associating per-resource metadata (such
   as HTTP headers) are not available.  This often leads to the usage of
   the root HTTP resource as a placeholder for host metadata that is not
   specific to the root resource, and often has nothing to do it.

   This memo registers the "well-known" URI suffix 'host-meta' in the
   Well-Known URI Registry established by [I-D.nottingham-site-meta],
   and specifies a simple, general-purpose metadata document for hosts,
   to be used by multiple Web-based protocols.  The name 'host-meta' was
   chosen based on the HTTP 'Host' header defined by [RFC2616].

   Please discuss this draft on the [1] mailing

1.1.  Example

   A simple host-meta document for the '' and
   '' authorities and 'http' scheme with a link providing
   scope-wide copyright information and a link template providing a URI
   for obtaining resource-specific metadata for each resource within the
   host-meta document scope:

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    <?xml version='1.0' encoding='UTF-8'?>
    <XRD xmlns=''


            <Title xml:lang='en-us'>Site License Policy</Title>
            <Title xml:lang='en-us'>Resource Descriptor</Title>

1.2.  Namespace and Version

   The host-meta document uses the XRD 1.0 XML namespace URI

   The XML namespace URI for the host-meta specific extension elements
   defined in this specification is:

1.3.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This specification uses the namespace prefix "host-meta:" for the
   extension Namespace URI identified in Section 1.2.  Note that the
   choice of namespace prefix is arbitrary and not semantically
   significant.  Element names without a namespace prefix belong to the
   XRD 1.0 XML namespace identified in Section 1.2.

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [RFC5234].  Additionally, the following rules are included from
   [RFC3986]: reserved, unreserved, authority, and scheme.

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2.  Metadata Scope

   The scope of each host-meta document is defined by one or more
   scheme-authority pairs, each applies to the collection of resources
   identified by URIs with the same scheme and authority as defined by
   [RFC3986].  The scope MUST be expressed explicitly within the
   document by using the one or more 'host-meta:Scope' elements
   (Section 3.1).

   The host-meta scope does not apply to a subset, nor does it apply to
   other combinations of scheme and authority (e.g., using another
   scheme, port, or a different hostname in the same domain) not
   explicitly declared.  For example, resources with URIs beginning with
   '', '',
   '', and '' all have
   different and non-overlapping scopes.

   The scope declared within the host-meta document MUST match the
   scheme and authority used to obtain (Section 4) the document.
   However, protocols MAY use a different scheme to obtain the document
   than the one they are interested in, as long as both scheme and
   authority combinations are declared.  For example, a protocol can use
   HTTP to obtain metadata about a scheme other than 'http' for which a
   host-meta document cannot be obtained using the method described in
   Section 4.

   Protocols MAY place additional requirements on protocol-specific
   metadata retrieved from multiple host-meta documents.  For example,
   protocols can require that protocol-specific metadata obtained for
   the '' authority to be the same in both host-meta
   documents for the 'http' and 'https' schemes.

   Any changes in scope, the retrieval of metadata for one scheme using
   another, and other requirements should only occur after a careful
   consideration of their security implications and authoritativeness.

3.  The host-meta Document Format

   The host-meta document uses the XRD 1.0 document format as defined by
   [OASIS.XRD-1.0], which provides a simple and extensible XML-based
   format for describing resources.  Since the host-meta XRD document
   does not describe a single resource but a collection of resources,
   this memo defines additional XRD elements and processing rules.  The
   XRD elements not mentioned in this memo are allowed and used as
   defined in [OASIS.XRD-1.0].

   host-meta document SHOULD NOT include the 'Subject' or 'Alias' XRD

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   elements since these elements require a valid URI to identify the
   resource being described, which is not available for the host-meta
   scope.  The use of these elements in host-meta is undefined.
   Instead, host-meta defines the 'host-meta:Scope' element
   (Section 3.1) for declaring document scope.

   The subject (or "context resource" as defined by
   [I-D.nottingham-http-link-header]) of the XRD 'Type' and 'Link'
   elements is the entire collection of resources included in the
   document scope, with the exception of 'Link' elements with a
   'URITemplate' child element as defined in Section 3.2.

3.1.  The 'host-meta:Scope' Element

   The 'host-meta:Scope" element is used to declare the scope of the
   host-meta document and is defined as a child element of the root
   'XRD' element.  The parent 'XRD' element MUST include one but MAY
   include more 'host-meta:Scope' elements (order does not matter).

   The element value syntax ABNF:

       Scope  =  scheme "://" authority

   If the authority component does not include a port number, the port
   is implicitly set to the default port number for the given scheme,
   and does not include any other port number.

   For example, these two scope declarations are equivalent:


3.2.  The 'Link' Element

   The XRD 'Link' element, when used with the 'URI' child element,
   conveys a link relation between the collection of resources included
   in the host-meta document scope and a common target URI.

   For example, the following link declares a common author for the
   entire scope:


   However, there are cases in which individual resources within the

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   same scope do not share the same target URI, but follow a common
   pattern in how the target URI is constructed.  A 'Link' element with
   a 'URITemplate' child element conveys relations whose context are
   individual resources within the host-meta document scope, and whose
   target is constructed by applying the context URI to a template.

   For example, a blog with multiple authors can provide information
   about each article's author by appending a suffix (such as ';by') to
   the URI of each article.  Each article has a unique author, but all
   share the same pattern of where that information is located:


3.2.1.  Template Syntax

   host-meta defines a simple template syntax and a vocabulary for URI
   transformation.  A template is a string containing brace-enclosed
   ("{}") variable names marking the parts of the string that are to be
   substituted by the corresponding variable values.

   Before substituting template variables, any value character other
   than reserved and unreserved (as defined by [RFC3986]) MUST be
   percent-encoded per [RFC3986].  If a variable name is not prefixed by
   a '+' character, any reserved character SHALL also be percent-

   To construct a URI using a template, the context URI is parsed into
   its URI components and each component value assigned to a variable
   name.  The variable set is based on the URI vocabulary defined by
   [RFC3986] section 3 and includes: 'scheme', 'authority', 'path',
   'query', 'fragment', 'userinfo', 'host', and 'port'.  In addition,
   this memo defines the 'uri' variable as the entire context URI
   excluding the fragment component and the '#' fragment separator.

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       \_/   \______________________/\_________/ \_________/ \__/
        |              |                  |           |        |
       scheme      authority             path       query   fragment

             \_____/ \_________/ \__/
                |         |        |
            userinfo     host     port


   Protocols MAY define additional variables and syntax rules, but
   SHOULD only do so for protocol-specific relation types, and MUST NOT
   change the meaning of the variables defined above.  If a template
   uses an unknown syntax or contains unknown variable names, the parent
   'Link' element SHOULD be ignored.

   The template syntax ABNF:

       URI-Template  =  *( uri-char | variable )
       variable      =  "{" [ "+" ] var-name "}"
       uri-char      =  ( reserved | unreserved )
       var-name      =  uri-var | ( 1*var-char )
       uri-var       =  "scheme" | "authority" | "path"     |
                        "query"  | "fragment"  | "userinfo" |
                        "host"   | "port"      | "uri"
       var-char      =  ALPHA / DIGIT / "." / "_"

   For example, given the input URI '',
   each of the following templates will produce the associated output

       {+uri}&test -->{uri} -->

       http://meta.{host}:8080{+path}?{+query} -->

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4.  Obtaining host-meta Documents

   The host-meta document for a given scheme-authority pair is
   identified by a URI constructed using the desired scheme, authority,
   and '/.well-known/host-meta' as the URI path.  For example, given the
   'http' scheme and '' authority, the host-meta URI

   The host-meta document is obtained by dereferencing the request host-
   meta URI.  The semantics of the protocol used to obtain the host-meta
   document apply.  Therefore, if the server indicates that the host-
   meta resource is located elsewhere (in HTTP, 3xx response status
   codes), the client MUST try to obtain the resource from the location
   provided.  This means that the host-meta document for one authority
   MAY be retrieved from a different authority.  Likewise, if the
   resource is not available or exists (in HTTP, the 404 or 410 response
   status codes), the client SHOULD infer that metadata is not available
   via this mechanism.

   If a representation is successfully obtained, but is not in the
   format described above, clients SHOULD infer that the authority is
   using this URI for other purposes, and not process it as a host-meta
   document.  To aid in this process, authorities using this mechanism
   SHOULD correctly label host-meta responses with the "application/
   xrd+xml" internet media type.

5.  Security Considerations

   The metadata returned by the host-meta resource is presumed to be
   under the control of the appropriate authority and representative of
   all the resources described by it.  If this resource is compromised
   or otherwise under the control of another party, it may represent a
   risk to the security of the server and data served by it, depending
   on what protocols use it.

   Scoping metadata to a single scheme-authority pair is the default
   host-meta scope.  Protocols that change the scope without careful
   consideration can incur security risks.

6.  IANA Considerations

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6.1.  The host-meta Well-Known URI

   This memo registers the 'host-meta' well-known URI in the Well-Known
   URI Registry as defined by [I-D.nottingham-site-meta].

   URI suffix:  host-meta

   Change controller:  IETF

   Specification document(s):  [[ this document ]]

   Related information:  None

Appendix A.  Acknowledgments

   This memo was initially based on [I-D.nottingham-site-meta].

   The author would like to acknowledge the contributions of everyone
   who provided feedback and use cases for this memo; in particular,
   Dirk Balfanz, DeWitt Clinton, Blaine Cook, Breno de Medeiros, Brad
   Fitzpatrick, Will Norris, Mark Nottingham, John Panzer, and Drummond

Appendix B.  Document History

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


   o  Changed Scope element syntax from attributes to URI-like string


   o  Editorial rewrite.

   o  Redefined scope as a scheme-authority pair.

   o  Added document structure section.


   o  Initial draft.

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7.  Normative References

              Nottingham, M., "Web Linking",
              draft-nottingham-http-link-header-06 (work in progress),
              July 2009.

              Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              URIs", draft-nottingham-site-meta-03 (work in progress),
              September 2009.

              Hammer-Lahav, E. and W. Norris, "Extensible Resource
              Descriptor (XRD) Version 1.0 (work in progress)", <http://

   [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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

              Marchiori, M., "The Platform for Privacy Preferences 1.0
              (P3P1.0) Specification", World Wide Web Consortium
              Recommendation REC-P3P-20020416, April 2002,

              Hollander, D., Bray, T., and A. Layman, "Namespaces in
              XML", World Wide Web Consortium FirstEdition REC-xml-
              names-19990114, January 1999,

   [1]  <>

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Author's Address

   Eran Hammer-Lahav


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