Network Working Group                                    E. Hammer-Lahav
Internet-Draft                                                    Yahoo!
Intended status: Informational                         November 10, 2009
Expires: May 14, 2010

                      host-meta: Web Host Metadata


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

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   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 BSD License.

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 'hm:Host' Element  . . . . . . . . . . . . . . . . . .  5
     3.2.  The 'Link' Element . . . . . . . . . . . . . . . . . . . .  6
       3.2.1.  Template Syntax  . . . . . . . . . . . . . . . . . . .  6
   4.  Obtaining host-meta Documents  . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  The host-meta Well-Known URI . . . . . . . . . . . . . . .  8
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . .  9
   7.  Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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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 entity
   controlling the collection of resources identified by URIs with a
   common host 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 storing
   its metadata in a common location used by 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
   overloading of the root HTTP resource (e.g. '')
   with host metadata that is not specific to the root resource (e.g. a
   home page or web application), and which 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.

   Please discuss this draft on the [1] mailing

1.1.  Example

   A simple host-meta document for the '' and
   '' hosts with a link providing host-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 "hm:" 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, and host.

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2.  Metadata Scope

   Each host-meta document describes one or more hosts.  The scope MUST
   be expressed explicitly within the document using the 'hm:Host'
   elements (Section 3.1).  The host-meta scope does not apply to any
   other hostname (or sub-domain) not explicitly declared.  For example,
   '', '', and '' all have
   different and non-overlapping scopes.

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
   schema for describing resources.  This memo defines additional
   elements and processing rules needed to describe hosts.  XRD elements
   not explicitly mentioned in this memo are permitted and used as
   defined in [OASIS.XRD-1.0].

   The host-meta document root MUST be an 'XRD' element.  The document
   SHOULD NOT include a 'Subject' element, as at this time no URI is
   available to identify hosts.  The use of the 'Alias' element in host-
   meta is undefined and NOT RECOMMENDED.

   This memo defines the 'hm:Host' 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 are the hosts included in the document scope, with the
   exception of 'Link' elements with a 'URITemplate' child element for
   which the subject are individual resources included in the document
   scope as defined in Section 3.2.

3.1.  The 'hm:Host' Element

   The 'hm:Host" 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 'hm:
   Host' elements (order does not matter).  If a host-meta document
   includes more than one 'hm:Host' element, it does not signify any
   relationship between the individual hosts other than sharing the same

   The element value syntax ABNF:

       Host-Element-Value  =  host

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3.2.  The 'Link' Element

   The XRD 'Link' element, when used with the 'URI' child element,
   conveys a link relation between the host (or hosts) described by the
   document and a common target URI.

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


   In addition, 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 providing an endpoint with a parameter
   set 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

   This memo defines a simple template syntax 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 unreserved (as defined by [RFC3986]) MUST be percent-encoded per

   This memo defines a single variable, 'uri', as the entire context
   URI.  Protocols MAY define additional relation-specific variables and
   syntax rules, but SHOULD only do so for protocol-specific relation
   types, and MUST NOT change the meaning of the 'uri' variable.  If a
   client is unable to successfully process a template (due to unknown
   variable names, or unknown or incompatible syntax) the parent 'Link'
   element SHOULD be ignored.

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   The template syntax ABNF:

       URI-Template  =  *( uri-char | variable )
       variable      =  "{" var-name "}"
       uri-char      =  ( reserved | unreserved )
       var-name      =  "uri" | ( 1*var-char )
       var-char      =  ALPHA / DIGIT / "." / "_"

   For example:


4.  Obtaining host-meta Documents

   The host-meta document is obtained by making an HTTP [RFC2616] GET
   request to the host's port 80, or an HTTPS [RFC2818] GET request to
   the host's port 443 for the '/.well-known/host-meta' path.

   Servers MUST support at least one but SHOULD support both ports
   unless restricted by other considerations.  If both ports are
   supported, they MUST serve the same document.  Clients MAY attempt to
   obtain the host-meta document from either port, and SHOULD attempt
   the other port if the first fails, unless restricted by other

   For example, the following request is used to obtain the host-meta
   document for the '' host:

       GET /.well-known/host-meta HTTP/1.1

   If the server response indicates that the host-meta resource is
   located elsewhere (a 301, 302, or 307 response status codes), the
   client SHOULD try to obtain the resource from the location provided
   in the response.  This means that the host-meta document for one host
   MAY be retrieved from a different host.  Likewise, if the resource is
   not available or exists (the 404 or 410 response status codes), the
   client SHOULD infer that metadata is not available via this

   If a representation is successfully obtained, but is not in the
   format described above, clients SHOULD infer that the path is being
   used 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"

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   internet media type.

   The scope declared within the host-meta document MUST match the
   desired host.

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.

   The host-meta scope is explicitly declared by the 'hm:Host' elements
   listed in the document.  Clients SHOULD evaluate the authority of a
   host-meta document obtained from one host to describe any other host.
   Protocols that change the scope from the one declared in the document
   without careful consideration can incur security risks.

   Protocols using host-meta templates SHOULD evaluate the construction
   of their templates as well as any protocol-specific variables or
   syntax to ensure that the templates cannot be abused by an attacker.
   For example, a client can be tricked into following a malicious link
   due to a poorly constructed template which produces unexpected
   results when its variable values contain unexpected characters.

   Protocols MAY restrict document retrieval to HTTPS based on their
   security needs.  Protocols utilizing host-meta documents obtained via
   other methods not described in this memo SHOULD consider the security
   and authority risks associated with such methods.

6.  IANA Considerations

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

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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, James Manger, Will Norris, Mark Nottingham, John Panzer,
   and Drummond Reed.

Appendix B.  Document History

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

   o  Corrected the <hm:Host> example.

   o  Changed scope to an entire host (per RFC 3986).
   o  Simplified template syntax to always percent-encode values and
      vocabulary to a single 'uri' variable.
   o  Changed document retrieval to always use HTTP(S).
   o  Added security consideration about the use of templates.
   o  Explicitly defined the root element to be 'XRD'.

   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.

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

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              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.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

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

   [1]  <>

Author's Address

   Eran Hammer-Lahav


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