Network Working Group                                    E. Hammer-Lahav
Internet-Draft                                                    Yahoo!
Intended status: Informational                              May 11, 2010
Expires: November 12, 2010

                      host-meta: Web Host Metadata


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

Status of this Memo

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   This Internet-Draft will expire on November 12, 2010.

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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 '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.  host-meta XML Schema  . . . . . . . . . . . . . . . .  9
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . .  9
   Appendix C.  Document History  . . . . . . . . . . . . . . . . . .  9
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11

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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 Universal
   Resource Identifiers (URI) 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 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, 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 [RFC5785], 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
   list. ]]

1.1.  Example

   The following is 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 author information for each resource within the
   host-meta document scope:

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


           <Link rel='license'
               <Title xml:lang='en-us'>Site License Policy</Title>
           <Link rel='author'
               <Title xml:lang='en-us'>Author Profile</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].

   Examples in this specification uses the namespace prefix "hm:" for
   the extension Namespace URI identified in Section 1.2.  The "hm:"
   namespace prefix is arbitrary and not is 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, where a 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], across all ports and schemes.

   The scope MUST be expressed explicitly within the document using the
   "hm:Host" element as described in 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.  Documents
   MAY include any XRD element not explicitly excluded.

   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 for declaring document scope.
   The subject (or "context resource" as defined by
   [I-D.nottingham-http-link-header]) of the XRD "Property" and "Link"
   elements consists of the hosts included in the document scope.
   However, the subject of "Link" elements with a "template" attribute
   is the individual resources (included in the document scope) applied
   to the link template as described 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
   metadata included in the document.

   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 "href" attribute, conveys
   a link relation between the hosts described by the document and a
   common target URI.

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

       <Link rel='author' href='' />

   However, a "Link" element with a "template" attribute conveys
   relations whose context are the individual resources within the host-
   meta document scope, and whose target is constructed by applying each
   context resource URI to the template.  The template string MAY
   contain a URI string without any variables to represent a resource-
   level relation that is identical for every individual resource.

   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:

       <Link rel='author' template='{uri}' />

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
   resource 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
   (e.g. unknown variable names, 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

   Clients obtain the host-meta document for a given host by making an
   HTTPS [RFC2818] GET request to the host's port 443 for the
   "/.well-known/host-meta" path.  If the request fails to produce a
   valid host-meta document, clients make an HTTP [RFC2616] GET request
   to the host's port 80 for the "/.well-known/host-meta" path.

   Servers MUST support at least one but SHOULD support both ports.  If
   both ports are supported, they MUST serve the same document.  Clients
   MAY attempt to obtain the host-meta document from either port, SHOULD
   attempt using port 443 first, and SHOULD attempt the other port if
   the first fails.

   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 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 the response 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.

   If the server response indicates that the host-meta resource is
   located elsewhere (a 301, 302, or 307 response status code), 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 another host.  Likewise, if the resource is
   not available or does not exist (indicated respectively, by the 404

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   and 410 response status codes) at both ports, the client should infer
   that metadata is not available via this mechanism.

   The scope declared within the host-meta document MUST match the
   desired host.  Clients MUST NOT use host-meta documents when the
   desired host (used to obtain the document) is not listed in the

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 another 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 [RFC5785].
   URI suffix:  host-meta

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   Change controller:  IETF
   Specification document(s):  [[ this document ]]
   Related information:  None

Appendix A.  host-meta XML Schema

   The following is the XML schema for the host-meta XRD extension

     <?xml version="1.0" encoding="UTF-8"?>

       <element name="Host" type="hm:HostType"/>

       <complexType name="HostType">
           <extension base="string">
             <anyAttribute namespace="##other" processContents="lax"/>


Appendix B.  Acknowledgments

   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, Eve Maler, Breno de
   Medeiros, Brad Fitzpatrick, James Manger, Will Norris, Mark
   Nottingham, John Panzer, Drummond Reed, and Peter Saint-Andre.

Appendix C.  Document History

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


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   o  Minor editorial clarifications.
   o  Added XML schema for host-meta extension.
   o  Updated XRD reference to the latest draft (no normative changes).

   o  Updated well-known reference to RFC 5785.
   o  Minor editorial changes.
   o  Made HTTPS a higher priority (SHOULD) over HTTP.

   o  Adjusted syntax to the latest XRD schema.
   o  Added note about using a link template without variables.

   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.

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

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   [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.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

              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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