Network Working Group                                    E. Hammer-Lahav
Internet-Draft                                                    Yahoo!
Intended status: Standards Track                           June 15, 2010
Expires: December 17, 2010

                           Web Host Metadata


   This memo describes a method for locating host metadata as well as
   information about individual resources controlled by the host.

Editorial Note (to be removed by RFC Editor)

   Please discuss this draft on the [1] mailing

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 17, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.1.  Processing Resource-Specific Information . . . . . . .  5
     1.2.  Notational Conventions . . . . . . . . . . . . . . . . . .  6
   2.  Obtaining host-meta Documents  . . . . . . . . . . . . . . . .  6
   3.  The host-meta Document Format  . . . . . . . . . . . . . . . .  7
     3.1.  The 'Link' Element . . . . . . . . . . . . . . . . . . . .  8
       3.1.1.  Template Syntax  . . . . . . . . . . . . . . . . . . .  8
   4.  Processing host-meta Documents . . . . . . . . . . . . . . . .  9
     4.1.  Host-Wide Information  . . . . . . . . . . . . . . . . . .  9
     4.2.  Resource-Specific Information  . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  The 'host-meta' Well-Known URI . . . . . . . . . . . . . . 11
     6.2.  The 'lrdd' Relation Type . . . . . . . . . . . . . . . . . 11
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 12
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 12
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15

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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 Uniform
   Resource Identifiers (URI) with a common URI host [RFC3986].

   While web protocols have a wide range of metadata needs, they often
   use metadata that is concise, has simple syntax requirements, and can
   benefit from storing their metadata in a common location used by
   other related protocols.

   Because there is no URI or representation 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 or relevant to the root
   resource itself.

   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 format for hosts, to be
   used by multiple web-based protocols.

   In addition, there are times when a host-wide scope for policy or
   metadata is too coarse-grained. host-meta provides two mechanisms for
   providing resource-specific information:

   o  Link Templates - links using a URI template instead of a fixed
      target URI, providing a way to define generic rules for generating
      resource-specific links by applying the individual resource URI to
      the template.

   o  Link-based Resource Descriptor Documents (LRDD, pronounced 'lard')
      - descriptor documents providing resource-specific information,
      typically information that cannot be expressed using link
      templates.  LRDD documents are linked to using link templates with
      the "lrdd" relation type.

1.1.  Example

   The following is a simple host-meta document including both host-wide
   and resource-specific information for the '' host:

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

     <!-- Host-wide Information -->

     <Property type=''>1.0</Property>

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

     <!-- Resource-specific Information -->

     <Link rel='hub'
      template='' />

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

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


   The host-wide information which applies to host in its entirety
   provided by the document includes:

   o  A "" host property with a value
      of "1.0".

   o  A link to the host's copyright policy ("copyright").

   The resource-specific information provided by the document includes:

   o  A link template for receiving real-time updates ("hub") about
      individual resources.  Since the template does not include a
      template variable, the target URI is identical for all resources.

   o  A LRDD document link template ("lrdd") for obtaining additional
      resource-specific information contained in a separate document for
      each individual resource.

   o  A link template for finding information about the author of
      individual resources ("author").

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1.1.1.  Processing Resource-Specific Information

   When looking for information about the an individual resource, for
   example, the resource identified by '', the
   resource URI is applied to the templates found, producing the
   following links:

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

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

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

   The LRDD document for '' is obtained using an
   HTTP "GET" request:

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


       <Property type=''>red</Property>

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

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

   Together, the information available about the individual resource
   (presented as an XRD document for illustration purposes) is:

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


    <Property type=''>red</Property>

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

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

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

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


   Note that the order of links matters and is based on their original
   order in the host-meta and LRDD documents.  For example, the "hub"
   link obtained from the host-meta link template has a higher priority
   than the link found in the LRDD document because the host-meta link
   appears before the "lrdd" link.

   On the other hand, the "author" link found in the LRDD document has a
   higher priority than the link found in the host-meta document because
   it appears after the "lrdd" link.

1.2.  Notational Conventions

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

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

2.  Obtaining host-meta Documents

   The client obtains 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

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   valid host-meta document, the client makes an HTTP [RFC2616] GET
   request to the host's port 80 for the "/.well-known/host-meta" path.

   The server MUST support at least one but SHOULD support both ports.
   If both ports are supported, they MUST serve the same document.  The
   client 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 the server response indicates that the host-meta resource is
   located elsewhere (a 301, 302, or 307 response status code), the
   client MUST 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 another host.  Likewise, if the resource is not
   available or does not exist (e.g. a 404 or 410 response status codes)
   at both ports, the client should infer that metadata is not available
   via this mechanism.

   The host-meta document SHOULD be server with the
   "application/xrd+xml" media type. [[ media type registration pending

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
   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.

   The subject (or "context resource" as defined by
   [I-D.nottingham-http-link-header]) of the XRD "Property" and "Link"
   elements is the host described by the host-meta document.  However,
   the subject of "Link" elements with a "template" attribute is the

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   individual resource whose URI is applied to the link template as
   described in Section 3.1.

3.1.  The 'Link' Element

   The XRD "Link" element, when used with the "href" attribute, conveys
   a link relation between the host described by the document and a
   common target URI.

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

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

   However, a "Link" element with a "template" attribute conveys a
   relation whose context is an individual resource within the host-meta
   document scope, and whose target is constructed by applying the
   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}' />

3.1.1.  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"

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   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.

   The template syntax ABNF:

     URI-Template =  *( uri-char / variable )
     variable     =  "{" var-name "}"
     uri-char     =  ( reserved / unreserved / pct-encoded )
     var-name     =  %x75.72.69 / ( 1*var-char ) ; "uri" or other names
     var-char     =  ALPHA / DIGIT / "." / "_"

   For example:


4.  Processing host-meta Documents

   Once the host-meta document has been obtained, the client processes
   its content based on the type of information desired: host-wide or

   Clients usually look for a link with a specific relation type or
   other attributes.  In such cases, the client does not need to process
   the entire host-meta document and all linked LRDD documents, but
   instead, process the various documents in their prescribed order
   until the desired information is found.

   Protocols using host-meta must indicate whether the information they
   seek is host-wide or resource-specific.  For example, "obtain the
   first host-meta resource-specific link using the 'author' relation
   type".  If both types are used for the same purpose (e.g. first look
   for resource-specific, then look for host-wide), the protocol must
   specify the processing order.

4.1.  Host-Wide Information

   When looking for host-wide information, the client MUST ignore any
   "Link" elements with a "template" attribute, as well as any link
   using the "lrdd" relation type.  All other elements are scoped as

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4.2.  Resource-Specific Information

   Unlike host-wide information which is contained solely within the
   host-meta document, resource-specific information is obtained from
   host-meta link templates, as well as from linked LRDD documents.

   When looking for resource-specific information, the client constructs
   a resource descriptor by collecting and processing all the host-meta
   link templates.  For each link template:

   1.  The client applies the URI of the desired resource to the
       template, producing a resource-specific link.

   2.  If the link's relation type is other than "lrdd", the client adds
       the link to the resource descriptor in order.

   3.  If the link's relation type is "lrdd":

       3.1  The client obtains the LRDD document by following the
            scheme-specific rules for the LRDD document URI.  If the
            document URI scheme is "http" or "https", the document is
            obtained via an HTTP "GET" request to the identified URI.
            If the HTTP response status code is 301, 302, or 307, the
            client MUST follow the redirection response and repeat the
            request with the provided location.

       3.2  The client adds any link found in the LRDD document to the
            resource descriptor in order, except for any link using the
            "lrdd" relation type (processing is limited to a single
            level of inclusion).  When adding links, the client SHOULD
            retain any extension attributes and child elements if
            present (e.g. <Property> or <Title> elements).

       3.3  The client adds any resource properties found in the LRDD
            document to the resource descriptor in order (e.g. <Alias>
            or <Property> child elements of the LRDD document <XRD> root

   A detailed example is provided in Section 1.1.1.

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

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   on what protocols use it.

   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

   Change controller:  IETF

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

   Related information:  None

6.2.  The 'lrdd' Relation Type

   This specification registers the "lrdd" relation type in the Link
   Relation Type Registry defined by [I-D.nottingham-http-link-header]:

   Relation Name:  lrdd

   Description:  "lrdd" (pronounced 'lard') is an acronym for Link-based
      Resource Descriptor Document.  It is used by the host-meta
      document processor to locate resource-specific information about
      individual resources.  When used elsewhere (e.g.  HTTP "Link"
      header fields or HTML <LINK> elements), it operates as an include
      directive, identifying the location of additional links and other
      metadata.  Multiple links with the 'lrdd' relation indicate
      multiple sources to include, not alternative sources of the same
      information.  An "application/xrd+xml" representation MUST be
      available, and this media type MAY appear in a link's "type"
      attribute.  Additional representations MAY be available (using

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      HTTP content negotiation), in which case the link's 'type'
      attribute SHOULD be omitted.

   Reference:  [[ This specification ]]

Appendix A.  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 B.  Document History

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


   o  Changed to standard track.

   o  Added note about host-meta media type.


   o  Clarified use of media type, simplified processing flow.


   o  Editorial clarifications.


   o  Integrated LRDD into the memo, dropping the multiple sources and
      using only host-meta for LRDD processing.


   o  Removed the <hm:Host> element due to lack of use cases (protocols
      with signature requirements can define their own way of declaring
      the document's subject for this purpose).

   o  Minor editorial changes.

   o  Changed following redirections to MUST.

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   o  Updated references.


   o  Fixed typo.


   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'.


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   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-10 (work in progress),
              May 2010.

              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.

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

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   [1]  <>

Author's Address

   Eran Hammer-Lahav


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