RFC 6415

Internet Engineering Task Force (IETF)              E. Hammer-Lahav, Ed.
Request for Comments: 6415                                       B. Cook
Category: Standards Track                                   October 2011
ISSN: 2070-1721

                           Web Host Metadata


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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2011 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
   (http://trustee.ietf.org/license-info) 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

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

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 (URIs) with a common URI host [RFC3986], which
   can be served by one or more servers.

   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., 'http://example.com/')
   with host metadata that is not specific or relevant to the root
   resource itself.

   This document defines a lightweight metadata document format for
   describing hosts (thus the name "host-meta"), intended for use by
   web-based protocols.  This document also registers the well-known URI
   suffix "host-meta" in the Well-Known URI Registry established by

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   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 resources or host-meta
      documents 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 'example.com' host:

   <?xml version='1.0' encoding='UTF-8'?>
   <XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>

     <!-- Host-Wide Information -->

     <Property type='http://protocol.example.net/version'>1.0</Property>

     <Link rel='copyright'
      href='http://example.com/copyright' />

     <!-- Resource-specific Information -->

     <Link rel='hub'
      template='http://example.com/hub' />

     <Link rel='lrdd'
      template='http://example.com/lrdd?uri={uri}' />

     <Link rel='author'
      template='http://example.com/author?q={uri}' />


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

   o  An "http://protocol.example.net/version" 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").

1.1.1.  Processing Resource-Specific Information

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

    <Link rel='hub'
     href='http://example.com/hub' />

    <Link rel='lrdd'
     href='http://example.com/lrdd?uri=http%3A%2F%2Fexample.com%2Fxy' />

    <Link rel='author'
     href='http://example.com/author?q=http%3A%2F%2Fexample.com%2Fxy' />

   The LRDD document for 'http://example.com/xy' (obtained via an HTTP
   "GET" request):

     <?xml version='1.0' encoding='UTF-8'?>
     <XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>


       <Property type='http://spec.example.net/color'>red</Property>

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       <Link rel='hub'
        href='http://example.com/another/hub' />

       <Link rel='author'
        href='http://example.com/john' />

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

   <?xml version='1.0' encoding='UTF-8'?>
   <XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>


    <Property type='http://spec.example.net/color'>red</Property>

    <Link rel='hub'
     href='http://example.com/hub' />

    <Link rel='hub'
     href='http://example.com/another/hub' />

    <Link rel='author'
     href='http://example.com/john' />

    <Link rel='author'
     href='http://example.com/author?q=http%3A%2F%2Fexample.com%2Fxy' />


   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",
   "OPTIONAL" in this document are to be interpreted as described in

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   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 sending
   an HTTP [RFC2616] or an HTTPS [RFC2818] GET request to the host for
   the "/.well-known/host-meta" path, using the default ports defined
   for each protocol (e.g., port 80 for HTTP and port 443 for HTTPS).
   The scope and meaning of host-meta documents obtained via other
   protocols or ports is undefined.

   The server MUST support at least one protocol but MAY support both.
   If both protocols are supported, they MUST produce the same document.

   The decision as to which protocol is used to obtain the host-meta
   document has significant security ramifications, as described in
   Section 5.

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

     GET /.well-known/host-meta HTTP/1.1
     Host: example.com

   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 another host.  Likewise, if the resource is not
   available or does not exist (e.g., a 404 or 410 response status
   code), the client SHOULD infer that metadata is not available via
   this mechanism.

   The host-meta document SHOULD be served with the
   "application/xrd+xml" media type.

3.  The host-meta Document

   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 specification defines
   additional processing rules needed to describe hosts.  Documents MAY
   include any elements included in the XRD 1.0 schema that are not
   explicitly excluded by this specification.

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   The server MAY offer alternative representations of any XRD document
   it serves (host-meta, LRDD, or other XRD-based documents).  The
   client MAY request a particular representation using the HTTP
   "Accept" request header field.  If no "Accept" request header field
   is included with the request, or if the client requests an
   "application/xrd+xml" representation, the server MUST respond using
   the REQUIRED XRD 1.0 XML representation described in Section 3.1.

   Applications using the host-meta document MAY require the server to
   provide a specific alternative representation in addition to the
   XRD 1.0 XML representation when explicitly requested by the client.

   A JavaScript Object Notation (JSON) Resource Descriptor, known as
   JRD, is described in Appendix A.  It is RECOMMENDED that servers
   offer the JRD representation in addition to the XRD representation.

3.1.  XML Document Format

   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 IRI", as defined by [RFC5988]) 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 individual resource whose URI is applied to the link
   template, as described in Section 3.1.1.

3.1.1.  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='http://example.com/copyright' />

   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.

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   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='http://example.com/author?article={uri}' />  Template Syntax

   This specification 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, values MUST be encoded using
   UTF-8, and any character other than unreserved (as defined by
   [RFC3986]) MUST be percent-encoded per [RFC3986].

   This specification 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.

   The template syntax ABNF follows:

    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:

    Input:    http://example.com/r?f=1
    Template: http://example.org/?q={uri}
    Output:   http://example.org/?q=http%3A%2F%2Fexample.com%2Fr%3Ff%3D1

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

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   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

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.

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       3.2.  The client adds any links 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 element).

5.  Security Considerations

   The host-meta document is designed to be used by other applications
   explicitly "opting-in" to use the facility.  Therefore, any such
   application MUST review the specific security implications of using
   host-meta documents.  By itself, this specification does not provide
   any protections or guarantees that any given host-meta document is
   under the control of the appropriate entity as required by each

   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 the applications using it.

   Applications utilizing the host-meta document where the authenticity
   of the information is necessary MUST require the use of the HTTPS
   protocol and MUST NOT produce a host-meta document using other means.
   In addition, such applications MUST require that any redirection
   leading to the retrieval of a host-meta document also utilize the
   HTTPS protocol.

   Since the host-meta document is authoritative for the entire host,
   not just the authority (combination of scheme, host, and port) of the
   host-meta document server, applications MUST ensure that using a
   host-meta document for another URI authority does not represent a
   potential security exploit.

   Protocols using host-meta templates must 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 that produces unexpected results
   when its variable values contain unexpected characters.

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6.  IANA Considerations

6.1.  The "host-meta" Well-Known URI

   This specification 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):  RFC 6415

   Related information:  The "host-meta" documents obtained from the
      same host using the HTTP and HTTPS protocols (using default ports)
      MUST be identical.

6.2.  The "host-meta.json" Well-Known URI

   This specification registers the "host-meta.json" well-known URI in
   the Well-Known URI Registry as defined by [RFC5785].

   URI suffix:  host-meta.json

   Change controller:  IETF

   Specification document(s):  RFC 6415

   Related information:  The "host-meta.json" documents obtained from
      the same host using the HTTP and HTTPS protocols (using default
      ports) MUST be identical.

6.3.  The "lrdd" Relation Type

   This specification registers the "lrdd" relation type in the Link
   Relation Type Registry defined by [RFC5988]:

   Relation Name:  lrdd

   Description:  Refers to further information about the link's context,
      expressed as a LRDD ("Link-based Resource Descriptor Document")
      resource.  See RFC 6415 for information about processing this
      relation type in host-meta documents.  When used elsewhere, it
      refers to additional links and other metadata.  Multiple instances
      indicate additional LRDD resources.  LRDD resources MUST have an
      "application/xrd+xml" representation, and MAY have others.

   Reference:  RFC 6415

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Appendix A.  JRD Document Format

   The JRD document format -- a general-purpose XRD 1.0 representation
   -- uses the JavaScript Object Notation (JSON) format defined in
   [RFC4627].  JRD uses the same elements and processing rules described
   in Section 3.1.  The JRD format is designed to include the same base
   functionality provided by the XML format, with the exception of
   extensibility, as extensibility is beyond the scope of this

   The client MAY request a JRD representation using the HTTP "Accept"
   request header field with a value of "application/json".  The server
   MUST include the HTTP "Content-Type" response header field with a
   value of "application/json".  Any other "Content-Type" value (or lack
   thereof) indicates that the server does not support the JRD format.

   Alternatively, the client MAY request a JRD representation by
   requesting the "host-meta.json" well-known document, by making a GET
   request for "/.well-known/host-meta.json", following the same process
   used for "/.well-known/host-meta".  If the server does not support
   serving a JRD representation at this location, the server MUST
   respond with an HTTP 404 (Not Found) status code.

   XRD elements are serialized into a JSON object as follows:

   o  The XML document declaration and "XRD" element are discarded.

   o  The "Subject" element is included as a name/value pair with the
      name 'subject', and value included as a string.

   o  The "Expires" element is included as a name/value pair with the
      name 'expires', and value included as a string.

   o  "Alias" elements are included as a single name/value pair with the
      name 'aliases', and value a string array containing the values of
      each element in order.

   o  "Property" elements are included as a single name/value pair with
      the name 'properties', and value an object with each element
      included as a name/value pair with the value of the "type"
      attribute as name, and element value included as a string value.
      The values of properties with empty values (i.e., using the
      REQUIRED "xsi:nil='true'" attribute) are included as null.  If
      more than one "Property" element is present with the same "type"
      attribute, only the last instance is included.

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   o  "Link" elements are included as a single name/value pair with the
      name 'links', and value an array with each element included as an
      object.  Each attribute is included as a name/value pair with the
      attribute name as name, and value included as a string.

   o  "Link" child "Property" elements are included using the same
      method as XRD-level "Property" elements using a name/value pair
      inside the link object.

   o  "Link" child "Title" elements are included as a single object with
      the name 'titles', and value an object with each element included
      as a name/value pair with the value of the "xml:lang" attribute as
      name, and element value included as a string value.  The names of
      elements without an "xml:lang" attribute are added with the name
      'default'.  If more than one "Title" element is present with the
      same (or no) "xml:lang" attribute, only the last instance is

   o  The conversion of any other element is left undefined.

   For example, the following XRD document...

    <?xml version='1.0' encoding='UTF-8'?>
    <XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'



      <Property type='http://blgx.example.net/ns/version'>1.2</Property>
      <Property type='http://blgx.example.net/ns/version'>1.3</Property>
      <Property type='http://blgx.example.net/ns/ext' xsi:nil='true' />

      <Link rel='author' type='text/html'
        <Title>About the Author</Title>
        <Title xml:lang='en-us'>Author Information</Title>
        <Property type='http://example.com/role'>editor</Property>

      <Link rel='author' href='http://example.com/author/john'>
        <Title>The other guy</Title>
        <Title>The other author</Title>

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      <Link rel='copyright'
            template='http://example.com/copyright?id={uri}' />

   ...is represented by the following JRD document:




            "default":"About the Author",
            "en-us":"Author Information"
            "default":"The other author"

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   Note that the "Subject" and "Alias" elements are NOT RECOMMENDED in
   the context of host-meta documents, and are included in the example
   for completeness only.

Appendix B.  Acknowledgments

   The authors would like to acknowledge the contributions of everyone
   who provided feedback and use cases for this specification -- in
   particular, Dirk Balfanz, DeWitt Clinton, Eve Maler, Breno de
   Medeiros, Brad Fitzpatrick, James Manger, Will Norris, Mark
   Nottingham, John Panzer, Drummond Reed, and Peter Saint-Andre.

Normative References

              Hammer-Lahav, E., Ed., and W. Norris, Ed., "Extensible
              Resource Descriptor (XRD) Version 1.0", November 2010,

   [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.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [RFC5234]  Crocker, D., Ed., 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.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

Hammer-Lahav & Cook          Standards Track                   [Page 15]

RFC 6415                        host-meta                   October 2011

Authors' Addresses

   Eran Hammer-Lahav (editor)

   EMail: eran@hueniverse.com
   URI:   http://hueniverse.com

   Blaine Cook

   EMail: romeda@gmail.com
   URI:   http://romeda.org

Hammer-Lahav & Cook          Standards Track                   [Page 16]