    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

    Abstract

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

    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
       http://www.rfc-editor.org/info/rfc6415.

    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
       [RFC5785].

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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'
          type='application/xrd+xml'
          template='http://example.com/lrdd?uri={uri}' />

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

       </XRD>

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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'
         type='application/xrd+xml'
         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'>

           <Subject>http://example.com/xy</Subject>

           <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' />
         </XRD>

       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'>

        <Subject>http://example.com/xy</Subject>

        <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' />

       </XRD>

       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",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       [RFC2119].

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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}' />

    3.1.1.1.  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
       resource-specific.

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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
       host-wide.

    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
       application.

       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
       specification.

       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
          included.

       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'
             xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>

          <Subject>http://blog.example.com/article/id/314</Subject>
          <Expires>2010-01-30T09:30:00Z</Expires>

          <Alias>http://blog.example.com/cool_new_thing</Alias>
          <Alias>http://blog.example.com/steve/article/7</Alias>

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

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

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

       ...is represented by the following JRD document:

        {
          "subject":"http://blog.example.com/article/id/314",
          "expires":"2010-01-30T09:30:00Z",

          "aliases":[
            "http://blog.example.com/cool_new_thing",
            "http://blog.example.com/steve/article/7"],

          "properties":{
            "http://blgx.example.net/ns/version":"1.3",
            "http://blgx.example.net/ns/ext":null
          },

          "links":[
            {
              "rel":"author",
              "type":"text/html",
              "href":"http://blog.example.com/author/steve",
              "titles":{
                "default":"About the Author",
                "en-us":"Author Information"
              },
              "properties":{
                "http://example.com/role":"editor"
              }
            },
            {
              "rel":"author",
              "href":"http://example.com/author/john",
              "titles":{
                "default":"The other author"
              }
            },
            {
              "rel":"copyright",
              "template":"http://example.com/copyright?id={uri}"
            }
          ]
        }

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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

       [OASIS.XRD-1.0]
                  Hammer-Lahav, E., Ed., and W. Norris, Ed., "Extensible
                  Resource Descriptor (XRD) Version 1.0", November 2010,
                  <http://docs.oasis-open.org/xri/xrd/v1.0/xrd-1.0.html>.

       [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]