Extensible Resource Descriptor (XRD) Version 1.0

Working Draft 06, 04 September 2009

Document identifier:
xrd-1.0-wd06 (XML, HTML, PDF)
Locations:
Persistent version: http://docs.oasis-open.org/xri/xrd/v1.0/xrd-1.0.html
Current version: http://docs.oasis-open.org/xri/xrd/v1.0/wd06/xrd-1.0-wd06.html
Previous version: http://docs.oasis-open.org/xri/xrd/v1.0/wd05/xrd-1.0-wd05.html
Technical committee:
OASIS eXtensible Resource Identifier (XRI) TC
Chairs:
Peter Davis, NeuStar Inc.
Drummond Reed, Cordance
Editors:
Eran Hammer-Lahav, Yahoo!
Will Norris, Internet2
Declared XML Namespace:
  • http://docs.oasis-open.org/ns/xri/xrd-1.0

Abstract:

This document defines XRD, a simple generic format for describing resources.

Related Work:

This specification replaces or supersedes:

  • Extensible Resource Identifier (XRI) Resolution Version 2.0, Committee Draft 03, February 2008

This specification is related to:

  • Extensible Resource Identifier (XRI) Version 3.0, Committee Draft 01, May 2009

Status:

This document was last revised or approved by the XRI Technical Committee on the above date. The level of approval is also listed above. Check the current location noted above for possible later revisions of this document. This document is updated periodically on no particular schedule.

Technical Committee members should send comments on this specification to the Technical Committee's email list. Others should send comments to the Technical Committee by using the "Send A Comment" button on the Technical Committee's web page at http://www.oasis-open.org/committees/xri.

For information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the Technical Committee web page ( http://www.oasis-open.org/committees/xri/ipr.php).

The non-normative errata page for this specification is located at http://www.oasis-open.org/committees/xri.

Notices:

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Table of Contents

1. Introduction
1.1. Terminology
1.2. Normative References
1.3. Schema Organization and Namespaces
1.4. Common Data Types
1.4.1. String Values
1.4.2. URI Values
1.4.3. Time Values
2. XRD Document Structure
2.1. XRD Elements
2.1.1. Element <XRD>
2.1.2. Element <Expires>
2.1.3. Element <Subject>
2.1.4. Element <Extensions>
2.1.5. Element <Alias>
2.1.6. Element <Type>
2.2. Linked Resource Elements
2.2.1. Element <Link>
2.2.2. Element <Rel>
2.2.3. Element <MediaType>
2.2.4. Element <URI>
2.2.5. Element <URITemplate>
3. XRD Extensibility
3.1. Identifier Extension
3.2. Schema Extension
4. Processing XRD Documents
4.1. Linked Resource Selection
4.2. Linked XRD Documents
4.3. Processing Order
5. XRD Signature
5.1. Signing Formats and Algorithms
5.2. References
5.3. Canonicalization
5.4. Transforms
5.5. KeyInfo
6. XRD Sequence
7. Conformance
7.1. XRD Consumer
7.2. XRD Provider

Appendixes

A. Acknowledgments (Non-Normative)
B. Example XRD
C. Media Type Definition for application/xrd+xml
D. Revision History

1. Introduction

This document defines XRD, a simple generic format for describing resources. Resource descriptor documents provide machine-readable information about resources (resource metadata) for the purpose of promoting interoperability and assist in interacting with unknown resources that support known interfaces.

For example, a web page about an upcoming meeting can provide in its descriptor document the location of the meeting organizer's free/busy information to potentially negotiate a different time. The descriptor for a social network profile page can identify the location of the user's address book as well as accounts on other sites. A web service implementing an API with optional components can advertise which of these are supported.

1.1. Terminology

The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be interpreted as described in [RFC 2119].

1.2. Normative References

[Exclusive Canonicalization] J. Boyer, et al Exclusive XML Canonicalization. W3 Recommendation. 2002

[HTML 4.01] D. Raggett HTML 4.01 Specification. W3 Recommendation. 1999

[HTTP Link Header] M. Nottingham Link Relations and HTTP Header Linking. IETF (Internet Engineering Task Force) Draft. 2009.

[RFC 2046] N. Freed, N. Borenstein Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types. IETF (Internet Engineering Task Force). 1996.

[RFC 2119] S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. IETF (Internet Engineering Task Force). 1997.

[RFC 2606] E. Eastlake, A. Panitz Reserved Top Level DNS Names. IETF (Internet Engineering Task Force). 1999.

[RFC 2616] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee Hypertext Transfer Protocol -- HTTP/1.1. IETF (Internet Engineering Task Force). 1999.

[RFC 3023] M. Murata, S. St.Laurent, D. Kohn XML Media Types. IETF (Internet Engineering Task Force). 2001.

[RFC 3986] T. Berners-Lee, R. Fielding, L. Masinter Uniform Resource Identifiers (URI): Generic Syntax. IETF (Internet Engineering Task Force). 2005.

[RFC 4287] M. Nottingham The Atom Syndication Format. IETF (Internet Engineering Task Force). 2005.

[RFC 4288] N. Freed, J. Klensin Media Type Specifications and Registration Procedures. IETF (Internet Engineering Task Force). 2005.

[XML 1.0] T. Bray Extensible Markup Language (XML) 1.0 W3 Recommendation. 2008

[XML Schema] H. Thompson XML Schema Part 1: Structures Second Edition. W3C Recommendation. 2004

[XML Schema Datatypes] P. Biron XML Schema Part 2: Datatypes Second Edition. W3 Recommendation. 2004

[XML Signature] D. Eastlake, et al XML Signature Syntax and Processing. W3 Recommendation. 2008

[XRI Resolution 2.0] G. Wachob Extensible Resource Identifier (XRI) Resolution V2.0. February 2008.

[xml:id] J. Marsh, et al xml:id. W3 Recommendation. 2005

1.3. Schema Organization and Namespaces

The XRD document structure is defined in a schema associated with the following XML namespace:

http://docs.oasis-open.org/ns/xri/xrd-1.0

The schema for [XML Signature] is imported into the XRD schema, which is associated with the following XML namespace:

http://www.w3.org/2000/09/xmldsig#

Also imported into the XRD schema is the schema for [XML 1.0] (the "xml:" namespace), which is associated with the following XML namespace:

http://www.w3.org/XML/1998/namespace

The following [XML Schema] fragment defines the XML namespaces and other header information for the XRD schema:

<schema targetNamespace="http://docs.oasis-open.org/ns/xri/xrd-1.0"
  xmlns="http://www.w3.org/2001/XMLSchema"
  xmlns:xrd="http://docs.oasis-open.org/ns/xri/xrd-1.0"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  elementFormDefault="unqualified"
  attributeFormDefault="unqualified"
  blockDefault="substitution"
  version="1.0">

    <import namespace="http://www.w3.org/2000/09/xmldsig#"
        schemaLocation="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/xmldsig-core-schema.xsd"/>
    <import namespace="http://www.w3.org/XML/1998/namespace"
        schemaLocation="http://www.w3.org/2001/xml.xsd"/>

    <annotation>
        <documentation>
            Document identifier: xrd-schema-1.0
            Location: http://docs.oasis-open.org/xri/xrd/v1.0/
        </documentation>
    </annotation>
...
</schema>

The location of the normative XML Schema file for an XRD document as defined by this specification is: http://docs.oasis-open.org/xri/xrd/v1.0/wd06/xrd-1.0-wd06.xsd

The following URI will always reference the latest version of this file: http://docs.oasis-open.org/xri/xrd/v1.0/xrd-1.0.xsd

1.4. Common Data Types

The following sections define how to use and interpret common data types that appear throughout the XRD schema.

1.4.1. String Values

All XRD string values have or extend the type xs:string, which is built in to the W3C [XML Schema Datatypes] specification. Unless otherwise noted in this specification or particular profiles, all strings in XRD documents MUST consist of at least one non-whitespace character (whitespace is defined in section 2.3 of [XML 1.0]).

The following schema fragment defines the xrd:string complex type, which extends xs:string to allow for artibtrary attributes (see Section 3.2, “Schema Extension”):

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

1.4.2. URI Values

All XRD URI reference values have or extend the type xs:anyURI, which is built in to the W3C [XML Schema Datatypes] specification. Unless otherwise noted in this specification or particular profiles, all URIs in XRD documents MUST consist of at least one non-whitespace character.

The following schema fragment defines the xrd:anyURI complex type, which extends xs:anyURI to allow for artibtrary attributes (see Section 3.2, “Schema Extension”):

<complexType name="anyURI">
    <simpleContent>
        <extension base="anyURI">
            <anyAttribute namespace="##other" processContents="lax"/>
        </extension>
    </simpleContent>
</complexType>

1.4.3. Time Values

All XRD time values have the type xs:dateTime, which is built in to the W3C [XML Schema Datatypes] specification, and MUST be expressed in UTC form, with no time zone component. XRD providers MUST NOT generate time instants that specify leap seconds.

2. XRD Document Structure

XRD provides a simple and extensible XML format for describing resources. An XRD document may describe the properties of the resource itself, as well as the relationship the resource has with other resources. XRD builds directly on the typed link relations framework defined by [HTTP Link Header], and used by HTML, Atom, and other protocols.

The XRD schema defines only the basic elements necessary to support the most common use cases, with the explicit intention that applications will extend XRD as defined in Section 3, “XRD Extensibility” to include any other metadata about the resources they describe.

2.1. XRD Elements

XRD defines elements used to describe the properties of the XRD document itself as well as properties of the resource it describes. These elements provide administrative information as to how the information should be cached, as well as information for the authentication and trust verification of XRD documents. They additionally provide information and properties about the resource that the XRD documet is describing.

2.1.1. Element <XRD>

The <XRD> element encapsulates the entire resource descriptor, and is most commonly the root element of the document. It contains the following attributes and elements:

xml:id [Optional]

This attribute, of type xs:ID, is defined by [xml:id]. It provies a unique identifier for this XRD, and is most commonly used as a signature reference.

<Expires> [Zero or One]

Specifies when this document expires, as described in Section 2.1.2, “Element <Expires>.

<Subject> [Zero or One]

Provides the canonical identifier for the resource described by this XRD. When <Subject> appears as a child element of <XRD>, it identifies the resource the XRD document is about. See Section 2.1.3, “Element <Subject>.

<ds:Signature> [Zero or One]

This is an XML Signature, included from the [XML Signature] schema, that protects the integrity of the document, as described in Section 5, “XRD Signature”.

<Extensions> [Zero or One]

Provides extensions point for additional elements to be attached at the <XRD> level. See Section 2.1.4, “Element <Extensions>.

<Alias> [Zero or More]

Provides an additional identifier for the resoure described by this XRD. See Section 2.1.5, “Element <Alias>.

<Type> [Zero or More]

Declares a property of the resource described by this XRD, as described in Section 2.1.6, “Element <Type>.

<Link> [Zero or More]

Identifies another resource which is related to the resource described by this XRD, and describes the semantics of that relationship. See Section 2.2.1, “Element <Link>.

The following schema fragment defines the <XRD> element and its XRDType complex type:

<element name="XRD" type="xrd:XRDType"/>
<complexType name="XRDType">
    <sequence>
        <element ref="xrd:Expires" minOccurs="0"/>
        <element ref="xrd:Subject" minOccurs="0"/>
        <element ref="ds:Signature" minOccurs="0"/>
        <element ref="xrd:Extensions" minOccurs="0"/>
        <choice minOccurs="0" maxOccurs="unbounded">
            <element ref="xrd:Alias"/>
            <element ref="xrd:Type"/>
            <element ref="xrd:Link"/>
        </choice>
    </sequence>
    <attribute ref="xml:id" use="optional"/>
    <anyAttribute namespace="##other" processContents="lax"/>
</complexType>

2.1.2. Element <Expires>

This dateTime value indicates the time instant after which the document is no longer valid. The value MUST use the UTC "Z" time zone, as specified in Section 1.4.3, “Time Values”, and MUST NOT use fractional seconds. The semantics of this element apply to the metadata available in the XRD document and are independent of the caching semantics of any transport protocol used to retrieve the document.

The following schema fragment defines the <Expires> element and its ExpiresType complex type:

<element name="Expires" type="xrd:ExpiresType"/>
<complexType name="ExpiresType">
    <simpleContent>
        <extension base="dateTime">
            <anyAttribute namespace="##other" processContents="lax"/>
        </extension>
    </simpleContent>
</complexType>

2.1.3. Element <Subject>

<Subject> is a URI value which identifies a resource. This value MUST be an absolute URI.

The following schema fragment defines the <Subject> element.

<element name="Subject" type="xrd:anyURI"/>

2.1.4. Element <Extensions>

The <Extensions> element provides an extension point from which XRD providers may attach child elements from arbitrary namespaces (except for the XRD namespace). See Section 3.2, “Schema Extension”.

The following schema fragment defines the <Extensions> element and its ExtensionsType complex type:

<element name="Extensions" type="xrd:ExtensionsType"/>
<complexType name="ExtensionsType">
    <sequence>
        <any namespace="##other" processContents="lax" maxOccurs="unbounded"/>
    </sequence>
</complexType>

2.1.5. Element <Alias>

This URI value provides an additional non-canonical identifier for the resource described by the XRD. This value MUST be an absolute URI.

The following schema fragment defines the <Alias> element:

<element name="Alias" type="xrd:anyURI"/>

2.1.6. Element <Type>

The <Type> element is a URI which declares a property of the resource described by the XRD. The meaning of the <Type> value is application-specific, and is used by the XRD provider to describe the resource to consumers familiar with the type identifier. <Type> contains the following attributes:

required [Optional]

The required attribute is a boolean used to indicate to a consumer that some pre-defined knowledge is required in order to interact with the resource, without which undefined or potentially harmful side-effects can occur.

If the required attribute is omitted or explicitly set to false, an XRD consumer SHOULD ignore any <Type> elements with values it does not recognize, and interact with the resource based on the values it does recognize. If the required attribute is set to true, an XRD consumer MUST NOT interact with the resource if it does not recognize the type identifier. The required attribute SHOULD NOT be used unless such harmful side-effects are likely.

The following schema fragment defines the <Type> element and its TypeType complex type:

<element name="Type" type="xrd:TypeType"/>
<complexType name="TypeType">
    <simpleContent>
        <extension base="xrd:anyURI">
            <attribute name="required" type="boolean" use="optional"/>
        </extension>
    </simpleContent>
</complexType>

2.2. Linked Resource Elements

One of the primary uses of XRD is to describe the relationship the resource described by the XRD has with other resources. The following elements identify and describe these related resources.

2.2.1. Element <Link>

The <Link> element serves as a container for metadata about the related resource, and carries similar semantics as the HTML Link element, the ATOM Link element, and the HTTP Link Header. The one distinction is that link relationships described by the <Link> element are between the resource described by the XRD (the context resource) and the linked resources (the target resources), and not between the XRD document itself and the linked resource. <Link> has the following elements and attributes:

<Subject> [Zero or One]

When <Subject> appears as a child element of a <Link>, it identifies the linked resource. This asserts the value that should be expected for the <Subject> in the linked XRD. See Section 2.1.3, “Element <Subject>.

<Extensions> [Zero or One]

Provides extensions point for additional elements to be attached at the <Link> level. See Section 2.1.4, “Element <Extensions>.

<Rel> [Zero or More]

Defines the semantics of the link relationship. See Section 2.2.2, “Element <Rel>.

<MediaType> [Zero or More]

Provides a hint at the media type of the linked resource. See Section 2.2.3, “Element <MediaType>.

<URI> [Zero or More]

Identifies how the linked resource can be retrieved. See Section 2.2.4, “Element <URI>.

<URITemplate> [Zero or More]

Provides a template which can be used to obtain a URI for retrieving the resource. See Section 2.2.5, “Element <URITemplate>.

<ds:KeyInfo> [Zero or More]

KeyInfo is included from the [XML Signature] schema, and provides the digital signature metadata necessary to validate interaction with the linked resource. The specifics of this validation process are left to other specifications to define.

The following schema fragment defines the <Link> element and its LinkType complex type:

<element name="Link" type="xrd:LinkType"/>
<complexType name="LinkType">
    <sequence>
        <element ref="xrd:Subject" minOccurs="0"/>
        <element ref="xrd:Extensions" minOccurs="0"/>
        <choice minOccurs="0" maxOccurs="unbounded">
            <element ref="xrd:Rel"/>
            <element ref="xrd:MediaType"/>
            <element ref="xrd:URI"/>
            <element ref="xrd:URITemplate"/>
            <element ref="ds:KeyInfo"/>
        </choice>
    </sequence>
    <anyAttribute namespace="##other" processContents="lax"/>
</complexType>

2.2.2. Element <Rel>

This URI value defines the semantics of the relationship between the resource described by the XRD and the linked resource. <Rel> is semantically equivalent to the Link Relationship Types defined in [HTTP Link Header]. It is important to note that this value does not identify any property of the linked resource. Rather, it describes only how the linked resource is related to the resource described by the XRD.

The following schema fragment defines the <Rel> element:

<element name="Rel" type="xrd:anyURI"/>

2.2.3. Element <MediaType>

This string value provides a hint as to the media type of the linked resource. The value of this element MUST be of the form of a media type defined in [RFC 2046]. The IANA media types registry can be found at http://www.iana.org/assignments/media-types/.

The following schema fragment defines the <MediaType> element:

<element name="MediaType" type="xrd:string"/>

2.2.4. Element <URI>

The <URI> element provides the URI of the linked resource. If no <URI> element is defined, it is assumed the URI can be obtained from a <URITemplate> element or by other means not specified in this specification.

The following schema fragment defines the <URI> element:

<element name="URI" type="xrd:anyURI"/>

2.2.5. Element <URITemplate>

URI Templates provide a mechanism for URI construction, taking a list of variables as input and producing a URI string as output. 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. If a variable name is prefixed by a "%" character, any character in the variable value other than unreserved MUST be percent-encoded per [RFC 3986].

The template mechanism is completely dependent upon the link relation types as indicated by the various <Rel> child elements of the corresponding <Link>. Relation types that wish to utilize the template mechanism need to define their variable vocabulary, as well as any additional or template parsing rules. If a link contains multiple relation types with conflicting vocabularies or template parsing rules, or if a template contains unknown variable names, the <URITemplate> SHOULD be ignored.

The following schema fragment defines the <URITemplate> element:

<element name="URITemplate" type="xrd:string"/>

3. XRD Extensibility

The XRD schema defines only the basic elements necessary to support the most common use cases, with the explicit intention that applications will extend XRD to include any other metadata about the resources they describe. XRD documents can be extended by providing custom, meaningful values for certain URI-based elements, as well as by extending the XML elements directly.

3.1. Identifier Extension

XRD uses URI-based identifiers for describing resources as well as for describing the relationships between resources. It is expected that applications will use appropriate established URI identifiers for these purposes, or define new identifiers as necessary. It is RECOMMENDED that any new identifiers be defined in a formal specification of use. In no case should the meaning of a given URI used as such an identifier significantly change, or be used to mean two different things.

New relation types between resources must follow the extensibility and registration requirements defined in [HTTP Link Header].

3.2. Schema Extension

The XRD schema allows for the inclusion of attributes from arbitrary namespaces (except for the XRD namespace) in almost all XRD elements. Additionally, the <XRD> and <Link> elements contain an <Extensions> element which allows for the inclusion of child elements from arbitrary namespaces (except for the XRD namespace).

XML extensions MUST NOT require new interpretation of elements defined in this document. If an extension attribute or element is present, an XRD consumer MUST be able to ignore it and still correctly process the XRD document.

4. Processing XRD Documents

XRD documents provide a list of resource properties and a list of linked resources. Once an XRD document has been obtained, the consumer typically inspects the list of resource properties looking for familiar identifiers, and performs resource selection to find the links relevant to it. The selection process involves iterating through the list of linked resource descriptions and filtering them based on various metadata.

4.1. Linked Resource Selection

Based on the XRD consumer's needs, the application defines a selection criteria based on the presence (or lack of) certain resource relationship values, media-types, and schema extensions. The selection criteria may be any combination of metadata describing the linked resources such as <Rel>, <MediaType>, <URI>, or non-XRD extension elements or attributes. For example, an application might look for all linked resources with an image media-type, a linked resource with a specific authentication relationship, or a linked resource with a URI matching a pattern.

Selection based on multiple criteria can be handled by performing multiple selections. Each selection is assigned preference order based on the consumer's needs. The selection results are compared to determine the most desired set. For example, if an application is looking for all image resources, giving higher preference to the JPEG formats over PNG, it will perform two selection processes, one for each media-type, and assign the resources in the JPEG set a higher preference value.

The XRD consumer performs the following steps in order to select the desired linked resource descriptions:

  • Each <Link> element is compared against the selection criteria by comparing the values of the <Link> child elements and attributes to those defined by the selection criteria.

  • If more than one <Link> element is matched, the consumer MUST process them in document order as defined in Section 4.3, “Processing Order”.

  • Within a matching <Link> element, if more than one <URI> or <URITemplate> elements are present, the consumer MUST process the combined list of elements to determine the URI fo the linked resource as defined in Section 4.3, “Processing Order”.

  • If no <Link> elements meet the selection criteria, the consumer SHOULD look for linked XRD documents, using the search criteria defined in Section 4.2, “Linked XRD Documents”. If more than one linked XRD is found, they MUST be processed in document order, as defined in Section 4.3, “Processing Order”. If the linked resource is a valid XRD document, the consumer SHOULD repeat this entire resource selection flow on the linked XRD. If the linked resource is not a valid XRD document, or a matching <Link> element cannot be found in the linked XRD, the consumer SHOULD continue with the next linked XRD. An XRD consumer MAY choose to limit the depth to which it will follow linked XRD documents for performance or other reasons.

4.2. Linked XRD Documents

The XRD document for a resource may assert that a different XRD document should be considered an additional valid descriptor for the same described resource. This allows for distributed management of descriptor documents, both within and across authority boundaries. A linked XRD is identified by a <Link> containing a <Rel> value of http://docs.oasis-open.org/xri/xrd/v1.0#see-also, as demonstrated in the following example.

<Link>
    <Rel>http://docs.oasis-open.org/xri/xrd/v1.0#see-also</Rel>
    <URI>http://example.net/xrd.xml</URI>
</Link>

4.3. Processing Order

XRD allows the <Link>, <URI>, and <URITemplate> elements to appear multiple times within the same parent element. When these elements appear more than once within the same parent, XRD consumers MUST process them in document order. Therefore, XRD providers MAY indicate element priority by placing them in a specific order.

If more than one element matches the selection criteria of the XRD consumer, the consumer MUST process the elements in the order in which they appear in the XRD document. If the first element is subsequently disqualified from the set of qualified elements, the consumer SHOULD attempt to select the next qualified element in document order. This process SHOULD be continued for all other instances of the qualified elements until success is achieved or all instances are exhausted.

The URI for a linked resource can be expressed using two different elements, <URI> and <URITemplate>, which differ only in the fact that templates require additional processing in order to obtain the final URI. Therefore, XRD Consumers MUST process the combined list of <URI> and <URITemplate> elements in document order when determining the URI for a given linked resource. In determining the URI of the linked resource in the following example, an XRD consumer MUST attempt to use the URI that results from processing the <URITemplate> before attempting to use the explicit <URI> value:

<Link>
    <Rel>http://spec.example.net/protocol/1.0</Rel>
    <URITemplate>http://example.com/{varname}</URITemplate>
    <URI>http://example.com/service/endpoint</URI>
</Link>

5. XRD Signature

An XRD provider MAY digitally sign an XRD document in order to enable XRD consumers to verify the authenticity and integrity of the document. The [XML Signature] specification defines a general XML syntax for signing data with flexibility and many options. This section details constraints on these options so that XRD consumers do not have to implement the full generality of XML Signature processing.

5.1. Signing Formats and Algorithms

While XML Signature has three ways of relating a signature to a document (enveloping, enveloped, and detached), XRD documents MUST use enveloped signatures when signing. Any algorithm defined for use with the XML Signature specification MAY be used.

5.2. References

XRD documents MUST supply a value for the xml:id attribute on the root element of the XRD being signed. The XRD's root element may or may not be the root element of the actual XML document containing the signed XRD (e.g., it might be included within another document schema).

Signatures MUST contain a single <ds:Reference> containing a same-document reference to the xml:id attribute value of the root element of the XRD being signed. For example, if the xml:id attribute value is foo, then the URI attribute in the <ds:Reference> element MUST be #foo.

5.3. Canonicalization

XRD implementations MUST use [Exclusive Canonicalization], with or without comments, both in the <ds:CanonicalizationMethod> element of <ds:SignedInfo>, and as a <ds:Transform> algorithm. Use of Exclusive Canonicalization facilitates the verification of signatures created over XRD instances when placed into a different XML context than present during signing.

Note that use of this algorithm alone does not guarantee that a particular signed object can be moved from one context to another safely, nor is that a requirement of signed XRD instances in general, though it MAY be required by particular profiles.

5.4. Transforms

Signatures in XRD documents MUST NOT contain transforms other than the enveloped signature transform (with the identifier http://www.w3.org/2000/09/xmldsig#enveloped-signature) or the exclusive canonicalization transforms (with the identifier http://www.w3.org/2001/10/xml-exc-c14n# or http://www.w3.org/2001/10/xml-exc-c14n#WithComments).

5.5. KeyInfo

XML Signature defines usage of the <ds:KeyInfo> element. XRD does not require the use of <ds:KeyInfo>, nor does it impose any restrictions on its use. Therefore, <ds:KeyInfo> MAY be absent.

6. XRD Sequence

In cases where an application requires a sequence of <XRD>s in a single XML document, this specification defines an alternate top-level element, <XRDS>. Its semantics are very simple: it contains a sequence of <XRD> elements, one optional attribute, and is not otherwise extensible.

ref [Optional]

This attribute, of type xs:anyURI, identfies the resource described by the sequence of <XRD> elements.

<XRD> [One or More]

See Section 2.1.1, “Element <XRD>.

The following schema fragment defines the <XRDS> element and its XRDSType complex type:

<element name="XRDS" type="xrd:XRDSType"/>
<complexType name="XRDSType">
    <sequence>
        <element ref="xrd:XRD" minOccurs="0" maxOccurs="unbounded"/>
    </sequence>
    <attribute name="ref" type="anyURI"/>
</complexType>

IMPORTANT: In past practice, <XRDS> was often used as the root element even if contained only a single <XRD>. That practice is now deprecated. The <XRDS> element SHOULD NOT be used unless it contains a sequence of two or more <XRD> elements.

7. Conformance

An implementation is a conforming XRD consumer if the implementation meets the conditions in Section 7.1, “XRD Consumer”. An implementation is a conforming XRD provider if the implementation meets the conditions in Section 7.2, “XRD Provider”. An implementation may serve as both an XRD consumer and provider.

7.1. XRD Consumer

An implementation conforms to this specification as an XRD consumerif it meets the following conditions:

  1. It MUST implement parsing of XRD documents which conform to the XRD schema as specified in Section 2, “XRD Document Structure”.

  2. It MUST conform to the processing rules as specified in Section 4, “Processing XRD Documents”.

7.2. XRD Provider

An implementation conforms to this specification as an XRD provider if it meets the following conditions:

  1. Any published XRD documents MUST conform to the XRD schema as specified in Section 2, “XRD Document Structure”.

A. Acknowledgments (Non-Normative)

The editors would like to thank the following current and former members of the OASIS XRI TC for their particular contributions to this and previous versions of this specification:

  • Dirk Balfanz, Google

  • John Bradley

  • Scott Cantor, Internet2

  • Brian Eaton, Google

  • George Fletcher, AOL

  • Joseph Holsten, Cordance

  • Nika Jones, Ouno

  • Breno de Medeiros, Google

  • Bob Morgan, Internet2

  • Markus Sabadello, XDI.org

  • Nat Sakimura, NRI

  • Tatsuki Sakushima, NRI

The editors would also like to acknowledge the contributions of the other members of the OASIS XRI Technical Committee, whose other voting members at the time of publication were:

  • Bill Barnhill, Booz Allen Hamilton

  • Giovanni Bartolomeo, University of Rome "Tor Vergata"

  • Les Chasen, NeuStar

  • Steven Churchill, XDI.org

  • Owen Davis, Planetwork

  • David Fuelling, Cordance

  • Victor Grey, Planetwork

  • Jeff Hodges

  • Fen Labalme, Planetwork

  • Ben Laurie, Google

  • XiaoDong Lee, China Internet Network Information Center

  • Michael Mel, Cordance

  • Nick Nicholas, Australian Department of Education

  • Marty Schleiff, The Boeing Company

  • William Tan, NeuStar

  • Paul Trevithick

  • Gabe Wachob, Socialtext

B. Example XRD

Following is an example of a signed XRD document. Line breaks have been added for readability; the signatures are not valid and cannot be successfully verified.

<XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0" xml:id="foo">
    <Expires>1970-01-01T00:00:00Z</Expires>
    <Subject>http://example.com/gpburdell</Subject>
    <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> 
        <ds:SignedInfo> 
            <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/> 
            <ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/> 
            <ds:Reference URI="#foo">
                <ds:Transforms> 
                    <ds:Transform 
                        Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/> 
                    <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"> 
                        <InclusiveNamespaces PrefixList="#default xrd ds xs xsi" 
                            xmlns="http://www.w3.org/2001/10/xml-exc-c14n#"/> 
                    </ds:Transform> 
                </ds:Transforms> 
                <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> 
                <ds:DigestValue>TCDVSuG6grhyHbzhQFWFzGrxIPE=</ds:DigestValue>
            </ds:Reference> 
        </ds:SignedInfo>
        <ds:SignatureValue> 
            x/GyPbzmFEe85pGD3c1aXG4Vspb9V9jGCjwcRCKrtwPS6vdVNCcY5rHaFPYWkf+5 
            EIYcPzx+pX1h43SmwviCqXRjRtMANWbHLhWAptaK1ywS7gFgsD01qjyen3CP+m3D 
            w6vKhaqledl0BYyrIzb4KkHO4ahNyBVXbJwqv5pUaE4= 
        </ds:SignatureValue> 
        <ds:KeyInfo> 
            <ds:X509Data> 
                <ds:X509Certificate> 
                    MIICyjCCAjOgAwIBAgICAnUwDQYJKoZIhvcNAQEEBQAwgakxCzAJBgNVBAYTAlVT 
                    MRIwEAYDVQQIEwlXaXNjb25zaW4xEDAOBgNVBAcTB01hZGlzb24xIDAeBgNVBAoT 
                    F1VuaXZlcnNpdHkgb2YgV2lzY29uc2luMSswKQYDVQQLEyJEaXZpc2lvbiBvZiBJ 
                    bmZvcm1hdGlvbiBUZWNobm9sb2d5MSUwIwYDVQQDExxIRVBLSSBTZXJ2ZXIgQ0Eg 
                    LS0gMjAwMjA3MDFBMB4XDTAyMDcyNjA3Mjc1MVoXDTA2MDkwNDA3Mjc1MVowgYsx
                    CzAJBgNVBAYTAlVTMREwDwYDVQQIEwhNaWNoaWdhbjESMBAGA1UEBxMJQW5uIEFy
                    Ym9yMQ4wDAYDVQQKEwVVQ0FJRDEcMBoGA1UEAxMTc2hpYjEuaW50ZXJuZXQyLmVk 
                    dTEnMCUGCSqGSIb3DQEJARYYcm9vdEBzaGliMS5pbnRlcm5ldDIuZWR1MIGfMA0G 
                    CSqGSIb3DQEBAQUAA4GNADCBiQKBgQDZSAb2sxvhAXnXVIVTx8vuRay+x50z7GJj 
                    IHRYQgIv6IqaGG04eTcyVMhoekE0b45QgvBIaOAPSZBl13R6+KYiE7x4XAWIrCP+ 
                    c2MZVeXeTgV3Yz+USLg2Y1on+Jh4HxwkPFmZBctyXiUr6DxF8rvoP9W7O27rhRjE 
                    pmqOIfGTWQIDAQABox0wGzAMBgNVHRMBAf8EAjAAMAsGA1UdDwQEAwIFoDANBgkq 
                    hkiG9w0BAQQFAAOBgQBfDqEW+OI3jqBQHIBzhujN/PizdN7s/z4D5d3pptWDJf2n 
                    qgi7lFV6MDkhmTvTqBtjmNk3No7v/dnP6Hr7wHxvCCRwubnmIfZ6QZAv2FU78pLX 
                    8I3bsbmRAUg4UP9hH6ABVq4KQKMknxu1xQxLhpR1ylGPdiowMNTrEG8cCx3w/w== 
                </ds:X509Certificate>
            </ds:X509Data>
        </ds:KeyInfo>
    </ds:Signature>
    <Link>
        <Rel>http://spec.example.net/auth/1.0</Rel>
        <URI>http://services.example.com/auth</URI>
    </Link>
</XRD>

C. Media Type Definition for application/xrd+xml

This section is prepared in anticipation of filing a media type registration meeting the requirements of [RFC 4288].

Type name:

application

Subtype name:

xrd+xml

Required parameters:

None

Optional paramters:

None

Encoding considerations:

Identical to those of application/xml as described by [RFC 3023]

Security considerations:

As defined in this specification. In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in [RFC 3023], Section 10.

Interoperability considerations:

There are no known interoperability issues.

Published specification:

This specification

Applications that use this media type:

Applications conforming to this specification use this media type.

Person & email address to contact for further information:

Drummond Reed, OASIS XRI Technical Committee Co-Chair, drummond.reed@cordance.net

Intended usage:

COMMON

Restrictions on usage:

None

Author

OASIS XRI Technical Comittee

Change controller:

OASIS XRI Technical Comittee

D. Revision History

Table D.1. 

RevisionDateEditorChanges Made
Working Draft 0604 September 2009willnorris
  • Combine "Document Property Elements" and "Resource Property Elements" into "XRD Elements"

  • Move schema and references to first section

  • Promote "XRD Extensions" section, and move schema fragment

  • Add example for URI / URITemplate processing order

  • Move XRD Example into an appendix

  • Various minor rewording

Working Draft 0501 September 2009willnorris
  • Remove priority attribute on Link, URI, and URITemplate elements. Instead, element priority is implied by document order. Additionally, requirement for consumers to respect priority strengthened from "should" to "must".

  • New "Extensions" element added to XRD and Link elements as the sole location to extend XRD with arbitrary child elements.

  • Define "XRDS" element to contain a sequence of XRD elements.

  • Removed "match" attribute from Subject.

  • Added requirement to follow normal rules for Rel values (either use a URI, or register value with IANA)

  • Switched from Relax NG to XSD as the authoritative schema language for the XRD Schema. (Primarily due to the lack of a Relax NG schema for XML DSig)

  • Clarify language regarding URIs and URI Templates

  • Define "Common Data Types" for XRD

  • Various minor editorial and grammatical changes

Working Draft 0412 August 2009willnorris
  • Remove XRD Trust section, pushing that work to separate trust profiles. Move XRD Signature section up one level.

  • Remove requirement for explicit Link Subject on linked XRDs

  • Use non-information URI for rel value to designate linked XRD

  • Flesh out subject matching rules

  • Remove "must not be used" from Expires element description

Working Draft 0304 August 2009willnorris
  • Revert to previous processing flow for related resources -- first filter, then sort by priority

  • Add media type definition for "application/xrd+xml"

  • Clarify text for URI templates

  • Strengthen requirement to use excl-c14n from "should" to "must"

  • Move Signature element to bottom of the document for readability

  • Add conformance section

  • Add "match" attribute to Subject element. Also add stub section for subject matching.

  • Add XSD schema (in addition to RELAX NG)

  • Various editorial and grammatical changes.

Working Draft 0203 July 2009willnorris
  • Remove XRD Trust namespace and elements (TargetSubject replaced by Subject, TargetAuthority replaced by ds:KeyInfo)

  • Section added for XML Digital Signature, primarily copied from SAML 2.0, which chnages as necessary

  • Language clarified on priority attribute values ('null' is not a valid value)

  • Add section for XRD Extensibility

  • Only require XML element order for elements with cardinality of "zero or one"

  • Add section for defining linked XRD documents

  • Processing rules changed for related resources to first sort by priority, then filter. Also add processing rule for linked XRD documents.

  • Various editorial and grammatical changes.

Working Draft 0109 May 2009willnorrisInitial Publication