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The XML format used by XRDS was originally developed in 2004 by the OASIS XRI (extensible resource identifier) Technical Committee as the resolution format for XRIs. The acronym XRDS was coined during subsequent discussions between XRI TC members and OpenID developers at first Internet Identity Workshop held in Berkeley, CA in October 2005.
The protocol for discovering an XRDS document from a URL was formalized as the Yadis specification published by Yadis.org in March 2006. Yadis became the service discovery format for OpenID 1.1.
A common discovery service for both URLs and XRIs proved so useful that in November 2007 the XRI Resolution 2.0 specification formally added the URL-based method of XRDS discovery (Section 6). This format and discovery protocol subsequently became part of OpenID Authentication 2.0.
In early 2008, work on OAuth discovery by Eran Hammer-Lahav led to the development of XRDS Simple, a profile of XRDS that restricts it to the most basic elements and introduces some extensions to support OAuth discovery and other protocols that use specific HTTP methods. In late 2008, XRDS Simple has been cancelled and merged back into the main XRDS specification resulting in the upcoming XRD 1.0 format.
Besides XRI resolution, examples of typical XRDS usage include:
Following is an example of an XRDS document for the fictional XRI i-name =example. This document would typically be requested from a Web server via HTTP or HTTPS using the content type application/xrds+xml. Note that the outer container <XRDS> element serves as a container for one or more <XRD> (Extensible Resource Descriptor) elements. Most simple XRDS documents have only one XRD. Other services like XRI resolution may construct a sequence of XRDs within a single XRDS document to reflect a chain of metadata about linked resources.
<?xml version="1.0" encoding="UTF-8"?><xrds:XRDSxmlns:xrds="xri://$xrds"xmlns="xri://$xrd*($v*2.0)"xmlns:openid="http://openid.net/xmlns/1.0"><XRDref="xri://=example"><Query>*example</Query><Statusceid="off"cid="verified"code="100"/><Expires>2008-05-05T00:15:00.000Z</Expires><ProviderID>xri://=</ProviderID><!-- synonym section --><LocalIDpriority="10">!4C72.6C81.D78F.90B2</LocalID><EquivIDpriority="10">http://example.com/example-user</EquivID><EquivIDpriority="15">http://example.net/blog</EquivID><CanonicalID>xri://=!4C72.6C81.D78F.90B2</CanonicalID><!-- service section --><Service><!-- XRI resolution service --><ProviderID>xri://=!F83.62B1.44F.2813</ProviderID><Type>xri://$res*auth*($v*2.0)</Type><MediaType>application/xrds+xml</MediaType><URIpriority="10">http://resolve.example.com</URI><URIpriority="15">http://resolve2.example.com</URI><URI>https://resolve.example.com</URI></Service><!-- OpenID 2.0 login service --><Servicepriority="10"><Type>http://specs.openid.net/auth/2.0/signon</Type><URI>http://www.myopenid.com/server</URI><LocalID>http://example.myopenid.com/</LocalID></Service><!-- OpenID 1.0 login service --><Servicepriority="20"><Type>http://openid.net/server/1.0</Type><URI>http://www.livejournal.com/openid/server.bml</URI><openid:Delegate>http://www.livejournal.com/users/example/</openid:Delegate></Service><!-- untyped service for access to files of media type JPEG --><Servicepriority="10"><Typematch="null"/><Pathselect="true">/media/pictures</Path><MediaTypeselect="true">image/jpeg</MediaType><URIappend="path">http://pictures.example.com</URI></Service></XRD></xrds:XRDS>XRDS documents can assert zero or more synonyms for a resource. In this context, a synonym is another identifier (a URI or XRI) that identifies the same target resource. For instance, the example XRDS document above asserts four synonyms:
For full details of XRDS synonym support, see XRI Resolution 2.0, Section 5.
The other main purpose of XRDS documents is to assert the services associated with a resource, called service endpoints or SEPs. For instance, the example XRDS document above asserts four service endpoints for the represented resource:
For full details of XRDS service endpoints, see XRI Resolution 2.0, Sections 4.2 and 13.
In XRDS documents, a service is identified using a URI or XRI. Following are listings of well-known service types. See also XRDS Type, an open community effort begun in May 2008 to provide a catalog of XRDS service types.
| Common Name | URI or XRI | Source | Established |
|---|---|---|---|
| Authority Resolution | xri://$res*auth*($v*2.0) | XRI Resolution 2.0 | March 2005 |
| Proxy Resolution | xri://$res*proxy*($v*2.0) | XRI Resolution 2.0 | March 2005 |
| Common Name | URI or XRI | Source | Established |
|---|---|---|---|
| OpenID 1.0 | http://openid.net/server/1.0 | OpenID Authentication 2.0, Section 14.2.1 | June 2005 |
| OpenID 1.1 | http://openid.net/server/1.1 | OpenID Authentication 2.0, Section 14.2.1 | May 2006 |
| OpenID 2.0 – Standard Login | http://specs.openid.net/auth/2.0/signon | OpenID Authentication 2.0, Section 7.3.2.1.2 | December 2007 |
| OpenID 2.0 – OP Identifier Login | http://specs.openid.net/auth/2.0/server | OpenID Authentication 2.0, Section 7.3.2.1.1 | December 2007 |
| OpenID Attribute Exchange 1.0 | http://openid.net/srv/ax/1.0 | OpenID Attribute Exchange 1.0, Section 2 | December 2007 |
| Common Name | URI or XRI | Source | Established |
|---|---|---|---|
| OAuth Discovery | http://oauth.net/discovery/1.0 | OAuth Discovery Draft 2 | March 2008 |
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