/**
* Implementation of the Server Dialback method as defined by the RFC3920.
*
* The dialback method follows the following logic to validate the remote server:
* <ol>
* <li>The Originating Server establishes a connection to the Receiving Server.</li>
* <li>The Originating Server sends a 'key' value over the connection to the Receiving
* Server.</li>
* <li>The Receiving Server establishes a connection to the Authoritative Server.</li>
* <li>The Receiving Server sends the same 'key' value to the Authoritative Server.</li>
* <li>The Authoritative Server replies that key is valid or invalid.</li>
* <li>The Receiving Server informs the Originating Server whether it is authenticated or
* not.</li>
* </ol>
*
* By default a timeout of 20 seconds will be used for reading packets from remote servers. Use
* the property <b>xmpp.server.read.timeout</b> to change that value. The value should be in
* milliseconds.
*
* @author Gaston Dombiak
*/