Web services are simple, self-contained applications that perform functions, from simple requests to complicated business processes. The "web services" model uses WSDL, UDDI, and SOAP/XMLP. A WSDL description is retrieved from the UDDI directory. WSDL descriptions allow the software systems of one business to extend to use those of the other directly. The services are invoked over the World Wide Web using the SOAP/XMLP protocol. Each of the components are XML based. Where two agencies know about each other's web services they can link their SOAP/XMLP interfaces - provided all security concerns are managed appropriately. It is only where services are going to have unknown users that they need to be formally described by a language such as WSDL and entered into a directory such as UDDI.