<P> A web service protocol stack is a protocol stack (a stack of computer networking protocols) that is used to define, locate, implement, and make Web services interact with each other . A Web service protocol stack typically stacks four protocols: </P> <Ul> <Li> (Service) Transport Protocol: responsible for transporting messages between network applications and includes protocols such as HTTP, SMTP, FTP, as well as the more recent Blocks Extensible Exchange Protocol (BEEP). </Li> <Li> (XML) Messaging Protocol: responsible for encoding messages in a common XML format so that they can be understood at either end of a network connection . Currently, this area includes such protocols as XML - RPC, WS - Addressing, and SOAP . </Li> <Li> (Service) Description Protocol: used for describing the public interface to a specific Web service . The WSDL interface format is typically used for this purpose . </Li> <Li> (Service) Discovery Protocol: centralizes services into a common registry so that network Web services can publish their location and description, and makes it easy to discover what services are available on the network . Universal Description Discovery and Integration (UDDI) was intended for this purpose, but it has not been widely adopted . </Li> </Ul>

The xml protocol fills what level of the web services protocol stack