While with all the previously treated protocols we can rely on the session management the TCP layer provides us, there are many protocols that will not give us such a facility --Protocols based on UDP, such as NFS, ICQ and all RPC portmapper-controlled protocols. UDP (User Datagram Protocol) was designed to work for services that need to be resistant to changes in the network topology, have fast recovery times, and do not need explicit session management.
Working with UDP would force us to define timeouts and open even more our system to DoS attacks, which would be extremely simple to carry out: Sending a large number of requests would perfectly do it. Although this can be done, the suggested implementation would perform poorly.