Time constraints are common in network protocols and are important features for real-time requirements.It is necessary to test time constraints in protocol implementations. Approaches in testing realtime are reviewed and the disadvantages are given. Some assumptions in the existing methods are revised to fit the need of our practice on testing routing protocols. We modified the theory of timed automata to convert time constraints to state variables and time events

which result in a nondetenninistic finite state machine. Abstract test suite is generated by Wp method and should be parameterized in execution. Example from RIP protocol is used to demonstrate the procedure to construct TTCN test cases.