Created attachment 593 [details]
Example script

When StopApplication is called in UdpEchoClient, the socket is closed but not reset to zero.  Therefore, when StartApplication is called again, it will think the socket is still in place and fail to bind a new socket.  Essentially if you start, stop, and start UdpEchoClient during a simulation, it won't work.  Attached is a modified version of first.cc which shows this.  The end 3 packets are not sent (it actually says sent, but DoSendTo is never called).  I will also attach a patch with what I think is the simple solution.