Hi,
the manual and the tutorial can help you for sure. Google as well (this topic has been discussed in the past).
I'd check in particular the Flowmonitor module and the examples/udp/udp-echo.cc script. As an hint, UdpClient and UdpServer apps use a nice method to know when a packet has been sent (and received).
I hope I caught the meaning of your message, as "tcMessage last time" really doesn't mean anything to me.
Cheers,
T.