I don't think you can get a 'generic initial UDP datagram that will illicit a response'. Whether an application responds to a UDP packet is a function of each specific protocol. The 'are you still alive' protocol the we implemented for our server farm is a datagram to the monitoring port on a Heart-Beat machine. The server sends a packet every 60 seconds, and does not expect any response from the other side. (After all, this is UDP, if it didn't get there, oh well....)