Several of the "reliable UDP" protocols I have worked on in the past have had a heartbeat mechanism that is specifically for detecting this. If you haven't sen

Several of the "reliable UDP" protocols I have worked on in the past have had a heartbeat mechanism that is specifically for detecting this. If you haven't sent a packet down the wire in 10-100 milliseconds, you will send an extra packet just to say you're still there. It's very useful to do this in intra-datacenter protocols.