We are getting protocol 73 in the ip cache flow and not able to conform what actually the protocol is meant for. After googling we come to know that it is a heartbeat protocol. But now question comes in mind what is heartbeat and why it is coming in l2tpv3. Actually it is the keepalive used by l2tpv3 for checking the remote destination whether it is dead or alive.
SO if you are getting 73 in your cache flow no need to worry simply check whether you are running l2tpv3 or not.