Time To Live
When you send a packet accross a network, it bounces around routers until it reaches its destination. However, sometimes, it might get stuck and keep between routers, say, because of a loop in the network. In order to make sure the packet “dies” after a while, each IP packet has a Time To Live value which gets decremented after each jump and the packet is deleted once TTL is 0. Once a packet dies, this is notified to the sender via ICMP.