 The TTL field is set by the sender of the datagram, and reduced by every router on the route to its destination.If the TTL field reaches zero before the datagram arrives at its destination, then the datagram is discarded and an Internet Control Message Protocol (ICMP) error datagram (11 - Time Exceeded) is sent back to the sender.The purpose of the TTL field is to avoid a situation in which an undeliverable datagram keeps circulating on an Internet system, and such a system eventually becoming swamped by such "immortals".