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'. 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 TTL field is set by the sender of the datagram, and reduced by every router on the route to its destination. The time-to-live value can be thought of as an upper bound on the time that an IP datagram can exist in an Internet system. The maximum TTL value is 255, the maximum value of a single octet. In the IPv6 header, it is the 8th octet of 40. In the IPv4 header, TTL is the 9th octet of 20. Under the Internet Protocol, TTL is an 8-bit field. The time to live can be thought of as a self destruct time limit. If the time to live reaches zero before the internet datagram reaches its destination, the internet datagram is destroyed. It is set by the sender of the datagram and reduced at the points along the route where it is processed. The Time to Live is an indication of an upper bound on the lifetime of an internet datagram. The original DARPA Internet Protocol's RFC document describes TTL as:

Wireshark cheat sheet packet life