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Every IP datagram contains a field called "time to live" or TTL. On each hop along the path to the destination, the TTL field is decremented by one. When the value of the TTL field equals zero, the datagram is discarded to prevent the datagram from floating around the network forever. The gateway may also notify the source host via the time exceeded message.


Because hosts along a network path may not have the same ammount of memory for buffering data, it is sometimes necessary to fragment a packet into smaller pieces. These fragments must later be reassembled. If a host is missing fragments, and is unable to reassemble the datagram, and the TTL has expired, an ICMP message can be sent to the transmitting host.

Addendum: Traceroute

Traceroute actually uses the TTL exceeded message to track the path through the network from source to destination. Traceroute sets the TTL on it's first set of packets to 1 and waits for the TTL exceeded response, which returns with the sender's IP address (this is how both round trip time to that device, and its IP address are aquired).

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