A field in a datagram, or IP packet header. It specifies how long, in seconds, the datagram is allowed to remain in the system. The computer sets a maximum time wheb injecting a datagram into the internet; the routers and hosts that process the datagram decrement this field (by 1) with each hop and remove it when the field reaches zero. If the packet is discarded, the router sends an error message back to the source. Acts as a hop cont. TTL was originally specified in seconds, but this is difficult to implement. The recommended default TTL is 64, though values like 15 and 32 are occasionally used.