Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 33
Time to Live (TTL)

This field seems to confuse many people, so let’s state what it does up front. Time to Live (TTL) indicates the amount of time that a datagram is allowed to stay on the network. It is not used by the routers to count up to 16 to know when to discard a packet. There are two functions for the TTL field: to limit the lifetime of a TCP segment (transmitted data) and to end routing loops.

The initial TTL entry is set by the originator of the packet, and it varies. To be efficient, a routing update will set this field to a 1 (RIP will). Why set it to anything else, when that update is sent only to its local segments? Multicast protocols set it to many different sizes to limit the scope of the multicast. For normal usage, many applications set it to 32 or 64 (2 and 4 times the size of a RIP network). Time to live is a field that is used by routers to ensure that a packet does not endlessly loop around the network. This field (currently defined as the number of seconds) is set at the transmitting station and then, as the datagram passes through each router, it will be decremented. With the speed of today’s routers, the usual decrement is 1. One algorithm is that the receiving router will notice the time a packet arrives, and then, when it is forwarded, the router will decrement the field by the number of seconds the datagram sat in a queue waiting for forwarding. Not all algorithms work this way. A minimum decrement will always be 1. The router that decrements this field to 0 will discard the packet and inform the originator of the datagram (through the ICMP protocol) that the TTL field expired and the datagram did not make it to its destination.

Time to Live (TTL)

The time–to–live field may also be set to a certain time (i.e., initialized to a low number like 64) to ensure that a packet stays on the network for only a set time. Some routers allow the network administrator to set a manual entry to decrement. This field may contain any number from 0 to 255 (an 8–bit field).