Traceroute Program

The Traceroute program was written by Van Jacobson. When an IP datagram moves from one host to another, Traceroute program lets us see the route that IP datagram follows. It is not always true that two consecutive IP datagrams follow the same route when they move from the same source to the same destination but most of the time they follow the same route. We can be aware of this with Traceroute program.
Operation of Traceroute program:
This program send UDP datagrams starting with a TTL (time to live) of 1. This TTL with value 1 finds a router and gets destroyed returning 'ICMP time exceeded' error. Now another datagram with TTL 2 is sent. It lives till it encounters 2 routers returning same error. Now another TTL with value 3 is sent. Thus each time the TTL is increased by 1, to locate each router in the path. An 'ICMP time exceeded' is returned by each router when it discards the UDP datagram, and an 'ICMP port unreachable' is generated by the final destination.