Understanding Network – How Traceroute works?

The program was written by Van Jacobson and others. It is based on a clever use of the Time-To-Live (TTL) field in the IP packet’s header. The TTL field is used to limit the life of a packet. When a router fails or is mis-configured, a routing loop or circular path may result. The TTL field prevents packets from remaining on a network indefinitely should such a routing loop occurs. A packet’s TTL field is decremented each time the packet crosses a router on its way through a network. When its value reaches 0, the packet is discarded rather forwarded. When discarded, the ICMP TIME_EXCEEDED message is sent back to the packet’s source to inform the source that the packet was discarded. By manipulating the TTL field original packet, the program traceroute uses information from these ICMP messages to discover paths through a network. Read the rest of this entry »