The utility program traceroute (Windows tracert) is a network management tool that is usually run from the command line. It's purpose is to determine the path from the host at which is initiated to a destination host. It does this by sending probe (ICMP echo) datagrams into the network which are addressed to the IP address of the host that you find the route for. The algorithm relies on the fact that there is a field in an IP datagram called time to live (TTL) which is decremented by 1 by each router that it is passed through. If the TTL reaches 0, the router will send an ICMP reply message that Time to Live is Exceeded.

The first three datagrams have a time to live (TTL) field that is set to 1. Each packet is transmitted and a timer is started. Because TTL = 1, the datgram will only reach the first router and it will send back the TTL execeeded message. On receipt of these messages, the program will stop the timers and will have a time for the return journey to the first router, and the IP address of its interface on the next network. The second group of packets will be sent with a TTL of 2 and so on. Eventually, the TTL will be large enough to reach the destination, which because the message is an Echo request, it will reply to say that the message has been received. In this way, an indication of each router and each network that the datagrams move through will emerge, and the total end-to-end propagation delay of the route.

Traceroute relies on the routers returning the TTL exceeded message. If they do not send back the message, or the trace-packet or its reply is lost, part of the route will remain unknown. If the final destination host cannot be reached, one of the routers will eventually reply with a “host unreachable” message.

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