Internet Core Protocols: The Definitive Guide

Notes on traceroute

Just as ping allows you to verify connectivity between different devices, traceroute allows you to identify the route that datagrams are taking on their way to a remote device. This is achieved by sending a series of packets with incrementally larger Time-to-Live values, and then monitoring for ICMP Time Exceeded error messages as the packets expire on the way to the destination system.

The first packet sent by traceroute will have a Time-to-Live value of 1. When the packet is received by the first-hop router on the local network, the router will be unable to forward the datagram without the Time-to-Live value reaching zero, so the router will discard the datagram and send an ICMP Time Exceeded error message back to the sender.

traceroute records the IP address of the router that returned the datagram, and then sends another datagram with the Time-to-Live field set to 2. This time the datagram makes it past the first-hop router to the next router in the path. However, since the Time-to-Live value will have been set to 1 by the first-hop router during the forwarding process, the next-hop router will reject the packet and send an ICMP Time Exceeded error message back to the sending system.

This process is repeated over and over, until the final destination system has been reached. Once that occurs, the local system will have received ICMP Time Exceeded error messages from every router between itself and the final destination system, and will therefore have a complete ...