traceroute issues packet(s) for the destination with RTT set as 1 on random ports which, on reaching the first router, expires and ICMP expired message is received back. Now RTT is set to 2 and more packets are sent. They expire on the second router and ICMP messages are returned back. This is done to map the reachability of source to the destination. This methodology is gradually extended until we reach the destination (and ICMP messages starts coming back from them).