 The router sends an ICMP Time Exceeded message back to the source.The next set of packets are given a TTL value of two, so the first router forwards the packets, but the second router drops them and replies with ICMP Time Exceeded.Proceeding in this way, traceroute uses the returned ICMP Time Exceeded messages to build a list of routers that packets traverse, until the destination is reached and returns an ICMP Destination Unreachable message if UDP packets are being used or an ICMP Echo Reply message if ICMP Echo messages are being used.