Traceroute and random UDP ports

Hi, The outgoing packets from traceroute are sent towards the destination using UDP and very high port numbers, typically in the range of 32,768 and higher. This is because no one is gernally expected to run UDP services up there, so when the packet finally reaches the destination, traceroute can tell that it got to the end (because the ICMP changes from "TTL exceeded" to "port unreachable"). My question is: What if the receiver is actually listening on one of the "random" UDP ports? What would happen in such cases? Also, why do we increase the UDP port number with each subsequent traceroute packet that is sent? Thanks, Glen