Date:        Mon, 20 May 2002 10:02:08 +0100
    From:        David Laight <david@l8s.co.uk>
    Message-ID:  <20020520100208.A447@snowdrop.l8s.co.uk>

  | IIRC traceroute stops when it receives a 'port unreachable' resonse
  | from the intended target.

Yes.  (Several other unreachables stop it as well).

  | Maybe it should send out the udp packet with a TTL of 255 first.
  | If it gets a port unreachable response then it can increase the TTL
  | from one until that happens again, if it doesn't get one then
  | stopping after a number of non-replies.

No.  While traceroute is "fun" to find out what route your packets
are using, its real utility is to find out where the packets are going
when they don't arrive.

If the actual route trace weren't done if the target wasn't replying,
most of the real purpose of traceroute would be lost.

kre