diff -urpb nmap-3.78/scan_engine.cc nmap-3.78mm/scan_engine.cc
--- nmap-3.78/scan_engine.cc	2004-11-24 21:13:01.000000000 +0100
+++ nmap-3.78mm/scan_engine.cc	2004-12-19 13:30:43.000000000 +0100
@@ -1475,7 +1475,11 @@ static void ultrascan_port_update(UltraS
 
   probe = *probeI;
 
-  if (rcvdtime) ultrascan_adjust_times(USI, hss, probe, rcvdtime);
+  /* ICMP unreachables are often rate-limited (RFC 1812)
+   * or generated by host in-between.
+   * If we can get the result without them, do not slow down (or up) */
+  if (rcvdtime && !noresp_open_scan && newstate != PORT_FILTERED)
+      ultrascan_adjust_times(USI, hss, probe, rcvdtime);
 
   if (probe->type == UltraProbe::UP_IP) {
     if (USI->prot_scan)
@@ -1570,9 +1574,11 @@ static void ultrascan_port_update(UltraS
   }
 
   /* The rcvdtime check is because this func is called that way when
-     we give up on a probe because of too many retransmissions. */
+     we give up on a probe because of too many retransmissions. 
+     We don't care ICMP/PORT_FILTERED in some type of scans because
+     they are often rate-limited and we don't need them anyway */
   if (oldstate != newstate && probe->tryno > hss->max_successful_tryno
-      && rcvdtime) {
+      && rcvdtime && !noresp_open_scan && newstate != PORT_FILTERED) {
     hss->max_successful_tryno = probe->tryno;
     if (o.debugging)
       log_write(LOG_STDOUT, "Increased max_successful_tryno for %s to %d (packet drop)\n", hss->target->targetipstr(), hss->max_successful_tryno);