Date: Wed, 25 Sep 1996 18:01:50 -0600 From: vjs@mica.denver.sgi.com (Vernon Schryver)

At R=100 SYNs/sec, RTT=250, and L=382, ((L-1)/L)^(RTT*R) = 93%, which is not so bad. Drop-oldest is better with those three numbers, since it works 100% (modulo ordinary problems), but its performance falls off the cliff to 0% at R=L/RTT. If you have a short queue and care about long RTT's, random drop is better than drop-oldest.

Agreed. Note that 93% is not bad for a human-initiated telnet, but is disastrous for a Web browser which initiates a dozen tcp sessions to retrieve one page, because the browser will probably not retry at all if it gets a reset, but instead report failure to retrieve the page to the user, who can only ask it to start over from the beginning. So I think that it's better to accept the limited-radius-under-attack property of drop-oldest, gaining the immunity from interference within the safe radius. If it were possible to set the syn-rcvd timeout with sub-second granularity, this "fix" would not even take any kernel code mods - but of course it does not adjust the safe radius dynamically as the attack rate changes. What's absolutely clear is that any method of queue pruning is better than none, and a big queue is required for survival. Barney Wolff <barney@databus.com>