ZyXEL Communications ZyXEL ZyWALL 2WE User Manual page 166

ZyWALL 2 and ZyWALL 2WE
When the rate of new connection attempts rises above a threshold (one-minute high), the ZyWALL starts
deleting half-open sessions as required to accommodate new connection requests. The ZyWALL continues to
delete half-open sessions as necessary, until the rate of new connection attempts drops below another
threshold (one-minute low). The rate is the number of new attempts detected in the last one-minute sample
period.
TCP Maximum Incomplete and Blocking Time
An unusually high number of half-open sessions with the same destination host address could indicate that a
Denial of Service attack is being launched against the host.
Whenever the number of half-open sessions with the same destination host address rises above a threshold
(TCP Maximum Incomplete), the ZyWALL starts deleting half-open sessions according to one of the
following methods:
1. If the Blocking Time timeout is 0 (the default), then the ZyWALL deletes the oldest existing half-open
session for the host for every new connection request to the host. This ensures that the number of half-
open sessions to a given host will never exceed the threshold.
2. If the Blocking Time timeout is greater than 0, then the ZyWALL blocks all new connection requests to
the host giving the server time to handle the present connections. The ZyWALL continues to block all
new connection requests until the Blocking Time expires.
The ZyWALL also sends alerts whenever TCP Maximum Incomplete is exceeded. The global values
specified for the threshold and timeout apply to all TCP connections. Click the Attack Alert tab to bring up
the next screen.
15-4 Firewall Configuration