ZyXEL Communications 70 Series - Manual (Page 228)

ZyWALL 5/35/70 Series User’s Guide
Chapter 11 Firewall Screens 228
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.
11.10.2.1 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
SECURITY, FIREWALL and then the Threshold tab to bring up the next screen.