ZyXEL Communications ZyXEL ZyWALL 5 - Manual (Page 186)

ZyWALL 5 User’s Guide
Chapter 10 Firewall Screens 185
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
FIREWALL link and then the Threshold tab to bring up the next screen.
Figure 76 Firewall Threshold
The following table describes the labels in this screen.
Table 55 Firewall Threshold
LABEL DESCRIPTION
Denial of Service
Thresholds
One Minute Low This is the rate of new half-open sessions that causes the firewall to stop deleting
half-open sessions. The ZyWALL continues to delete half-open sessions as
necessary, until the rate of new connection attempts drops below this number.