ZyXEL Communications 650 Series - Manual (Page 133)

Prestige 650 series User’s Guide
Firewall Configuration 11-5
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 Prestige starts deleting half-open sessions according to one of the
following methods:
1. If the Blocking Time timeout is 0 (the default), then the Prestige 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 Prestige blocks all new connection requests to
the host giving the server time to handle the present connections. The Prestige continues to block all new
connection requests until the Blocking Time expires.
The Prestige also sends alerts whenever TCP Maximum Incomplete is exceeded. The global values
specified for the threshold and timeout apply to all TCP connections. Click Advanced Setup, Firewall, and
Alert to bring up the next screen.