ZyXEL Communications Prestige 652H series, User Manual

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Prestige 652H/HW Series User’s Guide
Firewall Screens 13-25
The Prestige measures both the total number of existing half-open sessions and the rate of session
establishment attempts. Both TCP and UDP half-open sessions are counted in the total number and rate
measurements. Measurements are made once a minute.
When the number of existing half-open sessions rises above a threshold ( max-incomplete high ), the Prestige
starts deleting half-open sessions as required to accommodate new connection requests. The Prestige
continues to delete half-open requests as necessary, until the number of existing half-open sessions drops
below another threshold (
max-incomplete low ).
When the rate of new connection attempts rises above a threshold ( one-minute high ), the Prestige starts
deleting half-open sessions as required to accommodate new connection requests. The Prestige 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 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 the Firewall link and then the
Threshold tab to bring up the next screen.