ZyXEL Communications 802.11g Wireless ADSL2+ 4-port VoIP IAD P-2602HWNLI - Manual (Page 228)

P-2602HWNLI User’s Guide
228 Chapter 14 Firewall Configuration
• The maximum number of opened sessions.
• The minimum capacity of server backlog in your LAN network.
• The CPU power of servers in your LAN network.
• Network bandwidth.
• Type of traffic for certain servers.
If your network is slower than average for any of these factors (especially if you have servers
that are slow or handle many tasks and are often busy), then the default values should be
reduced.
You should make any changes to the threshold values before you continue configuring firewall
rules.
14.10.2 Half-Open Sessions
An unusually high number of half-open sessions (either an absolute number or measured as
the arrival rate) could indicate that a Denial of Service attack is occurring. For TCP, "half-
open" means that the session has not reached the established state-the TCP three-way
handshake has not yet been completed (see
Figure 105 on page 203 ). For UDP, "half-open"
means that the firewall has detected no return traffic.
The ZyXEL Device 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 ZyXEL Device starts deleting half-open sessions as required to accommodate new
connection requests. The ZyXEL Device 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
ZyXEL Device starts deleting half-open sessions as required to accommodate new connection
requests. The ZyXEL Device 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.
14.10.2.1 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 ZyXEL Device starts deleting half-open
sessions according to one of the following methods: