ZyXEL Communications P-2802H(W)(L)-I Series - Manual (Page 171)

Chapter 11 Firewalls
P-2802H(W)(L)-I Series User’s Guide
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Figure 97 Firewall Example: Rules: MyService
11.8 Firewall Thresholds
For DoS attacks, the ZyXEL Device uses thresholds to determine when to start dropping
sessions that do not become fully established (half-open sessions). These thresholds apply
globally to all sessions.
For TCP, half-open means that the session has not reached the established state-the TCP three-
way handshake has not yet been completed. Under normal circumstances, the application that
initiates a session sends a SYN (synchronize) packet to the receiving server. The receiver
sends back an ACK (acknowledgment) packet and its own SYN, and then the initiator
responds with an ACK (acknowledgment). After this handshake, a connection is established.
Figure 98 Three-Way Handshake
For UDP, half-open means that the firewall has detected no return traffic. An unusually high
number (or arrival rate) of half-open sessions could indicate a DOS attack.