ZyXEL Communications G-2000 Plus - Manual (Page 183)

ZyAIR G-2000 Plus User’s Guide
Chapter 14 Firewalls 182
Figure 75 Three-Way Handshake
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.
a SYN Attack floods a targeted system with a series of SYN packets.
Each packet causes the targeted system to issue a SYN-ACK
response. While the targeted system waits for the ACK that follows
the SYN-ACK, it queues up all outstanding SYN-ACK responses on
what is known as a backlog queue. SYN-ACKs are moved off the
queue only when an ACK comes back or when an internal timer
(which is set at relatively long intervals) terminates the three-way
handshake. Once the queue is full, the system will ignore all
incoming SYN requests, making the system unavailable for
legitimate users.