Chapter 7 Firewall for FortiOS 5.0 : Network defense : Defending against DoS attacks : SYN flood
  
SYN flood
When a client sends a SYN packet to a server, the server creates an entry in its session table to keep track of the connection. The server then sends a SYN+ACK packet expecting an ACK reply and the establishment of a connection.
An attacker intending to disrupt a server with a denial of service (DoS) attack can send a flood of SYN packets and not respond to the SYN+ACK packets the server sends in response. Networks can be slow and packets can get lost so the server will continue to send SYN+ACK packets until it gives up, and removes the failed session from the session table. If an attacker sends enough SYN packets to the server, the session table will fill completely, and further connection attempts will be denied until the incomplete sessions time out. Until this happens, the server is unavailable to service legitimate connection requests.
Figure 173: A single client launches a SYN flood attack
SYN floods are seldom launched from a single address so limiting the number of connection attempts from a single IP address is not usually effective.