A SYN-ACK flood is an attack method that involves sending a target server spoofed SYN-ACK packet at a high rate. Because a server requires significant processing power to understand why it is receiving such packets out-of-order (not in accordance with the normal SYN, SYN-ACK, ACK TCP three-way handshake mechanism), it can become so busy handling the attack traffic, that it cannot handle legitimate traffic and hence the attackers achieve a
denial-of-service condition.