I. Introduction
The Internet and local area networks are growing larger in recent years. As about 1,463,632,361 people all over the world use the Internet, they are unconsciously encountering the number of security threats such as viruses, worms and attacks from hackers [1], [2]. Now firewalls, anti-virus software, message encryption, secured network protocols, password protection and so on are not sufficient to guarantee the security in computer networking, which some intrusions take advantages of weaknesses in computer systems. Therefore, intrusion detection is becoming a more and more important technology which monitors network traffic and identifies network intrusion such as anomalous network behaviors, unauthorized network access, and malicious attacks to computer systems [3].