I. Introduction
The Internet has witnessed an incredible growth in its pervasive use. People are enjoying unprecedented convenience brought by the Internet boom. However, at the same time when Internet users benefit from the Internet based services, they are more and more troubled by the increasing amount of unwanted traffic, such as spam, malware, vicious intrusions, and so on. For example, spam accounts for 14.5 billion messages globally per day, i.e., it makes up 45% of all emails [3]. According to South Korea's National Police Agency, the computer networks of three major South Korea banks and three television networks went offline almost simultaneously at 2pm Seoul time on 20th March 2013, caused by a malware attack [1]. The websites and corporate networks at Bank of America, JPMorgan Chase and Citigroup suffered from distributed denial-of-service (DDoS) attacks both in 2011 and 2012, resulting in hundreds of complaints from their customers [2]. Such incidents undoubtedly increase public worries on network security. Thus, working out an efficient solution to control the unwanted traffic in the Internet has become a crucial task that brooks no delay.