1. College of Computer Science and Technology, Harbin Engineering University, Harbin, Heilongjiang 150001, P. R. China
2. Key Laboratory of Network Assessment Technology, Institute of Information Engineering, Chinese Academy of Sciences, Beijing 100093, P. R. China
3. Palo Alto Networks, Inc., 3000 Tannery Way, Santa Clara, CA 95054, USA
4. Department of Computing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, P. R. China
5. MOE Key Laboratory for Intelligent Networks and Network Security, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China