1. School of Information and Software Engineering, University of Electronic Science and Technology of China, Chengdu 610054 China, and also with the Network and Data Security Key Laboratory of Sichuan Province, Chengdu, 610054, China, with the State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications, Beijing 100876 China.
2. Department of Computer Science, University of California Davis, Davis, CA 95616 USA.
3. College of Computer Science and Engineering, Shandong University of Science and Technology, Qingdao 266590 China.