1. School of Information Science & Technology, Qingdao University of Science & Technology, Qingdao, Shandong 266061, China
2. School of Information Science and Engineering, Shandong University, Jinan, Shandong 250100, China
3. China United Network Communications Corporation, Qingdao Branch, Qingdao 266071, China
4. Shandong Provincial Key Laboratory of Computer Network, Jinan, Shandong 250101, China