1. National Computer Network Emergency Response Technical Team/Coordination Center of China (CNCERT/CC); Beijing 100029 China
2. Department of Electronic Engineering; Tsinghua University; Beijing 100084 China
3. Department of Electrical and Computer Engineering; University of Florida; Gainesville, FL 32611 U.S.A.
4. National Mobile Communications Research Laboratory; Southeast University; Nanjing 210096 China