1. School of Computer Engineering and Applied Mathematics; Changsha University; Changsha 410081 China
2. Department of Computer Science; Hunan University; Changsha 410082 China
3. School of Software; Central South University; Changsha 410083 China
4. School of Software; Henan University; Kaifeng 475001 China
5. CERNET Limited Company; Beijing 100084 China
6. State Key Laboratory of Networking and Switching Technology; Beijing University of Posts and Telecommunications; Beijing 100876 China