1. School of Software Engineering, Beijing Jiaotong University, Beijing, China
2. State Key Laboratory of Networking and Switching Technology, Beijing, China
3. School of Computer Science and Engineering, Beihang University, Beijing, China
4. National Key Laboratory of Science and Technology on Information System Security, Beijing, China
5. Department of Electrical and Information Engineering and the Consorzio Nazionale Interuniversitario per le Telecomunicazioni (CNIT), Politecnico di Bari, Bari, Italy