1. Key Laboratory of Trustworthy Distributed Computing and Service, Beijing University of Posts and Telecommunications (BUPT), Ministry of Education, Beijing, China
2. Department of Computer Science and Technology, Tsinghua University, Beijing, China
3. Beijing National Research Center for Information Science and Technology, Tsinghua University, Beijing, China
4. Department of Electrical and Computer Engineering and the Institute for Systems Research, University of Maryland, College Park, MD, USA