Design of a PI Rate Controller for Mitigating SIP Overload

Recent collapses of SIP servers in the real carrier networks indicate that the built-in SIP overload control mechanism cannot mitigate overload effectively. In this paper, we investigate the root cause of SIP server crash by studying the impact of the retransmission on the queuing delay of the overloaded server. The transient overload may introduce the excessive queuing delay, thus triggering unnecessary retransmissions to exacerbate the overload. Therefore, we adopt a control-theoretic approach that models the overloaded downstream server and its upstream server as a feedback control system. Then we design a PI rate controller to restrict the retransmission rate based on the queuing delay. We derive the guidelines for choosing PI controller gains to ensure the system stability. Our OPNET simulation results demonstrate that our proposed control theoretic approach can mitigate the SIP overload effectively, thus preventing the SIP network collapse.