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Recent collapses of SIP servers in the carrier networks (e.g., Skype outage) indicate that the built-in SIP overload control mechanism cannot mitigate overload effectively. In this paper, by employing a control-theoretic approach that models the interaction between an overloaded downstream server and its upstream server as a feedback control system, the authors investigate the root cause of SIP server crash by studying the impact of the retransmission on the queuing delay of the overloaded server. Then they design a PI rate controller to mitigate the overload by regulating the retransmission rate based on the round trip delay. They derive the guidelines for choosing PI controller gains to ensure the system stability.
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