Retransmission mechanism helps SIP maintain its reliability, but it can also make an overload worse. Recent server collapses due to emergency-induced call volume in carrier networks indicate that the built-in overload control mechanism cannot handle overload conditions effectively. Since the retransmissions caused by the overload are redundant, we suggest mitigating the overload by controlling redundant message ratio to an acceptable level. Using control-theoretic approach, we model the interaction of an overloaded downstream server with its upstream server as a feedback control system. Then we develop an adaptive PI control algorithm to mitigate the overload at the downstream server by controlling the retransmission message rate of its upstream servers. By performing OPNET simulations on two typical overload scenarios, we demonstrate that: (1) without overload control algorithm applied, the overload at the downstream server may propagate to its upstream servers; (2) our control-theoretic solution not only mitigate the overload effectively, but also achieve a satisfactory target redundant message ratio.
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