Controlling Retransmission Rate for Mitigating SIP Overload

With rapidly growing deployment, SIP has become a main signaling protocol for IP telephony and multimedia sessions in the Internet. SIP employs a retransmission mechanism to maintain its reliability. Recent server collapse due to emergency-induced call volume in carrier networks indicates that message retransmissions triggered by various SIP timers make the overload worse. The built-in overload control mechanism cannot handle overload conditions effectively. Since the retransmissions caused by the overload introduce more overhead rather than reliability, we suggest mitigating the overload by reducing the retransmission rate. We propose a novel algorithm to detect the potential overload at the downstream servers and control retransmission message rate from upstream servers to mitigate the overload at the downstream servers. We investigate two typical overload scenarios caused by demand burst and server slow down respectively. OPNET simulations demonstrate that (1) the proposed solution can help the overloaded downstream server to cancel its overload effectively after it resumes its normal operation status; (2) without the overload control algorithm applied, the overload at the downstream server may propagate or migrate to its upstream servers.