Stability condition for SIP retransmission mechanism: Analysis and performance evaluation

SIP (Session Initiation Protocol) has been widely adopted as a signaling protocol to establish, modify and terminate multimedia sessions between end-users in the Internet. SIP introduces a retransmission mechanism to ensure the reliability of its real-time message delivery. However, retransmission makes server overload worse, as indicated by the recent server crashes in the real carrier networks. In this paper, we use a discrete time model to describe the queuing dynamics of an overloaded SIP server with the retransmission mechanism. We then derive a sufficient stability condition that a SIP server can handle the overload effectively under the retransmission mechanism. Discrete time model allows us to run fluid-based Matlab simulation directly to evaluate the overload performance. This approach is much simpler than event-driven simulation. Event-driven OPNET simulation was also conducted to observe the transient behaviour of an overloaded server in a SIP network. Our simulation results demonstrate that: (1) The sufficient stability bound is quite tight. The bound indicates that effective CPU utilization as low as 20% can still lead to an unstable system after a short period of demand burst or a temporary server slowdown. Resource overprovisioning is not a viable solution to the server crash problem; (2) By satisfying the stability condition, the initial queue size introduced by a transient overload can avoid a system crash. Such stability condition can help the operator to determine whether and when to activate overload control mechanism in case of heavy load.