Application Layer Feedback-based SIP Server Overload Control

A SIP server may be overloaded by emergency-induced call volume, “American Idol” style flash crowd effects or denial of service attacks. The SIP server overload problem is interesting especially because the cost of serving and rejecting a SIP session could be in the same neighborhood. For this reason, the built-in SIP overload control mechanism based on generating rejection messages could not prevent the server from entering congestion collapse at heavy load. The SIP overload problem calls for a pushback control solution in which the potentially overloaded receiving server may notify its upstream sending servers to have them send only the amount of load within the receiving server’s processing capacity. The pushback framework can be achieved by SIP application layer rate-based feedback or window-based feedback. We propose three new window-based feedback algorithms and evaluate them together with two existing rate-based feedback algorithms. We compare the different algorithms in terms of number of tuning parameters and performance under both steady and dynamic load. Furthermore, we identify two categories of fairness requirements for SIP overload control, namely, user-centric and provider-centric fairness. With the introduction of a new double-feed SIP overload control architecture, we show how the algorithms can meet those fairness criteria.