SIP proxy high-load detection by continuous analysis of response delay values

The 3GPP has chosen the Session Initiation Protocol as signalling protocol for the IP Multimedia Subsystem; therefore, it is expected that telecom operators will widely use it for their systems. SIP relies on an underlying transport protocol, like, e.g., TCP, UDP or SCTP. In the case of UDP, SIP has to ensure itself that messages will be reliably delivered. For this purpose, retransmission timers within the SIP transaction state machine are used. On the other hand, retransmissions can lead to congestion or even cause a congestion collapse if traffic load becomes too high, and services of the operator may become unavailable. It is therefore important to detect an imminent collapse and to act accordingly in order to keep the users' perceived quality high. We propose to continuously measure response delay values to detect high-load situations that can lead to a collapse in order to be able to reduce the traffic load early enough for avoiding congestion situations. We validate this approach by means of dedicated simulations.