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C. Egger, M. Hirschbichler, P. Reichl:
"Enhancing SIP Performance by Dynamic Manipulation of Retransmission Timers";
Talk: 6. GI/ITG-Workshop MMBnet 2011, Universität Hamburg; 09-15-2011 - 09-16-2011; in: "Leistungs-, Zuverlässigkeits- und Verlässlichkeitsbewertung von Kommunikationsnetzen und verteilten Systemen", B. Wolfinger, K. Heidtmann (ed.); Universität Hamburg, Fachbereich Informatik, Vogt-Kölln-Straße 30, D-22527 Hamburg, Bericht 298 (2011), 47 - 54.

The 3GPP has chosen the Session Initiation Protocol (SIP) as signalling protocol for the IP Multimedia Subsystem (IMS); 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. In this paper we propose to dynamically increase SIP retransmission timer(s) in high load situations, in order to lower the probability of a congestion collapse. We validate this approach by means of dedicated simulations, and show how manipulating the SIP retransmission timers increases the success rate of SIP requests. Finally, we propose to introduce an additional SIP header which allows an overloaded server to signal its current state to the next upstream server or user agent.

SIP, UDP, Simulative Performance Evaluation, Retransmission Timer, Non-INVITE Transaction