The extent and diversity of systems provided by IP networks have lead various technologies to approach integrating various types of access networks and converting to next generation network. On account of features as being in text form, end-to-end connection, independence from the type of transmitted data, and supporting various forms of transmission, is an appropriate choice for signaling protocol in order to make connection between two IP network users. These advantages have made SIP be considered as a signaling protocol in IMS, a proposed signaling platform for next generation networks. Despite having all these advantages, SIP protocol is in lacking of appropriate mechanism for addressing overload. In this paper we try to improve a window-based overload control in RFC 6537. In window-based overload control method, a window is used to limit the number of messages that are sent to an overloaded SIP proxy, simultaneously. In this paper we first use fuzzy logic to regulate the accurate size of window and then we develop, implement, and evaluate it on an Asterisk open-source proxy. Simulation results show that this method could maintain throughput under overload conditions practically, change the maximum window size dynamically.
