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In this paper, we study how to build a front-end flow management system for SIP application servers. This is challenging because of some special characteristics of SIP and SIP applications. 1) SIP flows are well organized into sessions. The session structure should be respected when managing SIP flows. 2) SIP has been adopted by telecom industry, whose applications have more critical QoS requirements than WEB ones and requirement for service differentiation. 3) SIP message retransmissions exacerbate the overload situation in case of load bursts; moreover, they may trigger persistent retransmission phenomenon, which retains large response times even after the original burst disappears. To address the combination of these challenges, we propose a novel front-end SIP flow management system FEFM. FEFM integrates flow rate limiting, message scheduling and admission control to achieve overload protection, service differentiation, and performance management. It also devises some techniques such as response time prediction, multi-queue scheduling, and retransmission removal to accomplish SLA-oriented improvement, reduce the call rejection rate and banish the persistent retransmission phenomenon. Intensive experiments show that FEFM achieves overload protection in burst period, improves performance significantly, and has the ability to compromise different tradeoffs between throughput and SLA satisfaction.