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SIP-based systems inherently have NAT traversal and QoS problems because of the limited IPV4 addresses and the best-effort service on the Internet. Using relay server to relay media stream can solve NAT traversal in SIP systems, but have less scalability. In this paper, we propose a SIP+P2P architecture for SIP-based systems to solve the above problems. In this novel architecture, light weighted SIP signaling is still client/server based, but media stream is transmitted through P2P network. We embed relay service into some SIP UAs who can be peers, design a P2P overlay network to organize those peer nodes, and provide a fast and efficient relay nodes retrieving mechanism. We also present a detouring technology to select multi good performance paths in P2P overlay and switch real transmitting path between them according to the path performance. We implemented the whole design, and preliminary experiments show that our approach enhanced path reliability and QoS performance of end-to-end communications significantly.