IP-multicast is a bandwidth efficient transmission mechanism for group communications. Reliability in IP-multicast, however, poses a set of significant challenges. To address the reliability and scalability issues in IP-multicast, this paper proposes a novel overlay retransmission architecture that exploits path-diversity by taking advantages of both IP multicast and an overlay network. We show that the proposed path diversity overlay retransmission architecture has the potential to significantly improve the reliability, delay, playback quality, and scalability of IP-multicast based multimedia applications. The general concept of using P2P overlay networks to help improve the QoS performance of multimedia applications as illustrated in this paper is expected to have significant impact on the deployment of next generation multimedia services.