In this paper, an end-to-end real-time adaptive protocol for multimedia transmission is presented. The transmission rate is determined by the quadratic probing algorithm that can obtain the maximal utilization of the client buffer and minimal allocation of the network bandwidth. It is also coupled with a congestion control mechanism that can effectively decrease the packet loss rate during network congestion. We investigate the performance of our quadratic probing algorithm in different congestion levels under both Local Area Net (LAN) and Internet environment. Performance analysis reveals that our approach is more robust in avoiding overflow and underflow in different network congestion levels, and adapting to the changing network delays. Comparisons are made with the fixed rate approach and the rate by playback requirement approach. Experimental results show that the proposed real time protocol with rate adjusting quadratic probing algorithm is efficient in utilizing the network resources and decreasing the packet loss ratios.