As a greater number of people begin to access the Internet through high speed connections, the content offered is expanding. One method of broadcasting video and audio streams over the Internet is Multicasting. However, as use of multitask protocols and acknowledgement signals for each packet sent increases, so does congestion over the internet. Therefore, there exists a need in the art for a system and method of congestion control for multicast transmissions that is implemented entirely at the source of the transmission without any modifications to the receivers or routers. This invention provides a method of congestion control for multicast transmissions that is entirely implemented at the source of the transmission. Various types of filters as well as a round trip time estimator are used to determine when the rate of the multicast transmission should be reduced to alleviate congestion.