Congestion control of multicast real-time flows in the Internet has different requirements and poses further difficulties than common end-to-end unicast congestion control. In this paper we present a novel congestion control protocol, the layered datagram protocol (LDP), aimed at solving such problems. LDP has been designed to be fair against competing TCP traffic and, at the same time, exhibit smoothness in the instantaneous throughput, making it attractive for real-time multimedia transmission. Furthermore, the open-loop approach makes the protocol highly scalable, as the server load is not affected by the number of clients present in the transmission. Our simulations show that the protocol exhibits the expected TCP fairness while still being highly tunable to smooth the instantaneous throughput.