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Most of the traffic in the Internet nowadays is transmitted using TCP, or transport control protocol. The stability of the Internet depends on the congestion control being performed by this protocol, as well as equivalent mechanisms employed by other protocols. The ECN technique (explicit congestion notification), in which packets forwarded by routers are marked whenever congestion is about to occur (or already occurring), allows a transmitter to reduce the sending rate accordingly without relying on packet drops. ECN has not been fully explored, particularly regarding multicast protocols. This paper presents models of poll-based reliable multicast protocols and extends them with a new single-rate congestion control mechanism that harnesses ECN. Simulation results show that it provides fairness between flows from multiple sources, and is TCP-friendly. Further, they demonstrate the substantial efficiency gain obtained with the use of ECN in multicast.