Currently, the new transport protocol, Stream Control Transmission Protocol (SCTP), is not ECN-capable. The ECN-capable SCTP is proposed in this paper, which is bandwidth-efficient and robust to non-congestion losses. A SCTP source needs to adjust its congestion window when receiving ECN messages. We find the optimal value of the congestion window for a SCTP source in response to ECN messages, and develop a simple and practical method to achieve this optimal congestion window. Our simplified method in achieving the optimal congestion window is attractive because the total goodput performance of SCTP associations or the bottleneck link utilization is not sensitive to the window reduction policies when the network load is heavy. Using complicated methods to fine-tune SCTP or TCP?s congestion window in response to congestion indications may be not worth the increase in complexity of the protocol.