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There is an urgent need to find the best congestion-control mechanism for space Internet. This article presents an experimental study of rate-based congestion control, window-based congestion control, and a hybrid of the two in a simulated small-satellite environment using the space-to-ground link simulation (SGLS) testbed. The study is done by examining the throughput performance and transmission behavior of pure rate-control and a variant of each of the window-based transmission control protocol (TCP) selective acknowledgment (TCP-SACK) and TCP-Vegas congestion-control mechanisms, with and without the channel-rate control being hybridized. The study reveals that the traffic-shaping mechanism of a rate-based transmission mechanism is more effective than the bursty flow of window-based mechanisms in error-prone space environments with a long link delay. Pure rate-control is preferable to other mechanisms in space in which channel resource reservation is available. The performance differences arise from their different behaviors in controlling data transmission.