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Multi-rate multicast has been proposed as a scalable solution to transmitting video over the Internet to receivers with heterogeneous and dynamic rate requirements. The applicability of an equation-based mechanism to congestion control for a protocol which bases its join and leave actions on the calculation of the TCP response function is investigated. We focus on the rate calculation algorithm proposed in TFRC (TCP-friendly rate control), as it is currently a very promising and mature approach to calculating a TCP-compatible rate. By means of a network simulator and an adjusted TFRC protocol implementation, we show that the TCP-compatible rate calculated with the algorithm as originally proposed tends to be biased when applied in environments with a low degree of statistical multiplexing. To improve the performance of the basic algorithm, we propose a simple heuristic approach.