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Layered multicast protocol (LMP) is designed for simultaneous and real-time multimedia content distribution to large number of disparate receivers across the heterogeneous Internet. In order to behave properly in the Internet, in particular to be friendly towards TCP, LMP control their rate by adapting to the estimated target rate. TCP-equation model and packet-pair probe are two popular techniques used to estimate target rate, and are used in most of the current LMPs. This paper reports a comparative study of the two rate estimators used in LMPs. Simulation technique with NS2 as the network simulator is used to conduct the study, and the network topology is designed to represent network heterogeneity of the Internet. The results suggest that in heterogeneous network environment, the protocol with TCP-equation model is friendly toward TCP flows but cannot compete fairly with TCP in the session with long RTT. On the other hand, the protocol with packet-pair probe is not friendly towards TCP and is not affected by long RTT.