DualPats exploits the strong correlation between TCP throughput and flow size, and the s tatistical stability of Internet path characteristics to accurately predict the TCP throughput of large transfers using active probing. We propose additional mechanisms to explain the correlation, andthen analyze why traditional TC P benchmarking fails to predict the throughput of large transfers well. We characterize stability and develop a dynamic sampling rate adjustment algorithm so that we probe a path based on its stability. Our analysis, desi gn, and evaluation is based on a large-scale measurement study.