Sorry for a mistake in the previous mail, it should be: when I use $ns at 0 "$tcp1 tracevar cwnd_" $ns at 0 "$tcp2 tracevar cwnd_" in .tcl file, then the flow 2 is better. while, when I use $ns at 0 "$tcp2 tracevar cwnd_" $ns at 0 "$tcp1 tracevar cwnd_" in .tcl file, then the flow 1 is better.