[ns] TCP source behaviour

I have a problem with ns-2 TCP source. In ns-2, if the window size of the TCP is greater than the BW*Delay product of the link, it leads to packet drops in the source. Is this expected behaviour for any source in commercial TCP implemetations ? Or the TCP implemetations are expected to refrain from sending when they cannot send any more ? Does anybody have any information about any commercially available TCP source and how they behave in this scenario ? How does unix or windows implementations behave ? Is use of large buffers at the source is advisable ?