libwww flow control?

On NT, if I have one or more threads that continuously pump GET requests
into libwww setup with HTProfile_newNoCacheClient using non-blocking
sockets, most requests except the first few come back with status =
HT_INTERRUPTED.  It because the request queue is jammed?  How do I know if
the libwww is busy, and not send it more requests?  Thanks.