Hello!

Actually, it may think slow start is being done - there was enough 
small packet back and forth on the connection before the "heavy 
transfer" to get cwnd opened

If receiver sent an ACK it still does not mean that sender used it
to increase its cwnd. Particularly, small packet exchange definitely
does not inflate cwnd.

output.  All the stacks with ACK avoidance with which I am familiar do 
not make the assumption that the sender is not doing slow-start.  They 
make sure to send enough ACKs at the beginning (or after packet loss) 
to allow the sender's cwnd to grow.

Well, we do similar thing with delayed ACKs. And it took a few of runs
of testing to understand that we cannot detect even packet loss reliably
enough. :-)

Actually, those receivers could use the first delayed ACK event as
a sign of failure of their heuristics and block stretching acks for
this connection.

Alexey