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.

Is that in general, or in Linux?

quoted

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. :-)

I never claimed it was easy :)

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.

The ones with which I am familiar do - after N delayed ACK events where 
N is something other than one though.  And they still send immediate 
ACKs to the senders upon out of order data and all that.

rick jones
Wisdom teeth are impacted, people are affected by the effects of events