Re: BSD 4.2 style TCP keepalives

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Wed, 06 Jan 2010 00:23:28 -0800 (PST)

> Special casing the seq == end_seq == tp->rcv_wup case using
> something like:
> 
> 		(after(end_seq, tp->rcv_wup) ||
> 		 (end_seq == tp->rcv_wup && seq == end_seq)) &&
> 
> might work, but I'm not confident that's exactly what we want at the
> moment, as it partially defeats what this code is trying to do (let us
> accept URG/FIN/RST after seq and end_seq are truncated to the window).

I did some more research and everything I've said here turns
out to be moot.

We should be ACK'ing these things anyways.  Here is why:

1) if tcp_sequence() accepts the sequence we continue on in
   tcp_established()

2) We make it to tcp_data_queue() unless tcp_ack() finds that the
   ACK sequence is invalid (it covers data we never sent).

3) tcp_data_queue() should make it to, and hit, this conditional:

	if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {

   which will schedule an ACK the same exact way we would if
   tcp_sequence() rejected the sequence range.

So it's a mystery why we aren't responding to Windows 2000's
BSD 4.2 style zero window probes.

Can someone please validate my analysis?

Someone with access to a system exhibiting this will probably need to
do some diagnostics to figure out what's going on.