Re: Possible BUG in IPv4 TCP window handling, all recent 2.4.x/2.6.xkernels

[Ion Badulescu]

Hi Alexey,

On Fri, 2 Sep 2005, Alexey Kuznetsov wrote:

> > This is where things start going bad. The window starts shrinking from
> > 15340 all the way down to 2355 over the course of 0.3 seconds. Notice the
> > many duplicate acks that serve no purpose
>
> These are not duplicate, TCP_NODELAY sender just starts flooding
> tiny segments, and those are normal ACKs acking those segments, note
> ACK field is not the same.

Well, take a look at the double acks for 84439343, 84440447 and 84441059, 
they seem pretty much identical to me.

> I still do not know how the value of 184 is possible in your case,
> I would expect 730 as an absolute possible minumum. I see 9420 (2355*4).

The numbers I mentioned are straight from the tcpdump and are not scaled, 
so they need to be multiplied by 4. But even 9420, combined with a RTT of 
20ms, results in a total usable bandwidth of about 3.75 Mbps, not enough 
for this real-time stream at peak times.

Besides, it often gets even worse than 2355, all it takes is a few 
application slowdowns.

> Anyway, ignoring this puzzle, the following patch for 2.4 should help.
>
>
> --- net/ipv4/tcp_input.c.orig	2003-02-20 20:38:39.000000000 +0300
> +++ net/ipv4/tcp_input.c	2005-09-02 22:28:00.845952888 +0400
> @@ -343,8 +343,6 @@
> 			app_win -= tp->ack.rcv_mss;
> 		app_win = max(app_win, 2U*tp->advmss);
>
> -		if (!ofo_win)
> -			tp->window_clamp = min(tp->window_clamp, app_win);
> 		tp->rcv_ssthresh = min(tp->window_clamp, 2U*tp->advmss);
> 	}
> }

That makes perfect sense...

I'll test it out on Tuesday, when I can connect again to the real-time 
streams that we use.

Thanks a lot!
-Ion
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