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

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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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