Date: Sat, 26 Mar 2011 23:43:40 +0100 From: Stefan `Sec` Zehl <sec@42.org> To: freebsd-net@freebsd.org Subject: Re: The tale of a TCP bug Message-ID: <20110326224340.GB23803@ice.42.org> In-Reply-To: <20110326140212.GB45402@ice.42.org> References: <4D8B99B4.4070404@FreeBSD.org> <201103250825.10674.jhb@freebsd.org> <20110325194109.GB25392@ice.42.org> <201103251640.16147.jhb@freebsd.org> <20110326140212.GB45402@ice.42.org>
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Hi,
> On Fri, Mar 25, 2011 at 16:40 -0400, John Baldwin wrote:
> > And the problem is that the code that uses 'adv' to determine if it
> > sound send a window update to the remote end is falsely succeeding due
> > to the overflow causing tcp_output() to 'goto send' but that it then
> > fails to send any data because it thinks the remote window is full?
On a whim I wanted to find out, how often that overflow is triggered in
normal operation, and whipped up a quick counter-sysctl.
--- sys/netinet/tcp_output.c.org 2011-01-04 19:27:00.000000000 +0100
+++ sys/netinet/tcp_output.c 2011-03-26 18:49:30.000000000 +0100
@@ -87,6 +87,11 @@
extern struct mbuf *m_copypack();
#endif
+VNET_DEFINE(int, adv_neg) = 0;
+SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, adv_neg, CTLFLAG_RD,
+ &VNET_NAME(adv_neg), 1,
+ "How many times adv got negative");
+
VNET_DEFINE(int, path_mtu_discovery) = 1;
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_RW,
&VNET_NAME(path_mtu_discovery), 1,
@@ -573,6 +578,10 @@
long adv = min(recwin, (long)TCP_MAXWIN << tp->rcv_scale) -
(tp->rcv_adv - tp->rcv_nxt);
+ if(min(recwin, (long)TCP_MAXWIN << tp->rcv_scale) <
+ (tp->rcv_adv - tp->rcv_nxt))
+ adv_neg++;
+
if (adv >= (long) (2 * tp->t_maxseg))
goto send;
if (2 * adv >= (long) so->so_rcv.sb_hiwat)
I booted my main (web/shell) box with (only) this patch:
11:36PM up 3:50, 1 user, load averages: 2.29, 1.51, 0.73
net.inet.tcp.adv_neg: 2466
That's approximately once every 5 seconds. That's way more often than I
suspected.
CU,
Sec
--
I wish there was a knob on the TV to turn up the intelligence.
There's a knob called "brightness", but it doesn't seem to work.
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