Re: TCP SACK issue, hung connection, tcpdump included

["Ilpo Järvinen" <ilpo.jarvinen@helsinki.fi>]

On Sun, 29 Jul 2007, Willy Tarreau wrote:

> On Sun, Jul 29, 2007 at 12:28:04PM +0300, Ilpo Järvinen wrote:
> 
> > [...snip...]
> > 
> > > BTW, some information are missing. It would have been better if the trace
> > > had been read with tcpdump -Svv. We would have got seq numbers and ttl.
> > > Also, we do not know if there's a firewall between both sides. Sometimes,
> > > some IDS identify attacks in crypted traffic and kill connections. It
> > > might have been the case here, with the connection closed one way on an
> > > intermediate firewall.
> > 
> > Yeah, firewall or some other issue, I'd say it's quite unlikely a bug in 
> > TCP because behavior to both directions indicate client -> sender 
> > blackhole independently of each other...
> 
> It would also be possible that something stupid between both ends simply
> drops packets with the SACK option set. Also something which sometimes
> happen is that some firewalls automatically translate sequence numbers
> but not necessarily SACK values, which could pretty well lead to this
> packet being received but ignored on the server side.

...One can toggle those off with /proc/sys/net/ipv4/tcp_dsack but I 
suspect DSACKs are not the cause because these retransmissions neither
are making it through (there are many of them also earlier in the log, 
just quoted the easiest ones :-) ):

> > > > 09:36:44.335591 IP CLIENT.50727 > SERVER.ssh: P 2991:3039(48) ack 18464 win 
> > > > 378 <nop,nop,timestamp 800227942 7692910>
> > > > 09:38:44.351950 IP CLIENT.50727 > SERVER.ssh: P 2991:3039(48) ack 18464 win 
> > > > 378 <nop,nop,timestamp 800257942 7692910>
> > > > 09:40:44.368172 IP CLIENT.50727 > SERVER.ssh: P 2991:3039(48) ack 18464 win 
> > > > 378 <nop,nop,timestamp 800287943 7692910>

...there are no SACKs involved in them, yet no cumulative ACK ever 
arrives from SERVER...


-- 
 i.