netdev - Re: NAT stops forwarding ACKs after PMTU discovery

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Date:	Mon, 19 Aug 2013 17:33:28 +0200
From:	Christoph Paasch <christoph.paasch@...ouvain.be>
To:	Phil Oester <kernel@...uxace.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Corey Hickey <bugfood-ml@...ooh.org>,
	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
	Linux Netdev List <netdev@...r.kernel.org>,
	netfilter-devel@...r.kernel.org
Subject: Re: NAT stops forwarding ACKs after PMTU discovery

On 19/08/13 - 07:35:09, Phil Oester wrote:
> On Mon, Aug 19, 2013 at 06:58:05AM -0700, Eric Dumazet wrote:
> > Yeah, but here, this is conntrack who is blocking the thing.
> > 
> > TCP receiver has no chance to 'fix' it.
> > 
> > See conntrack is one of those buggy middle box as well.
> > 
> > So if you want to properly handle this mess, you'll also have to fix
> > conntrack.
> 
> Better to fix the box which is randomizing the acks and ignoring
> the SACKs.  Usually these are older Cisco PIX/ASA devices which just
> need a code upgrade.  

I agree, the problem are these horrid middleboxes...

Unfortunately, they will hardly go away in the near futur. Rather the
opposite is the case.


If you have a public server running, I would be interested in the count of
invalid SACK-blocks received (netstat -s | grep TCPSACKDiscard). This is an
indication for such kind of middlebox between your server and the client,
implying that these connections cannot benefit from TCP-FastRetransmission
and each packet-loss will require an RTO to recover.


Cheers,
Christoph

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