netdev - Re: TCP fast retransmit issues

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Date:   Wed, 26 Jul 2017 07:32:12 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Willy Tarreau <w@....eu>
Cc:     Klavs Klavsen <kl@...n.dk>, netdev@...r.kernel.org
Subject: Re: TCP fast retransmit issues

On Wed, 2017-07-26 at 15:42 +0200, Willy Tarreau wrote:
> On Wed, Jul 26, 2017 at 06:31:21AM -0700, Eric Dumazet wrote:
> > On Wed, 2017-07-26 at 14:18 +0200, Klavs Klavsen wrote:
> > > the 192.168.32.44 is a Centos 7 box.
> > 
> > Could you grab a capture on this box, to see if the bogus packets are
> > sent by it, or later mangled by a middle box ?
> 
> Given the huge difference between the window and the ranges of the
> values in the SACK field, I'm pretty sure there's a firewall doing
> some sequence numbers randomization in the middle, not aware of SACK
> and not converting these ones. I've had to disable such broken
> features more than once in field after similar observations! Probably
> that the Mac doesn't advertise SACK support and doesn't experience the
> problem.

We need to check RFC if such invalid SACK blocks should be ignored (DUP
ACK would be processed and trigger fast retransmit anyway), or strongly
validated (as I suspect we currently do), leading to a total freeze.