netdev - Re: TCP keepalives ignored by kernel when the contain timestamps

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Message-ID: <1307724870.4044.20.camel@edumazet-laptop>
Date:	Fri, 10 Jun 2011 18:54:30 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Charles Bearden <Charles.F.Bearden@....tmc.edu>
Cc:	netdev@...r.kernel.org
Subject: Re: TCP keepalives ignored by kernel when the contain timestamps

Le vendredi 10 juin 2011 à 11:39 -0500, Charles Bearden a écrit :
> On 06/10/2011 11:17 AM, Eric Dumazet wrote:
> > Le vendredi 10 juin 2011 à 11:07 -0500, Charles Bearden a écrit :
> >
> >> You are correct: the checksums in the keepalives are broken, though they are
> >> correct in the other segments from the Win2008 server. I have updated the pastes
> >> linked to above with 'tcpdump -v' output. I apologize for missing that problem
> >> the first time around.
> >
> > Hmm, maybe its OK : If checksums are offloaded to NIC, tcpdump 'lies'
> > telling checksum is not OK, since tcpdump get a copy of the packet
> > before being handled by the NIC.
> >
> > You should take a tcpdump on the receiving machine (the machine that
> > apparently doesnt react to the keepalive probes)
> 
> I should have made this clear before: these dumps were captured on the machine 
> that is ignoring the keepalives ("Ubuntu.host" in the dumps) from the other host 
> ("Win200[38].host" in the dumps). If I understand tcp checksum offloading 
> correctly, it wouldn't play a role here.
> 

OK, thanks.

I am curious, could you check the payload byte included in the keepalive
probes has the value of last sent byte on the session ?

(linux send keepalive probes with no data, so implementing this
'feature' would need to keep track of this value in tcp socket)



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