netdev - Re: [regression] 2.6.37+ commit 0363466866d9.... breaks tcp ipv6

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Date:	Fri, 21 Jan 2011 20:55:45 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hans de Bruin <jmdebruin@...net.nl>
Cc:	Jesse Gross <jesse@...ira.com>, netdev <netdev@...r.kernel.org>
Subject: Re: [regression] 2.6.37+ commit 0363466866d9.... breaks tcp ipv6

Le vendredi 21 janvier 2011 à 20:47 +0100, Hans de Bruin a écrit :
> On 01/18/2011 11:03 PM, Eric Dumazet wrote:
> > Le mardi 18 janvier 2011 à 22:42 +0100, Hans de Bruin a écrit :
> >> On 01/18/2011 09:06 PM, Jesse Gross wrote:
> 
> ...
> 
> > You could try "tcpdump -i eth0 ip6 -v"
> >
> > I guess you receive frames with bad checksums
> 
> While you where staring at the code, I was fooling around with tcpdump. 
> And while the problem is fixed, I still have some questions:
> 
> Is there tool which shows whether a nic supports ipv6 checksum offload 
> or not?
> 
> I have captured http traffic (wget http://bootes/) between psion (my git 
> tree following laptop) and bootes (something running 2.6.33.7).
> Attached is a capture with psion running 2.6.37 and one with this 
> morning's git tree. Wat's with the 'chsum ... ( incorrect -> ' lines ?
> ifconfig does not show errors on either of the machines.
> 

tcpdump gets a copy of outgoing frames before NIC performs tx checksum
(if tx checksum handled by NIC), so it's normal to have "bad checksums"
on TX, unless you disable tx offloading (ethtool -K eth0 tx off)

I was referring to check with tcpdump incoming frames, because invalid
checksums in RX is sign that other peer sent wrong checksums



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