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

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Date:	Fri, 21 Jan 2011 14:15:49 -0800 (Pacific Standard Time)
From:	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>
To:	Hans de Bruin <jmdebruin@...net.nl>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jesse Gross <jesse@...ira.com>, netdev <netdev@...r.kernel.org>
Subject: Re: [regression] 2.6.37+ commit 0363466866d9.... breaks tcp ipv6



On Fri, 21 Jan 2011, Hans de Bruin wrote:
> > 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)
> 
> That seem reasonable but: the bug was triggered because my nic could not 
> offload checksumming, so what's tx=on if there's no support for it? I 
> have turned tx off and my tcpdump still shows bad checksums on outgoing 
> tcp/ip6 packets. I have tried 2.6.36: bad checksums, 2.6.35 and 
> surprise: good checksums with tx=on.

maybe you're still seeing confusing output on tx due to GSO, try turning 
that off with ethtool -K ethX gso off

GSO still software-segments packets, and maybe the tap for tcpdump is 
before GSO operation.

> > I was referring to check with tcpdump incoming frames, because invalid
> > checksums in RX is sign that other peer sent wrong checksums
> 
> Ok, thats clear, the receiving site is apparently a more reliable 
> checksummer than the sending site.
> 
> 
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