linux-kernel - Re: Question about TCP Checksum errors

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Message-ID: <1324165382.3323.59.camel@edumazet-laptop>
Date:	Sun, 18 Dec 2011 00:43:02 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Larry Finger <Larry.Finger@...inger.net>
Cc:	wireless <linux-wireless@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question about TCP Checksum errors

Le samedi 17 décembre 2011 à 17:18 -0600, Larry Finger a écrit :
> In https://bugzilla.novell.com/show_bug.cgi?id=718736, a user reports that his 
> network using an RTL8188CE becomes very slow under "heavy" load. This condition 
> started when he updated from openSUSE 11.4 to 12.1. The kernels involved are 
> 2.6.37 and 3.1. I am unable to duplicate the problem.
> 
> Packet capture with Wireshark shows that the slow down is due to retransmissions 
> due to TCP checksum errors. I found nothing to explain the situation with a 
> Google search, thus I am asking here.
> 
> I put heavy in quotes because the traffic is from a web browser to yahoo.com, 
> thus the potential throughput is not very high.
> 
> As far as I know, this checksum is generated in a higher layer of the network 
> stack, and that data is just passed through the 802.11 layer, and the wireless 
> driver. Is this correct? Anyone have any ideas on where to look for the problem?
> 

I see nothing special in the packet capture from this bug report.

(Only suboptimal application opening many concurrent flows in //)

Keep in mind most modern NICS compute checksums themselve.

Linux offloads this to NIC

This can be controled eventually by "ethtool -k ethX" (-K to change
settings)

# ethtool -k eth0
Offload parameters for eth0:
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp-segmentation-offload: on
udp-fragmentation-offload: off
generic-segmentation-offload: on
generic-receive-offload: on
large-receive-offload: off
rx-vlan-offload: on
tx-vlan-offload: on
ntuple-filters: off
receive-hashing: off

# ethtool -K eth0 tx off
# ethtool -k eth0
Offload parameters for eth0:
rx-checksumming: on
tx-checksumming: off
scatter-gather: off
tcp-segmentation-offload: off
udp-fragmentation-offload: off
generic-segmentation-offload: off
generic-receive-offload: on
large-receive-offload: off
rx-vlan-offload: on
tx-vlan-offload: on
ntuple-filters: off
receive-hashing: off


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