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From: Eric Dumazet <eric.dumazet@gmail.com>
To: Larry Finger <Larry.Finger@lwfinger.net>
Cc: wireless <linux-wireless@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: Question about TCP Checksum errors
Date: Sun, 18 Dec 2011 00:43:02 +0100	[thread overview]
Message-ID: <1324165382.3323.59.camel@edumazet-laptop> (raw)
In-Reply-To: <4EED232D.3010208@lwfinger.net>

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



  reply	other threads:[~2011-12-17 23:43 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-12-17 23:18 Question about TCP Checksum errors Larry Finger
2011-12-17 23:43 ` Eric Dumazet [this message]
2011-12-19 11:38 ` Question about IEEE80211_TX_CTL_NO_ACK in ath_9k 李刚
2011-12-19 12:33   ` Mohammed Shafi
2011-12-19 13:03     ` 李刚
2011-12-19 12:33   ` Daniel Smith
2011-12-19 13:00     ` 李刚
2011-12-19 13:43       ` Daniel Smith
2011-12-19 13:43         ` [ath9k-devel] " Daniel Smith
2011-12-30  8:15         ` Question: ieee80211_rx called twice after one ieee80211_subif_start_xmit call 李刚
2011-12-30  8:15           ` [ath9k-devel] " 李刚
2011-12-30  9:30           ` Adrian Chadd
2011-12-30  9:30             ` [ath9k-devel] " Adrian Chadd
2011-12-30 11:13             ` [ath9k-devel] 答复: " 李刚
2012-01-07  2:11         ` Question about duration of data frame in ath9k_htc 李刚
2012-01-07  2:11           ` [ath9k-devel] " 李刚
2012-01-07  5:31           ` Sujith Manoharan
2012-01-07  5:31             ` [ath9k-devel] " Sujith Manoharan
2011-12-19 13:07   ` Question about IEEE80211_TX_CTL_NO_ACK in ath_9k Helmut Schaa
2011-12-19 14:42   ` Sujith
2011-12-20  5:39     ` 李刚
2011-12-20 15:24       ` Sujith

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