netdev - Re: 2.6.20.7 mss negotiation and path mtu discovery mostly broken?

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Date:	Wed, 25 Apr 2007 13:16:02 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bristuccia@...rentnetworks.com
Cc:	netdev@...r.kernel.org, mchan@...adcom.com
Subject: Re: 2.6.20.7 mss negotiation and path mtu discovery mostly broken?

From: "Ristuccia, Brian" <bristuccia@...rentnetworks.com>
Date: Wed, 25 Apr 2007 16:11:51 -0400

> > I'm seeing a 
> > problem where the kernel attempts to send packets with a MSS 
> > larger than the one negotiated when the TCP connection is 
> > established. Even after ICMP "can't fragment" messages 
> > arrive, the kernel still attempts to increase the MSS rather 
> > aggressively. The end result is extremely poor throughput 
> > when sending to a network with a smaller MTU. 
> 
> I've tracked this problem to the TSO feature in the bnx2 driver. Turning
> off TSO with "ethtool -K eth1 tso off" seems to work around the problem.
> It appears that the bnx2 device is not using the correct mss when
> performing segmentation offload. 

Thanks for narrowing it down like that.

Michael can you have a look?  Is the bnx2 firmware using the MTU
setting in the device and ignoring the passed in MSS or something
like that?

Thanks.

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