PMTUDiscovery vs ClampMSS

[Rob Townley]

On Mon, Dec 13, 2010 at 2:57 PM, Guus Sliepen <guus at tinc-vpn.org> wrote:
> On Mon, Dec 13, 2010 at 02:06:43PM -0600, Rob Townley wrote:
>
>> >> Currently, i have nodes with PMTUDiscovery =yes and ClampMSS = yes.

An OpenVPN user on the vyatta.org forums had the problems with
excessively slow connections.

He found that either lowering the MTU to about one-third helped:
         router:~# ifconfig vtun0 mtu 516

But also found turning OFF TCP Segmentation Offload (TSO) on the
server speeded things up just as well.  As i understand it, the word
segment in this context refers to Maximum Segment Size which is broken
down to ethernet MTU frame size (usually 1500bytes).  So if you have
TSO enabled on your NICs but the tun device doesn't support TSO,
couldn't that create a problem?

         ethtool -K eth0 tso off


i have not tested it, but hope to..

echo "ethtool -K eth0 tso off" >> /etc/rc.local


DATA >> SEGMENTS  >> PACKETS  >> FRAMES  >>  BITS
Dont          Send                  Paul               For              Beer

Original posting on vyatta.org:
http://vyatta.org/forum/viewtopic.php?t=4419