[Openvpn-users] Improve tunnel performance

["Thomas Zangl - Home" <thomas@xxxxxxxx>]

Dear List!

I've a OpenVPN setup with one server and 8 clients. The server is
connected to the internet with a 2mbit symetric SDSL line. The clients
are connected either using xDSL (2mbit down/512kbit up) or ADSL (1mbit
down/256kbit up). Every client is behind a ADSL or xDSL broadband
router which handels the connection to the ISP. Every client itself
has the OpenVPN client for Win32 installed. 

Before I deployed the OpenVPN setup for all clients some of them where
using a Netscreen VPN setup (IPSEC w/o compression) because of
historical reasons. The Netscreen box died and we migrated all user to
OpenVPN. So far so good :-) 

We are using an application for main business which connects to a
firebird database resisting on the same server as the OpenVPN server is
(so the bandwith from firebird to OpenVPN is virtually unlimited). 

My problem now is, that the ADSL user complains about very low speed
(2-3 times slower) since we switched to OpenVPN. I know that firebird
performance depends on RTT. Firebird does a lot of small transfers  
with a lot of ACKs flowing around. What can I do to improve performance? 

I already use TCP for the tunnel connection, "comp-lzo" is activated.
Which MTU / MSSFIX settings are recommended for ADSL?  I doubt it
would help as line MTU is handled by each router seperatly. 

Another point might be: how to improve RTT when a datatransfer is
running over the tunnel? I actually see stable RTT of ~80ms to the
ADSL client but when the application does some data transfer, it goes
up to 300ms and more. How to get this more stable? 

TIA,
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