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Re: [Openvpn-users] Heavy packet loss with high traffic

  • Subject: Re: [Openvpn-users] Heavy packet loss with high traffic
  • From: Terry Dooher <tdooher.lists@xxxxxxxxxxxxxxxxx>
  • Date: Mon, 20 Dec 2004 11:20:26 +0000

Some kind of throttling on the part of the university is the most likely cause, but here's a couple of other possibilites.

Try adding the following option to both ends of the connection:

mssfix 1200

If this cures it, then it's an MTU discovery problem. You should be able to increase that value back up towards 1500 to find the highest stable point in order to maximise throughput. 1400 should be ok, but 1200 is a good starting point.

If not then it could be the adaptive retransmission timeout of the TCP connection failing:

Take a look at this page for a description of the issue:


I should say that I've never seen anyone actually having this issue before, but it might be the cause. Sadly, the only way around this is to switch to UDP, which you can't do.


Leonard Isham wrote:
On Mon, 20 Dec 2004 00:45:25 -0700, Eric Hokanson <hokanson@xxxxxxxxx> wrote:

I have a WinXP box at home running OpenVPN 2.0rc4 which is connect to
a high speed university network.  This network blocks all udp packets
and most tcp ports.

At my work place across town, I have a Linux box also running OpenVPN
2.0rc4.  It is connected directly to the Internet with a high speed

I have a tcp tunnel between the two machines with all my home traffic
being sent through to my work machine.  This way I was hoping I could
play games and stream video, etc.  When the connection isn't stressed
it appears to work great and a ping from my home machine to a reliable
site produces hundreds of pings with a 0% packet loss.  When I start
to send a high volume of packets through, such as an online game or
video stream, the first 5 seconds or so work great then I suddenly
start to get massive packet loss.  I've tried disabling encryption,
authentication, and compression with no change.  I've also tried
lowering my sndbuf size to reduce latency with no change.  I see no
obvious problems in the log window.  Below is part of my WinXP
configuration in case you spot anything abnormal.  Anybody know how to
fix this problem?

OK I'm going to guess that University = home.

Many universities have high bandwidth, but extremely high utilization.
 If this is the case then their Internet connection may be clogged.

Another possibility is that the university is intentionally minimizing
the available traffic to student housing.  Is this also a time when
most of your fellow students are home?

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