[GLLUG] Openvpn overhead?

James Courtier-Dutton james.dutton at gmail.com
Thu Apr 5 01:35:32 UTC 2018

On 4 April 2018 at 19:58, Tim Woodall via GLLUG
<gllug at mailman.lug.org.uk> wrote:
> Hi all,
> I have a wifi link between two (debian) firewalls that are in separate
> premises. I'd like to put a wired link in but that requires getting some
> permissions so it won't be quick (although it's simple to do)
> Over the wifi link I can scp data at at least 1MB/s which isn't great
> but it's not too bad. iwconfig says the connection is at 150Mb/s so in
> theory I should be able to do 10x that. (One end is an access point into
> a 100Mb ethernet link while the other end is a USB wifi dongle so I'm
> not expecting 15MB/s)

The problems you are likely to run into are the different link speeds.
Whenever data travels between links of different speeds, packets get lost.
Approximately, and 100Mbits/second Ethernet link can transfer data at
about 10Mbytes/second.
TCP does its best to backoff, and thus reduce packet loss to a minimum
but each time a packet is lost, a TCP transfer slows.
One work around to this is to create 2 TCP sessions across the same
link. While one is backing off, the other can continue.
So, if the Wifi link says it is 150Mbits/s, I would suggest putting in
1Gig Ethernet each end.
Radio transmissions will always have packet loss, and in general, the
best performance over radio network is using something called FEC. I
bit like sending the same packet twice, but don't care if one of those
packets is lost. In reality FEC is a bit more complex, and allows for
say you have 5 packets to send, the FEC creates 6 packets, and at the
receiving end, if only 1 packet is lost, the original 5 packets can be
Another option is to have 100Mbits/s Ethernet each end. USB wifi
dongle is terribly slow.
You should then be able to get maybe 10Mbytes/s of data across the link.
One TCP session is unlikely to get 10MB/s due to backoff on lost
packets, but shared between 2 TCP sessions, you might reach that.
I.e. You are seeing one TCP session transfer 1MB/s of data.
If you do two TCP session transfers at the same time, what does that total to?

Your USB wifi is probably the limiting factor here, maybe only able to
achieve 12 MBits/sec, or 1.2 MB/s.
To find out the maximum possible transfer rates, use netperf in UDP
mode. It lets packets get lost, but will report how much actually gets

Kind Regards


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