I'm trying to troubleshoot an extremely slow file transfer (5kb/s up) to a work server over the VPN.
A network admin at work has told me that I should expect to see a transfer rate reduced by up to 55% when I am connected to the VPN at work. I realize there are a lot of variables here, but is that figure typical? I did a speed test at BandwidthPlace before and after connecting the VPN and didn't see any appreciable change. Is that a reliable test?
We are using Forticlient SSL VPN.
Best Answer
Assuming you have a sufficiently fast processor on both ends of the device terminating the VPN, you shouldn't see much of a change in throughput at all. Throughput is the amount of data that can be transmitted during a certain amount of time.
Most VPNs do not really drastically change the size of the payload, and don't add that much additional overhead. An overhead of 10-15% might be reasonable, but a 55% overhead is not.
What is more likely is that your VPN is simply increasing the time it takes for a packet to be transmitted from the source to the destination. Additional latency can make certain protocols perform worse. Windows file sharing (CIFS aka SMB) is extremely sensitive to latency.
If you are trying to use CIFS over a VPN, you basically have to give up on the idea that you will be able to see your full capacity used.
Probably the best tool to test performance is iperf.