I'm having trouble with larger file transfers from one of our virtual servers to our office network. The virtual server acts as a VPN server, therefore we call it "VPN", but the usage of VPN is not of issue here, I used direct connections for the file transfer. The machine uses Ubuntu 12.04.5 LTS if that's of any matter.
Now, the upload speed of the VPN server is around 71,20 Mbit/s whereas the download speed for our internet gateway for our office network (called alix, runs on Debian 6) can handle around ~86,31 Mbit/s by default (It's a 100 Mbit line). I tested those speeds multiple times with using speedtest-cli, which uses speedtest.net servers. The ping is around 35-44 ms.
When I used iperf I got transfer rates between 20,7 Mbit/s and 35,7 Mbit/s. Multiple connections with iperf maxed out the available bandwidth. So the issue might be the TCP window size (as seen on other topics) and it's not that there's much traffic on the line by default. I used a window size calculator and now know I need roughly 400KB for the window size. So I changed the respecitve settings on both ends (net.core.rmem and wmem, net.ipv4.tcp_rmem and wmem), but it got even worse (~ 4 to 9Mbit/s). We're not having any packet drops or anything. TCP window scaling is enabled on both machines. There's no firewall throttling on both ends.
What can I do to further narrow down the cause or even already improve the throughput? Thanks for your help.
