I used mtu-test on the client and determined that the MTU size is 997. Using this, I tried the settings (using UDP) tun-mtu 1500. fragment 1000. mssfix. and I'm happy to report that the connection is stable, and fairly fast. Not as fast as the PPTP VPN, but not bad (about 2/3 the speed by unscientific and subjective measures)

May 31, 2018 wiki.ipfire.org - Troubleshooting MTU - Test. For possible performance problems it can be quite useful, to check the MTU settings, therefor OpenVPN offers a MTU-test. To create an empirical MTU test, it needs an additional entry in the OpenVPN configuration file before the connection starts, the following line must be added. INFO: OpenVPN and MTU | Netgate Forum The OpenVPN documentation says that using "fragment" adds 4 bytes to the OpenVPN header. We did indeed see that now we could ping to a maximum of 1287 bytes (compared to 1291 previously). Up to 1287 is fine, 1288 to 1308 bytes would not ping, 1309 and above worked. Presumably a 1309 byte raw packet translates into a 1400 byte OpenVPN encrypted OpenVPNに接続したときに、接続が不安定になる問題を解消する …

OpenVPN's mtu-test will try to automatically determine what the largest packet size that actually goes through is; you can then use its values to update your config file. That said, when you have a specific issue like this, "anything unusual" is a good thing to look for.

Adjusting MTU. MTU problems may result in degraded network service, but may not affect some users' abilities to access the required applications, so sometimes MTU problems go unreported.

Mar 23, 2016

I read that it is not recommended to lower the link-mtu or tun-mtu, so I don't think lowering to 1472 should be tried. I also know that -fragment option is not applicable to the OpenVPN for Android (the version I am using is 3.1.0.(4292). Expected Remote Options String (VER=V4): 'V4,dev-type tun,link-mtu 1550,tun-mtu 1500, daemon vpn writepid /var/run/openvpn-vpn.pid remote 4.3.2.1 dev tun0 proto udp port 1859 comp-lzo keepalive 10 60 ping-timer-rem persist-tun persist-key ifconfig 10.9.0.2 10.9.0.1 route 192.168.0.0 255.255.252.0 secret my-key.key log-append vpn.log mtu-test verb 5 Solving OpenVPN MTU issues By hambier On April 4th, 2016 In Linux Introduction For some time now I’ve systematically used an OpenVPN-connection whenever I was using an untrusted WLAN (at hotels, restaurants, etc.). Hello, we have tested the --mtu-disc directive for N2N connections on OpenVPN. Currently "fragment" and "mssfix" for UDP and the "tun-mtu" for TCP can be used to adjust the package size. OpenVPN Management Interface Notes ----- The OpenVPN Management interface allows OpenVPN to be administratively controlled from an external program via a TCP or unix domain socket. The interface has been specifically designed for developers who would like to programmatically or remotely control an OpenVPN daemon, and can be used when OpenVPN is