I raised OpenVPN on server for the first time. It worked fine until the server rebooted. The firewall configuration is unchanged. When I tcpdump from tun interface on the server, I always get TCP retransmissions on all TCP packets larger than a few bytes in Android.
Using a similar(read: same) configuration on my Linux box from where I'm posting this message now works just fine; no retransmissions, no loss.
It did work just fine before the server rebooted. What could possibly cause this? I already tried rebooting Android, it didn't help.
Client configuration, used on both Linux PC and Android tablet:
dev tun
client
remote 192.168.0.1
persist-key
persist-tun
nobind
pkcs12 user.p12
mssfix 1480
cipher AES-256-CBC
verb 3
1480 is below the mtu of uplink connection on server which is 1492.
I tried adding tun-mtu 1480 to Android but that didn't help either.
Best Answer
final attempt: i set both 'tun-mtu' and 'mssfix' to 1380 on both server and client. surprisingly, it worked.