I've been having a hell of a time with VPN tunnels lately. ATT uverse modems seem to be actively blocking IPSEC and almost every type of VPN tunnel except PPTP, which was working for several years until recently and it now no longer works. ATT released a firmware update to modem which blocks it. Now, I tried OVPN but the performance was TERRIBLE! We are pushing video from 10 live HD cameras and lots of packet loss and just strange dropouts that don't exist with PPTP. PPTP worked perfectly but OVPN didn't work reliably at all. I assume this is because the TCP-over-TCP problem. PPTP ping times were consistent 70ms. OVPN was 150ms-250ms all over the place. Internet is not the problem as there is very solid gigabit fiber at both sites, just the originating site has pesky ATT modem with blocking.
I finally got everything switched over to GRE which does indeed work through the ATT modem. Throughput seems fine, but I am noticing dropouts. This isn't nearly as bad as the OVPN, but every few minutes a camera will drop out for 5 seconds then come back online. This didn't happen with PPTP. We are also not using IPSEC, just straight GRE. Does GRE have the same TCP-over-TCP problem, or is there any way to get better performance from it?