As I said this HOWTO is mainly a quick memo on how I had set up a VPN. There are things in the configuration I didn't experiment yet. These things will go into their place when I try them, or anyone tells me "it works in the following way" The most important thing is that the connection ppp uses is not 8-bit yet. I believe it has something to do either with ssh configuration or the pty setup. In this configuration ssh uses the tilde (~) character as an escape character. It might stop or slow down the communication, as any newline-tilde sequence causes ssh to give a prompt. Ssh documentation said: <On most systems, setting the escape character to ``none'' will also make the session transparent even if a tty is used.> The corresponding flag to ssh is '-e', and you can also set it in the configuration file.
Creating anything virtual comes with utilization of real-world resources. A VPN eats up bandwidth and computing resources. The goal would be to get balance between the two. You can tune it with the '-C' switch or the 'CompressionLevel' option. You might try using another cipher, but I don't recommend it. Also note that the round-trip-time can be longer if you use better compression. Any experiments on it are welcome.