If server notice that it didn't received any UDP packet from one guest, it could auto-switch car position packets to TCP. This will be more elegant solution for problem about some ISPs blocking UDP packets.

Also adding possibility to set position packets permanently to TCP in options screen would be nice, because mostly same people have problems with UDP.

Why are so many ISPs blocking UDP?

What if the ISP isn't doing it but the router?

With China I have overall doubts by the way

Quote from Keling :

Why are so many ISPs blocking UDP?

Some services like VoIP and torrents use UDP and ISP's have been known to heavily throttle them at peak times, or in extreme cases, block ports completely in an attempt to prevent users hogging the available bandwidth.

UDP has a lower overhead than TCP, and is sometimes referred to as connection less, so it's the preferred method of connection where packet sequence order is less important. The client and server normally have their own methods of resolving out of order/lost packets, but any intervening routers (ISP) just see an increase in traffic. As they are unable to determine the nature of the data, and therefore do not have the methods to control the flow/route, limiting or blocking the ports makes the problem go away (for the ISP's)

The upshot of these sweeping limitations is that connections that use low bandwidth (LFS/other games), also suffer.

Quote from DANIEL-CRO :

If server notice that it didn't received any UDP packet from one guest, it could auto-switch car position packets to TCP. This will be more elegant solution for problem about some ISPs blocking UDP packets.

Also adding possibility to set position packets permanently to TCP in options screen would be nice, because mostly same people have problems with UDP.

I can understand your point, but I don't think this is such a good idea. UDP packets are always supposed to come through, if they don't, the Internet connection is broken somewhere. Working around the problem by switching to TCP is not a solution, the option should be only used for diagnostics.
Making it more automated or even default would IMHO cause more problems that it would solve in the long run.

Quote from cargame.nl :

What if the ISP isn't doing it but the router?

With China I have overall doubts by the way

Well, in China they block whatever the big brother doesn't like, so whether it's TCP or UDP doesn't matter.

For the rest of the world where traffic get blocked due to overloading reasons, what's the best option? Tunnel it through TCP and find the best bridge with try and error?