This is the code I wrote (or copied and modified) during the research I did for implementing multiplayer in my game.

Useful for defining custom message formats (with automatic serialization into binary and json). https://developers.google.com/protocol-buffers/docs/tutorials

A must read paper that actually made sense. Includes info on NAT routers, describes UDP and TCP hole punching AT LENGTH: https://bford.info/pub/net/p2pnat/

Python code people wrote based on that paper. It's not ideal, but serves as reference. https://github.com/dwoz/python-nat-hole-punching

UDP hole punching explanation: https://stackoverflow.com/a/11377330/9731532

UDP hole punching implementation: https://stackoverflow.com/a/53215243/9731532

I also cleaned up that code a little bit, see UDP_StackOverflow.

My simplified implementation, see UDP_My_Test.

Mostly used the aforementioned paper and the python code as reference.

How To TCP NAT Traversal. A simple technique with a not self hosted relay server is presented. Note that hosting a relay server is not a problem in my case. https://gist.github.com/mildred/b803e48801f9cdd8a4a8

TCP hole punching wiki: https://www.wikiwand.com/en/TCP_hole_punching