Reliable & unreliable messages over UDP. Robust message fragmentation & reassembly. P2P networking / NAT traversal. Encryption.
If you want to try a known library, I think Valve might have a good one (didn't test this myself, though): https://github.com/ValveSoftware/GameNetworkingSockets If you search for "C++ game development P2P library" you can find some suggesting on SO (but without having tested some and having an idea on what you really want, it will be hard to pick one).
A protocol for secure client/server connections over UDP
If you want to learn more about networking for game in C++, Glenn Fiedler has great articles from the basics to advanced topics: https://www.gafferongames.com/ He also created a library for client-server networking: https://github.com/networkprotocol/netcode (He has a big name in networking, totally recommend reading his articles if you want to learn more about networking)
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