LiteNetLib
A pure C# open networking library with reliable channels, ordering, fragmentation, NAT punch-through, lag simulation, and serialization helpers.
Why use it
Unity, Godot, and custom .NET clients share transport code without native plugin deployment.
Where it fits
Find engines, frameworks, code libraries, version control, and production automation.
What to check
It is not a state-sync framework; encryption, anti-cheat, server authority, and protocol versions remain custom work.