GameNetworkingSockets from Valve. Maybe not middleware enough? You have to implement the serialization and a state update strategy yourself.

Reliable & unreliable messages over UDP. Robust message fragmentation & reassembly. P2P networking / NAT traversal. Encryption.

https://github.com/ValveSoftware/GameNetworkingSockets

Yep, you will need to implement your own serialization and snapshot system with GNS.