GameNetworkingSockets is Valve's production-grade C++ networking library for real-time multiplayer games. In actual operation it provides connection-oriented and message-oriented transport over UDP, with reliable and unreliable message delivery, fragmentation, retransmission, AES-GCM-256 encryption, Curve25519/Ed25519 key exchange, certificate-based authentication, and a full connection state machine. The library also implements peer-to-peer connectivity through ICE/STUN/TURN, WebRTC transport integration, and Steam Datagram Relay (SDR) in Steam builds, plus custom signaling support for non-Steam rendezvous. It is not a standalone application; it is an embeddable SDK whose primary public surface is the Steamworks-compatible C++ interfaces in include/steam (ISteamNetworkingSockets, ISteamNetworkingMessages, ISteamNetworkingUtils) and a flat C ABI wrapper. The main implementation lives in src/steamnetworkingsockets/clientlib, where the SNP reliable protocol, socket thread, direct UDP transport, P2P routing, and connection state machines are implemented, supported by a substantial private utility layer (tier0/tier1 containers, serialization buffers, string utilities) and multiple interchangeable crypto backends.