kcp-go is a production-grade, high-performance reliable UDP library for Go, implementing the KCP (KCP Protocol) ARQ protocol. It provides a stream-based interface (net.Conn) over UDP with low latency, high throughput, and packet-level control. The library is widely used in gaming, streaming, and network acceleration (e.g., kcptun). Architecturally, it is layered: an application layer using net.Conn, a session layer (sess.go) managing encryption, FEC, and connection lifecycle, a pure protocol state machine (kcp.go) handling ARQ, retransmission, and congestion control, and a transport layer using raw UDP sockets. The data pipeline for outgoing packets is: App Write -> KCP Send -> KCP flush -> FEC Encode -> Encrypt -> UDP Socket. Incoming: UDP Socket -> Decrypt -> FEC Decode -> KCP Input -> App Read. Key features include: reliable ARQ with fast retransmit, Forward Error Correction (FEC) using Reed-Solomon codes, packet-level encryption (AES, TEA, Salsa20, etc., plus AEAD support), platform-specific optimizations (sendmmsg/recvmmsg on Linux), a global timer scheduler to avoid per-session goroutines, and a buffer pool to minimize GC pressure. The codebase demonstrates exceptional craftsmanship with hand-optimized CFB encryption (8x loop unrolling), a generic ring buffer with dynamic resizing, and an auto-tuning FEC decoder that adapts to peer parameters.
Fuso is a Rust-based intranet penetration and port forwarding tool that enables exposing local services behind NAT to the public internet via a relay server. It implements a client-server architecture with support for TCP forwarding, SOCKS5 proxy (including UDP association), KCP protocol for reliable UDP transport, LZ4 compression, and RSA+AES encryption. The system is built on a custom async runtime abstraction (supporting tokio and smol), with a modular design using provider and builder patterns. The server (fus) manages client connections and public listeners, while the client (fuc) registers port mappings and forwards traffic. Additional features include bridge mode for cascading proxies, heartbeat health checks, observer hooks for external notifications, and a configuration API. The codebase demonstrates strong Rust async patterns, protocol implementation skills, and a clean separation between core abstractions and runtime specifics.