MsQuic is Microsoft's production-grade, cross-platform implementation of the IETF QUIC transport protocol. Written primarily in C with Rust and C# bindings, it provides a high-performance, low-latency UDP-based transport with built-in TLS 1.3 encryption, multiplexed streams, and advanced congestion control (CUBIC, BBR). The library is designed for both user-mode and kernel-mode operation on Windows, Linux, and macOS, with extensive support for hardware offload (XDP, DPDK) and performance tracing via ETW/LTTng. Architecturally, MsQuic is a library with a minimal public API surface (MsQuicOpenVersion, MsQuicClose) and a rich internal state machine. Core modules include connection management, stream send/receive, packet building/parsing, loss detection, ACK tracking, and path validation. The platform abstraction layer (CxPlat*) provides OS-independent interfaces for sockets, TLS, storage, threading, and memory management. A worker thread pool with event queues handles asynchronous I/O, while partitioned global state enables lock-free concurrency. Key entry points are the public API functions in src/core/api.c, the platform-specific datapath implementations (epoll, io_uring, kqueue, raw sockets, XDP), and the TLS backends (OpenSSL, Schannel, quictls). The library is consumed by tools like secnetperf (performance testing), interop server, load balancer, and attack tool. Extensive unit tests, fuzz testing, and a debugger extension ensure reliability. External dependencies include OpenSSL/quictls for TLS, XDP for Windows kernel offload, and optional DPDK. The build system uses CMake, Cargo, and PowerShell scripts, with Docker support for cross-compilation. The codebase is actively maintained with frequent commits, CI/CD pipelines, and a comprehensive test suite.