smoltcp is a standalone, event-driven TCP/IP network stack designed for bare-metal and real-time embedded systems. It operates without a heap, using compile-time fixed-size buffers and `no_std` Rust. The stack implements a full suite of protocols: Ethernet, ARP, IPv4/IPv6, TCP, UDP, ICMP, DHCPv4, DNS, IGMP/MLD, 6LoWPAN, RPL, and IPsec (AH/ESP). It is structured into modular layers: `wire/` for packet parsing and emission, `iface/` for interface logic (routing, neighbor discovery, fragmentation, multicast), `socket/` for transport and application sockets, `phy/` for hardware abstraction (loopback, raw sockets, TUN/TAP, pcap, fault injection), and `storage/` for reusable data structures (ring buffers, assemblers). The architecture follows a `Packet`/`Repr` pattern for each protocol, enabling zero-copy parsing and emission. The stack is highly configurable via feature flags, allowing minimal builds for constrained devices. It includes extensive unit tests, integration tests (including a network simulator), fuzz targets, and benchmarks. The codebase demonstrates strong engineering practices with RFC compliance, pluggable congestion control (Reno, Cubic), and async support via waker registration. It is primarily a library for use in embedded projects, IoT devices, and any environment requiring a lightweight, reliable network stack without an operating system.