Netstack
IPv4 and IPv6 userland network stack
...The stack implements TCP, UDP, ICMP, neighbor discovery, and common socket behaviors, including options like MSS/MTU handling and congestion control hooks. Because it’s written in a memory-safe language with extensive unit tests, it is easier to audit and evolve than many kernel-bound alternatives. netstack is frequently used as the networking core for systems that need strict isolation guarantees and deterministic behavior without privileged code.