KA9Q Mirror
An impressive early implementation of TCP/IP
... and an IP packet router, and handle multiple client and server sessions.
KA9Q NOS attracted many contributors and became widely used throughout the 1980s and 1990s in amateur packet radio and various educational projects. In a way, it was the Linux of its day.
KA9Q NOS became the basis for several low-end commercial dialup terminal servers and routers. It influenced the development of the Internet protocols and implementations, including the Linux kernel.