Phoenix-RTOS network stack is based on LWIP. According to microkernel architecture philosophy it works as a server on the user level and provides socket interface. Sockets are implemented using native Phoenix-RTOS message passing mechanism. Socket are implemented in libphoenix library.
The source code of the POSIX emulation server could be obtained using the following command
git clone http://git.phoenix-rtos.com/phoenix-rtos-lwip