If a packet is either sent (i.e., written) to the ip address of the ip port out of which the packet is sent or sent to the loopback address, ip_send() calls ev_enqueue() with its second argument set to a reference of ip_process_loopb(). The next time that the event queue is processed, ip_process_loopb() will be called.
ip_process_loopb() calls ip_arrived() for every ip packet in the ip port's ip_loopb_head queue.