When an arp-request is sent out, a timer is set. If an arp-reply is not received for this arp-request within a half second, arp_timeout() is called. If fewer than four arp-requests have already been sent out, arp_timeout() simply calls setup_write() to send out another arp-request. If four arp-requests have already been sent out, the arp table entry is marked as unreachable and client_reply() is called to alert interested ip file descriptors.