If CONFIG_NET_ARP_SEND is enabled, then all ICMP, TCP, and UDP send operations...

If CONFIG_NET_ARP_SEND is enabled, then all ICMP, TCP, and UDP send operations will call arp_send() before attempting the real send operation.  arp_send() will check if the the IP address mapping is in the ARP table and, if not send ARP requests periodically until it is.  This eliminates losing the first outgoing message because there is not mapping in the ARP table.