eth_write_port(eth_port, pack) sends a message to the ethernet task (driver) requesting that the packet pack, eth_write_port()'s second parameter, be sent.
eth_write_port() is called only by eth_send(). By the time that eth_write_port() has been called, all layers (e.g., ethernet, ip, udp) have verified that the outgoing packet is valid and that the packet is destined for a remote system (i.e., is not destined for the local loopback or the address of the local port).