void down_to_networklayer(/* event, timer, data */)
{
    READ_APPLICATION(/* get destn, msg and length */);

    // ... fill in packet header info

    for(int link=1 ; link<=NLINKS ; ++link) {
	DOWN_TO_DATALINK(link, packet, len);
    }
}


void up_to_networklayer(/* packet, length */)
{
    if(packet_is_for_this_node) {
	if(packet_is_data) {
	    WRITE_APPLICATION(/* the msg in the packet */);

	    // ... prepare acknowledgement

	    for(int link=1 ; link<=NLINKS ; ++link) {
		DOWN_TO_DATALINK(link, ... /* send ACK */);
            }
	}
	else {
	    ;  // ACK received => prepare to send more data
        }
    }
    else {     // send packet out again
	for(int link=1 ; link<=NLINKS ; ++link) {
	    DOWN_TO_DATALINK(link, packet, len);
        }
    }
}