a little help here please, if_tun.c

I am supposed to program a virtual network device driver for openBSD. all the driver does is: display packets sent to it I started with if_loop.c and if_tun.c and I think I am lost. can any one please give me an idea what these functions below do?

how does the kernel assign an IP address to the network device driver? how does the kernel config(ifconfig) the network device driver? what functions got call when packets are handed to the network device driver? sorry for asking too many question, I have never written any device driver.

regards,
Tho.

void tunattach(int);

int	tunopen(dev_t, int, int, struct proc *);
int	tunclose(dev_t, int, int, struct proc *);
int	tun_ioctl(struct ifnet *, u_long, caddr_t);
int	tun_output(struct ifnet *, struct mbuf *, struct sockaddr *,
		        struct rtentry *rt);
int	tunioctl(dev_t, u_long, caddr_t, int, struct proc *);
int	tunread(dev_t, struct uio *, int);
int	tunwrite(dev_t, struct uio *, int);
int	tunselect(dev_t, int, struct proc *);
int	tunkqfilter(dev_t, struct knote *);