62 struct ifnet {  struct
63 	char	*if_name;		/* name, e.g. ``en'' or ``lo'' */
64 	short	if_unit;		/* sub-unit for lower level driver */
65 	short	if_mtu;			/* maximum transmission unit */
66 	short	if_flags;		/* up/down, broadcast, etc. */
67 	short	if_timer;		/* time 'til if_watchdog called */
91 	struct	ifnet *if_next;  argument
92 	struct	ifnet *if_upper;	/* next layer up */  argument
93 	struct	ifnet *if_lower;	/* next layer down */  argument
94 	int	(*if_input)();		/* input routine */
95 	int	(*if_ctlin)();		/* control input routine */
96 	int	(*if_ctlout)();		/* control output routine */
97 	struct map *if_memmap;		/* rmap for interface specific memory */