Normally, a variant of "struct sockaddr" is some kind of address.
Why is all this stuff crammed together in a sockaddr variant?

In <net/if_dl.h>:
/*
 * Structure of a Link-Level sockaddr:
 */
struct sockaddr_dl {
	u_char	  sdl_len;	/* Total length of sockaddr */
	u_char	  sdl_family;	/* AF_DLI */
	u_int16_t sdl_index;	/* if != 0, system given index for interface */
	u_char	  sdl_type;	/* interface type */
	u_char	  sdl_nlen;	/* interface name length, no trailing 0 reqd */
	u_char	  sdl_alen;	/* link level address length */
	u_char	  sdl_slen;	/* link layer selector length */
	char	  sdl_data[12];	/* minimum work area, can be larger;
				   contains both if name and ll address */
};


I would have expected to see some other interface structure with
all the above information, also containing a simpler sockaddr_dl
as one of the members, perhaps something like this:

struct sockaddr_dl {
	u_char	  sdl_len;	/* Total length of sockaddr */
	u_char	  sdl_family;	/* AF_DLI */
	u_char	  sdl_type;	/* interface type */
	u_char	  sdl_alen;	/* link level address length */
	char	  sdl_data[12];
};

struct if_info {
	u_int16_t ifi_index;	/* if != 0, system given index for interface */
	u_char	  ifi_hdrlen;	/* media header length */
	char	  ifi_xname[IFNAMSIZ];	/* external name (name + unit) */
	struct sockaddr_dl *ifi_sdl;
}

Could someone explain the reasoning behind the current design?

Gordon