struct ip6hdr {    union {	struct ip6hdrctl {	    u_int32_t ctl6_flow;	/* 24 bits of flow-ID */	    u_int16_t ctl6_plen;        /* payload length */	    u_int8_t  ctl6_nxt;         /* next header */	    u_int8_t  ctl6_hlim;        /* hop limit */	} un_ctl6;	u_int8_t un_vfc;		/* 4 bits version, 4 bits reserved */    } ip6_ctlun;    struct in6_addr ip6_src;		/* source address */    struct in6_addr ip6_dst;		/* destination address */};#define ip6_vfc		ip6_ctlun.un_vfc#define ip6_flow	ip6_ctlun.un_ctl6.ctl6_flow#define	ip6_plen	ip6_ctlun.un_ctl6.ctl6_plen#define	ip6_nxt		ip6_ctlun.un_ctl6.ctl6_nxt#define	ip6_hlim	ip6_ctlun.un_ctl6.ctl6_hlim#define	ip6_hops	ip6_ctlun.un_ctl6.ctl6_hlim