in_cksum_add
return (in_cksum_fini(in_cksum_add(addr, len, sum)));
extern uint32_t in_cksum_add(const void *, size_t, uint32_t);
cksum = in_cksum_add(&ip6->ip6_src,
cksum = in_cksum_add(&ip6->ip6_dst,
cksum = in_cksum_add(&ip->ip_src,
cksum = in_cksum_add(&ip->ip_dst,