ipf_cksum
ip->ip_sum = ipf_cksum((u_short *)ip, IP_HL(ip) << 2);
ip->ip_sum = ipf_cksum((u_short *)ip, sizeof(*ip));
icmp->icmp_cksum = ipf_cksum((u_short *)icmp,
extern u_short ipf_cksum __P((u_short *, int));