build_ip_csum
iph->check = build_ip_csum((void *)iph);
iph->check = build_ip_csum(iph);
iph->check = build_ip_csum((void *) iph, iph->ihl << 1, 0);
udph->check = build_ip_csum(header - (2 * alen), alen,
iph->check = build_ip_csum(buf, iph->ihl << 2, 0);
iph->check = build_ip_csum(buf, iph->ihl << 2, 0);