fix_cksum
ip->ip_sum = fix_cksum(ip->ip_sum, oaddr[0], naddr[0], 0);
ip->ip_sum = fix_cksum(ip->ip_sum, oaddr[1], naddr[1], 0);
*csum = fix_cksum(*csum, oaddr[0], naddr[0], udp);
*csum = fix_cksum(*csum, oaddr[1], naddr[1], udp);
*csum = fix_cksum(*csum, *old_port, new_port, udp);