checksum = inet_checksum(&pseudo, sizeof(pseudo), ~0);
checksum = inet_checksum(&ns, ns_size, checksum);
checksum = inet_checksum(&opt, opt_size, checksum);
checksum = inet_checksum(&pseudo, sizeof(pseudo), ~0);
checksum = inet_checksum(&ns, ns_size, checksum);
checksum = inet_checksum(&opt, opt_size, checksum);