nm_os_csum_fold
return nm_os_csum_fold(nm_os_csum_raw((uint8_t*)iph, sizeof(struct nm_iphdr), 0));
*check = nm_os_csum_fold(nm_os_csum_raw(data, datalen, 0));
*check = nm_os_csum_fold(nm_os_csum_raw(data, datalen, 0));
uint16_t nm_os_csum_fold(rawsum_t cur_sum);
*check = nm_os_csum_fold(csum);