checksum_nofold
sum = checksum_nofold(data, len, sum);
sum = checksum_nofold(&ph6, sizeof(ph6), 0);
sum = checksum_nofold(&ph4, sizeof(ph4), 0);
sum = checksum_nofold(data, len, sum);
sum = checksum_nofold(iph_addr_p, alen, 0);