csum_replace2
csum_replace2(&ipv4->check, ipv4->tot_len, newlen);
csum_replace2(&iph->check, iph->tot_len, htons(length));
csum_replace2(&iph->check, iph->tot_len, htons(length));
csum_replace2(&iph->check, iph->tot_len, htons(length));
csum_replace2(&iph->check, old_len, iph->tot_len);
csum_replace2(ptr, from, to);
csum_replace2(&iph->check, totlen, iph->tot_len);
csum_replace2(&iph->check, htons(oldtos), htons(iph->tos));
csum_replace2(&hdr4->check,
csum_replace2(&iph->check, htons(iph->ttl << 8),
csum_replace2(&iph->check, iph->tot_len, newlen);
csum_replace2(&nh->check, htons(old_ttl << 8),
csum_replace2(&nh->check, htons(nh->ttl << 8), htons(new_ttl << 8));