ipv4_csum
ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
ipv4_csum(iph, sizeof(struct iphdr), &csum);
icmph->checksum = ipv4_csum(icmph, ICMP_ECHO_LEN);
icmph->checksum = ipv4_csum(icmph, ICMP_ECHO_LEN);