in_cksum
ip->ip_sum = in_cksum(m, ip->ip_hl << 2);
icp.icmp_cksum = in_cksum(outpack, cc);
ip.ip_sum = in_cksum(outpackhdr, cc);
u_short in_cksum(u_char *, int);
icmpheader->icmp_cksum = in_cksum((u_short *)icmpheader, protlen);
sum[1] = in_cksum((u_short *)&ipo, sizeof(ipo)); /* pseudo ip hdr cksum */
sum[0] = in_cksum(data, cov); /* payload data cksum */
return (~in_cksum(sum, sizeof(sum)));
u_short in_cksum(u_short *, int);