wrapsum
ip.ip_sum = wrapsum(checksum((unsigned char *)&ip, sizeof(ip), 0));
udp.uh_sum = wrapsum(checksum((unsigned char *)&udp, sizeof(udp),
if (wrapsum(checksum(buf + bufix, ip_len, 0)) != 0) {
sum = wrapsum(checksum((unsigned char *)udp, sizeof(*udp),
u_int32_t wrapsum(u_int32_t);