in_cksumdata
sum += in_cksumdata(addr, mlen) << 8;
sum += in_cksumdata(addr, mlen);
sum += in_cksumdata((caddr_t) &ipov + 8, sizeof(ipov) - 8);
sum += in_cksumdata(addr, mlen) << 8;
sum += in_cksumdata(addr, mlen);