in_addword
iph_csum = in_addword(csum1, (~csum2 & 0xffff));
iph_csum = in_addword(iph_csum, (~xp[0] & 0xffff));
iph_csum = in_addword(iph_csum, (~xp[1] & 0xffff));
iph_csum = in_addword(iph_csum, xp[0]);
iph_csum = in_addword(iph_csum, xp[1]);
ipo_csum = in_addword(ipo_csum, *p);
iph_csum = in_addword(iph_csum, ipo_csum);
ipd_csum = in_addword(csum2, (~ipo_csum & 0xffff));
th->th_sum = in_addword(th->th_sum,