in4_cksum
sum = in4_cksum(m, IPPROTO_TCP, off, len);
int in4_cksum(struct mbuf *, u_int8_t, int, int);
ieh->ieh_cksum = in4_cksum(n, 0, off, sizeof(hdr) + len);
if (in4_cksum(m, 0, hlen, icmplen)) {
csum = in4_cksum(m, 0, offset, m->m_pkthdr.len - offset);
cksum = in4_cksum(m, IPPROTO_TCP, skip,
sum = in4_cksum(m, IPPROTO_TCP, iphlen, tlen);
uh->uh_sum = in4_cksum(m, IPPROTO_UDP,
icp->icmp_cksum = in4_cksum(m, 0, sizeof(*ip),