in_cksum_range
sum = in_cksum_range(m, p, off, len);
if (in_cksum_range(m, 0, *offp, sizeof(*ch))) {
__uint32_t in_cksum_range(const struct mbuf *m, int nxt, int offset, int bytes);
return(in_cksum_range(m, 0, 0, len));
return(in_cksum_range(m, 0, skip, len - skip));