sctp_calculate_sum
calc_check = sctp_calculate_sum(m, &mlen, iphlen);
csum = sctp_calculate_sum(m, &m->m_pkthdr.len, 0);
comp_cp->sh.checksum = sctp_calculate_sum(mout, NULL, offset_out);
abm->sh.checksum = sctp_calculate_sum(mout, NULL, iphlen_out);
val = sctp_calculate_sum(scm, NULL, 0);
u_int32_t sctp_calculate_sum(struct mbuf *, int32_t *, u_int32_t);
calc_check = sctp_calculate_sum(m, &mlen, iphlen);