sctp_calculate_cksum
checksum = sctp_calculate_cksum(m, offset);
uint32_t sctp_calculate_cksum(struct mbuf *, int32_t);
cksum_calculated = sctp_calculate_cksum(m, iphlen);
shout->checksum = sctp_calculate_cksum(mout, sizeof(struct ip) + sizeof(struct udphdr));
shout->checksum = sctp_calculate_cksum(mout, sizeof(struct ip6_hdr) + sizeof(struct udphdr));
sctphdr->checksum = sctp_calculate_cksum(m, sizeof(struct ip) + sizeof(struct udphdr));
sctphdr->checksum = sctp_calculate_cksum(m, sizeof(struct ip6_hdr) + sizeof(struct udphdr));
sctp->checksum = sctp_calculate_cksum(m, hlen);
sum = sctp_calculate_cksum(m, off);