conn_ipsec_length
extern int conn_ipsec_length(conn_t *);
mss = pmtu - conn_ipsec_length(sctp->sctp_connp);
mss = pmtu - connp->conn_ht_iphc_len - conn_ipsec_length(connp);
tcp->tcp_ipsec_overhead = conn_ipsec_length(connp);