rack_update_rtt
rack_update_rtt(tp, rack, rsm, to, cts, CUM_ACKED, th_ack);
rack_update_rtt(struct tcpcb *tp, struct tcp_rack *rack,
rack_update_rtt(tp, rack, nrsm, to, cts, SACKED, 0);
rack_update_rtt(tp, rack, rsm, to, cts, SACKED, 0);
rack_update_rtt(tp, rack, nrsm, to, cts, SACKED, 0);
rack_update_rtt(tp, rack, rsm, to, cts, SACKED, 0);