TCP_RTTVAR_SHIFT
(rtt << (TCP_RTTVAR_SHIFT - 1)));
delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
tp->t_rttvar = rtt << (TCP_RTTVAR_SHIFT - 1);
((TCPTV_RTOBASE - TCPTV_SRTTBASE) << TCP_RTTVAR_SHIFT) / 4;