[quicwg/base-drafts] The calc with rttvar. (#753)

[scw00 <[e-mail]>]

Hello

It seems we use RFC6298. But RFC used the absolutely value with rttvar.

```
   (2.3) When a subsequent RTT measurement R' is made, a host MUST set

            RTTVAR <- (1 - beta) * RTTVAR + beta * |SRTT - R'|
            SRTT <- (1 - alpha) * SRTT + alpha * R'

```

```   
OnAckReceived(ack):
     largest_acked_packet = ack.largest_acked
     // If the largest acked is newly acked, update the RTT.
     if (sent_packets[ack.largest_acked]):
       latest_rtt = now - sent_packets[ack.largest_acked].time
       if (latest_rtt > ack.ack_delay):
         latest_rtt -= ack.delay
       UpdateRtt(latest_rtt)
     // Find all newly acked packets.
     for acked_packet in DetermineNewlyAckedPackets():
       OnPacketAcked(acked_packet.packet_number)

     DetectLostPackets(ack.largest_acked_packet)
     SetLossDetectionAlarm()


   UpdateRtt(latest_rtt):
     // Based on {{?RFC6298}}.
     if (smoothed_rtt == 0):
       smoothed_rtt = latest_rtt
       rttvar = latest_rtt / 2
     else:
       rttvar = 3/4 * rttvar + 1/4 * (smoothed_rtt - latest_rtt)
       smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt

```

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