Re: [quicwg/base-drafts] Ignoring ACK delay can result in wrong RTT calculations (#3350)

kixelated <notifications@github.com> Wed, 15 January 2020 20:16 UTC

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Date: Wed, 15 Jan 2020 12:16:13 -0800
From: kixelated <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Ignoring ACK delay can result in wrong RTT calculations (#3350)
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Using the above example, where we expect `smoothed_rtt=100` and `rttvar=0.6`

Here is the problematic code. It produces `smoothed_rtt=116.6` and `rttvar=10.4`
```
    adjusted_rtt = latest_rtt
     if (latest_rtt > min_rtt + ack_delay):
       adjusted_rtt = latest_rtt - ack_delay
```

Like @ianswett said, an improvement would be to use the `ack_delay` while `adjusted_rtt >= min_rtt`:

``
    adjusted_rtt = min(latest_rtt - ack_delay, min_rtt)
``

It produces `smoothed_rtt=100.3` and `rttvar=0.4`. However, this will still over-estimate the smooth and under-estimate the variance.

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