Re: [quicwg/base-drafts] compensation of ack_delay is fragile against errors (#2060)

Kazuho Oku <> Thu, 29 November 2018 02:56 UTC

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From: Kazuho Oku <>
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Subject: Re: [quicwg/base-drafts] compensation of ack_delay is fragile against errors (#2060)
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> I think what you want is:
>    if (latest_rtt - min_rtt > ack_delay)
>        latest_rtt -= ack_delay
>    else
>        latest_rtt = min_rtt
> ```

Isn't that mathematically equivalent to `latest_rtt = max(latest_rtt - ack_delay, min_rtt)`?

> Basically if you have a bad clock at the receiver, this means that you constantly underestimate the latest RTT sample.

What type of behavior are you assuming for a "bad clock"?

The issue with the current approach is that it considers the true ack_delay to be zero when the value reported by the peer is huge.

Assuming that we cannot tell how good the clock of the peer (or the endpoint) is, _we should round ack_delay to the nearest reasonable value_. That is, if `ack_delay` is too large so that latest_rtt becomes below min_rtt, adjust ack_delay to `latest_rtt - min_rtt`. That is what the issue proposes.

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