Date: Wed, 19 Dec 2018 06:31:10 +0000 (UTC)
From: Marten Seemann <notifications@github.com>
Reply-To: quicwg/base-drafts
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To: quicwg/base-drafts <base-drafts@noreply.github.com>
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Message-ID: <quicwg/base-drafts/issues/2208@github.com>
Subject: [quicwg/base-drafts] persistent congestion can never happen (#2208)
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The collapse of the congestion window (which used to happen after an RTO was verified) happens in `CongestionEvent`, if `pto_count > kPersistentCongestionThreshold`.
`CongestionEvent` is called from `OnPacketsLost`, which is called from `DetectLostPackets`, which in the case of a tail is only called when a new ACK is received from `OnAckReceived`. However, `OnAckReceived` resets the `pto_count` to 0 *before* calling `DetectLostPackets`, so the condition that leads to the collapse of the congestion window can never occur.

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<p>The collapse of the congestion window (which used to happen after an RTO was verified) happens in <code>CongestionEvent</code>, if <code>pto_count &gt; kPersistentCongestionThreshold</code>.<br>
<code>CongestionEvent</code> is called from <code>OnPacketsLost</code>, which is called from <code>DetectLostPackets</code>, which in the case of a tail is only called when a new ACK is received from <code>OnAckReceived</code>. However, <code>OnAckReceived</code> resets the <code>pto_count</code> to 0 <em>before</em> calling <code>DetectLostPackets</code>, so the condition that leads to the collapse of the congestion window can never occur.</p>

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