Re: [quicwg/base-drafts] No RTT samples, no persistent congestion (#3889)

Marten Seemann <notifications@github.com> Wed, 22 July 2020 04:45 UTC

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Date: Tue, 21 Jul 2020 21:45:43 -0700
From: Marten Seemann <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] No RTT samples, no persistent congestion (#3889)
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@marten-seemann commented on this pull request.



> @@ -1581,6 +1588,10 @@ Invoked when DetectAndRemoveLostPackets deems packets lost.
 
 ~~~
    InPersistentCongestion(lost_packets):

After talking with @kazuho OOB, I managed to work out what our disagreement is about. Consider the following example:

If there’s an ACK that declares loss in 1-RTT space for packets sent between T=0 and T=5 (which, taken for itself would establish persistent congestion), but there was a Handshake packet acknowledged (in a separate ACK frame) sent at T=3, @kazuho wouldn't declare persistent congestion. I very much agree with that logic, and note that this would constitute a fine solution for #3831.

Pseudo-code-wise, I think we'll have to get a bit fancy here, by introducing two new global variables:
* `time_of_first_rtt_measurement`, the timestamp we were first able to establish an RTT measurement
* `largest_acked_packet_send_time`, the timestamp when the largest acknowledged packet was sent, across all packet number spaces (note that this value is only updated **after** calling `InPersistentCongestion`).

When checking for persistent congestion in `InPersistentCongestion(lost_packets)`, you'd only consider packets sent after `max(time_of_first_rtt_measurement, largest_acked_packet_send_time)`. How exactly "considering" here looks like will depend on the outcome of #3937.

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