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[quicwg/base-drafts] Persistent congestion threshold is unreliable during the early stages of a connection (#3875)

Kazuho Oku <notifications@github.com> Wed, 08 July 2020 10:45 UTC

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Date: Wed, 08 Jul 2020 03:45:49 -0700
From: Kazuho Oku <notifications@github.com>
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Subject: [quicwg/base-drafts] Persistent congestion threshold is unreliable during the early stages of a connection (#3875)
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Consider the following case:
* endpoint sends Initial
* after 1 second, endpoint sends Initial (PTO)
* after 1ms, the endpoint receives an ACK for Initial (PTO)

At this point, the endpoint would declare persistent congestion because the duration of congestion (1 seconds) is greater than the threshold (i.e. `(smoothed_rtt + 4 * rttvar + max_ack_delay) * kPersistentCongestionThreshold` where smoothed_rtt is 1ms, rttvar is 1ms, and max_ack_delay is zero).

The problem is that determining persistent congestion based on SRTT is not a good idea until the RTT estimate becomes stable.

I think the easiest fix (which is beneficial at this late stage) would be to disable persistent congestion until the handshake concludes.

Spawning off from #3831 as a dedicated issue.

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