Re: [quicwg/base-drafts] QUIC PTO is too conservative, causing a measurable regression in tail latency (#3526)

mjoras <> Fri, 29 May 2020 19:29 UTC

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Subject: Re: [quicwg/base-drafts] QUIC PTO is too conservative, causing a measurable regression in tail latency (#3526)
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> This probably shouldn't make me nervous, but it still does. But I still greatly appreciate the data.

@martinthomson For what it's worth, I did extensive testing of this change on smaller scale first, e.g. A/B tests on groups of individual hosts in different locales to ensure that nothing was fundamentally problematic. But indeed, there is good reason to be nervous when experimenting with recovery on the Internet ūüôā 

> can you confirm that you track both observed loss rates AND the rate of spurious retransmission? It strikes me that there would be high correlation between improvements in latency and real loss, so the loss rates probably don't change except where the faster transmission coincides with true congestion (which I would expect to be undetectable), and the same is likely also true for spurious retransmission, but I just wanted to confirm.

One thing I'll note here is that the vast majority of the newly-fired PTOs in this test (i.e. PTOs that previously would have fired later) were not retransmissions at all but rather PTOs using fresh application data. As such the retransmission rate, spurious or otherwise, was pretty much constant when compared to the control.

I think your intuition about the loss rate is totally warranted; in this test though there was not a statistically significant change in the observed loss rate, either the average or in the p95+ or p99 tail.

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