Re: [quicwg/base-drafts] Changing the Default QUIC ACK Policy (#3529)

Jana Iyengar <notifications@github.com> Fri, 29 May 2020 06:41 UTC

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Date: Thu, 28 May 2020 23:41:27 -0700
From: Jana Iyengar <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Changing the Default QUIC ACK Policy (#3529)
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@gorryfair : I understand your position. While I agree that ack thinning happens with TCP, it is not what I expect to find on a common network path. And as far as I know, the endpoints still implement the RFC 5681 recommendation of acking every other packet.

What you are arguing is that _with ack thinners in the network_ TCP's behavior on _those_ network paths is different. However, I do not believe we understand the performance effects of these middleboxes, that is, how they might reduce connection throughput when the asymmetry is not a problem. Importantly,  I don't think we should be specifying as default what some middleboxes might be doing without fully understanding the consequences of doing this. Please bear in mind that Chrome's ack policy of 1:10 works with BBR, not with Cubic or Reno. 

I do not believe there is a magic number at the moment, and as @kazuho noted, even 1:10 is not a small enough ratio under some conditions. Given this, and given that we know 1:2 is used by TCP endpoints, it makes the most sense to specify that as the suggested guidance. The new text proposed in #3706 describes the tradeoff involved, and the rationale for using 1:2. I am strongly against doing anything substantially different without _overwhelming information_.

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