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

Gorry Fairhurst <notifications@github.com> Tue, 24 March 2020 08:12 UTC

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Date: Tue, 24 Mar 2020 01:12:44 -0700
From: Gorry Fairhurst <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Changing the Default QUIC ACK Policy (#3529)
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QUIC ACKs are larger!
The smallest IPv4 QUIC ACK packet is 51 bytes. The ACK frame is 4 bytes, but is sent using a QUIC packet. For IPv4 this is: 20 (IP) + 8 (UDP) +3 (1+1+1 QUIC header ... assuming a 1 B CID) + 4 (minimum ACK frame size)+ 16 (crypto). Other types of frames may also be sent in the same packet, increasing this size. That size difference matters.

The point is that QUIC ACKs are consuming more capacity in terms of bytes on the wire,  and often also important: in more transmission opportunities at the physical layer. That makes QUIC perform more poorly than it needs to across some paths. 

The text "As an optimization, a receiver MAY process multiple packets before sending any ACK frames in response. In this case the receiver can determine whether an immediate or delayed acknowledgement should be generated after processing incoming packets." Is permissive to do something different - alas the effect of ordering of the processing depends on rate, timing, etc - The present text is not a recommendation to do something appropriate, which I think it needs to. If this were to describe that a receiver SHOULD reduce the ACK to data packet ratio to 1:10, then that would help. 

On low bandwidth links, then I agree that waiting for 10 packets would not be suitable, but then wouldn't the rule 4 ACKs/RTT still cover this case?

(I have no problems believing every an ACK 20 packets can be slightly better than 10 in some cases, but then we have chosen IW at 10 and that relates to bustiness and responsiveness of the basic spec, so I think 1:10 makes more sense as the recommendation. If the CC and path information suggest 1:20 - perhaps as the rate increases, then a sender can "tune" as per the transport parameter proposal. That I STILL assert that this is a different issue from the expected default.)

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