Re: [quicwg/base-drafts] Why min CWND of 2 instead of 1 (#3586)

Gorry Fairhurst <> Wed, 22 April 2020 09:19 UTC

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Date: Wed, 22 Apr 2020 02:19:39 -0700
From: Gorry Fairhurst <>
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Subject: Re: [quicwg/base-drafts] Why min CWND of 2 instead of 1 (#3586)
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@gorryfair commented on this pull request.

> @@ -245,6 +245,13 @@ QUIC specifies a time-based definition to ensure one or more packets are sent
 prior to a dramatic decrease in congestion window; see
+### Increase the min congestion window to 2 packets
+QUIC recommends a minimum congestion window of 2 packets instead of TCP's 1.
+2 packets avoid waiting for a delayed acknowledgement and allow the PTO to
+send 2 packets instead of 1, which can be particularly important during the

I see this as a more fundamental deviation to TCP than other changes, because although it will have some performance benefit in common cases, it also changes the flow aggressiveness in cases of extreme overload. As I udnerstand, the basis of TCP's back-off is that it scales back cwnd to 1 MSS, and then should scale back in the interval between sending 1 MSS (fractional MSS). If QUIC has a minimum of 2*MSS in QUIC, does it immediately change to 2*RTT interval, otherwise this appears to be significantly more aggressive flow under congestion collapse?

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