Re: [quicwg/base-drafts] Add some MUSTs to congestion control (#3978)

[mirjak <[e-mail]>]

@mirjak commented on this pull request.



> @@ -996,11 +998,12 @@ Reporting additional ECN-CE markings will cause a sender to reduce their sending
 rate, which is similar in effect to advertising reduced connection flow control
 limits and so no advantage is gained by doing so.
 
-Endpoints choose the congestion controller that they use.  Though congestion
-controllers generally treat reports of ECN-CE markings as equivalent to loss
-({{?RFC8311}}), the exact response for each controller could be different.
-Failure to correctly respond to information about ECN markings is therefore
-difficult to detect.
+Endpoints choose the congestion controller that they use. Congestion controllers
+respond to reports of ECN-CE by reducing their rate, but the response may vary.
+Markings can be treated as equivalent to loss ({{?RFC3168}}), but other
+responses can be specified, such as ({{?RFC8511}}) or ({{?RFC8311}}). Failure to
+correctly respond to information about ECN markings is therefore difficult to
+detect.

I actually don't really understand the intention of the last sentence. This is also true for lost base congestion control or even controller which are not based on loss at all.

>  
 ## Congestion Avoidance
 
-Slow start exits to congestion avoidance.  Congestion avoidance uses an
-Additive Increase Multiplicative Decrease (AIMD) approach that increases
-the congestion window by one maximum packet size per congestion window
-acknowledged.  When a loss or ECN-CE marking is detected, NewReno halves
-the congestion window, sets the slow start threshold to the new
-congestion window, and then enters the recovery period.
+Congestion avoidance uses an Additive Increase Multiplicative Decrease (AIMD)
+approach that typically increases the congestion window by one maximum datagram
+size per congestion window acknowledged, and MUST NOT increase the congestion

This MUST NOT kind of already says that QUICv1 cannot implement Cubic. Is that what we want?

> @@ -767,20 +767,22 @@ While in slow start, QUIC increases the congestion window by the
 number of bytes acknowledged when each acknowledgment is processed, resulting
 in exponential growth of the congestion window.
 
-QUIC exits slow start upon loss or upon increase in the ECN-CE counter.
-When slow start is exited, the congestion window halves and the slow start
-threshold is set to the new congestion window.  QUIC re-enters slow start
-any time the congestion window is less than the slow start threshold,
-which only occurs after persistent congestion is declared.
+QUIC MUST exit slow start and enter congestion avoidance upon loss or upon
+increase in the ECN-CE counter. When slow start is exited, the congestion
+window halves and the slow start threshold is set to the new congestion

Restating this here: With https://tools.ietf.org/html/rfc5681#section-4.3 ssthresh is reduced upon entering recover but cwnd is set to ssthresh when leaving recovery.

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