Add some MUSTs to congestion control

draft-ietf-quic-recovery.md

@@ -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
+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.
 
 ## 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.
+NewReno 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 window faster.  When a loss or ECN-CE marking is detected, the sender
+MUST reduce the congestion window. NewReno halves the congestion window, sets the
+slow start threshold to the new congestion window, and then enters the recovery
+period.
 
 ## Recovery Period
 
@@ -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.
 
 
 # IANA Considerations