use largest acked, not packet number

draft-ietf-quic-transport.md

@@ -3170,10 +3170,9 @@ use the following steps on receiving an ACK frame to validate ECN.
   vice versa).
 
 Processing ECN counts out of order can result in validation failure.  An
-endpoint SHOULD NOT perform this validation if the ACK frame is received in a
-packet with packet number lower than a previously received ACK frame.
-Validating based on ACK frames that arrive out of order can result in disabling
-ECN unnecessarily.
+endpoint SHOULD perform this validation only if the largest packet number
+acknowledged in this ACK frame is greater than the largest packet number
+acknowledged so far in this connection.
 
 An endpoint could miss acknowledgements for a packet when ACK frames are lost.
 It is therefore possible for the total increase in ECT(0), ECT(1), and CE counts