Exception in ECN detection when ACK loss happens (#1555)

* Adding exception due to ack loss.

* Fixing trailing spaces.

* Implementing Ian's proposals

* Addressing editorial from Ian

* Fixing spelling of comparisons

* Removed extra empty line.

draft-ietf-quic-transport.md

@@ -1778,6 +1778,14 @@ network, an endpoint verifies the following when an ACK_ECN frame is received:
 * The increase in ECT(0) and ECT(1) counters MUST be no greater than the number
   of packets newly acknowledged that were sent with the corresponding codepoint.
 
+* If the increase in the ECT(0), ECT(1) and CE counters are greater than
+  the number of newly acknowledged packets, and the received ACK's
+  smallest acknowledged is larger than the previous largest acknowledged,
+  then it's likely acknowledgements were missed, and the above comparison
+  MUST NOT be performed. Instead a new comparison point is stored by the
+  sender so that only changes after this point will be used in the future
+  comparisons.
+
 Upon successful verification, an endpoint continues to set ECT codepoints in
 subsequent packets with the expectation that the path is ECN-capable.