how to request one ack per rtt

fixes #227

draft-ietf-quic-ack-frequency.md

@@ -498,19 +498,20 @@ can delay a sender's response to network congestion or cause it to underutilize
 the available bandwidth.
 
 To limit the consequences of reduced acknowledgment frequency, a sender
-can cause a receiver to send an acknowledgment at least once per round trip
-when there are ack-eliciting packets in flight.
+can utilize the extension in this draft to request a receiver
+to send an acknowledgment at least once per round trip,
+when there are ack-eliciting packets in flight, the following ways:
 
-A sender can accomplish this by setting the Requested Max Ack
-Delay value to no more than the estimated round trip time.
+A sender can set the Requested Max Ack Delay value
+to no more than the estimated round trip time.
 The sender can also improve feedback and robustness to
 variation in the path RTT by setting the Ack-Eliciting Threshold
 to a value no larger than number of maximum-sized packets that fit
-into the current congestion window. Alternatively,
-a sender can accomplish this by sending an IMMEDIATE_ACK frame once each
-round trip time, although if the packet containing an IMMEDIATE_ACK is lost,
-detection of that loss will be delayed by the Reordering Threshold or Requested
-Max Ack Delay.
+into the current congestion window.
+Alternatively, a sender can send an IMMEDIATE_ACK frame if no acknowledgement
+has been receive for more than one round trip time.  Although if the
+packet containing an IMMEDIATE_ACK is lost, detection of that loss
+will be delayed by the Reordering Threshold or Requested Max Ack Delay.
 
 When setting the Requested Max Ack Delay as a function of the RTT, it is usually
 better to use the Smoothed RTT (smoothed_rtt) ({{Section 5.3 of QUIC-RECOVERY}}) or another