Document TCP RTO vs QUIC PTO

draft-ietf-quic-recovery.md

@@ -231,13 +231,19 @@ more accurate round-trip time estimate (see Section 13.2 of {{QUIC-TRANSPORT}}).
 ### Probe Timeout Replaces RTO and TLP
 
 QUIC uses a probe timeout (see {{pto}}), with a timer based on TCP's
-RTO timeout.  Unlike TCP's RTO, which collapses the congestion window upon
-expiry, QUIC does not change the congestion window and instead allows sending
-probe packets whenever the timer expires.  This is similar to TCP with F-RTO,
-but it does allow more packets to be sent when the congestion window was not
-fully utilized when the probe timeout expires. Though this is slightly more
-aggressive than TCP RTO, it's less aggressive than the congestion controller
-allowed.
+RTO timeout.  QUIC PTO includes the peer's max_ack_delay in the calculation,
+instead of relying upon a fixed minimum RTO.  Unlike TCP's RTO, which collapses
+the congestion window upon expiry, QUIC does not change the congestion window
+and instead allows sending probe packets whenever the timer expires.  This is
+similar to TCP with F-RTO, but it does allow more packets to be sent when the
+congestion window was not fully utilized when the probe timeout expires. Though
+this is slightly more aggressive than TCP RTO, it's less aggressive than the
+congestion controller allowed.
+
+A single packet loss at the tail does not indicate persistent congestion, so
+QUIC defines a time-based definition (see {{persistent-congestion}}) to
+ensure one or more packets are sent prior to a dramatic decrease in
+congestion window.
 
 
 # Estimating the Round-Trip Time {#compute-rtt}