Do not reset pto_count on Initial ACKs

draft-ietf-quic-recovery.md

@@ -501,7 +501,13 @@ and prevents a server from sending a 1-RTT packet on a PTO expiration before it
 has the keys to process an acknowledgement.
 
 When a PTO timer expires, the PTO period MUST be set to twice its current
-value. This exponential reduction in the sender's rate is important because
+value. The PTO is set back to the original value upon receiving an
+acknowledgement for a non-Initial packet. The PTO timer is not decreased when
+the client receives an Initial ACK to ensure the client's anti-deadlock timer
+does not fire too aggressively when the server does not yet have handshake data
+to send.
+
+This exponential reduction in the sender's rate is important because
 consecutive PTOs might be caused by loss of packets or acknowledgements due to
 severe congestion.  Even when there are ack-eliciting packets in-flight in
 multiple packet number spaces, the exponential increase in probe timeout
@@ -1139,8 +1145,9 @@ OnAckReceived(ack, pn_space):
     OnPacketAcked(acked_packet.packet_number, pn_space)
 
   DetectLostPackets(pn_space)
-
-  pto_count = 0
+  // Reset pto_count, unless the client receives an Initial ACK
+  if (endpoint is server || pn_space != Initial)
+    pto_count = 0
 
   SetLossDetectionTimer()
 
@@ -1541,6 +1548,7 @@ OnPacketNumberSpaceDiscarded(pn_space):
   // Reset the loss detection and PTO timer
   time_of_last_sent_ack_eliciting_packet[kPacketNumberSpace] = 0
   loss_time[pn_space] = 0
+  pto_count = 0
   SetLossDetectionTimer()
 ~~~