Add OnPacketReceived to pseudocode

draft-ietf-quic-recovery.md

@@ -1142,6 +1142,22 @@ Pseudocode for OnPacketSent follows:
      SetLossDetectionTimer()
 ~~~
 
+## On Receiving a Datagram
+
+Receiving a datagram might allow the server to send if the
+server was previously blocked by the anti-amplification limits.
+This is necessary even if none of the packets in the datagram
+are successfully processed.
+
+Pseudocode for OnDatagramReceived follows:
+
+~~~
+OnDatagramReceived(datagram):
+  // If this datagram unblocks the server, arm the
+  // PTO timer to avoid deadlock.
+  if (server was at anti-amplification limit):
+    SetLossDetectionTimer()
+~~~
 
 ## On Receiving an Acknowledgment