What to send on multiple PTOs

Fixes #2103

draft-ietf-quic-recovery.md

@@ -535,6 +535,12 @@ MAY use alternate strategies for determining the content of probe packets,
 including sending new or retransmitted data based on the application's
 priorities.
 
+When the PTO timer expires multiple times, implementations must choose between
+sending the same payload every time or sending different payloads.  Sending the
+same payload may be simpler and ensures the highest priority frames arrive
+first.  Sending different payloads each time reduces the changes of spurious
+retransmission.
+
 When a PTO timer expires, new or previously-sent data may not be available to
 send and packets may still be in flight.  A sender can be blocked from sending
 new data in the future if packets are left in flight.  Under these conditions, a