Update draft-ietf-quic-recovery.md

draft-ietf-quic-recovery.md

@@ -439,10 +439,10 @@ ack delay to avoid causing the peer spurious timeouts.  The maximum ack delay
 is communicated in the `max_ack_delay` transport parameter and the default
 value is 25ms.
 
-An acknowledgement SHOULD be sent immediately upon receiving two or more
-retransmitable full-sized packets, as TCP does {{?RFC5681}}.  QUIC recovery
-algorithms, including tail loss probe{{tail-loss-probe}}, assume the peer
-generates an acknowledgement immediately when receiving a second full-sized
+An acknowledgement SHOULD be sent for every second full-sized packet, as TCP
+does {{?RFC5681}}, or may be sent less frequently, as long as the delay does
+not exceed the maximum ack delay. QUIC recovery algorithms do not assume the
+peer generates an acknowledgement immediately when receiving a second full-sized
 packet.
 
 Out-of-order packets SHOULD be acknowledged more quickly, in order to accelerate