diff --git a/draft-ietf-quic-ack-frequency.md b/draft-ietf-quic-ack-frequency.md
index 815c01c8..f70abb7b 100644
--- a/draft-ietf-quic-ack-frequency.md
+++ b/draft-ietf-quic-ack-frequency.md
@@ -513,7 +513,7 @@ will be less responsive to changes in the path's RTT, resulting in either
 delayed or unnecessary packet transmissions.
 
 A sender might use timers to detect loss of PMTU probe packets
-({{Section 14 of QUIC-TRANSPORT}}). A sender SHOULD
+({{Section 14 of QUIC-TRANSPORT}}). A sender MAY
 bundle an IMMEDIATE_ACK frame with any PMTU probes to avoid triggering such
 timers.