diff --git a/PROTOCOL.md b/PROTOCOL.md
index 6e89cf32..e35a4cdd 100644
--- a/PROTOCOL.md
+++ b/PROTOCOL.md
@@ -604,7 +604,7 @@ After validating the message, the session associated with the connection SHOULD
 - Removing all messages from the send buffer that have an `seq` strictly less than the `ack` of the received message.
 - Setting the `ack` for the current session to be `seq + 1` of the received message.
 
-When sending messages to the other side, the session associated with the connection SHOULD:
+When sending messages to the other side, the session associated with the connection MUST:
 
 - Increment the `seq` for the session by 1.