I'm still unclear on a couple of things. How do you even know which send request the attacker attempted to attack if only the codeword identifies the send request, and the codeword was wrong? Also, what stops an attacker from trying again - alternatively, what stops an attacker from doing a denial-of-service by purposefully guessing the wrong codeword?

Or the attacker only needs to monitor the last free slot to see what shows up?