PPT Slide
At last consider the case where the maximum time delay J is known to both parties but is not common knowledge.
This simply means that at any moment both agents can conceive a state where one of them doesn㦙 know the maximum. In that state common knowledge on j can㦙 be reached and consequently it can㦙 be reached in all states where this state can be conceived of.
Unfortunately common knowledge on J can㦙 be established without common knowledge on J ...