Alice and Bob both want to send each other a message over an unreliable connection. They've agreed on a protocol whereby Alice will go first and keep sending her message until she gets a message from Bob. Likewise, Bob will wait until he receives a message from Alice then keep sending his message until he gets an acknowledgement message from Alice.
And so forth...
What is the minimum number of messages that need to be sent so both Alice and Bob are sure that both Alice and Bob are sure that their messages have been read?
Assume there's a 100% reliable connection.