[Twisol]

It only takes one round-trip for both sides to be confident that the first message was received. However, the second party can't be sure the first party knows they received the message without hearing an ACK from the first party. As far as they're concerned, the first party might think it's shouting into the void. This despite the first party (hypothetically) in fact receiving the second party's ACK.

Both parties can never agree on the status of all messages, because you have to generate a new message in order for an old one to advance its state. TCP works on this principle, but because not all messages are semantically relevant to the application, it can generate extra messages to push semantically meaningful messages into the agreed-upon prefix.

On the other hand, a TCP connection must always terminate with at least one message un-ACKed. The best we can do is guarantee that there are no more application-level messages in flight.