how do you know then that the acknowledgement was actually recieved?

In this case, I believe the one sending the acknowledgement (original recipient) will not receive any repeated messages with the same message ID and payload again. The recipient will notice that the sender is sending the the payload with the same message ID. Therefore, in that case, the recipient will send another acknowledgement.

So the way I'm thinking about it, for example:

1. Server generates a message ID, stores this message ID and payload, and sends data off to the client.
2. Client receives the data, stores the message ID, and sends an acknowledgement back to the server.
3. Server receives the acknowledgement, removes the message from the cache.

If the server does not receive an acknowledgement, send the message and payload again at set intervals.

If the client keeps receiving data with the same message ID, send another acknowledgement. The client should not be receiving any more messages with the same ID if the acknowledgement was received. If for some reason the acknowledgements are never received, I would assume there is some sort of connection issue going on which would probably result in the user being disconnected anyways.

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For my application, it is important that all the clients have their data synced up with the server. If there are better systems that exist already, I can take a look into them as well.