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The protocol is in fact quite simple:

1. Sender sends message.

2. If no acknowledgement from recipient is returned to sender, resend message until an acknowledgement is received from recipient.

3. If recipient receives message, check store to see if it's been received before.

4. If it's not in the store, store it, acknowledge receipt to sender, and process it.

5. If it's already in the recipient's store, acknowledge receipt to sender, and discard message.

What if the "acknowledge receipt to sender" message gets lost?

Server will keep trying to send the message. Each time the client responds with receipt. Eventually communication succeeds and protocol is done. The major drawback is requirement of persistent storage on the client.

That's not a drawback - it's an advantage. It enables store-and-forward, i.e. on- and offline use. The client remains functional without a connection to the server.

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