That (to quote you) means you have to keep them in the queue until the next time the client connects, which could be a very long time. To be clear, there's no

That (to quote you) means you have to keep them in the queue until the next time the client connects, which could be a very long time. To be clear, there's no real difference - in both cases you have to keep messages in some queue and potentially resend them until they've been acknowledged.