tcp - Consequences of the lack of acknowledgment on last message -

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Not long, I did not think deeply about the same limit that TCP is also known,

As I have stumbled due to this problem, I have started thinking that the outcomes that arise from the impossibility of solving the problem are. Consider the e-mail protocol. When AS sends a mail, A verifies A that B actually receives its message. Still, B does not end with knowing whether A is acceptable. Why is it not important? I am trying to understand why there is no possibility of consensus among day-to-day applications and we worry about it (I'm probably finishing things).

Another situation which I thought about ATM machines says that wants to withdraw $ 50, the transaction is accepted at the bank headquarters, so the money is reduced from the balance , But believe that the relationship between the headquarters and the ATM fails at this time. Money is discounted but machine is not paying. How are these situations handled?

Maybe I'm a little confused, but it's all new to me. It seems weird that during the conversation between people, the person who says the last sentence has no way of knowing that the other person Has really heard, because the last sentence is to be left without a receipt.

  • Application-to-application basis based on this business logic Is handled at:

    • SMTP: The worst thing is that the mail is sent twice. Today's mail server will see the duplicate ID and the other will drop the copy
    • ATM: Money is reserved only in the first stage, only after the withdrawal of the user's money it is converted from the refund to the reservation. This is actually a lot more complicated, because all the steps are logged locally on the device ("calculating money ... okay", "opening slot ... ok", "removing money out of the slot. .. OK ", ..) The machine, connection or power fails in a bad moment to be able to roll back or forward the event.

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