| Guaranteed
delivery means that a message transmitter is guaranteed to know
whether the intended recipient received the message or not. Guaranteed
delivery should not be confused with reliable
delivery. Guaranteed deliver doesn’t promise that the message
will get there, it only guarantees that the transmitter will know
whether the message got there.
Non-guaranteed delivery simply means that an attempt will be
made to send a message, but there’s no telling if it ever
made it to its recipient.
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One subtle point
to be understood about a protocol that is supposed to have guaranteed
delivery is whether the protocol has an end to end or only a single
hop guarantee. For instance, if a message is going to be transmitted
through several nodes over HTTP, the sender
will know whether the first hop received the message based on
a 200 or a 400 status code. However, if the message can’t
be sent on to a second hop, HTTP doesn’t have a built in
way to inform the original transmitter of that problem. A higher-level
protocol may be needed to give end-to-end guarantees.
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