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.

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.