Can you explain what is meant by ensuring message delivery in data transmission systems?

Ensuring message delivery in data transmission systems means guaranteeing that a message sent from the sender will be received by the recipient. Depending on the requirements, delivery can be:

• At most once — the message may be delivered once or lost.
• At least once — the message will be delivered, possibly with repetitions.
• Exactly once — the message is delivered exactly once, without losses or duplicates.

Various mechanisms are used for this:

• Acknowledgments (ACK) from the receiver.
• Resending messages when no acknowledgment is received.
• Message identifiers to detect duplicates.

For example, TCP protocol provides reliable data delivery, while messaging systems (like Kafka, RabbitMQ) employ their own methods to guarantee delivery.