By what mechanisms is data delivery guaranteed?

Mainly through acknowledgments (ACK) and retransmissions. The sender sends data and waits for acknowledgment from the receiver. If acknowledgment is not received within a certain time, the sender retransmits. Reliability is also ensured by checksums to detect corrupted data.

At higher levels, mechanisms like exactly-once processing in distributed systems or transactional support in databases are used.

Examples from protocols:

• TCP: Uses sequence numbers, ACK, and RTT (Round-Trip Time) to manage retransmissions and flow control.
• Message queue protocols (e.g., AMQP): Acknowledged delivery and mechanisms for redelivery or placing messages in a "dead-letter queue".

// Sender sends ACK  
// Receiver confirms receipt  
// If ACK is not received, sender retransmits