How can guaranteed data delivery over UDP protocol be…

Guaranteed delivery over UDP, being a connectionless protocol without delivery guarantees, requires implementing reliability mechanisms at the application level or using middleware. The main approaches include:

Acknowledgement mechanisms (ACK): The sender includes an identifier (e.g., sequence number) in each datagram. The receiver sends an acknowledgment for each successfully received datagram.
Retransmission mechanisms: The sender sets a timer after sending a datagram. If acknowledgment is not received before the timer expires, the datagram is considered lost and is retransmitted.
Duplicate control mechanisms: The receiver tracks the identifiers of received datagrams and discards duplicates if they appear as a result of retransmission.
Order control mechanisms: Although UDP does not guarantee order, the application can use sequence numbers to reorder datagrams on the receiver side.
Flow control: If necessary, a mechanism can be implemented to prevent buffer overflow at the receiver.

Implementing these mechanisms requires significant effort at the application level or using specialized libraries/protocols built on top of UDP, such as:

Reliable User Datagram Protocol (RUDP): An extension of UDP that adds reliability.
QUIC: A transport protocol developed by Google that operates over UDP and provides functionality similar to TCP (reliability, flow control, security), but with improved connection setup time and less impact from packet loss.
Real-time protocols with partial reliability: For example, Secure Real-time Transport Protocol (SRTP) can include optional reliability mechanisms.

An example schematic logic on the sender side:

# pseudocode for illustration
data_to_send = [...] # list of datagrams
sent_packets = {}    # dictionary: sequence number -> (data, send time)
next_seq_num = 0
ack_timeout = 0.1    # seconds
max_retransmissions = 5

for data in data_to_send:
    packet = (next_seq_num, data)
    send_udp(packet)
    sent_packets[next_seq_num] = (packet, time.time(), 0) # packet, send time, retransmission count
    next_seq_num += 1

while sent_packets:
    # Wait for acknowledgments
    acks = receive_acks() # simulate receiving acknowledgments
    for seq_num in acks:
        if seq_num in sent_packets:
            del sent_packets[seq_num] # Remove from waiting list

    # Check timeouts and retransmit
    current_time = time.time()
    packets_to_retransmit = []
    for seq_num, (packet, send_time, retransmit_count) in sent_packets.items():
        if current_time - send_time > ack_timeout:
            if retransmit_count < max_retransmissions:
                packets_to_retransmit.append(seq_num)
            else:
                # Packet is considered lost after max retransmissions
                handle_packet_loss(seq_num)
                del sent_packets[seq_num]

    for seq_num in packets_to_retransmit:
        packet, _, retransmit_count = sent_packets[seq_num]
        send_udp(packet)
        sent_packets[seq_num] = (packet, time.time(), retransmit_count + 1) # update time and count

This implementation essentially creates a protocol on top of UDP that mimics TCP's reliability features. The choice of approach depends on application requirements (e.g., latency, throughput). For most cases requiring reliability, it is preferable to use existing protocols (TCP, QUIC) or proven libraries.

How can guaranteed data delivery over UDP protocol be ensured?