4. Reliability and error control: data retransmission

In IP networks, the IP protocol offers only a datagram service, i.e. there is no guarantee that packets will arrive in the order they were sent (or at all). Sequence numbers, acknowledgements and the sliding window are mechanisms that regulate data exchange, enabling TCP to provide the receiving application with contiguous, ordered blocks of bytes. However, when packets can be lost, TCP ensures transfer reliability by implementing a retransmission mechanism for missing bytes. Packets can be lost for a variety of reasons: network congestion, transmission errors, router failure or malfunction, routing problems, etc. Whatever the reason, TCP detects the loss of the packet and retransmits it.