How do TCP and UDP differ in the way they guarantee packet delivery?

TCP and UDP both include checksums, but TCP adds reliability mechanisms that UDP does not provide. TCP uses acknowledgments so the sender knows what data arrived, retransmissions when data is lost, sequencing to put data back in order, and windowing to control the amount of outstanding data. UDP is deliberately simpler. It uses a checksum for error detection, but it does not establish a reliable session, acknowledge every segment, or retransmit lost data at the transport layer. Applications that use UDP can build their own recovery if they need it, but UDP itself does not guarantee delivery. Cisco CCNA 200-301 v1.1 Network Fundamentals expects this distinction to be automatic: TCP is connection-oriented and reliable; UDP is connectionless and best-effort. The other answer choices introduce parity checks or CRC as transport reliability methods, which is not the TCP/UDP comparison being tested. The correct statement is that TCP uses checksum, acknowledgments, and retransmissions, while UDP uses checksums only.