What is the key difference between TCP and UDP?

• A. TCP is connection-oriented and reliable; UDP is connectionless and best-effort with lower overhead.
• B. TCP is connectionless and unreliable; UDP is connection-oriented and reliable.
• C. Both are connection-oriented; UDP is slower.
• D. TCP is connectionless and no ordering; UDP guarantees in-order delivery.

Answer: (A)

The main distinction is how these protocols handle delivery guarantees and connections. TCP is connection-oriented: it establishes a connection before data transfer and guarantees reliable delivery, meaning data arrives intact and in the same order it was sent, with mechanisms for retransmission, sequencing, and flow/congestion control. UDP, on the other hand, is connectionless and best-effort: it sends datagrams without establishing a persistent connection and does not guarantee delivery or order, though this results in lower overhead and usually lower latency.

That’s why the description that TCP is connection-oriented and reliable, while UDP is connectionless and best-effort with lower overhead is the best fit. The other statements mix up these properties, incorrectly claiming UDP is connection-oriented, or that both are connection-oriented, or that TCP has no ordering guarantees.