How do UDP and TCP differ in terms of reliability and ordering?

• A. UDP provides no guaranteed delivery or ordering and is connectionless; TCP provides reliability and in-order delivery with a connection
• B. UDP guarantees in-order delivery; TCP does not
• C. UDP uses congestion control by default; TCP does not
• D. UDP and TCP provide identical reliability

Answer: (A)

Reliability and ordering distinguish UDP from TCP. UDP is connectionless and delivers data on a best-effort basis; there’s no built-in guarantee that a datagram will arrive, and there’s no inherent ordering, so packets can be lost, arrive out of order, or be duplicated. Applications that need reliability or strict ordering must handle it themselves or use a different protocol.

TCP, by contrast, is connection-oriented. It establishes a connection, then uses sequence numbers, acknowledgments, and retransmissions to ensure every byte is delivered reliably, and it delivers data to the receiving application in the same order as sent. It also manages flow and congestion control to adapt to network conditions.

So the correct view is that UDP offers no guaranteed delivery or ordering, while TCP provides reliable, in-order delivery over a connection. The other statements misstate UDP’s behavior or the baseline differences between the two protocols.