What is the difference between reliable delivery and best-effort delivery in the EPD Pilot's protocol design?

• A. Reliable ensures delivery and order (with retries); best-effort may drop or reorder messages.
• B. Reliable means messages are only encrypted.
• C. Best-effort guarantees delivery with retries.
• D. Both guarantee perfect ordering.

Answer: (A)

Delivery guarantees tell you how the protocol handles sending messages. Reliable delivery means every message reaches the recipient and comes in the same order as sent, thanks to mechanisms like acknowledgments and retransmissions with sequencing. Best-effort delivery, however, does not promise delivery or order—messages can be dropped, delayed, duplicated, or arrive out of order, and there are no built-in retries. So the statement that reliable ensures delivery and order (with retries) while best-effort may drop or reorder messages captures the essential difference in how these two approaches behave. Encryption is about privacy, not delivery guarantees, so it doesn’t address reliability. Best-effort does not guarantee delivery with retries, so that claim is inaccurate. And perfect ordering is not guaranteed by best-effort, so both guaranteeing perfect ordering is incorrect.