What is out-of-order delivery and how should a protocol handle it in EPD Pilot?

• A. Out-of-order delivery cannot occur.
• B. Frames may arrive scrambled; the receiver stores them, reorders based on sequence numbers, and delivers contiguous data to the application.
• C. Out-of-order delivery should be ignored by the receiver.
• D. Frames arriving out of order should be discarded immediately.

Answer: (B)

Out-of-order delivery happens when frames don’t arrive in the same sequence they were sent. To handle this, the receiver uses a reordering buffer and sequence numbers. As frames arrive, they’re stored in the buffer according to their sequence numbers. The receiver then delivers data to the application only when the next in-order block is available, producing a contiguous data stream. This preserves data integrity and the expected ordering despite varying network delays or paths.

Dropping or ignoring out-of-order frames would create gaps and lose data, and assuming out-of-order delivery can’t happen is incorrect because networks can and do deliver frames out of order. The reordering-and-delivery approach is the correct way to maintain a reliable, in-order stream.