What mechanism helps ensure message ordering in an unreliable channel?

• A. Sequence numbers.
• B. Random re-sending.
• C. Immediate closure after each message.
• D. Ignore duplicates.

Answer: (A)

Messages traveling through an unreliable channel can arrive out of order, be lost, or duplicate themselves. By attaching a sequence number to each message, the receiver gains a clear map of the intended order. As data arrives, it can be placed in the correct position, kept in a buffer until earlier pieces arrive, and checked for gaps. When a gap is detected, the system can trigger retransmission or wait for the missing piece, so the final delivery preserves the original sequence.

Other approaches don’t reliably ensure order. Random re-sending can create more disorder and still doesn’t provide a way to identify the correct sequence or missing pieces. Closing the channel after each message ends the stream without preserving order. Ignoring duplicates helps avoid duplicates but doesn’t ensure that subsequent data arrives in the intended order or that no gaps exist. Sequence numbers give a concrete, scalable method to maintain order and integrity.