How does selective retransmission improve performance in lossy networks?

• A. Retransmitting every packet on timeout.
• B. Retransmitting data in the order they were sent.
• C. Using forward error correction.
• D. Retransmitting only the missing segments reported via SACK, reducing unnecessary retransmissions.

Answer: (D)

Selective retransmission targets only the data that was actually lost, which keeps bandwidth available for what’s still arriving correctly and reduces congestion in lossy networks. When the receiver can report exactly which segments are missing (via SACK), the sender doesn’t waste time re-sending everything. It can retransmit only the missing pieces, repair the gaps quickly, and continue sending new data. This focused approach improves throughput and reduces both wasted transmissions and duplicate traffic.

In contrast, retransmitting every packet after a timeout wastes bandwidth because many of those packets are already fine. Re-sending data in the original send order doesn’t address the core inefficiency of guessing what was lost, and forward error correction adds fixed overhead rather than precisely repairing only the missing pieces.