Selective Acknowledgments (SACK) benefit in retransmission efficiency?

• A. It increases the TCP receive window
• B. It enables sending data faster using parallel paths
• C. It allows reporting of non-contiguous blocks of data received, enabling retransmission of only missing segments
• D. It eliminates the need for retransmission entirely

Answer: (C)

Selective Acknowledgments let the receiver report non-contiguous blocks of data that have arrived. In a TCP stream, a gap can cause the sender to assume everything after that gap is lost if only forward-looking cumulative acknowledgments are used. SACK communicates exactly which blocks beyond the gap have arrived, so the sender knows precisely which pieces still need to be resent. This lets retransmissions target only the missing segments, avoiding wasting bandwidth on data that’s already been received, and speeds up recovery after packet loss or reordering.

For example, if segments 1–40 and 70–100 have arrived while 41–69 are missing, the receiver can advertise the two received blocks. The sender then retransmits only 41–69, rather than resending data that’s already been received. SACK requires both ends to support the option during setup, and it improves retransmission efficiency, but it doesn’t increase the receive window, enable parallel paths, or eliminate retransmission entirely.