What is Selective Acknowledgments (SACK) and how do they benefit TCP?

• A. SACK allows the receiver to reorder data within the stream
• B. SACK eliminates the need for any retransmissions
• C. SACK reports the total bytes received, but not gaps
• D. SACK allows the receiver to report non-contiguous blocks of data received, enabling efficient retransmission of only missing segments

Answer: (D)

Selective Acknowledgments let the receiver tell the sender exactly which data blocks have arrived, including multiple non-adjacent blocks. This way the sender knows which parts of the data are still missing and can retransmit only those pieces, rather than tossing back a whole chunk after a single lost packet.

For example, if bytes 1–1000 and 2001–3000 have arrived but 1001–2000 are lost, the receiver can advertise the received blocks, and the sender can retransmit just 1001–2000. This targeted retransmission reduces duplicate data, speeds recovery after loss, and improves overall TCP throughput on networks with losses or large delays.

SACK is negotiated and used alongside the regular cumulative acknowledgments, but it doesn’t reorder data by itself, nor does it eliminate all retransmissions. It also doesn’t simply report total bytes received without gaps; it provides the actual received blocks and the gaps, enabling efficient retransmission of only the missing segments.