What is the purpose of SACK in TCP?

• A. To initiate a new connection with a single packet.
• B. To inform the sender of the next expected byte in order.
• C. Selective acknowledgment to inform the sender about additional blocks that have been received beyond the last contiguous byte.
• D. To perform congestion control.

Answer: (C)

Selective acknowledgment in TCP lets the receiver tell the sender exactly which blocks of data have arrived even if they’re out of order or there’s been a gap in what’s been received. With a normal cumulative ACK, the receiver only confirms up to the highest in-order byte received, which can hide which later sections have already made it to the receiver. SACK adds information about additional data blocks that have arrived beyond that last contiguous byte. This helps the sender avoid retransmitting everything after a gap and instead retransmits only the missing pieces, speeding recovery after losses and reducing needless data sent. It’s not about starting a connection, nor about indicating the next expected byte by itself (that’s the role of the cumulative ACK), and it’s not a congestion control mechanism, though it works in concert with TCP’s reliability and performance goals.