With which of the following forms of acknowledgment can the sender be informed by the data receiver about all segments that have arrived successfully?

Question

Options

• ABlock Acknowledgment
• BNegative Acknowledgment
• CCumulative Acknowledgment
• DSelective Acknowledgment

• A
  3% (1)
• C
  5% (2)
• D
  92% (34)

Explanation

Selective Acknowledgment (SACK) allows the receiver to explicitly report all successfully received segments to the sender - including out-of-order ones - so the sender only needs to retransmit specific missing segments rather than everything after the first gap. This makes it the only option that fully informs the sender about what arrived successfully, regardless of ordering.

• A (Block Acknowledgment) is used in 802.11 Wi-Fi for batched frame acknowledgment and is not a general mechanism for reporting all received TCP segments.
• B (Negative Acknowledgment / NAK) does the opposite - it tells the sender what failed to arrive, not what arrived successfully.
• C (Cumulative Acknowledgment) only acknowledges the highest in-order segment received, so if segment 4 is missing but 5 and 6 arrived, the receiver can only ACK up to segment 3 - those later segments go unreported.

Memory tip: Think "SACK = Shopping receipt" - just as a receipt lists every item you successfully purchased, SACK lists every segment successfully received. Cumulative ACK is like only being told "everything up to aisle 3 is confirmed."

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