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TCP SACK, DSACK & RACK: Modern Loss Recovery

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Why cumulative ACKs alone are insufficient

Classic TCP only tells the sender: 'I've received everything up to byte N.' When segment 2 out of 10 is lost, the sender gets ACK=N×3 and retransmits from N — including segments 3-10 that were already received.

SACK, DSACK, and RACK address three distinct problems: knowing what was received (SACK), detecting unnecessary retransmissions (DSACK), and finding loss faster than 3 duplicate ACKs (RACK).

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