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TCP SACK — Selective Acknowledgment

Medium

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The problem with cumulative ACKs

Standard TCP uses cumulative ACKs — the ACK number says "I've received everything up to byte N." When segment 2 of 4 is lost, the receiver can only say ACK=1000 even after receiving segments 3 and 4.

The sender doesn't know which segments arrived — must retransmit everything from 1000 onwards.

SACK (Selective Acknowledgment, RFC 2018) fixes this: the receiver reports exactly which byte ranges it has, so the sender retransmits only the gaps.

Packets in motion — click to inspect:

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