Selective ACK
Selective ACK (SACK) is a synonym for the SACK option, acknowledging the discontinuous data blocks the receiver already holds.
Purpose
It makes the sender retransmit only what was truly lost, avoiding redundant re-sends and improving efficiency on high-loss links.
Mechanism
The receiver attaches a SACK option to its ACK listing the boundaries of received blocks; the sender compares these against sent sequence numbers, identifies the gaps, and retransmits only those.
Use cases
- TCP recovery on high-loss links
- Wireless networks
- Large-window connections
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