What is SACK in TCP?

SACK = Selective Acknowledgment - the receiver tells the sender exactly which bytes (including discontinuous ones) it got, so only the truly missing segments are retransmitted.

SACK = Selective Acknowledgment

SACK allows the receiver to tell the sender exactly which bytes were received, so only missing segments are retransmitted.

Without SACK: Lost segment 5 → Retransmit 5, 6, 7, 8, 9, 10

With SACK: Lost segment 5 → Retransmit only segment 5

Benefits:

• Reduces unnecessary retransmissions
• Improves efficiency on lossy networks
• Negotiated during three-way handshake

Memory tip: SACK stands for Selective ACKnowledgment - the receiver is selective about which bytes it acknowledges, so only the missing pieces get resent.

Go deeper:

• RFC 2018 — TCP Selective Acknowledgment Options — the spec that lets the receiver name discontinuous received blocks so only lost bytes are resent.
• TCP — Selective acknowledgments — Wikipedia's summary of how SACK improves loss recovery.