In some protocols, the receiver knows a packet is missing before the sender’s timeout expires. A way to proactively tell the sender “I’m missing packet X” can speed up recovery.
A control message sent by the receiver to the sender indicating that a packet was lost or corrupted and needs to be retransmitted.
- Receiver detects missing packet (gap in sequence numbers)
- Receiver sends NAK for the missing packet sequence number
- Sender receives NAK and retransmits the missing packet immediately
- Avoids waiting for timeout to detect loss
- Used by Selective Repeat and some error correction protocols
- Explicitly signals packet loss (vs waiting for timeout)
- Speeds up retransmission compared to timeout-based recovery
- Used with Selective Repeat (not Go-Back-N which uses cumulative ACK)
- Can be combined with ACKs
- Contrasts with: Acknowledgment — positive vs negative feedback
- Built from: Selective Repeat ARQ — uses NAKs
- Related: Retransmission — NAK triggers this
- Related: Timeout — NAK avoids needing timeout for loss detection
- NAK loss can still require timeout-based recovery as backup
- Some protocols (TCP) don’t use explicit NAKs — use duplicate ACKs instead to signal loss
- NAK storms: if many packets lost, many NAKs can add to congestion