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acknowledgement

The receiver's short "got it" reply to a message.

An acknowledgement is a message the receiver sends back to confirm it received something; a sender that waits for it can retry when it doesn't come, which is what turns loss into duplicates instead of gaps.

Examples

  • When the laptop receives a reading it replies with a short ACK; if the sensor's timer runs out without one, it sends the reading again.

  • TCP's ACK is cumulative: ack=1023 means every byte up to 1022 has arrived, even if later segments are still missing.

See also

retransmission sequence number TCP
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