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Networking Fundamentals · Intermediate

Acknowledgment Number

The TCP field indicating the next byte the receiver expects, implicitly acknowledging all prior bytes.

Definition

TCP acknowledgments are cumulative: an acknowledgment number of 1501 confirms receipt of every byte through 1500. Duplicate acknowledgments signal a gap and can trigger fast retransmission. Selective acknowledgment (SACK) extends this to report non-contiguous blocks.

Example

A receiver that got bytes 1 to 1000 replies with ACK 1001.

Exam tip

The ACK value is the next expected byte, so it is one higher than the last byte received.

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