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Why the Ack number is bigger than the last Seq number?

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you see the Ack response for each two TCP packages.

but there I have a question, you see the Ack response for Seq=19997, Seq=22893 and Seq=25789.

but the 3 Ack are not by add 1. （ Seq:19997->Ack:21445, Seq:22893->Ack:24341, Seq:25789->Ack:27237）.

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    "Segments also carry an acknowledgment number which is the sequence number of the next expected data octet of transmissions in the reverse direction."
    – Ron Maupin ♦
    Commented Jun 18, 2019 at 12:30
  • 1
    ACK = ISN + all received data + 1
    – Nakrule
    Commented Jun 18, 2019 at 13:27

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ACK is always bigger than the last SEQ number because ACK is the next expected value.

Let's assume I received a packet which has a sequence number zero (SEQ=0) and let's say that the size of the packet is k bytes. Then my ack will be the last sequence number (0)+(k)+1

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