ACK is Start Plus Length - TCP Counts Bytes

The ACK value comes from the data sequence number plus the exact payload byte count.

highlighted = computed this step

The arithmetic is byte count

The data started at 1001 and carried 2 bytes.

1001+2=1003

The ACK header field is 1003, exactly one past the last byte carried by the two-byte payload.

\text{ack}=1003

The first byte is at 1001 and the second byte is at 1002. The next wanted byte is therefore 1003.

1001,1002\Rightarrow1003

The ACK number is recomputed from the starting sequence and exact payload byte length.

\text{ACK}=1003