ACK is FIN Plus One - TCP FIN Counts One

A FIN has no payload, but it still advances the sequence count by one.

highlighted = computed this step

ACK names the next control slot

The ACK field in the server segment is 1004. It names the next client-side sequence slot wanted.

\text{ACK field}=1004

The FIN used sequence 1003 and consumes 1 sequence number.

1003+1=1004

The bit strip's acknowledgment-number field shows 1004, matching the recomputed FIN rule.

\text{acknowledgment-number}=1004

A FIN with no payload is acknowledged as sequence plus one.

\text{FIN ACK}=1004