TCP Close Boundary - TCP FIN Counts One

This toy close shows FIN ACK arithmetic only, not the full close lifecycle.

highlighted = computed this step

What this toy close shows

This book shows one client FIN and one server ACK. It only models close arithmetic.

\text{FIN consumes}=1

Client FIN sequence 1003 plus one gives server ACK 1004.

1003+1=1004

NOTE: toy close arithmetic only; not simultaneous close, TIME_WAIT, retransmission, half-close APIs, RST, socket lifecycle, or real capture timing.

\text{toy FIN exchange}=1

FIN is a close signal, and TCP counts that signal as one sequence number.

\text{ACK}=1004