.# Control a regular door "Close it", "Open it", "Blow it out" : MESSAGE; OPEN, CLOSED, GONE : STATE; OPEN : "Close it" -> CLOSED; CLOSED : "Open it" -> OPEN; OPEN, CLOSED : "Blow it out" -> GONE;
and the ASCII-art version, produced by Shadok via Graph::Easy.
+--------------+
+--------> | CLOSED | -+
| +--------------+ |
| | |
| Close it | Open it |
| v |
| +--------------+ |
+--------- | OPEN | | Blow it out
+--------------+ |
| |
| Blow it out |
v |
+--------------+ |
| GONE | <+
+--------------+
and the PNG version, produced by
Shadok via Graphviz. .
TCP (Transmission Control Protocol) is described in RFC 793. Its state machine is more complicated
# The TCP state machine. RFC 793 3.2 "Terminology"
Title = "Transmission Control Protocol";
SYN-RCVD, SYN-SENT, FIN-WAIT-1, FIN-WAIT-2, ESTAB,
CLOSING, TIME-WAIT, CLOSED, CLOSE-WAIT,
LISTEN, LAST-ACK : STATE;
CLOSE, passive-OPEN, active-OPEN ,
rcv-SYN, rcv-ACK-of-FIN, rcv-ACK-of-SYN, rcv-SYN-ACK,
rcv-FIN, SEND, Timeout : MESSAGE;
LISTEN : CLOSE -> CLOSED : Delete-TCB ;
# ALTERNATIVE syntax:
# CLOSE: LISTEN -> CLOSED : Delete-TCB ;
# ALTERNATIVE syntax:
# (LISTEN, CLOSE) -> CLOSED : Delete-TCB ;
CLOSED : passive-OPEN -> LISTEN : Create-TCB;
LISTEN : rcv-SYN -> SYN-RCVD;
SYN-RCVD : CLOSE -> FIN-WAIT-1;
FIN-WAIT-1: rcv-ACK-of-FIN -> FIN-WAIT-2;
FIN-WAIT-1: rcv-FIN -> CLOSING;
FIN-WAIT-2 : rcv-FIN -> TIME-WAIT;
CLOSING : rcv-ACK-of-FIN -> TIME-WAIT;
CLOSED : active-OPEN -> SYN-SENT;
LISTEN : SEND -> SYN-SENT;
SYN-SENT : rcv-SYN -> SYN-RCVD;
SYN-RCVD : rcv-ACK-of-SYN -> ESTAB;
SYN-SENT : rcv-SYN-ACK-> ESTAB;
ESTAB : CLOSE -> FIN-WAIT-1;
ESTAB : rcv-FIN -> CLOSE-WAIT;
CLOSE-WAIT : CLOSE -> LAST-ACK;
TIME-WAIT : Timeout -> CLOSED;
LAST-ACK : rcv-ACK-of-FIN -> CLOSED;
And the ASCII-art output
+-----------------+ rcv-FIN
| CLOSE-WAIT | <---------------------+
+-----------------+ |
| |
| CLOSE |
v |
+-----------------+ |
| LAST-ACK | |
+-----------------+ |
| |
| rcv-ACK-of-FIN |
v |
+--------------------------------------+ |
+-----------> | CLOSED | <+-----------+
| +--------------------------------------+ | |
| | | | |
| CLOSE | passive-OPEN | | |
| v | | |
| +-----------------+ | | |
+------------ | LISTEN | -+---------------+ | |
+-----------------+ | | | |
| | | | |
| SEND | active-OPEN | | |
v | | | |
+-----------------+ | | | |
+------------ | SYN-SENT | <+ | | |
| +-----------------+ | | |
| | | | |
| | rcv-SYN | | |
| v | | |
| +-----------------+ rcv-SYN | | |
+-----+------------ | SYN-RCVD | <----------------+ | |
| | +-----------------+ | |
| | | | |
| | rcv-SYN-ACK | rcv-ACK-of-SYN | |
| | v | |
| | +-----------------+ | |
| +-----------> | ESTAB | ----------------------+ |
| +-----------------+ |
| | |
| | CLOSE +-----------+
| v |
+------------+ | CLOSE +-----------------+ |
+--------------> | FIN-WAIT-2 | +-----------------> | FIN-WAIT-1 | -----------------+ |
| +------------+ +-----------------+ | |
| | | | |
| | | rcv-FIN | |
| | v | |
| | +-----------------+ | |
| | | CLOSING | | |
| | +-----------------+ | |
| rcv-ACK-of-FIN | | | |
| | | rcv-ACK-of-FIN | |
| | v | |
| | rcv-FIN +-----------------+ | |
| +-------------------------------------> | TIME-WAIT | -----------------+----+
| +-----------------+ |
| |
+-----------------------------------------------------------------------------------------------+
And the more graphical output 
.
# Just a test, do not take it seriously.
Title = "Complicated state machine, to harass the processor";
Initial =
Limbo;
Final = Dead;
Limbo :
birth -> Childhood;
Limbo : RU486 -> Limbo;
Childhood : school ->
Manhood;
Manhood: wedding ->Honorability;
Honorability: divorce->
Manhood;
Childhood: "Reading of Eric Raymond's books" -> Geekness;
Geekness: "Using Microsoft software", aging -> Manhood;
Manhood, Honorability: retirment->Oldness;
Childhood, Manhood, Honorability, Oldness: death -> Dead;
RU486, birth, school, wedding, "Reading of Eric Raymond's books",
"Using Microsoft software", divorce,
aging, retirment, death : MESSAGE;
Limbo, Childhood, Manhood, Honorability, Geekness,
Oldness, Dead: STATE;
And the ASCII-art output
+----------------------------------+ RU486
| | ----------+
| Limbo | |
| | <---------+
+----------------------------------+
|
| birth
v
+----------------------------------+
+------ | Childhood | ---------------+
| +----------------------------------+ |
| | |
| | Reading of Eric Raymond's books |
| v |
| +----------------------------------+ |
| | Geekness | -+ |
| +----------------------------------+ | |
| | | |
| | Using Microsoft software | aging | school
| v v v
| +-----------------------------------------------------------+
+-----------+-----> | Manhood |
| | +-----------------------------------------------------------+
| | | | |
| divorce | | wedding | |
| | v | |
| | +----------------------------------+ | |
+-----------+------ | Honorability | -+-------------+----------+
| +----------------------------------+ | | |
| | | | |
| | retirment | retirment | |
| v | | |
| +----------------------------------+ | | |
| death | Oldness | <+ | |
| +----------------------------------+ | |
| | | |
| | death | |
| v | |
| +----------------------------------+ death | |
+-----> | Dead | <--------------+ |
+----------------------------------+ |
^ death |
+------------------------------------------------------------+
And the more graphical output 
.