A simplified TCP connection state machine (active-open path)

tcp_connection.cell

// A simplified TCP connection state machine (active-open path).
// closed -> syn_sent -> established -> fin_wait -> closed. This is a
// classic protocol automaton; the model checker proves that from any
// state the connection can return to `closed` (no half-open leaks) and
// that the handshake order cannot be skipped.

cell type TcpState {
    variants { Closed  SynSent  Established  FinWait  TimeWait }
}

cell TcpConnection {
    face {
        signal open_conn(id: String) -> String
        signal syn_ack(id: String) -> String
        signal close_conn(id: String) -> String
        signal timeout(id: String) -> String
        signal reset(id: String) -> String
        signal phase(id: String) -> String
    }

    state conn: TcpState {
        initial: Closed
        Closed      -> SynSent
        SynSent     -> Established
        Established -> FinWait
        FinWait     -> TimeWait
        TimeWait    -> Closed
        *           -> Closed
    }

    on open_conn(id: String)    { transition(id, SynSent)     return get_status(id) }
    on syn_ack(id: String)    { transition(id, Established) return get_status(id) }
    on close_conn(id: String) { transition(id, FinWait)     return get_status(id) }
    on timeout(id: String)    { transition(id, TimeWait)    return get_status(id) }
    on reset(id: String)      { transition(id, Closed)      return get_status(id) }
    on phase(id: String)      { return get_status(id) }
}

cell test TcpTests {
    rules {
        // full lifecycle
        assert open_conn("c1") == "SynSent"
        assert syn_ack("c1") == "Established"
        assert close_conn("c1") == "FinWait"
        assert timeout("c1") == "TimeWait"
        assert reset("c1") == "Closed"

        // a RST from any state returns to Closed (the * arrow)
        assert open_conn("c2") == "SynSent"
        assert reset("c2") == "Closed"

        // illegal: cannot reach Established without sending SYN first
        assert_fails syn_ack("c3")
        // illegal: cannot close a connection that was never established
        assert_fails close_conn("c4")
    }
}