/// Opaque token echoed in a keep-alive round trip to match request to response.
pubtypeCookie=u16;/// Keep-alive state machine state.
#[derive(Debug, PartialEq, Eq, Clone)]pubenumState{/// Client may send another ping (or close the protocol).
Client,/// Server owes a response carrying the given cookie.
Server(Cookie),/// Protocol is terminated.
Done,}/// Keep-alive protocol message.
#[derive(Debug, Clone)]pubenumMessage{/// Client → server: ping carrying a cookie to echo back.
KeepAlive(Cookie),/// Server → client: response echoing the cookie from the ping.
ResponseKeepAlive(Cookie),/// Client → server: terminate the protocol.
Done,}