Constant TRACK_PROPERTIES_23 

const TRACK_PROPERTIES_23: &str = "Track Properties are populated in TRACK_STATUS_OK and \
                                   FETCH_OK; they are empty in PUBLISH_OK, REQUEST_UPDATE_OK, \
                                   SUBSCRIBE_NAMESPACE_OK and PUBLISH_NAMESPACE_OK. If an \
                                   endpoint receives Track Properties in one of these messages it \
                                   MUST close the session with INVALID_FIELD.";