static String closeCodeExceptionMessage(int code) {
        if (code < 1000 || code >= 5000) {
            return "Code must be in range [1000,5000): " + code;
        } else if ((code >= 1004 && code <= 1006) || (code >= 1012 && code <= 2999)) {
            return "Code " + code + " is reserved and may not be used.";
        } else {
            return null;
        }
    }
Used when an empty close frame was received (i.e., without a status code).
netbare-core/src/main/java/com/github/megatronking/netbare/ws/WebSocketProtocol.java