code: public static ChannelFuture sendUnsupportedVersionResponse(Channel channel, ChannelPromise promise) {
          HttpResponse res = new DefaultFullHttpResponse(
                  HttpVersion.HTTP_1_1,
                  HttpResponseStatus.UPGRADE_REQUIRED);
          res.headers().set(HttpHeaderNames.SEC_WEBSOCKET_VERSION, WebSocketVersion.V13.toHttpHeaderValue());
          HttpUtil.setContentLength(res, 0);
          return channel.writeAndFlush(res, promise);
      }
docs: Return that we need cannot not support the web socket version