我正在尝试从aws iot core发布和订阅aws mqtt代理。我使用的代码示例来自flutter mqtt_client包。我将所有必要的占位符替换为我的法律特定的id, url，…我还在aws中创建了证书，并在我的代码中链接了它们。但是当我尝试连接到代理时，它尝试连接三次，之后它打印一个错误并断开连接。有人知道是什么问题吗?
下面是文本，从第二次尝试中打印到控制台中:
I/flutter ( 2969): 1-2021-10-01 10:46:16.075339 -- SynchronousMqttServerConnectionHandler::internalConnect - initiating connection try 2, auto reconnect in progress false
I/flutter ( 2969): 1-2021-10-01 10:46:16.075769 -- SynchronousMqttServerConnectionHandler::internalConnect - secure selected
I/flutter ( 2969): 1-2021-10-01 10:46:16.076389 -- SynchronousMqttServerConnectionHandler::internalConnect - calling connect
I/flutter ( 2969): 1-2021-10-01 10:46:16.076777 -- MqttSecureConnection::connect - entered
I/flutter ( 2969): 1-2021-10-01 10:46:16.216762 -- MqttSecureConnection::connect - securing socket
I/flutter ( 2969): 1-2021-10-01 10:46:16.217531 -- MqttSecureConnection::connect - start listening
I/flutter ( 2969): 1-2021-10-01 10:46:16.218088 -- MqttServerConnection::_startListening
I/flutter ( 2969): 1-2021-10-01 10:46:16.219200 -- SynchronousMqttServerConnectionHandler::internalConnect - connection complete
I/flutter ( 2969): 1-2021-10-01 10:46:16.219674 -- SynchronousMqttServerConnectionHandler::internalConnect sending connect message
I/flutter ( 2969): 1-2021-10-01 10:46:16.220373 -- MqttConnectionHandlerBase::sendMessage - MQTTMessage of type MqttMessageType.connect
I/flutter ( 2969): Header: MessageType = MqttMessageType.connect, Duplicate = false, Retain = false, Qos = MqttQos.atMostOnce, Size = 22
I/flutter ( 2969): Connect Variable Header: ProtocolName=MQTT, ProtocolVersion=4, ConnectFlags=Connect Flags: Reserved1=false, CleanStart=false, WillFlag=false, WillQos=MqttQos.atMostOnce, WillRetain=false, PasswordFlag=false, UserNameFlag=false, KeepAlive=20
I/flutter ( 2969): MqttConnectPayload - client identifier is : MyIotThing
I/flutter ( 2969): 1-2021-10-01 10:46:16.224343 -- SynchronousMqttServerConnectionHandler::internalConnect - pre sleep, state = Connection status is connecting with return code of noneSpecified and a disconnection origin of none
I/flutter ( 2969): 1-2021-10-01 10:46:18.277033 -- MqttConnectionBase::_onDone - calling disconnected callback
I/flutter ( 2969): 1-2021-10-01 10:46:21.227646 -- SynchronousMqttServerConnectionHandler::internalConnect - post sleep, state = Connection status is connecting with return code of noneSpecified and a disconnection origin of none
I/flutter ( 2969): 1-2021-10-01 10:46:21.229027 -- SynchronousMqttServerConnectionHandler::internalConnect failed
I/flutter ( 2969): MQTT client exception - mqtt-client::NoConnectionException: The maximum allowed connection attempts ({3}) were exceeded. The broker is not responding to the connection request message (Missing Connection Acknowledgement?
I/flutter ( 2969): 1-2021-10-01 10:46:21.237212 -- MqttConnectionHandlerBase::disconnect - entered
I/flutter ( 2969): 1-2021-10-01 10:46:21.237719 -- MqttConnectionHandlerBase::_performConnectionDisconnect entered
I/flutter ( 2969): 1-2021-10-01 10:46:21.240652 -- MqttConnectionKeepAlive::stop - stopping keep alive
I/flutter ( 2969): ERROR MQTT client connection failed - disconnecting, state is MqttConnectionState.disconnected
I/flutter ( 2969): Sleeping....
I/flutter ( 2969): Disconnecting
确保在AWS IoT Core控制台上为Thing分配了连接/断开/发布/订阅策略。