MQTT

MQTT is a lightweight and widely adopted messaging protocol that is designed for constrained devices. AWS IoT support for MQTT is based on the MQTT v3.1.1 specification, with some differences. For information about how AWS IoT differs from the MQTT v3.1.1 specification, see AWS IoT differences from MQTT version 3.1.1 specification.

AWS IoT Core supports device connections that use the MQTT protocol and MQTT over WSS protocol. The AWS IoTDevice SDKs support both protocols and are the recommended ways to connect devices to AWS IoT. The AWS IoT Device SDKs support the functions necessary for devices and clients to connect to and access AWS IoT Core services and they support the authentication protocols that the AWS IoT services require. For information about how to connect to AWS IoT using the AWS Device SDKs and links to examples of AWS IoT in the supported languages, see Connecting with MQTT using the AWS IoT Device SDKs. For more information about authentication methods and the port mappings for MQTT messages, see プロトコル、ポートマッピング、認証.

While we recommend using the AWS IoT Device SDKs to connect to AWS IoT, they are not required. If you do not use the AWS IoT Device SDKs, however, you must provide the necessary connection and communication security. Clients must send the Server Name Indication (SNI) TLS extension in the connection request. Connection attempts that don't include the SNI are refused. 詳細については、「AWS IoT のトランスポートセキュリティ」を参照してください。Clients that use IAM users and AWS credentials to authenticate clients must provide the correct Signature Version 4 authentication.

Connecting with MQTT using the AWS IoT Device SDKs

This section contains links to the AWS IoT Device SDKs and to the source code of sample programs that illustrate how to connect a device to AWS IoT. The sample apps linked here show how to connect to AWS IoT using the MQTT protocol and MQTT over WSS.

C++

Using the AWS IoT C++ Device SDK to connect devices

• Source code of a sample app that shows an MQTT connection example in C++

• AWS IoT C++ Device SDK v2 on GitHub

Python

Using the AWS IoT Device SDK for Python to connect devices

• Source code of a sample app that shows an MQTT connection example in Python

• AWS IoT Device SDK for Python v2 on GitHub

JavaScript

Using the AWS IoT Device SDK for JavaScript to connect devices

• Source code of a sample app that shows an MQTT connection example in JavaScript

• AWS IoT Device SDK for JavaScript v2 on GitHub

Java

Using the AWS IoT Device SDK for Java to connect devices

• Source code of a sample app that shows an MQTT connection example in Java

• AWS IoT Device SDK for Java v2 on GitHub

Embedded C

Using the AWS IoT Device SDK for Embedded C to connect devices

重要

This SDK is intended for use by experienced embedded-software developers.

• Source code of a sample app that shows an MQTT connection example in Embedded C

• AWS IoT Device SDK for Embedded C on GitHub

MQTT Quality of Service (QoS) options

AWS IoT and the AWS IoT Device SDKs support the MQTT Quality of Service (QoS) levels 0 and 1. The MQTT protocol defines a third level of QoS, level 2, but AWS IoT does not support it. Only the MQTT protocol supports the QoS feature. HTTPS does not support QoS.

This table describes how each QoS level affects messages published to and by the message broker.

With a QoS level of...	The message is...	コメント
QoS level 0	Sent zero or more times	This level should be used for messages that are sent over reliable communication links or that can be missed without a problem.
QoS level 1	Sent at least one time, and then repeatedly until a PUBACK response is received	The message is not considered complete until the sender receives a PUBACK response to indicate successful delivery.

Using MQTT persistent sessions

Persistent sessions store a client’s subscriptions and messages, with a quality of service (QoS) of 1, that have not been acknowledged by the client. When a disconnected device reconnects to a persistent session, the session resumes, its subscriptions are reinstated, and subscribed messages received prior to the reconnection and that have not been acknowledged by the client are sent to the client.

Creating a persistent session

You create an MQTT persistent session by sending a CONNECT message and setting the cleanSession flag to 0. If no session exists for the client sending the CONNECT message, a new persistent session is created. If a session already exists for the client, the client resumes the existing session.

Operations during a persistent session

Clients use the sessionPresent attribute in the connection acknowledged (CONNACK) message to determine if a persistent session is present. If sessionPresent is 1, a persistent session is present and any stored messages for the client are delivered to the client immediately after the client receives the CONNACK, as described in Message traffic after reconnection to a persistent session. If sessionPresent is 1, there is no need for the client to resubscribe. However, if sessionPresent is 0, no persistent session is present and the client must resubscribe to its topic filters.

After the client joins a persistent session, it can publish messages and subscribe to topic filters without any additional flags on each operation.

Message traffic after reconnection to a persistent session

A persistent session represents an ongoing connection between a client and an MQTT message broker. When a client connects to the message broker using a persistent session, the message broker saves all subscriptions that the client makes during the connection. クライアントの接続が切断されると、クライアントがサブスクライブしているトピックにパブリッシュされた未確認の QoS 1 メッセージと新しい QoS 1 メッセージが保存されます。Messages are stored according to account limit, messages that exceed that limit will be dropped. For more information about persistent message limits, see AWS IoT Core endpoints and quotas. When the client reconnects to its persistent session, all subscriptions are reinstated and all stored messages are sent to the client at a maximum rate of 10 messages per second.

After reconnection, the stored messages are sent to the client, at a rate that is limited to 10 stored messages per second, along with any current message traffic until the Publish requests per second per connection limit is reached. Because the delivery rate of the stored messages is limited it will take several seconds to deliver all stored messages if a session has more than 10 stored messages to deliver after reconnection.

Ending a persistent session

The following conditions describe how persistent sessions can end.

• When the persistent session expiration time elapses. The persistent session expiration timer starts when the message broker detects that a client has disconnected, either by the client disconnecting or the connection timing out.

• When the client sends a CONNECT message that sets the cleanSession flag to 1.

注記

The stored messages waiting to be sent to the client when a session ends are discarded; however, they are still billed at the standard messaging rate, even though they could not be sent. For more information about message pricing, see AWS IoT Core Pricing. You can configure the expiration time interval.

Reconnection after a persistent session has expired

If a client doesn't reconnect to its persistent session before it expires, the session ends and its stored messages are discarded. When a client reconnects after the session has expired with a cleanSession flag to 0, the service creates a new persistent session. Any subscriptions or messages from the previous session are not available to this session because they were discarded when the previous session expired.

Persistent session message charges

Messages are charged to your AWS account when the message broker sends a message to a client or an offline persistent session. When an offline device with a persistent session reconnects and resumes its session, the stored messages are delivered to the device and charged to your account again. For more information about message pricing, see AWS IoT Core pricing - Messaging.

The default persistent session expiration time of one hour can be increased by using the standard limit increase process. Note that increasing the session expiration time might increase your message charges because the additional time could allow for more messages to be stored for the offline device and those additional messages would be charged to your account at the standard messaging rate. The session expiration time is approximate and a session could persist for up to 30 minutes longer than the account limit; however, a session will not be shorter than the account limit. For more information about session limits, see AWS Service Quotas.

Using connectAttributes

ConnectAttributes allow you to specify what attributes you want to use in your connect message in your IAM policies such as PersistentConnect and LastWill. With ConnectAttributes, you can build policies that don't give devices access to new features by default, which can be helpful if a device is compromised.

connectAttributes でサポートされる機能は以下のとおりです。

PersistentConnect

Use the PersistentConnect feature to save all subscriptions the client makes during the connection when the connection between the client and broker is interrupted.

LastWill

Use the LastWill feature to publish a message to the LastWillTopic when a client unexpectedly disconnects.

By default, your policy has non-persistent connection and there are no attributes passed for this connection. You must specify a persistent connection in your IAM policy if you want to have one.

For ConnectAttributes examples, see Connect Policy Examples.

AWS IoT differences from MQTT version 3.1.1 specification

The message broker implementation is based on the MQTT v3.1.1 specification, but it differs from the specification in these ways:

• AWS IoT supports MQTT quality of service (QoS) levels 0 and 1 only. AWS IoT doesn't support publishing or subscribing with QoS level 2. When QoS level 2 is requested, the message broker doesn't send a PUBACK or SUBACK.

• AWS IoT では、QoS レベル 0 でトピックにサブスクライブすると、メッセージが 0 回以上配信されます。メッセージは複数回配信される場合があります。複数回配信されるメッセージは、異なるパケット ID を使用して送信される場合があります。これらの場合、DUP フラグは設定されません。

• 接続リクエストに応答するとき、メッセージブローカーは CONNACK メッセージを送信します。このメッセージには、接続で前のセッションを再開するかどうかを示すフラグが含まれます。

• クライアントがトピックにサブスクライブすると、メッセージブローカーは SUBACK を送信してから、クライアントが新しい一致するメッセージの受信を開始するまでに、遅延が生じる場合があります。

• The MQTT specification provides a provision for the publisher to request that the broker retain the last message sent to a topic and send it to all future topic subscribers. AWS IoT doesn't support retained messages. ブローカーがメッセージを保持するようにリクエストされた場合、接続は切断されます。

• メッセージブローカーは、クライアント ID を使用して、各クライアントを識別します。クライアント ID は MQTT ペイロードの一部としてクライアントからメッセージブローカーに渡されます。Two clients with the same client ID can't be connected concurrently to the message broker. あるクライアントが別のクライアントのクライアント ID を使用してメッセージブローカーに接続すると、新しいクライアント接続が受け入れられ、以前に接続されたクライアントは切断されます。

• まれに、メッセージブローカーは、パケット ID が異なる同じ論理 PUBLISH メッセージを再送信する場合があります。

• The message broker doesn't guarantee the order in which messages and ACK are received.