FAQs
Last updated:2026-01-30 15:02:26
What Is MQTT?
MQTT is a lightweight publish/subscribe protocol designed to minimize network bandwidth and device resource requirements. MQTT is commonly used in scenarios such as vehicles, logistics, smart homes, consumer electronics, and transportation. For more scenarios, see use cases. The OASIS organization provides three standard protocol specifications: v3.1, v3.1.1, and v5.
Differences between TDMQ for MQTT and Standards
TDMQ-MQTT is developed based on the two standard protocols MQTTv3.1.1 and MQTTv5, but some features are not yet supported:
1. TDMQ-MQTT supports QoS Publish and Subscribe, but cannot guarantee Exactly Once semantics in extreme scenarios.
2. TDMQ-MQTT does not support AUTH control messages temporarily.
3. TDMQ-MQTT does not support Server Redirect.
4. In extreme scenarios, Message Ordering cannot be guaranteed.
How Does the Server Re-Delivery Mechanism Work When a Client Fails to Acknowledge Message Consumption in Time While Subscribing Via QoS1?
When using the v3.1 or v3.1.1 protocol to access TDMQ-MQTT, the default transmission window is 16, meaning up to 16 messages can be in-flight. Messages enter the window and start timing. If corresponding PUBACK or PUBREC messages are not received within 120 seconds, the server will retransmit the messages, with the retransmitted messages having the corresponding DUP bit set.
When using the v5 standard protocol for access, the window size is controlled by Receive Maximum. The retransmission interval is also 120 seconds.
Support Which TLS Versions?
TDMQ supports SSLv1, TLSv1.1, TLSv1.2, and TLSv1.3. National Cryptography Support includes TLCPv1.0. SSLv3 can be consulted via submit a ticket for specific details.
IoV Scenarios Typically Use One Vehicle, One Certificate. Is This Supported?
TDMQ-MQTT product supports the "one device, one certificate" usage scenario and can reuse user PKI facilities. The product also supports activating, deactivating, and revoking device certificates through TencentCloud API to achieve the objective of the OCSP protocol specification.
Low-Computation Scenarios Cannot Use "One Device, One Certificate". Does It Support Independent Authentication and Authorization for Each Device?
TDMQ-MQTT product supports the "one device, one key" scenario. Users can register each low-computation device in the cloud via Shared Access Token, and authenticate by issuing tokens with pre-shared keys. Users can also integrate existing JWT, HTTP Service, etc., to achieve independent authentication and authorization for each device.
How Is TDMQ for MQTT Billed?
Enter the Purchase Page from the console, then select the corresponding specifications based on your actual business volume level. MQTT supports both hourly postpaid and prepaid monthly subscription payment modes.
Whether TDMQ for MQTT Supports Public Network Access?
Support. When purchasing a TDMQ MQTT edition cluster, you can enable public network access. After purchasing the cluster, you can enable or disable public network connection at any time.
Usage Limitations of TDMQ for MQTT
Tencent Cloud MQTT is sold in different specifications. Different cluster specifications support different TPS levels and number of client connections.
Except for specification limits, each MQTT Session supports a maximum of 30 subscriptions; if the quota is exceeded, Subscription requests will fail, SUBACK return code is 0x80.
Supported Authentication and Authorization Modes for Send/Receive Messages Using TDMQ for MQTT?
Authentication and authorization using username and password: TDMQ MQTT validates the username and password of the role used by the connected client. Different roles have different permissions (such as sending or consuming messages), which you can freely and flexibly define.
X.509 certificate authentication: Enable server-side verification using the default certificate. You can also use your self-owned certificate for mutual authentication between client and server.
Custom JWT Authentication and Authorization: Supports the use of customers' own JWT services for authentication and authorization.
Subsequently, TDMQ MQTT will combine customer needs to launch more diverse authentication and authorization methods.
How to Calculate the TPS Specification of TDMQ for MQTT?
When using the MQTT protocol to send/receive messages, the settings of QoS and cleanSession parameters impact the calculation of TPS specification. The sending/receiving of each message and client online/offline actions are billed at a basic unit of 1, multiplied by the corresponding multiplication rate based on specific transmission quality and settings. The message count and multiplication rates are as follows:
QoS=0 and cleanSession=true, the conversion multiplier is 1;
QoS=0 and cleanSession=false, the conversion multiplier is 1;
QoS=1 and cleanSession=true, the conversion multiplier is 2;
QoS=1 and cleanSession=false, the conversion multiplier is 5;
QoS=2 and cleanSession=true, the conversion multiplier is 5;
QoS=2 and cleanSession=false, the conversion multiplier is 5;
Messages containing Retain Flag and Will Flag have a conversion multiplier of 10.
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