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    IoT Platform:MQTT 5.0 device-cloud interaction enhancements

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    IoT Platform:MQTT 5.0 device-cloud interaction enhancements

    Last Updated:Jun 08, 2026

    IoT Platform supports MQTT 5.0 features that improve transparency and interoperability between devices and the server, including connection feedback, server-sent DISCONNECT packets, and enhanced error codes.

    Overview

    MQTT 5.0 enhances the following device-cloud interactions:

    Feedback on connection establishment

    Connection feedback is a negotiation mechanism. The device sends connection parameters during connection, and the server returns a CONNACK packet that confirms the supported features and final parameter values.

    CONNACK property

    Description and value range

    Response from IoT Platform

    Retain Available

    • 1: Retained messages are supported.

    • 0: Retained messages are not supported.

    1

    Wildcard Subscription Available

    • 1: Wildcard Subscriptions are supported.

    • 0: Wildcard Subscriptions are not supported.

    1

    Subscription Identifiers Available

    • 1: Subscription Identifiers are supported.

    • 0: Subscription Identifiers are not supported.

    0

    Shared Subscription Available

    • 1: Shared Subscriptions are supported.

    • 0: Shared Subscriptions are not supported.

    1

    Topic Alias

    • 1: The server accepts Topic Aliases on this connection.

    • 0: The server does not accept Topic Aliases on this connection.

    1

    Maximum QoS

    • 2: QoS 2.

    • 1: QoS 1.

    • 0: QoS 0.

    1

    Server Keep Alive

    The keep-alive period during which the server must send heartbeat or data packets. If no packet is sent within this period, the connection is disrupted.

    Valid values: 30 to 1200.

    Returns the keep-alive period specified by the device.

    If the device-specified value exceeds the valid range, the connection is disrupted.

    Session Expiry Interval

    The session retention period after the connection is disrupted.

    Valid values: 0 to 0xFFFFFFFF.

    Returns the session retention period specified by the device.

    Default: 0 (if not specified by the device).

    Maximum Packet Size

    Valid values: 0 to 0xFFFFFFFF.

    Always returns 262144 (256 KB).

    Topic Alias Maximum

    Valid values: 0 to 0xFFFFFFFF.

    Always returns 20.

    Server-sent DISCONNECT packets

    In MQTT 3.1.1, the server closes a device connection without explanation when a rule is violated.

    In MQTT 5.0, the server can send a DISCONNECT packet to the device before closing the connection. This packet contains a reason code and reason string, which simplifies troubleshooting.

    Enhancements in error codes

    Enhancement

    MQTT 3.1

    MQTT 5.0

    Quantity increase

    5 error codes

    20 error codes

    Reason string

    Not supported

    Reason String property added for detailed error diagnostics.

    Error codes for more types of packets

    Only CONNECT packets return error codes.

    Error codes returned for CONNECT, UNSUBACK, PUBACK, DISCONNECT, and other packets.

    Error codes

    Decimal

    Hexadecimal

    Name

    Packet type

    0

    0x00

    Success

    CONNACK, PUBACK, PUBREC, PUBREL, PUBCOMP, UNSUBACK, and AUTH

    128

    0x80

    Unspecified error

    CONNACK, PUBACK, PUBREC, SUBACK, UNSUBACK, and DISCONNECT

    129

    0x81

    Malformed Packet

    CONNACK and DISCONNECT

    130

    0x82

    Protocol Error

    CONNACK and DISCONNECT

    132

    0x84

    Unsupported Protocol Version

    CONNACK

    136

    0x88

    Server unavailable

    CONNACK

    137

    0x89

    Server busy

    CONNACK and DISCONNECT

    138

    0x8A

    Banned

    CONNACK

    140

    0x8C

    Bad authentication method

    CONNACK and DISCONNECT

    141

    0x8D

    Keep Alive timeout

    DISCONNECT

    144

    0x90

    Topic Name invalid

    CONNACK, PUBACK, PUBREC, and DISCONNECT

    147

    0x93

    Receive Maximum exceeded

    DISCONNECT

    148

    0x94

    Topic Alias invalid

    DISCONNECT

    149

    0x95

    Packet too large

    CONNACK and DISCONNECT

    150

    0x96

    Message rate too high

    DISCONNECT

    151

    0x97

    Quota exceeded

    CONNACK, PUBACK, PUBREC, SUBACK, and DISCONNECT

    152

    0x98

    Administrative action

    DISCONNECT

    153

    0x99

    Payload format invalid

    PUBACK, PUBREC, and DISCONNECT

    154

    0x9A

    Retain not supported

    CONNACK and DISCONNECT

    155

    0x9B

    QoS not supported

    CONNACK and DISCONNECT

    156

    0x9C

    Use another server

    CONNACK and DISCONNECT

    157

    0x9D

    Server moved

    CONNACK and DISCONNECT

    158

    0x9E

    Shared Subscription not supported

    SUBACK and DISCONNECT

    159

    0x9F

    Connection rate exceeded

    CONNACK and DISCONNECT