Reason code and ACK - MQTT 5.0 new features

MQTT v3.1.1

MQTT v3.1.1 protocol has only 10 return codes. These return codes can represent little meaning, and the same return code value can have different meanings in different messages.


There are only 6 return codes in the CONNECT packet. Only when the return code of the CONNACK packet sent back by the server is 0, the connection is successfully established.

返回码 描述
0 0x00 Connection accepted Connection accepted
1 0x01 Connection rejected for unsupported protocol version The server does not support the MQTT protocol requested by the client.
2 0x02 Connection rejected for rejected client ID The client ID is the correct UTF-8 string, but is not allowed by the server
3 0x03 Connection rejected for unavailable server Network connection has been established, but MQTT service is unavailable
4 0x04 Connection rejected for damaged username or password The data in the username or password is in the wrong format
5 0x05 Connection rejected for unauthorized Client connection is not authorized
6-255 Reserved for future


There are only 4 return codes in the SUBACK packet.

Value Return code Description
0 0x00 success Maximum granted QoS 0 message
1 0x01 success Maximum granted QoS 1 message
2 0x02 success Maximum granted QoS 2 message
128 0x80 Failure Failure

There are 4 return codes in the SUBACK packet. Except that the return code 0x80 indicates failure, the other return codes indicate that the subscription is successful, and the three values 0, 1, 2 represent the maximum QoS value of the subscribed message.

MQTT v5.0

The MQTT v5.0 protocol renames the return code to a reason code, adding a reason code to indicate more types of errors.

The following table is a list of reason codes that represent the value of the reason code and the control packet containing the reason code:

Decimal Hexadecimal Name Packet
0 0x00 Granted QoS 0 SUBACK
1 0x01 Granted QoS 1 SUBACK
2 0x02 Granted QoS 2 SUBACK
4 0x04 Disconnect with Will Message DISCONNECT
16 0x10 No matching subscribers PUBACK, PUBREC
17 0x11 No subscription existed UNSUBACK
24 0x18 Continue authentication AUTH
25 0x19 Re-authenticate AUTH
129 0x81 Malformed Packet CONNACK, DISCONNECT
130 0x82 Protocol Error CONNACK, DISCONNECT
131 0x83 Implementation specific error CONNACK, PUBACK, PUBREC, SUBACK, UNSUBACK, DISCONNECT
132 0x84 Unsupported Protocol Version CONNACK
133 0x85 Client Identifier not valid CONNACK
134 0x86 Bad User Name or Password CONNACK
136 0x88 Server unavailable CONNACK
137 0x89 Server busy CONNACK, DISCONNECT
138 0x8A Banned CONNACK
139 0x8B Server shutting down DISCONNECT
140 0x8C Bad authentication method CONNACK, DISCONNECT
141 0x8D Keep Alive timeout DISCONNECT
142 0x8E Session taken over DISCONNECT
143 0x8F Topic Filter invalid SUBACK, UNSUBACK, DISCONNECT
144 0x90 Topic Name invalid CONNACK, PUBACK, PUBREC, DISCONNECT
145 0x91 Packet Identifier in use PUBACK, PUBREC, SUBACK, UNSUBACK
146 0x92 Packet Identifier not found PUBREL, PUBCOMP
147 0x93 Receive Maximum exceeded DISCONNECT
148 0x94 Topic Alias invalid DISCONNECT
149 0x95 Packet too large CONNACK, DISCONNECT
150 0x96 Message rate too high DISCONNECT
152 0x98 Administrative action DISCONNECT
153 0x99 Payload format invalid PUBACK, PUBREC, DISCONNECT
154 0x9A Retain not supported CONNACK, DISCONNECT
155 0x9B QoS not supported CONNACK, DISCONNECT
156 0x9C Use another server CONNACK, DISCONNECT
157 0x9D Server moved CONNACK, DISCONNECT
158 0x9E Shared Subscription not supported SUBACK, DISCONNECT
159 0x9F Connection rate exceeded CONNACK, DISCONNECT
160 0xA0 Maximum connect time DISCONNECT
161 0xA1 Subscription Identifiers not supported SUBACK, DISCONNECT
162 0xA2 Wildcard Subscription not supported SUBACK, DISCONNECT

The reason code is a single-byte unsigned value used to indicate the result of the operation. The reason code less than 0x80 indicates that the result of the operation is successful. Under normal circumstances, the reason code value returned by the operation is 0. If the reason code returned is greater than or equal to 0x80, the operation has failed.

The reason codes for CONNACK, PUBACK, PUBREC, PUBREL, PUBCOMP, DISCONNECT and AUTH control packets are stored in the variable header. The SUBACK and UNSUBACK packets contain a list of reason codes in the payload.

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