CONNACK Acknowledge connection request

The CONNACK message is the message sent by the broker in response to a CONNECT request from a client.

Fixed header

The fixed header format is shown in the table below.

bit 7 6 5 4 3 2 1 0
byte 1 Message type (2) DUP flag QoS flags RETAIN
  0 0 1 0 x x x x
byte 2 Remaining Length (2)
  0 0 0 0 0 0 1 0

The DUP, QoS and RETAIN flags are not used in the CONNACK message.

Variable header

The variable header format is shown in the table below.

  Description 7 6 5 4 3 2 1 0
Topic Name Compression Response
byte 1 Reserved values. Not used. x x x x x x x x
Connect Return Code
byte 2 Return Code                

The values for the one byte unsigned Connect return code field are shown in the table below.

Enumeration HEX Meaning
0 0x00 Connection Accepted
1 0x01 Connection Refused: unacceptable protocol version
2 0x02 Connection Refused: identifier rejected
3 0x03 Connection Refused: broker unavailable
4-255   Reserved for future use

Return code 2 (identifier rejected) is sent if the unique client identifier is not between 1 and 23 characters in length.

Payload

There is no payload.

Related concepts
WebSphere MQ Telemetry Transport

Related reference
WebSphere MQ Telemetry Transport fixed header
WebSphere MQ Telemetry Transport variable header
CONNECT Client requests a connection to a broker