Refer to the exhibit. Which two statements about MQTT messaging are true? (Choose two.)

Question

Exhibit

Options

• AResult code 5 means the connection is accepted; result code 0 means the connection is "refused,
• BRedundant ClientIDs are supported by MQTT brokers.
• CMQTT does not support a "keep alive" functionality because it runs on top of TCP.
• DResult code 0 means the connection is accepted; result code 5 means the connection is "refused,
• EBrokers can send the Last Will and Testament messages on behalf of the client.

• A
  3% (1)
• B
  15% (5)
• C
  9% (3)
• D
  73% (24)

Explanation

D and E are correct. In MQTT's CONNACK packet, result code 0 means the connection is accepted, and result code 5 means "Connection Refused, not authorized" - the reverse of what option A claims, making A a classic trap answer. Regarding E, brokers do store and send the Last Will and Testament (LWT) message on behalf of a client when that client disconnects unexpectedly - this is a core MQTT feature.

Why the distractors are wrong:

• A has the result codes backwards - 0 = accepted, 5 = refused.
• B is false: MQTT brokers enforce unique ClientIDs; if a new client connects with a duplicate ID, the broker typically disconnects the existing client.
• C is false: MQTT does support keep-alive - clients send periodic PINGREQ messages and the broker responds with PINGRESP to maintain the connection, even though TCP provides the transport layer.

Memory tip: Think "zero = go" - result code 0 means you're good to go (accepted), and the broker acts as your posthumous messenger for LWT when you drop off unexpectedly.

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