QoS levels (0, 1, 2) in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding QoS 0 in MQTT

What is the main characteristic of QoS level 0 in MQTT?

A The message is stored on the broker until the subscriber is online.

B The message is delivered at most once, with no confirmation.

C The message is delivered exactly once, using a four-step handshake.

D The message is delivered at least once, with confirmation required.

💡 Hint

Think about the simplest delivery method without retries.

QoS 0 means the message is sent once without any guarantee of delivery or confirmation. It is the fastest but least reliable.

🧠 Conceptual

intermediate

Behavior of QoS 1 in MQTT

Which statement best describes QoS level 1 in MQTT?

A Messages are delivered at least once, with acknowledgment required.

B Messages are delivered exactly once with a two-step handshake.

C Messages are delivered at most once without acknowledgment.

D Messages are queued indefinitely on the broker.

💡 Hint

Consider the delivery guarantee that ensures the message arrives but may duplicate.

QoS 1 ensures messages are delivered at least once by requiring acknowledgment, but duplicates can occur.

🧠 Conceptual

advanced

Exact Delivery Guarantee of QoS 2

What mechanism does QoS level 2 use to ensure messages are delivered exactly once?

A A single acknowledgment from the receiver after message receipt.

B A simple send and forget approach with no handshake.

C A four-step handshake involving message exchange and release confirmation.

D Storing messages on the client until the broker confirms delivery.

💡 Hint

Think about a multi-step process that avoids duplicates.

QoS 2 uses a four-step handshake (PUBLISH, PUBREC, PUBREL, PUBCOMP) to guarantee exactly one delivery.

💻 Command Output

advanced

Output of MQTT QoS 1 Message Flow

Given the following MQTT client log snippet for a QoS 1 message, what is the final status?

IOT Protocols

Client sends PUBLISH message with packet ID 10
Broker responds with PUBACK for packet ID 10
Client logs: 'Message 10 delivered successfully'

A Message delivery failed due to missing PUBREC.

B Message was delivered exactly once using four-step handshake.

C Message was delivered at most once, no confirmation.

D Message was delivered at least once, confirmed by PUBACK.

💡 Hint

Look at the acknowledgment type and packet ID matching.

PUBACK confirms receipt for QoS 1 messages, ensuring at least one delivery.

❓ Troubleshoot

expert

Troubleshooting Duplicate Messages in QoS 1

A subscriber using QoS 1 receives duplicate messages. What is the most likely cause?

A The subscriber is not sending PUBACK, causing the broker to resend messages.

B The broker is using QoS 0, which does not guarantee delivery.

C The subscriber is using QoS 2, which duplicates messages by design.

D The network is dropping PUBREL packets, causing duplicates.

💡 Hint

Consider what happens if acknowledgments are missing in QoS 1.

If PUBACK is not received, the broker resends the message, causing duplicates.