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.

Practice

1. What does QoS level 0 mean in IoT messaging?

easy

A. The message is stored indefinitely until acknowledged.

B. The message is delivered at least once, with confirmation.

C. The message is delivered exactly once, with a handshake.

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

Solution

1. Step 1: Understand QoS 0 delivery

   QoS 0 means the message is sent once without waiting for any acknowledgment.

2. Step 2: Compare with other QoS levels

   QoS 1 and 2 involve confirmations; QoS 0 does not guarantee delivery.

3. Final Answer:

   The message is delivered at most once, with no confirmation. -> Option D

4. Quick Check:

   QoS 0 = no confirmation, at most once delivery [OK]

Hint: QoS 0 means no delivery guarantee, just send once [OK]

Common Mistakes:

• Confusing QoS 0 with guaranteed delivery
• Thinking QoS 0 waits for acknowledgment
• Mixing QoS 0 with QoS 1 or 2 features

2. Which QoS level requires a four-step handshake to ensure message delivery?

easy

A. QoS 0

B. QoS 2

C. QoS 1

D. none of the above

Solution

1. Step 1: Recall handshake details for QoS levels

   QoS 2 uses a four-step handshake to guarantee exactly once delivery.

2. Step 2: Differentiate from other QoS levels

   QoS 0 has no handshake; QoS 1 uses a simpler acknowledgment.

3. Final Answer:

   QoS 2 -> Option B

4. Quick Check:

   Four-step handshake = QoS 2 [OK]

Hint: Four-step handshake means QoS 2 for exactly once delivery [OK]

Common Mistakes:

• Confusing QoS 1's single acknowledgment with four-step handshake
• Assuming QoS 0 has any handshake
• Choosing 'none of the above' incorrectly

3. Given a device publishes a message with QoS 1, what is the expected behavior?

medium

A. The message is sent at least once, with acknowledgment.

B. The message is sent once without acknowledgment.

C. The message is sent exactly once, with a four-step handshake.

D. The message is queued indefinitely until delivered.

Solution

1. Step 1: Understand QoS 1 delivery

   QoS 1 ensures the message is delivered at least once by requiring acknowledgment.

2. Step 2: Compare with QoS 0 and 2

   QoS 0 sends once without acknowledgment; QoS 2 ensures exactly once with handshake.

3. Final Answer:

   The message is sent at least once, with acknowledgment. -> Option A

4. Quick Check:

   QoS 1 = at least once with ack [OK]

Hint: QoS 1 means message sent at least once, confirmed by ack [OK]

Common Mistakes:

• Mixing QoS 1 with QoS 0's no acknowledgment
• Confusing QoS 1 with QoS 2's exactly once guarantee
• Assuming indefinite queuing for QoS 1

4. A developer sets QoS 2 but notices duplicate messages received. What is the likely cause?

medium

A. The handshake process was interrupted or not completed.

B. QoS 2 does not guarantee exactly once delivery.

C. QoS 0 was actually set by mistake.

D. The broker does not support QoS levels.

Solution

1. Step 1: Understand QoS 2 guarantees

   QoS 2 guarantees exactly once delivery only if handshake completes successfully.

2. Step 2: Analyze duplicate message cause

   If handshake is interrupted, duplicates can occur due to retries.

3. Final Answer:

   The handshake process was interrupted or not completed. -> Option A

4. Quick Check:

   Incomplete handshake causes duplicates in QoS 2 [OK]

Hint: Incomplete handshake causes duplicates in QoS 2 [OK]

Common Mistakes:

• Believing QoS 2 never allows duplicates
• Assuming wrong QoS level was set without checking
• Ignoring broker capabilities

5. An IoT system must send critical sensor data exactly once without loss or duplication. Which QoS level should be used and why?

hard

A. QoS 0, because it is fastest and sufficient for critical data.

B. QoS 1, because it guarantees delivery but may duplicate messages.

C. QoS 2, because it guarantees exactly once delivery with handshake.

D. None, critical data should be sent manually.

Solution

1. Step 1: Identify critical data requirements

   Critical data needs no loss and no duplication, so exactly once delivery is required.

2. Step 2: Match QoS level to requirement

   QoS 2 provides exactly once delivery using a four-step handshake, ensuring no duplicates or loss.

3. Final Answer:

   QoS 2, because it guarantees exactly once delivery with handshake. -> Option C

4. Quick Check:

   Critical data needs QoS 2 for exactly once delivery [OK]

Hint: Critical data needs QoS 2 for no loss or duplicates [OK]

Common Mistakes:

• Choosing QoS 0 for critical data due to speed
• Confusing QoS 1's at least once with exactly once
• Ignoring handshake importance