CoAP request-response model in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

CoAP Message Types Purpose

Which CoAP message type is used to confirm that a message was received successfully?

A Reset (RST)

B Non-confirmable (NON)

C Confirmable (CON)

D Acknowledgement (ACK)

💡 Hint

Think about which message type acts as a reply to confirm receipt.

The Acknowledgement (ACK) message is sent to confirm that a Confirmable (CON) message was received successfully.

💻 Command Output

intermediate

CoAP Client Request Output

What is the expected output when a CoAP client sends a Confirmable GET request to a server and receives a successful response?

IOT Protocols

coap-client -m get coap://example.com/resource

A 2.05 Content: <payload data>

B 4.04 Not Found

C 2.01 Created

D Error: Timeout waiting for ACK

💡 Hint

Look for the success response code for GET requests in CoAP.

A successful GET request returns a 2.05 Content response code with the payload.

🔀 Workflow

advanced

CoAP Request-Response Sequence

Arrange the steps in the correct order for a CoAP Confirmable GET request and response exchange.

A 1,2,3,4

B 1,3,2,4

C 2,1,3,4

D 1,2,4,3

💡 Hint

Think about the natural flow of request, processing, response, and receipt.

The client first sends the CON request, the server processes it, then sends an ACK with the response, and finally the client receives and processes the ACK.

❓ Troubleshoot

advanced

CoAP Timeout Cause

A CoAP client sends a Confirmable request but never receives an Acknowledgement. What is the most likely cause?

A Client's network is blocking UDP port 5683

B Client sent a Non-confirmable message instead

C Server did not receive the request or is down

D Server sent a Reset (RST) message instead of ACK

💡 Hint

Consider what happens if the server never replies to a Confirmable message.

If the server does not receive the request or is down, the client will not get an ACK and will timeout.

✅ Best Practice

expert

Choosing CoAP Message Type for Sensor Data

For a battery-powered sensor sending frequent temperature updates where reliability is not critical, which CoAP message type is best to minimize power and network usage?

A Confirmable (CON) to ensure delivery

B Non-confirmable (NON) to reduce overhead

C Acknowledgement (ACK) to confirm receipt

D Reset (RST) to reject unwanted messages

💡 Hint

Think about saving power and network traffic when reliability is less important.

Non-confirmable (NON) messages do not require ACKs, reducing power and network use, suitable for frequent sensor updates where some loss is acceptable.

Practice

1. What is the main purpose of the CoAP request-response model?

easy

A. To create user interfaces for IoT devices

B. To store data permanently on a server

C. To send a request and receive a response between devices

D. To encrypt messages between devices

Solution

1. Step 1: Understand CoAP communication basics

   CoAP is designed to allow devices to send requests and get responses, similar to asking a question and getting an answer.

2. Step 2: Identify the purpose of request-response

   The request-response model is about communication, not storage, encryption, or UI creation.

3. Final Answer:

   To send a request and receive a response between devices -> Option C

4. Quick Check:

   Request-response = send and receive [OK]

Hint: Request-response means ask and answer communication [OK]

Common Mistakes:

• Confusing request-response with data storage
• Thinking CoAP handles encryption by default
• Assuming CoAP creates user interfaces

2. Which CoAP method is used to retrieve data from a device?

easy

A. GET

B. POST

C. DELETE

D. PUT

Solution

1. Step 1: Recall CoAP methods

   CoAP uses methods similar to HTTP: GET, POST, PUT, DELETE.

2. Step 2: Identify method for retrieving data

   GET is used to request and retrieve data from a device.

3. Final Answer:

   GET -> Option A

4. Quick Check:

   GET = retrieve data [OK]

Hint: GET method always fetches data [OK]

Common Mistakes:

• Using POST to retrieve data instead of send
• Confusing PUT with GET
• Thinking DELETE retrieves data

3. Given a CoAP GET request to a sensor, what is the expected response code if the data is successfully retrieved?

medium

A. 2.05 Content

B. 4.04 Not Found

C. 5.00 Internal Server Error

D. 3.01 Moved Permanently

Solution

1. Step 1: Understand CoAP response codes

   CoAP uses numeric codes similar to HTTP. 2.xx codes mean success, 4.xx client error, 5.xx server error.

2. Step 2: Identify success code for GET response

   2.05 Content means the request succeeded and the content is returned.

3. Final Answer:

   2.05 Content -> Option A

4. Quick Check:

   Success code for GET = 2.05 Content [OK]

Hint: 2.xx codes mean success in CoAP [OK]

Common Mistakes:

• Choosing 4.04 which means resource not found
• Confusing 5.00 server error with success
• Thinking 3.01 means success instead of redirect

4. A CoAP client sends a POST request but receives no response. What is the most likely cause?

medium

A. The server does not support POST method

B. The request was sent as non-confirmable and got lost

C. The client used an invalid URI format

D. The server returned a 2.05 Content response

Solution

1. Step 1: Understand CoAP message types

   CoAP supports confirmable (CON) and non-confirmable (NON) messages. NON messages may be lost without retry.

2. Step 2: Analyze no response scenario

   If a POST is sent as NON and network drops it, no response is received.

3. Final Answer:

   The request was sent as non-confirmable and got lost -> Option B

4. Quick Check:

   NON messages can be lost without response [OK]

Hint: Non-confirmable messages may get lost silently [OK]

Common Mistakes:

• Assuming server always supports POST
• Confusing invalid URI with no response
• Thinking 2.05 Content means no response

5. You want to update a device's configuration using CoAP. Which method should you use and what response code indicates success?

hard

A. DELETE method with 2.02 Deleted response

B. POST method with 4.00 Bad Request response

C. GET method with 2.05 Content response

D. PUT method with 2.04 Changed response

Solution

1. Step 1: Identify method for updating data

   PUT is used to update or replace a resource on the device.

2. Step 2: Find success response code for update

   2.04 Changed means the update was successful.

3. Final Answer:

   PUT method with 2.04 Changed response -> Option D

4. Quick Check:

   Update = PUT + 2.04 Changed [OK]

Hint: PUT updates; 2.04 means update success [OK]

Common Mistakes:

• Using GET to update data
• Confusing 4.00 error as success
• Mixing DELETE with update operation