CoAP request-response model in IOT Protocols - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CoAP stand for and what is its main use?

CoAP stands for Constrained Application Protocol. It is mainly used for simple devices in IoT to communicate over the internet using a lightweight request-response model.

intermediate

In CoAP, what are the four types of messages used in the request-response model?

The four types of CoAP messages are: Confirmable (CON), Non-confirmable (NON), Acknowledgement (ACK), and Reset (RST).

beginner

What is a Confirmable (CON) message in CoAP?

A Confirmable (CON) message requires an acknowledgement (ACK) from the receiver to confirm the message was received.

intermediate

How does CoAP handle reliability in its request-response model?

CoAP uses Confirmable messages that require acknowledgements to ensure reliability. If no ACK is received, the message is retransmitted.

intermediate

What is the difference between a Non-confirmable (NON) and a Confirmable (CON) message in CoAP?

A Non-confirmable (NON) message does not require an acknowledgement and is used for messages where reliability is not critical, unlike Confirmable (CON) messages which require ACK.

Which CoAP message type requires an acknowledgement?

A Confirmable (CON)

B Non-confirmable (NON)

C Reset (RST)

D Empty message

Confirmable (CON) messages require an acknowledgement (ACK) to confirm receipt.

What happens if a Confirmable message is not acknowledged in CoAP?

A The connection is closed

B The message is ignored

C The message is retransmitted

D A Reset message is sent

If no ACK is received, CoAP retransmits the Confirmable message to ensure delivery.

Which CoAP message type is used when reliability is not critical?

A Non-confirmable (NON)

B Confirmable (CON)

C Acknowledgement (ACK)

D Reset (RST)

Non-confirmable (NON) messages do not require acknowledgement and are used when reliability is less important.

What is the purpose of an Acknowledgement (ACK) message in CoAP?

A To request data

B To send an error

C To reset the connection

D To confirm receipt of a Confirmable message

ACK messages confirm that a Confirmable message was received successfully.

Which CoAP message type indicates an error or rejection?

A Non-confirmable (NON)

B Reset (RST)

C Confirmable (CON)

D Acknowledgement (ACK)

Reset (RST) messages indicate that a message was received but could not be processed.

Explain the CoAP request-response model and how it ensures message delivery.

Think about how CoAP uses message types to confirm delivery or send without confirmation.

Describe the difference between Confirmable and Non-confirmable messages in CoAP.

Consider when you want to be sure a message arrives versus when speed is more important.

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