CoAP request-response model in IOT Protocols - Step-by-Step Execution

Process Flow - CoAP request-response model

Client sends CoAP Request

↓

Server receives Request

↓

Server processes Request

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Server sends CoAP Response

↓

Client receives Response

↓

Client processes Response

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END

This flow shows how a CoAP client sends a request to a server, the server processes it, sends back a response, and the client processes the response.

Execution Sample

IOT Protocols

Client -> Server: GET /sensors/temp
Server -> Client: 2.05 Content "22.5 C"

Client requests temperature data; server responds with current temperature.

Process Table

Step	Action	Message Type	Code	Payload	Result
1	Client sends request	Confirmable (CON)	GET	/sensors/temp	Request sent to server
2	Server receives request	Confirmable (CON)	GET	/sensors/temp	Request received
3	Server processes request	-	-	-	Reads temperature sensor
4	Server sends response	Acknowledgement (ACK)	2.05 Content	"22.5 C"	Response sent to client
5	Client receives response	Acknowledgement (ACK)	2.05 Content	"22.5 C"	Response received and processed
6	END	-	-	-	Request-response cycle complete

💡 Response received by client, completing the CoAP request-response cycle

Status Tracker

Variable	Start	After Step 1	After Step 4	Final
Request Message	None	GET /sensors/temp (CON)	GET /sensors/temp (CON)	None (sent)
Response Message	None	None	2.05 Content "22.5 C" (ACK)	2.05 Content "22.5 C" (ACK) received

Key Moments - 2 Insights

Why does the client send a Confirmable (CON) message instead of a Non-confirmable (NON)?

Because the client wants to ensure the server receives the request and responds, as shown in Step 1 and Step 4 where CON and ACK messages are exchanged.

What does the code '2.05 Content' mean in the response?

It means the server successfully processed the GET request and is returning the requested content, as seen in Step 4 and Step 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the message type of the client's request at Step 1?

A Confirmable (CON)

B Non-confirmable (NON)

C Acknowledgement (ACK)

D Reset (RST)

💡 Hint

Check the 'Message Type' column at Step 1 in the execution table.

At which step does the server send the response back to the client?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the 'Server sends response' action in the execution table.

If the client used a Non-confirmable (NON) message instead of Confirmable (CON), what would change in the execution table?

A The server would send an ACK message

B The client would not wait for an ACK response

C The response code would change

D The payload would be empty

💡 Hint

Consider the difference between CON and NON message types in the 'Message Type' column.

Concept Snapshot

CoAP request-response model:
- Client sends a request (usually Confirmable) to server
- Server processes and replies with response (ACK with code)
- Confirmable messages require ACK; Non-confirmable do not
- Response codes like 2.05 mean success with content
- Payload carries requested data (e.g., sensor reading)

Full Transcript

The CoAP request-response model starts with the client sending a Confirmable GET request to the server. The server receives and processes this request, then sends back an Acknowledgement response with a success code and the requested data payload. The client receives and processes the response, completing the cycle. Confirmable messages ensure reliability by requiring acknowledgements. The response code 2.05 indicates successful content delivery. This simple exchange allows constrained devices to communicate efficiently.

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