CoAP Interview Questions and Answers for IoT Engineers

This page covers top CoAP interview questions and answers to help interviewees pass job interviews for CoAP engineer positions. These CoAP questions are written by specialists in the CoAP IoT (Internet of Things) domain.

Question -1: What is CoAP ?

Answer -1: CoAP stands for Constrained Application Protocol. It’s a web protocol developed for constrained nodes or networks, e.g., WSN, M2M, IoT, etc., with lower power and memory requirements. It is defined in RFC 7252. The GET, POST, PUT, and DELETE methods are used by CoAP.

Question -2: What are the benefits or advantages of CoAP ?

Answer -2: Advantages of CoAP are as follows:

• Less overhead due to operation over UDP.
• It offers secure communication using IPSec or DTLS.
• It offers lower latency than HTTP.
• It consumes less power than HTTP.
• It uses ACK like HTTP and avoids unnecessary retransmissions.
• It does not require synchronous communication.
• It offers a long battery life due to short wake-up states and long sleepy states.

Question -3: What is the difference between CoAP and MQTT ?

Answer -3: The following table mentions the difference between CoAP and MQTT.

Question -4: What is the difference between CoAP and HTTP ?

Answer -4: The following table mentions the difference between CoAP and HTTP.

Question -5: Explain the CoAP message format and mention the size of the header used in the CoAP message.

Answer -5: A CoAP message contains a 4-byte fixed-size header with contents as mentioned in the table below.

Question -6: Does CoAP use TCP or UDP ?

Answer -6: It lies between UDP and the application layer.

Question -7: Which security protocols are used by CoAP ?

Answer -7: CoAP uses DTLS-based PSK, RPK, and certificate security protocols.

Question -8: Which model is used by CoAP client and CoAP server ?

Answer -8: CoAP uses the Request/Response model like HTTP. The CoAP endpoint (i.e., “client”) sends one or more CoAP requests to the “server,” which services the requests by sending CoAP responses. Unlike HTTP, requests and responses are not sent over a previously established connection but exchanged asynchronously over CoAP messages.

Question -9: Explain the CoAP architecture and its elements.

Answer -9: The figure depicts the CoAP Architecture. As shown, it extends normal HTTP clients to clients having resource constraints. These clients are known as CoAP clients. A proxy device bridges the gap between constrained environments and typical internet environments based on HTTP protocols. The same server takes care of both HTTP and CoAP protocol messages.

Question -10: What are the drawbacks or limitations or disadvantages of CoAP protocol ?

Answer -10: The disadvantages of CoAP are as follows:

• It is unreliable due to the use of the UDP protocol.
• It acknowledges each receipt of the message and increases processing time, but it does not verify the decoding of the message at the other end.
• CoAP faces communication issues for devices behind NAT.
• It is an unencrypted protocol like MQTT and uses DTLS to provide security at implementation cost.

This set of CoAP interview questions and answers is useful for freshers and experienced-level job positions.