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CoAP Interview questions and answers | IoT

This page covers top CoAP interview questions and answers will help interviewee pass the job interview for CoAP engineer job position. This CoAP questions are written by specialists in CoAP IoT (Internet of Things) domain.

Question -1: What is CoAP ?
Answer -1: It stands for Constrained Application Protocol. It is a web protocol developed for constrained nodes or networks e.g. WSN, M2M, IoT etc. having less power and less 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 avoid unnecessary retransmissions.
• It does not require synchronous communication.
• It offers 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: Following table mentions difference between CoAP and MQTT.

difference between CoAP and MQTT

Question -4: What is the difference between CoAP and HTTP ?
Answer -4: Following table mentions difference between CoAP and HTTP.

difference between CoAP and HTTP

Question -5: Explain CoAP message format and mention size of header used in the CoAP message.
Answer -5: CoAP message contans 4 bytes fixed size header with contents as mentioned in the below table.

CoAP message format

Question -6: Is CoAP use TCP or UDP ?
Answer -6: It lies between UDP and 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 Request/Response model as HTTP. The CoAP end-point ( i.e. "client" ) sends one or more CoAP requests to "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.

CoAP message exchange

Question -9: Explain CoAP architecture and its element.
Answer -9: The figure depicts CoAP Architecture. As shown it extends normal HTTP clients to clients having resource constraints. These clients are known as CoAP clients. Proxy device bridges gap between constained environment and typical internet environment based on HTTP protocols. Same server takes care of both HTTP and CoAP protocol messages.

CoAP Architecture

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

This set of CoAP interview questions and answers are useful for freshers and experienced level of job positions. Refer following links on CoAP for more information.

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