# Enhancements and Challenges in CoAP--A Survey
## 7 Conclusions  
The CoAP developed for the lightweight communication in IoT domains for the constrained devices has been applied in many fields ranging from home automation to disaster and health management systems. The use of CoAP in more applications has exhibited the shortcomings in CoAP architecture such as inefficient congestion control mechanism, poor interoperability with other devices, etc. This led to many enhancements in the basic CoAP according to specific applications; for example, many advance congestion control mechanisms were developed over time to improve the issues in the default congestion control mechanism. Similarly, enhancements in other operations such as security, interoperability, etc. were also made. This survey paper presents some applications where CoAP is implemented in various domains and covers in detail the enhancements in congestion control mechanism as well as some application specific enhancements over time. A tabular comparison is provided for the congestion control mechanisms discussed in the survey. In addition, the survey also provides a qualitative and quantitative analysis of the enhanced congestion control mechanisms of CoAP. the qualitative and quantitative analysis can help the researchers to identify when it is better to use one scheme or another, depending on the application requirements. After discussing the enhancements in the basic architecture of CoAP, the survey discusses the open research challenges in CoAP implementation in the fields of security, interoperability, resource discovery, energy efficiency and congestion control. The current architecture of CoAP may perform inefficiently in the future as the number of internet devices are continuously increasing. Therefore, the highlighted research gaps provide insight to future research directions in order to enable CoAP for performing better in the dense network scenarios.  
Conceptualization, M.A.T., D.K. and M.T.R.K; methodology, M.A.T.; software, M.A.T., validation, M.A.T., M.K. and D.K.; formal analysis, M.A.T. and M.K.; investigation, M.A.T. and M.T.R.K.; resources, D.K.; data curation, M.A.T.; writing--original draft preparation, M.A.T.; writing--review and editing, M.A.T., visualization, M.T.R.K; supervision, D.K.; All authors have read and agreed to the published version of the manuscript.  
The Korea Research Fellowship Program funded by the Ministry of Science and ICT through the National Research Foundation of Korea under Grant 2019H1D3A1A01102987.  
This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (2016R1D1A3B01015510).  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} IETF & Internet of Engineering Task \\ CoAP & Constrained Application Protocol \\ IoT & Internet of Things \\ IoUT & Internet of Underwater Things \\ IoE & Internet of Everything \\ WSN & Wireless Sensor Networks \\ REST & Representational State Transfer \\ UDP & User Datagram Protocol \\ TCP & Transmission Control Protocol \\ BEB & Binary Exponential Backoff \\ CoCoA & Congestion Control/ Advance \\ RTT & Round Trip Time \\ CON & Confirmable \\ NON & Non-confirmable \\ RTO & Retransmission Timeout \\ DTLS & Datagram Transport Layer Security \\ MQTT & Message Queuing Telemetry Transport \\ OSF & Open Smart Grid Protocol \\ \end{tabular}