# Enhancements and Challenges in CoAP--A Survey
## 1 Introduction
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Scheme Name** & **Adopted Mechanism** & **Comm. Type** & **Burst Traffic Support** & **Complexity Level** & **Issues Countered** & **Shortcomings** \\ \hline Default CoAF [2] & Binary Exponential & & & & & & \\ Backoff (BEB) & Reliable & No & Low & Basic Congestion Control & & \\  & & & & & & \\  & & & & & & \\ \hline CoCo-RED [22] & Buffer management using & & & & & & \\  & Revised Random & & & & & & \\  & Early Detection & & & & & \\ \hline CoCoA [6] & RTT measurements + & & & & & & \\  & Variable Backoff & & & & & \\  & & & & & & \\ \hline \multirow{2}{*}{4-state Estimator [23]} & \multirow{2}{*}{Statebased Variable Backoff} & \multirow{2}{*}{Reliable} & \multirow{2}{*}{No} & \multirow{2}{*}{Low} & \multirow{2}{*}{\begin{tabular}{} \end{tabular} } & \multirow{2}{*}{\begin{tabular}{} \end{tabular} } & \multirow{2}{*}{\begin{tabular}{} \end{tabular} } \\  & & & & & & \\ \hline Adaptive Congestion & & & & & & & \\ Control [4] & Transmission count-based & Reliable & No & Low & \begin{tabular}{} \end{tabular} & \
\begin{tabular}{} \end{tabular} &
\begin{tabular}{} \end{tabular} \\ \hline CoCoA+ [7] & Modifications in CoCoA & & & & & & \\  & RTO calculations & & & & & & \\  & & & & & & \\ \hline Improved Adaptive Congestion Control [24] & Packet loss ratio-based & & & & & & \\  & & & & & & \\  & & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline \multirow{3}{*}{CACC [25]} & RTT Estimators + & & & & & & & \\ Retransmission Count based & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline FASOR [26] & RTO Estimators + & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of Congestion Control Mechanisms. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Scheme Name** & **Adopted Mechanism** & **Comm. Type** & **Burst Traffic Support** & **Complexity Level** & **Issues Countered** & **Shortcomings** \\ \hline \multirow{4}{*}{pCoCoA [8]} & Transmission Count + Modifications in RTO Estimation + Estimation + Estimation of Max mean deviation of RTO & \multirow{2}{*}{Reliable} & \multirow{2}{*}{Yes} & \multirow{2}{*}{High} & \multirow{2}{*}{ \begin{tabular}{c} Spurious retransmissions, \\ vanishing BTIVAR due to \\ similar RTT sampling \\ \end{tabular}  } & \multirow{2}{*}{Not included} \\  & Delay Gradient based calculation + probabilistic backoff & & & & & & \\ \cline{1-1} \cline{5-7}  & & Delay Gradient based calculation + probabilistic backoff & & & & & \\ \cline{1-1} \cline{5-7}  & & Delay Gradient based calculation + probabilistic backoff & & & & & \\ \cline{1-1} \cline{5-7}  & & Delay Gradient based calculation + probabilistic backoff & & & & & \\ \cline{1-1} \cline{5-7}  & & Delay Gradient based calculation + probabilistic backoff & & & & & \\ \cline{1-1} \cline{5-7}  & & Delay Gradient based calculation + probabilistic backoff & & & & & \\ \cline{1-1} \cline{5-7}  & & Delay Gradient based calculation + probabilistic backoff & & & & & \\ \hline \multirow{4}{*}{Genetic CoCoA++ [28]} & CoCoA++-based + Genetic algorithm & \multirow{2}{*}{Reliable} & \multirow{2}{*}{No} & \multirow{2}{*}{High} & \multirow{2}{*}{ \begin{tabular}{c} Issues of CoCoA+ including \\ (Accurate transmission) RTO measurement (i) Large \\ changes in RTO estimates \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} RTT observation time is limited, \\ burst traffic is not considered \\ \end{tabular}  } \\ \cline{1-1} \cline{5-7}  & Message Loss Feedback-based [29] & \multirow{2}{*}{Reliable/Unreliable} & \multirow{2}{*}{No} & \multirow{2}{*}{Low} & \multirow{2}{*}{ \begin{tabular}{c} Congestion detection in \\ default CoAP \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} Loss rate of transmission cannot \\ be found if number of CON and \\ bound if number of CON and \\ number as equal \\ \end{tabular}  } \\ \cline{1-1} \cline{5-7}  & Content Freshness based [30] & \multirow{2}{*}{ \begin{tabular}{c} Congestion window \\ size control \\ \end{tabular}