# A Survey on Congestion Control for RPL-Based Wireless Sensor Networks
## 4 Congestion Control Schemes for RPL
### Schemes Exploiting Path Diversity
#### H-Rpl
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Main Feature** & **Routing Metrics \& Constraints** & **Evaluation Tool** & **Evaluation Metrics** \\ \hline BRPL [31] & Switching smoothly and adaptively between the RPL or with backpressure routing. Considering mobility. & Queue backlogs, ETX & Cooja simulation, FIT IoT-LAB testbed & Packet loss, end-to-end delay, communication overhead \\ \hline [PERSON] et al. [32] & Opportunitiesly forwarding packets over multiple parents satisfying the delay constraint on a per-packet basis & Delay to the sink, deadline constraints, ETX & WSNet/Worldsens & PDR, incurred delay, overhead \\ \hline ORPL [27] & Opportunistic routing protocol supporting upward, downward, and any-to-any & EDC (Estimated Duty Cycled wake-ups) & Indriya testbed experiment & PDR, latency, duty cycle \\ \hline ORPL-LB [33] & Opportunistic forwarding. Adjusting the wake-up interval depending on the traffic load & EDC, average duty cycle & Indriya testbed experiment & PDR, latency, duty cycle, \\ \hline ORPL-DT [34] & Opportunistic forwarding. Improving the reliability of routes by updating the routing metric using both upward and downward traffic & EDC & Real testbed experiment & Uplink/downlink packet reception ratio (PRR), PRR fairness, no. of routing messages \\ \hline LB-RPL [55] & Forwarding data using multiple parent nodes. Setting the DD message transmission timer in proportion to workload & Packet drop probability, buffer utilization counter & N\textgreater{}2 simulation & Packet delivery ratio, packet loss, packet delivery delay delay delay \\ \hline [PERSON] et al. [36] & Equalizing traffic load share between parents of an equal depth from the sink while minimizing transmission cost & Hop count, ETX & Simulation using MATLAB, ONNET++ & Network life time, throughput, collissions, link drops \\ \hline [PERSON] et al. [37] & Splitting traffic between multiple parents to spread traffic load uniformly among energy bottleneck nodes. & ELT (Expected Lifetime) & WSNet & PDR, energy consumption, network lifetime, no. of parent changes \\ \hline [PERSON] [38] & Adopting a cross-layer approach using RSSI and superframe distance. & RSSI, superframe distance, remaining energy, load & WSNet & PDR, Delay, Lifetime \\ \hline M-RPL [22] & Initially establishing a single path but providing temporary multipath routing during period of congestion. & Cooja simulation & Throughput, end-to-end delay, energy consumption \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Protocols forwarding packets via multiple parents. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Main Feature** & **Routing Metrics \& Constraints** & **Evaluation Tool** & **Evaluation Metrics** \\ \hline [PERSON] et al. [39] & Using a bird flocking model & Buffer filling ratio & Cooja simulation & Latency, Packet loss ratio \\ \hline CA-RPL [40] & Using a delay metric calculated considering the duty cycle at the MAC layer. & ETX, minimized delay to the & & \\  & & DQDAG root, & Cooja simulation & Throughput, packet loss ratio, \\  & Splitting traffic over two best parents & no. of received packets & & \\ \hline H-RPL [41] & Allowing heterogeneous, mixed, adaptive & & & \\  & routing types based on available/required & & & \\  & resource. Shifting routing workload from & & & \\  & nodes with less resource to nodes with more & & & \\  & resources. & & & \\  & Load balancing based on queue utilization. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._