# Routing Protocols for Mobile Internet of Things (IoT): A Survey on Challenges and Solutions
## 4 Solutions to the Issues Faced by RPL during Mobility
### Miscellaneous Solutions
#### 4.5.1 Sink-to-Sink Coordination Framework (SSCF)  
The authors [56] proposed a Sink-to-Sink Coordination Framework (SSCF) in which multiple sinks are considered in the network. Therefore, a single DAG consists of multiple  
\begin{table}
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**Category** & **Working Principle** & **Issues Addressed** & **Advantages** & **Disadvantages** \\ \hline \multirow{6}{*}{KP-RPL} & Kalman filter takes into account the velocity estimates by enhancing mobile node position accuracies resulting in better routing decisions. & \multirow{6}{*}{Lack of positioning information. KP-RPL without Kalman filtering.} & Robust to channel conditions, and reliable routing, which increases the network reliability by 25\% as compared to KP-RPL without Kalman filtering. & Routing among the mobile nodes alone and how it will affect the positioning information is not discussed. \\ \hline \multirow{6}{*}{MP-RPL [67]} & MP-RPL uses the SNSC model to detect node movements and constructs DODAG by selecting the least mobile nodes as Preferred Parents to ensure route stability. & \multirow{6}{*}{Lack of positioning information. FPD remains unchanged even if the node number increases.} & \multirow{6}{*}{This approach provides scalability as the gap such as energy consumption, throughput, and PDR are not discussed.} \\ \cline{1-1} \cline{3-3}  & & & & \\ \hline \hline \end{tabular}
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Table 4: Summary of Position-based Solutions.  
DODAG and each DODAG has a different sink ID that is included in the control messages. A node receiving DIO messages from sub-DODAG is said to be _border_ node.  
In this framework, _border_ nodes receive and process the Network Optimization Metric (NOM) values used for coordination among sink nodes in the DIO message. Network Coordination Interval (NCI) is included in this framework in which every border node updates its sink node about the NOM value of its neighboring sub-DODAG. NOMs information is saved in the parent table of the _border_ nodes. Sub-DODAG ID and NOM values are sent in DAO messages by the _border_ node to its respective sink node. The sink node checks the version number to detect the latest DAO sent by the _border_ node. In case, _border_ router does not receive any DIO message from its neighboring sub-DODAG in three consecutive DIO intervals, the sub-DODAG is removed from its parent table and informs the sink node about the unavailability of sub-DODAG. Sink nodes are required to save NOM values of neighboring DODAGs and this information is saved in Network Optimization Metric Table (NOMT). Similarly, the 'Stale' timer is used i.e., defining maximum time after which the entry is considered as out of date and is equal to three DIO intervals.  
The simulation results show that data in RPL is sent by event reporting nodes to their nearest sink nodes, packet drop is seen as the paths getting congested as traffic converges at nodes near the sink. However, the event traffic is distributed among both sinks in SCCF by forming different paths with lesser congestion and lower traffic loads. Hence, throughput observed at SCCF is higher than the basic RPL by approximately 2.0 Kbps for packet intervals of 0.5 s, 1 s, 2 s, 4 s, 6 s, and 8 s. The packet drop ratio of RPL is observed as more than SSCF since the congestion is more severe in basic RPL resulting in high packet drops. The latency of both the protocols are high as nodes spent most of the time in sleep mode as compared to active mode. Energy consumption is slightly higher in SCCF as, we note that, RPL does not drop many packets if the packet interval is high and the data rate is low, therefore, energy consumption per bit is low in lower packet generation intervals.