# Study of Mobility Enhancements for RPL in Convergecast Scenarios
## 4 The RSSI, Rank, and Dynamic+ (RRD+) Mechanism
### Loop Avoidance and Rank Update
CO-RPL also suffers from the same problem. The reason for CO-RPL is due to the fact that not updating C_ID would increase the number of hops during transmission, and hence, increase the delay. In addition, as we only compute delays for received packets, packets generated from lower rank nodes will have a low average end-to-end delay compared to packets generated from higher rank nodes. As with RRD+, higher rank nodes are able to successfully deliver their packets; this makes the average end-to-end delay of RRD+ higher. Figure 4: \(Cont\). Figure 3: Number of dropped packets. (**a**) Number of dropped packets (all nodes are mobile); (**b**) Number of dropped packets (75% of nodes are mobile); (**c**) Number of dropped packets (50% of nodes are mobile); (**d**) Number of dropped packets (25% of nodes are mobile). #### 5.3.4 Number of Control Packets

Figure 5 shows the number of control packets of different RPL variants based on different degrees of mobility. Results show that RRD+ outperforms CO-RPL and ME-RPL in all scenarios. This is mainly due to the fact that ripple control message management helps reduce overhead according to rank updating. Since RRD+ does not have a mechanism to distinguish static nodes and mobile nodes, there is no significant difference in overhead when the degree of mobility changes. CO-RPL periodically broadcasts DIO, therefore there is also no significant difference in overhead for different degrees of mobility. OR-RPL has the lowest overhead cost. This is mainly due to the fact that trickle algorithm, which is used in static networks, helps reduce overhead. ME-RPL suffers from a huge overhead cost when all nodes are mobile and gradually decreases the overhead when number of static nodes increases. This is mainly due to the fact that ME-RPL introduces a dynamic DIS management and ME-RPL will suffer from a huge number of DIS messages when there is more mobile nodes in the network. RT-RPL outperforms RRD+, CO-RPL and ME-RPL. The reason is that the reverse trickle algorithm is a modified trickle algorithm, which can deal with mobility and reduces overhead at the same time. However, it cannot deal with random movement, and thus compared with RRD+ it reduces overhead but does not increase the packet delivery ratio. Figure 4: Average end-to-end delay. (**a**) Average end-to-end delay (all nodes are mobile); (**b**) Average end-to-end delay (75% of nodes are mobile); (**c**) Average end-to-end delay (50% of nodes are mobile); (**d**) Average end-to-end delay (25% of nodes are mobile). Figure 5: _Cont._

## 6 Conclusions

Dealing with mobility in WSNs and LLNs is a challenging task for which a compromise should be found between efficiency and complexity. One of the most dominant routing protocols designed for LLNs, RPL, was an original designed without special support for mobility. This is a major drawback that prevents many applications from using it. In this paper, we proposed a mobility enhancement mechanism called RRD+, designed for convergecast scenarios. RRD+ can easily be applied to the RPL routing protocol. RRD+ monitors RSSI values and updates rank values accordingly in order to avoid loops, and it also dynamically manages the interval of control messages. RRD+ monitors the direction of moving nodes based on the variation of RSSI values. When a node detects that one of its potential next-hops in the parents set is moving away, it will anticipate a link failure and try to use another node from the parents set. This helps nodes update their next-hop choice in a timely manner in mobile scenarios. We compared RRD+ to other existing mobility enhancements for RPL and standard RPL. Simulation results show that the RRD+ mechanism enhances RPL on many levels: successfully delivered packets, packet loss, number of dropped packets, end-to-end delay, and overhead. However, results also show that RRD+ is suited for networks where more than 25% of nodes are mobile nodes. When there are fewer mobile nodes in the network, RRD+ has a limited contribution to network performance.