# Efficient Route Management Method for Mobile Nodes in 6 TiSCH Network
## 2 Background
### Rpl  
The routing protocol for low-power and lossy networks (IETF) [6] corresponds to a layer 3 protocol in a 6 TiSCH network stack. RPL forms a DODAG in a wireless sensor network environment and has a tree-type topology in which a root node acts as a gateway by collecting sensors from each node as a single destination. RPL uses rank to prevent network cycles or loops. Rank represents the relative distance between each node and the root node, and rank calculation depends on the objective function (OF). OF used as a standard in RPL is OF0, and rank is determined based on the expected transmission count (ETX) and received signal strength indication (RSSI). ETX is the estimated number of transmissions and is a measure of the quality of a link between two nodes. The method for calculating this is shown in Equation (1). Slots can be shared or used as dedicated slots for a single node. Shared slots follow the CSMA/CA method. Dedicated slots are formed through a schedule capable of performing one-way communication through negotiation between two nodes. For example, MAC layer broadcast packets are sent using shared slots, and unicast packets are sent through dedicated slots. \[ \begin{array}{l}\text{Rank(N)}=\text{Rank(P)}+\text{Rank\_increase}\\ \text{Rank\_increase}=\text{(Step)}\times\text{MinHopRank}\end{array}  \tag{1}\]  
The root node that forms a network and relays each node has a rank value of 0. The child nodes of the root node follow Equation (1). Rank(N) is the rank of the current node, and Rank(P) is the rank of the preferred parent node. Rank_increase consists of various parameters. Figure 1: 6 TiSCH stack structure. Step calculates a value from the properties between the parent node, child node, and neighbor. MinHopRank is an index representing the distance between ranks, and the calculation method varies according to the OF. To form and maintain the topology, the RPL utilizes control messages that are periodically broadcast. The nodes participating in the network to form the topology propagate the DODAG information solicitation (DIS) message to the RPL network. This is a request message requesting a DODAG information object (DIO) message. The node receiving the DIS message broadcasts the DIO. The sensor node receiving the DIO message calculates the OF, selects the node with the lowest rank value as its preferred parent node, and forms a DODAG. The node that calculates the rank sends the information by putting information in the rank item in the DIO message field and propagates its information to neighboring nodes within the communication distance. The preferred parent node is selected, and the node that forms the DODAG transmits the destination advertisement object (DAO) message from the fixed cell of the TSCH network to the root node, which is the routing table based on the data stored in the DAO message. It stores the address information and forms a routing path. A downlink path can be formed by referring to the DAO message received from the root node and the formed routing table. Several techniques exist as RPL protocols for mobile nodes. RPL protocols use control messages to form and maintain networks, and there are existing investigations into techniques to improve the trickle timer algorithm, which specifically reduces the broadcast cycle of DIO messages, to avoid conflicts. In [11], the author proposed the trickle-F algorithm. This gives the RPL nodes priority to send DIOs. As a result, the longer the node does not send DIO, the more likely it is to transmit in the next transport frame. However, mobile nodes may not be able to receive control messages in the process of selecting a parent node in an RPL network by simply moving in any way in the network and improving the trickle timer algorithm, which can increase packet loss and energy consumption. Therefore, in [12], the author proposes an algorithm that dynamically adjusts the transmission cycle of messages based on their own moving speed and direction to select the parent node. This suggests mobility and sends request messages in a timely manner.