# Optimizing DODAG Build with RPL Protocol

Authors: Xin Niu
Journal: Mathematical Problems in Engineering
 (2021)

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## Abstract

In the lossy wireless network routing system based on RPL technology, an improved DODAG construction optimization scheme is proposed for sensor nodes with low-power dissipation in the interference environment. If a node discovers that all paths between it and its neighbors have failed, the node reset action begins. Once the node reset is complete, the DODAG system build process is resumed. This prevents the DODAG root from initiating a global fix by incrementing the DODAG VersionNumer to produce a new version of the DODAG in a disruptive environment. The power loss caused by this global repair operation is avoided. The performance of DODAG in interference environment is enhanced, and the data retransmission rate is reduced.


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## Body

## 1. Introduction

LLNs (low-power lossy networks), which are restricted in routing and connection performance, have the characteristics of high loss rate, relatively low data transmission rate, and unstable transmission performance. LLNs consist of dozens to thousands of routes that support the flow of traffic including point to point (between devices in the LLN), single point to multipoint (from central control point to child devices in the LLN) [1], and multipoint to single point (from LLN devices to a single central control point).With the progress of the times and the rapid development of the Internet of Things, battery-powered sensor devices are gradually connected to the Internet, triggering the rapid development of LLNs. The 6 LoWPAN protocol stack connects the IPv6 Internet well with the nodes of the low-power lossy information network. The RPL routing protocol enables it to connect the nodes of various types of IOT systems. The 6 LoWPAN has become the most widely used protocol standard for the IOT. Signal transfer protocols such as ZigBee and Bluetooth support the RPL routing protocol. Information data quality sensing technology can reduce redundant transmission of invalid data based on the quality of information data collected by sensors of the Internet of Things system and only transmit the relatively valuable information in the sensor network system, which can reduce the energy consumption of the system under the condition of guaranteeing the quality of data transmission [2]. Studying and analyzing RPL routing protocol and building DODAG based on information quality theory are the way to enhance the performance of DODAG in interference environment and reduce the energy consumption of nodes.RPL distance vector routing protocol, as a part of 6 LoWPAN protocol stack, based on IPV6 address, runs in LLNs, 6 LoWPAN adapter layer is in IPV6 protocol network layer, and 6 LoWPAN protocol stack is seamlessly combined in IPV6 protocol. RPL distance vector routing protocol uses Objective Function (target function, OF) and measure set to construct Oriented Directed Acyclic Graph for data flow to destination [3], DODAG (directed acyclic graph for destination), and its optimal path algorithm is calculated by OF constraint condition of target function and the set of function using measure. OF: routing metrics for DODAG systems based on the RPL distance vector routing protocol, target Target optimization, and how the related function Function is used to calculate Rank are defined by the target function OF. The OF defines the method of route measurement, the algorithm of Target optimization, and the process and method of calculating Rank by using the function. The metric algorithm is used to find out which path costs the least when the route is calculated and selected, and it is used as the standard of route selection. In a wired or wireless network, the “cost” is related to bandwidth, delay, hop number, and other factors, while in a network using RPL distance vector routing protocol, the “cost” is related to LQI link quality. Metrics are algorithms that Rank computes. OF is also, in some ways, a function of a certain type, such as Cost = OF (Metrics1, Metrics2, …), so that the metric Metrics can be calculated on that basis. Many factors must be considered in routing. In routing algorithm, we can only use a certain value and use the objective function of OF to calculate a lot of factors synthetically to get a certain value.RPL is short for IPv6 Routing Protocol for Low-Power and Lossy Networks. LLN is a kind of network in which both internal links and routers are limited. The processor function, memory, and system power consumption (battery power) of the routers under this network may be greatly limited, and the network connection within it is also characterized by high packet loss rate, low data transmission rate, and instability. The number of nodes that make up the network can vary from just a few to tens of thousands of nodes in a network.6 LoWPAN technology can be used in wireless sensor network IPv6 packet transmission application transmission and packet transmission needs routing. LLN IPv6 routing protocol is specialized in routing 6 LoWPAN packets. Its routing topology is DODAG.


## 2. Network Model of RPL Protocol and DODAG Elements

The three types of nodes constitute the network model of RPL protocol.(1)
LBRs (Low-Power and Lossy Border Routers): the root node of a DODAG system that can build a DAG or serve as a gateway between the Internet and the LLN(2)
Host: as a leaf node, this type of sensor node generates traffic that cannot be forwarded to a terminal device(3)
Network router: network equipment capable of forwarding and generating traffic and capable of running and configuring and managing various types of network protocols


### 2.1. DODAG Building Blocks Based on the RPL Protocol

In LLNs, the network topology often changes with the change of sensor, network router, and other nodes. The state of different nodes at different times determines the network topology. RPL routing protocol is used to exchange control management messages and routing information with other nodes, and DODAG network topology is constructed after successful connection between nodes [4].After the network topology is constructed, the state of nodes changes with time, so DODAG needs to maintain the dynamic parameters. The following basic parameters are required for the construction and dynamic maintenance of RPL-based DODAGs:(1)
RPL Instance ID: The RPL Instance ID, a collection of individual or multiple DODAGs that share the same RPL Instance ID, forms an RPL Instance.(2)
DODAG VersionNumber: The DODAG VersionNumber is a counter that is counted sequentially, increments from the root, and is capable of forming a completely new version of the DODAG. DODAG Version is uniquely identified by RPLInstanceID, DODAGID, DODAG VersionNumber, and so on [5].(3)
DODAGID: The DODAGID is the identifier of the DODAG root. DODAGID has unique properties in a range of RPL instances within LLN. The tuple (RPLInstanceID, DODAGID, and DODAG VersionNumber) uniquely identifies and defines DODAG Version information.(4)
Rank: The Rank is attribute of the node within the DODAG system, indicating the location of the node relative to the root of the DODAG system. Rank can only be used within a version of the DODAG and not in different versions of the DODAG. In the direction of moving away from the DODAG root, Rank increases progressively and decreases progressively. Use OF (target function) to perform precise calculations on Rank [6].RANK value can be obtained by using different path generation values, which means that Rank values can be obtained through different path generation values, as shown in Figure1.Figure 1
Relationship between DODAG roots.


### 2.2. A Network Using the RPL Protocol Includes Several Different RPL Instances

Each RPL Instance is composed of a single or several DODAGs that are different from each other, and each DODAG within each RPL Instance system has the same RPL Instance ID. Therefore, each RPL Instance ID can determine a unique RPL Instance in the system, and RPL Instance is composed of different random numbers to forms. In other words [7], All DODAGs in the same RPL Instance use the same OF. Different RPL Instance systems have different random Numbers, as shown in Figure 2.Figure 2
Relationships between various elements in the RPL protocol.


## 2.1. DODAG Building Blocks Based on the RPL Protocol

In LLNs, the network topology often changes with the change of sensor, network router, and other nodes. The state of different nodes at different times determines the network topology. RPL routing protocol is used to exchange control management messages and routing information with other nodes, and DODAG network topology is constructed after successful connection between nodes [4].After the network topology is constructed, the state of nodes changes with time, so DODAG needs to maintain the dynamic parameters. The following basic parameters are required for the construction and dynamic maintenance of RPL-based DODAGs:(1)
RPL Instance ID: The RPL Instance ID, a collection of individual or multiple DODAGs that share the same RPL Instance ID, forms an RPL Instance.(2)
DODAG VersionNumber: The DODAG VersionNumber is a counter that is counted sequentially, increments from the root, and is capable of forming a completely new version of the DODAG. DODAG Version is uniquely identified by RPLInstanceID, DODAGID, DODAG VersionNumber, and so on [5].(3)
DODAGID: The DODAGID is the identifier of the DODAG root. DODAGID has unique properties in a range of RPL instances within LLN. The tuple (RPLInstanceID, DODAGID, and DODAG VersionNumber) uniquely identifies and defines DODAG Version information.(4)
Rank: The Rank is attribute of the node within the DODAG system, indicating the location of the node relative to the root of the DODAG system. Rank can only be used within a version of the DODAG and not in different versions of the DODAG. In the direction of moving away from the DODAG root, Rank increases progressively and decreases progressively. Use OF (target function) to perform precise calculations on Rank [6].RANK value can be obtained by using different path generation values, which means that Rank values can be obtained through different path generation values, as shown in Figure1.Figure 1
Relationship between DODAG roots.


## 2.2. A Network Using the RPL Protocol Includes Several Different RPL Instances

Each RPL Instance is composed of a single or several DODAGs that are different from each other, and each DODAG within each RPL Instance system has the same RPL Instance ID. Therefore, each RPL Instance ID can determine a unique RPL Instance in the system, and RPL Instance is composed of different random numbers to forms. In other words [7], All DODAGs in the same RPL Instance use the same OF. Different RPL Instance systems have different random Numbers, as shown in Figure 2.Figure 2
Relationships between various elements in the RPL protocol.


## 3. Optimization of DODAG Construction in Interference Environment Based on RPL Protocol

### 3.1. DODAG Building Based on RPL Protocol

RPL distance vector network routing protocol uses parameter control technology to limit the frequency and number of messages used in network control management in the system and constructs a network topology with DODAG structure to reduce system power consumption and network maintenance cost. Each node within a DODAG based on the RPL protocol can perform DODAG discovery, build, and maintenance. RPL network protocol can manage ICMP V6 control message and management information and can exchange DODAG information between nodes. Based on this, a new DODAG network topology can be constructed. The node uses a DIS (DODAG Information Solicitation (DODAG Information Request)) to detect a nearby DODAG [8]. DIO (DODAG Information Object (DODAG Information Object)) carries the RPL Instance information of the node, obtains the configuration parameters of the DODAG, selects the set of parent nodes in the DODAG system, and stores the information about its DODAG. The DAO (Destination Advertisement Object) can pass the reverse routing information of the DODAG system. The Rank value for each node indicates the distance of the node from other nodes. The Rank value is strictly incremented along the routing path, away from the root node. The Rank value is strictly decreasing along the routing path, close to the root node [9].Start the DODAG system built from the DODAG root node, as shown in Figure3. The root node of DODAG transmits DIO messages by broadcasting to the nodes directly connected with it to transmit DODAG information. Upon receiving the DIO message, the neighbor node that is directly connected to the root node processes the received DIO message and then forwards the DIO message to the next node. By analogy, DIO messages gradually spread to the other nodes of the DODAG system. If the node receiving the DIO message does not add any DODAG, the node calculates its path overhead to the sending DIO message node [10], calculates the optimal path through the OF constraint of the target function and the set of metrics, and then decides whether to join the DODAG system. When this node joins the DODAG, it calculates the route to the DODAG root node, and the node that sends the DIO message to this node becomes the parent of the DODAG system of that node. Next, the node computes its Rank to the DODAG root node based on the target function OF in the DODAG and then replies back to its parent with the DAO.Figure 3
Under normal circumstances, the DODAG system is built to control the transfer of information.If the node does not join any DODAG system, nor does it receive any DIO message, it periodically loops a DIS message to its direct-connected neighbor, requests DODAG information from the surrounding direct-connected node, computes it, and so on, until the node joins a DODAG system [11].The DODAG root node broadcasts the DIO message carrying the DODAG information. The node X receives the DIO message broadcasted by the root node, joins the DODAG system, and after processing the DAO message with the node X prefix information passes it to the DODAG root node. DODAG node X sends DIO message containing DODAG information to the node directly connected with node X. After receiving the message, node Y directly connected with node X joins the DODAG system and replies the DAO message to node X, node X becomes the parent of node Y. Node Y receives the DODAG information request information DIS from node Z, but node Y will not reply to any information until node Y joins a DODAG system. After node Y joins a DODAG system, node Y sends a DIO message to node Z and invites node Z to join the DODAG system. After receiving the message, Z, which is directly connected with Y node, joins the DODAG system and replies the DAO message to Y node, and Y node becomes the parent of Z. After receiving the message from node Z, node Y adds its own routing information and fuses it into the received message and then sends the DAO message to the preferred parent node X directly with node Y. By this analogy, the DODAG root node receives and calculates the DAO messages from each node in the DODAG system [12] and obtains the prefix information of each node in the DODAG system step by step.


### 3.2. Improved Algorithm for Building DODAG Systems in a Noisy Environment

In the interference environment, the loss of DIO messages will lead to the change of the network topology. In the process of DODAG system initialization, RPL distance vector network routing protocol is used to send control management messages such as DIO, DIS, and DAO to each other to build DODAG system. Under the influence of environment interference, some control management messages will be lost in the process of DODAG construction. In this case, the child node may not get the DIO message of the preferred parent node [13], so that its DODAG edge length is increased, The routing protocol for low power lossy networks (RPL) uses the objective function (OF) to construct a target-oriented directed acyclic graph (DoDAG). DoDAG is built depending on routing metrics and routing constraints adopted by the target function. The Rank attribute of the node within the DODAG system indicates the location of the node relative to the root of the DODAG system. Rank can only be used within a version of the DODAG and not in different versions of the DODAG. In the direction of moving away from the DODAG root, Rank increases progressively and decreases progressively. Use OF (target function) to perform precise calculations on Rank. In this paper , at the interference environment, there is no direct neighbor relationship between node X and node Z, so it is impossible to transmit data directly. Data communication between node X and node Z can only pass through node Y. At this time, the delay of data transmission and path in DODAG system increases, so does the network power consumption.In an uncluttered environment, the process for building a DODAG system is shown in Figure4. Node X broadcasts DIO messages to its neighbors. Nodes Z and Y receive DIO messages from node A successively. Node X will gradually become the priority parent of node Z. At the same time, node Z and node Y, as the children directly connected with node X, have joined the DODAG system. This completes the construction of the DODAG system in a noninterference environment.Figure 4
Basic process of building a DODAG in a nondisruptive environment.Figure5 illustrates the causes and effects of network topology in a noisy environment. In the graph, X, Y, and Z are neighbor nodes that are directly connected to each other. Node X is a node that has been added to a DODAG system. When the data are transmitted between nodes, it is disturbed and the data packet is lost [14].Figure 5
Basic process of building a DODAG in a disrupted environment.When starting to build DODAG system, the node of DODAG system is not joined. Broadcast is used to send DIS message to the node of DODAG system to check whether the node of DODAG system has joined. A node that has joined a DODAG system will broadcast a DIO message to all nodes directly connected to it after it receives a request for DIS information, and the DODAG system information of the node is contained in the DIO message. All nodes that are directly connected to this DIO message sender receive their DIO message, calculate their path overhead to the sending DIO message node, and then decide to join the DODAG system or leave it as it is. If a node that has joined the DODAG system does not receive a DIS request from a direct node for a period of time, the node broadcasts a DIO message after a delay of some time. The Trickle algorithm calculates the latency wait time [15].When the DODAG system encounters noise interference, some control management messages are lost as a result, as shown in Figure5. When node X broadcasts DIO messages to its neighbors, node Y receives the DIO messages sent by node X. Due to the noise of interference environment, node Z does not receive the DIO messages sent by node X in time. Node Y does not join DODAG system because node Z does not receive DIO messages from node X. After Y joins DODAG system, it is delayed for a certain time and broadcasts DIO messages to the neighboring nodes, and Y notifies them to join DODAG system. In this case, node Z receives the DIO message from node Y by broadcasting. After calculation, node Y becomes the priority parent of node Z, and node Z joins the DODAG system. At this time, the network topology of DODAG system deviates greatly from that of the noninterference environment. This is because node Z received interference and no DIO message from node X broadcast was received. In the interference environment, there is no direct neighbor relationship between node X and node Z, so it is impossible to transmit data directly. Data communication between node X and node Z can only pass through node Y. At this time, the delay of data transmission and path in DODAG system increases, so does the network power consumption [16].The reason for this situation is that node Z cannot receive the DIO message from node X by broadcasting in time. As a result, the direct neighbor relationship between node X and node Z cannot be established. The data communication between node X and node Z needs to pass through node Y. As a result, the energy consumption and transmission latency of DODAG systems have increased.


## 3.1. DODAG Building Based on RPL Protocol

RPL distance vector network routing protocol uses parameter control technology to limit the frequency and number of messages used in network control management in the system and constructs a network topology with DODAG structure to reduce system power consumption and network maintenance cost. Each node within a DODAG based on the RPL protocol can perform DODAG discovery, build, and maintenance. RPL network protocol can manage ICMP V6 control message and management information and can exchange DODAG information between nodes. Based on this, a new DODAG network topology can be constructed. The node uses a DIS (DODAG Information Solicitation (DODAG Information Request)) to detect a nearby DODAG [8]. DIO (DODAG Information Object (DODAG Information Object)) carries the RPL Instance information of the node, obtains the configuration parameters of the DODAG, selects the set of parent nodes in the DODAG system, and stores the information about its DODAG. The DAO (Destination Advertisement Object) can pass the reverse routing information of the DODAG system. The Rank value for each node indicates the distance of the node from other nodes. The Rank value is strictly incremented along the routing path, away from the root node. The Rank value is strictly decreasing along the routing path, close to the root node [9].Start the DODAG system built from the DODAG root node, as shown in Figure3. The root node of DODAG transmits DIO messages by broadcasting to the nodes directly connected with it to transmit DODAG information. Upon receiving the DIO message, the neighbor node that is directly connected to the root node processes the received DIO message and then forwards the DIO message to the next node. By analogy, DIO messages gradually spread to the other nodes of the DODAG system. If the node receiving the DIO message does not add any DODAG, the node calculates its path overhead to the sending DIO message node [10], calculates the optimal path through the OF constraint of the target function and the set of metrics, and then decides whether to join the DODAG system. When this node joins the DODAG, it calculates the route to the DODAG root node, and the node that sends the DIO message to this node becomes the parent of the DODAG system of that node. Next, the node computes its Rank to the DODAG root node based on the target function OF in the DODAG and then replies back to its parent with the DAO.Figure 3
Under normal circumstances, the DODAG system is built to control the transfer of information.If the node does not join any DODAG system, nor does it receive any DIO message, it periodically loops a DIS message to its direct-connected neighbor, requests DODAG information from the surrounding direct-connected node, computes it, and so on, until the node joins a DODAG system [11].The DODAG root node broadcasts the DIO message carrying the DODAG information. The node X receives the DIO message broadcasted by the root node, joins the DODAG system, and after processing the DAO message with the node X prefix information passes it to the DODAG root node. DODAG node X sends DIO message containing DODAG information to the node directly connected with node X. After receiving the message, node Y directly connected with node X joins the DODAG system and replies the DAO message to node X, node X becomes the parent of node Y. Node Y receives the DODAG information request information DIS from node Z, but node Y will not reply to any information until node Y joins a DODAG system. After node Y joins a DODAG system, node Y sends a DIO message to node Z and invites node Z to join the DODAG system. After receiving the message, Z, which is directly connected with Y node, joins the DODAG system and replies the DAO message to Y node, and Y node becomes the parent of Z. After receiving the message from node Z, node Y adds its own routing information and fuses it into the received message and then sends the DAO message to the preferred parent node X directly with node Y. By this analogy, the DODAG root node receives and calculates the DAO messages from each node in the DODAG system [12] and obtains the prefix information of each node in the DODAG system step by step.


## 3.2. Improved Algorithm for Building DODAG Systems in a Noisy Environment

In the interference environment, the loss of DIO messages will lead to the change of the network topology. In the process of DODAG system initialization, RPL distance vector network routing protocol is used to send control management messages such as DIO, DIS, and DAO to each other to build DODAG system. Under the influence of environment interference, some control management messages will be lost in the process of DODAG construction. In this case, the child node may not get the DIO message of the preferred parent node [13], so that its DODAG edge length is increased, The routing protocol for low power lossy networks (RPL) uses the objective function (OF) to construct a target-oriented directed acyclic graph (DoDAG). DoDAG is built depending on routing metrics and routing constraints adopted by the target function. The Rank attribute of the node within the DODAG system indicates the location of the node relative to the root of the DODAG system. Rank can only be used within a version of the DODAG and not in different versions of the DODAG. In the direction of moving away from the DODAG root, Rank increases progressively and decreases progressively. Use OF (target function) to perform precise calculations on Rank. In this paper , at the interference environment, there is no direct neighbor relationship between node X and node Z, so it is impossible to transmit data directly. Data communication between node X and node Z can only pass through node Y. At this time, the delay of data transmission and path in DODAG system increases, so does the network power consumption.In an uncluttered environment, the process for building a DODAG system is shown in Figure4. Node X broadcasts DIO messages to its neighbors. Nodes Z and Y receive DIO messages from node A successively. Node X will gradually become the priority parent of node Z. At the same time, node Z and node Y, as the children directly connected with node X, have joined the DODAG system. This completes the construction of the DODAG system in a noninterference environment.Figure 4
Basic process of building a DODAG in a nondisruptive environment.Figure5 illustrates the causes and effects of network topology in a noisy environment. In the graph, X, Y, and Z are neighbor nodes that are directly connected to each other. Node X is a node that has been added to a DODAG system. When the data are transmitted between nodes, it is disturbed and the data packet is lost [14].Figure 5
Basic process of building a DODAG in a disrupted environment.When starting to build DODAG system, the node of DODAG system is not joined. Broadcast is used to send DIS message to the node of DODAG system to check whether the node of DODAG system has joined. A node that has joined a DODAG system will broadcast a DIO message to all nodes directly connected to it after it receives a request for DIS information, and the DODAG system information of the node is contained in the DIO message. All nodes that are directly connected to this DIO message sender receive their DIO message, calculate their path overhead to the sending DIO message node, and then decide to join the DODAG system or leave it as it is. If a node that has joined the DODAG system does not receive a DIS request from a direct node for a period of time, the node broadcasts a DIO message after a delay of some time. The Trickle algorithm calculates the latency wait time [15].When the DODAG system encounters noise interference, some control management messages are lost as a result, as shown in Figure5. When node X broadcasts DIO messages to its neighbors, node Y receives the DIO messages sent by node X. Due to the noise of interference environment, node Z does not receive the DIO messages sent by node X in time. Node Y does not join DODAG system because node Z does not receive DIO messages from node X. After Y joins DODAG system, it is delayed for a certain time and broadcasts DIO messages to the neighboring nodes, and Y notifies them to join DODAG system. In this case, node Z receives the DIO message from node Y by broadcasting. After calculation, node Y becomes the priority parent of node Z, and node Z joins the DODAG system. At this time, the network topology of DODAG system deviates greatly from that of the noninterference environment. This is because node Z received interference and no DIO message from node X broadcast was received. In the interference environment, there is no direct neighbor relationship between node X and node Z, so it is impossible to transmit data directly. Data communication between node X and node Z can only pass through node Y. At this time, the delay of data transmission and path in DODAG system increases, so does the network power consumption [16].The reason for this situation is that node Z cannot receive the DIO message from node X by broadcasting in time. As a result, the direct neighbor relationship between node X and node Z cannot be established. The data communication between node X and node Z needs to pass through node Y. As a result, the energy consumption and transmission latency of DODAG systems have increased.


## 4. Topology Repair Scheme for Two DODAG Systems in Interference Environment

In the interference environment, the loss of DIO messages will lead to the change of the network topology. The DODAG system based on RPL protocol can be used to repair the topology in two ways.The first scenario: the root node of the DODAG uses incremental DODAG VersionNumer, initiates a global repair operation of the DODAG system, traverses the nodes of the system, and produces a brand new version of DODAG VersionNumer. In the new DODAG system, nodes that are not restricted by the old version of Rank value can recalculate their position in the DODAG system based on OF, Rank, and so on. This approach rebuilds the DODAG. Because of its global repair operation, the power loss caused by the system is relatively large.The second scheme: RPL network protocol also supports the local topology repair mechanism in DODAG Version. You can set the necessary parameters for DIO messages when you implement system configuration and control using the DODAG root policy.At this time, a method of repairing network topology is proposed to solve this problem. RPL network protocol can use an external mechanism to detect whether neighbors are no longer reachable. The goal of this mechanism is to maintain routing proximity with minimal overhead. Examples of similar mechanisms include the Neighbor Unreachability Detection (neighbor inaccessibility detection [RFC4861]) mechanism in IPv6 [17].The process by which a node detects and determines whether it is reachable with a neighbor is called a Neighbor Unreachability Detection. The neighbor unreachable detection mechanism describes the reachability of the node and the direct neighbor by the state whether the node and neighbor can reach each other.In the DODAG system, the nodes communicate with other nodes through the neighbor nodes which are directly connected with them. Under the interference of the system environment, the nodes will interrupt and fail due to data loss and control information failure. If the destination node fails, the data transmission cannot be resumed and communication with the destination node fails; if the data transmission path fails, it is possible to resume communication. Therefore, nodes actively track the reachability status of packets when communicating with their neighbors.Neighbor unreachability detection exists in all paths between nodes, including data communication from node to node, from node to router, and from router to node. In this way, the fault of neighbor node or forward path of neighbor node is detected.If a node in a DODAG system recently receives an acknowledgment and the neighbor’s IP layer has received the packet that was recently sent to it, the neighbor is reachable. The RPL is only used for routing in DODAG systems, and neighbor inaccessibility detection can be confirmed in two ways: one is a prompt from the upper protocol that provides a “connection processing” confirmation, and the other is a node sending a unicast neighbor request message that receives a reply neighbor notification message. In order to reduce unnecessary network traffic, probe messages are only sent to the neighbor nodes directly connected to them.In DODAG system, there are two tasks simultaneously: neighbor inaccessibility detection (NUD) and a node sending packets to neighbor nodes. In the process of neighbor node reachability verification, the node continues to continuously send packet information to the neighbor node at the address of the cache link layer; if there is no packet information sent to the neighbor node in the system, the neighbor nonreachable detection (NUD) will not send either.When the DODAG system is disturbed, Neighbor Unreachability Detection is used to enable the node to detect whether all paths between the node and its neighbor are reachable or not. If a node discovers that all paths between it and its neighbors have failed, the node reset action begins. Once the node reset is complete, the DODAG system build process is resumed. At this point, the DODAG root is prevented from initiating a global repair operation by incrementing the DODAG VersionNumer to avoid regenerating a new version of the DODAG. At the same time, the power loss caused by this global repair operation is avoided. The performance of DODAG in interference environment is enhanced, and the data retransmission rate is reduced.In DODAG system, controlling the energy consumption of message transmission and data information calculation is an important part of routing algorithm consumption. The energy consumption of data information transmission is far more than that of data calculation [18].In the Internet of Things, all kinds of objects around people can join the communication network, connect with the traditional Internet, and communicate with each other freely and exchange information. The "content" in the Internet of things refers to the intelligent device capable of communication and network, the intelligent equipment has a large number and frequent switching characteristics, such as the limited resources, how to manage, organize the efficient, safe network between smart devices, and merge with the traditional Internet, you need to design special routing protocols. In view of the above Internet of Things networking requirements, IETF specially designed a lightweight IPv6 networking routing protocol for Internet of Things devices, RPL routing protocol. RPL routing protocol can efficiently utilize the energy and computing resources of intelligent devices, form a flexible topology structure, and realize data routing.In the Internet of Things, all kinds of objects around us can join the communication network, connect with the traditional Internet, communicate with each other freely, and exchange information. The “content” in the Internet of Things refers to the intelligent device capable of communication and network, the intelligent equipment has a large number, and frequent switching characteristics, such as the limited resources, how to manage, organize the efficient, safe network between smart devices, and merge with the traditional Internet, you need to design special routing protocols. In view of the above Internet of Things networking requirements, IETF specially designed a lightweight IPv6 networking routing protocol for Internet of Things devices, RPL routing protocol. RPL routing protocol can efficiently utilize the energy and computing resources of intelligent devices, form a flexible topology structure, and realize data routing.The low-power and lossy nature of LLNS necessitate the use of additional packets for RPL’s on-demand loop detection. Because of the sporadic nature of data traffic, maintaining a routing topology that varies with the physical topology can result in wasted energy. In physical connections, LLNs typically show instantaneous and unimpeded changes in traffic, but tracking these changes closely from the control plane is costly. Transient and occasional changes in connections do not need to be processed by RPL until data are sent. The design of RPL in this regard borrows from the existing LLN protocol for high-frequency use, as well as a large number of experiments and deployments demonstrating its efficacy. The RPL packet information is sent along with the packet and contains the sender’s Rank information. Inconsistencies in routing decisions (up or down) and the Rank relationship between the two nodes indicate a possible loop. If such data is received, the node initiates a local repair operation.


## 5. Conclusions

In this paper, an improved DODAG construction optimization scheme is proposed. The nodes of DODAG can use the Neighbor Unreachability Detection mechanism to perceive the survival of the neighbors directly connected to it. If a node discovers that all paths between it and its neighbors have failed, the node reset action begins. Once the node reset is complete, the DODAG system build process is resumed. This prevents the DODAG root from initiating a global fix by incrementing the DODAG VersionNumer to produce a new version of the DODAG in a disruptive environment. The power loss caused by this global repair operation is avoided. The performance of DODAG in interference environment is enhanced, and the data retransmission rate is reduced.In DODAG system, there are two tasks simultaneously: neighbor inaccessibility detection (NUD) and a node sending packets to neighbor nodes. When the DODAG system is disturbed, Neighbor Unreachability Detection is used to enable the node to detect whether all paths between the node and its neighbor are reachable or not. If a node discovers that all paths between it and its neighbors have failed, the node reset action begins. Once the node reset is complete, the DODAG system build process is resumed. At this point, the DODAG root is prevented from initiating a global repair operation by incrementing the DODAG VersionNumer to avoid regenerating a new version of the DODAG. At the same time, the power loss caused by this global repair operation is avoided. The performance of DODAG in interference environment is enhanced, and the data retransmission rate is reduced.



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*Source: 5579564-2021-04-02.xml*