# Study of Mobility Enhancements for RPL in Convergecast Scenarios
## 1 Introduction  
Wireless sensor networks (WSNs) have been widely developed and deployed in the last decade. Their ease of deployment and auto-configuration features have helped them gain this success. Typical WSNs are mostly static and nodes rarely change positions. However, with emergence of new applications, mobility has become an important requirement. In some scenarios, the monitoring nodes are mobile, such as in disaster response applications where a forest-keeper should be able to request event-related data from sensor nodes while moving inside a forest [1]. In other scenarios, objects being monitored need to be mobile, for example in animal monitoring applications. Sensor nodes attached to cows moving inside a field send information towards sink nodes [1]. In these scenarios, the degree of mobility varies in various applications. In some applications, only some of the network nodes need to be mobile, such as in farming equipment monitoring [2]. On a farm, some equipment is installed at fixed points and some equipment is mobile, for example ploughs. In some applications it is required that all nodes be mobile, such as in health-care monitoring. In health-care monitoring systems all patients are equipped with sensor nodes to transmit data to a base station and are free to move inside a hospital [3]. In this paper, we concentrate on highly-mobile scenarios where a significant part of the network or all nodes are free to move and send data to a fixed sink node. When only few nodes are mobile in a highly-connected network, there will not be a great impact on the topology. Indeed, when only few nodes change positions, most nodes do not need to rebuild theroutes to reach the destination. However, in the case of a high degree of mobility, the changes in the topology have a greater impact on routes. Consequently, routing protocols need to react fast to adapt to the movement before making a decision for a node to select the next-hop.  
The Routing Protocol for Low-Power and Lossy Networks (RPL) is a routing protocol, designed in 2012 by the IETF ROLL (Internet Engineering Task Force Routing Over Low power and Lossy networks) working group for Low-Power and Lossy Networks (LLNs). The RPL is designed to meet the requirements of many applications, which are mainly suitable for static networks [4]. RPL defined a numerical metric, called the rank metric, that represents the topological position of a node with respect to the sink in order to avoid routing loops. A node is not allowed to send data packets to neighbours with lower ranks. In mobility scenarios, due to the movement of nodes, the position of a node will change. However, RPL specifications do not offer methods to update the rank in a timely manner. This causes loops when a parent node becomes a descendant node. The trickle algorithm is used in RPL to disseminate control information over the network [5]. This algorithm is used in static networks in order to reduce overhead. Indeed, it reduces control traffic generation rate when the topology is stable. However, in mobility scenarios, the trickle algorithm cannot disseminate information in a timely manner and fails to cope with the changes of the topology.  
In this paper, we only study convergecast scenarios to different mobile degrees. In these scenarios, there is only one data collection sink node that is fixed and does not change its position. Other nodes of the network are mobile or static according to the degree of mobility and generate constant bit rate traffic towards the sink node.  
The paper is organized as follows. In Section 2, an overview of RPL protocol is presented. In Section 3, we present the related work concerning mobility in RPL. Section 4 describes our mobility support mechanism. In Section 5, we analyse the results obtained when our mobility support mechanism is integrated in RPL. Finally, we conclude the paper along with future investigations in Section 6.