# Adaptive Downward/Upward Routing Protocol for Mobile-Sensor Networks
## 2 Related Work  
Downward routing is less studied than upward routing in WSNs. It can be mainly classified into three categories: broadcast-based, unicast-based, and broadcast- and unicast-based. Most of these protocols are proposed for static scenarios and only few of them can cope with mobile scenarios. Glossy [5] and LWB [6] are broadcast-based routing protocols that use flooding as the basic dissemination method for propagating data to the entire network. [PERSON] et al. [5] proposed a flooding and time synchronized protocol for WSNs called Glossy. Glossy exploits the flooding mechanism to implicitly synchronize the network. According to clock values embedded in flooding packets, all receivers synchronize relatively to the clock of the initiator. Each packet also embeds a relay counter value, which represents how many times a packet has been relayed. Nodes always concurrently transmit packets with the same relay counter. Benefiting from concurrent transmission and synchronization, Glossy avoids interference during flooding and profit from constructive interference. However, Glossy highly relies on concurrent transmissions and actuation of initiators, and this results in Glossy not supporting data-collection scenarios, since too many initiators cause serious congestion in the network and congestion induces inaccurate synchronization. In order to obtain precise synchronization, Glossy also has a very strict packet-size limit during transmission and this makes the propagation of variable-sized packets impossible. Low-Power Wireless Bus (LWB) is an updated version of Glossy, which concurrently supports one-to-many, many-to-one, and many-to-many traffic. Unlike Glossy, which is simply driven by the events of initiators, LWB appoints a node in the network to work as a controller and it sends schedules of each initiator. Although LWB reduces congestion using a central control method, it still meets the same problems of a limited number of initiators and packet sizes as Glossy. In addition, neither Glossy nor LWB supports mobility. RPL is a unicast-based routing protocol for Lossy and Low Power Networks (LLNs) that supports upward and downward traffic patterns. RPL supports two modes of downward traffic: Nonstoring mode and Storing mode. In Nonstoring mode, Destination Advertisement Object (DAO) messages are directly sent to the Destination-Oriented Directed Acyclic Graph (DODAG) root along a default route. The root establishes source-routing table entries for destinations learned from DAOs. Before sending a data packet, the root uses source routing to completely specify the route of the packet. In Storing mode, DAOs are sent to all parent nodes to inform them of the existence of a child node. Once a data packet is sent by the DODAG root, it must be sent to all one-hop neighbors first. Afterward, the data packet is sent hop by hop until it reaches its destination or hop limit. In our previous work [4],we already showed that RPL does not support upward routing well in mobility. It is also hard for RPL to support downward routing well in mobility. Due to topology changes in mobility, parent nodes change frequently. This requires RPL to send DAOs on time in order to keep the downward routing table up to date, which costs too much overhead. Otherwise, packets are lost due to outdated path information. [PERSON] et al. [7] proposed a new mechanism to improve RPL downward route updating of Nonstoring mode in mobility. Instead of sending a no-path DAO to a mobile parent node, a child node sends a DAO containing no-path information to a static parent node to reach root. However, this method suffers from high overhead and it relies on the presence of fixed nodes. This means that this method cannot operate in a scenario where all nodes are mobile. Due to the inefficiency of DAOs in downward routing, [PERSON] et al. [8] proposed an opportunistic routing protocol called Opportunistic RPL (ORPL), which is built on-top of RPL. ORPL deactivates DAO and simply uses the broadcasting of DODAG Information Object (DIO).