# Collision Avoidance Geographic P2P-RPL in Multi-Hop Indoor Wireless Networks
## 2 Research Background  
A large number of LLN applications, such as home automation and building control applications, require point-to-point (P2P) routing support. RPL has been defined by IETF as a standard routing protocol for LLNs [1]. RPL organizes a tree-like network topology called Destination-Oriented Directed Acyclic Graph (DODAG). When two non-root nodes need to communicate, the data packets must be routed only along the pre-established DODAGs. A DODAG is built to optimize the routing cost to reach the root node. Hence, such P2P routing may lead to a longer end-to-end delay, low packet delivery ratio, and severe traffic congestion near the root node [4; 5]. Complementary to these shortcomings, a reactive discovery of Point-to-Point Routes in Low-Power and Lossy Networks (P2P-RPL) is standardized [6]. P2P-RPL reactively discovers P2P paths by constructing the temporary DODAG. The temporary DODAG is solely used for route discovery. P2P-RPL uses the same mechanism of the basic RPL to for m the DODAG. The source node acts as the root node and it originates P2P mode DIO (P2P-DIO) for temporary DODAG construction. The P2P-DIO piggybacks a P2P Route Discovery Option to discover the route for a source-destination pair. Each node receiving the P2P-DIO joins the temporary DODAG and disseminates the P2P-DIO periodically with the trickle algorithm. Once the destination node receives the P2P-DIO, it replies P2P Discovery Reply Object (P2P-DRO) to the source node along the discovered route. The discovered route is used for data packet transmission from the source node to destination node. RPL and P2P-RPL adopt trickle algorithm to control the transmission rate of the DIO control packet with the goal of a fast network convergence time and low energy consumption of the nodes in the network. The two conflicting goals are achieved by adaptive transmission periods and message suppression mechanism [7]. A node exponentially increases the transmission period when a network is in a stable state. To detect the state of a network, a node has a listen-only period before each transmission period. When receiving an inconsistent message (e.g., a route advertised by the message is better than the existing route in terms of the routing metrics in use), a node decreases the transmission period to the minimum value to quickly resolve the inconsistency. When a node receives a consistent message and the number of consistent messages is more than predefined value \(k\), the node suppresses a DIO transmission. However, the non-deterministic message suppression forces some nodes to remain silent for a long time, resulting in some routes remaining undiscovered [12]. This non-deterministic nature of the trickle algorithm can lead to sub-optimal route formation when high message suppression is performed (e.g., \(k=1\)). Moreover, the listen-only period imposed by the trickle algorithm delays a transmission at every hop, resulting in an increase of the network convergence time [8]. P2P-RPL constantly broadcasts DIO control packets throughout the whole network using trickle algorithm. The network-wide broadcast approach could lead to serious medium contention known as the broadcast storm problem, especially in dense networks [13; 14]. The Lightweight On-demand Ad hoc Distance-vector routing protocol-next generation (LOADng) uses a flooding mechanism to discover and maintain routes only on-demand. LOADng is a reactive routing protocol based on AODV and was extended for use in LLNs. When a new route is required, LOADng disseminates route discovery packets throughout the entire network. A node forwards route discovery packets until they reach the destination node according to the flooding operation. This classic flooding mechanism uses reduced relay sets [15] or the Trickle algorithm [16]. LOADng suffers from broadcast storms caused by flooding route discovery packets and does not include mechanisms to stop the spread of control packets after the target is found [17]. The LOADng protocol is suitable under few concurrent traffic flows in the network.