# Collision Avoidance Geographic P2P-RPL in Multi-Hop Indoor Wireless Networks
## 3 Collision Avoidance Geographic Point-to-Point RPL
### Location-Aided P2P-RPL with Location-Constrained Forwarding Zone  
From the result of cooperative self localization, all nodes have up-to-date position states. This position information is piggybacked on the standard RPL control messages (e.g., DIO and DAO). A location-constrained forwarding zone is defined to limit the propagation area for route discovery messages. A node within the location-constrained forwarding zone joins the temporary DAG formation procedure and forwards a route discovery packet (i.e., P2P-DIO) to its neighbors. This results in a great reduction of the routing overhead and energy consumption, especially when the distance between the source and destination nodes is short [27]. We consider when a source node (say, node S) tries to find an optimal P2P path to a destination node (say, node D) as shown in Figure 2. Node S measures the path cost reaching node D by sending a measurement request along the pre-established global DAG. The measurement request carries the accumulated path cost (ETX). On receiving the measurement request, an intermediate node updates the path cost and forwards it to the next hop. When node D receives the measurement request, node D sends a Measurement Reply back through the reverse path to node S. The measurement reply carries the cost of the route and the B-box state of node D with the location option. Upon receiving the measurement reply, and if the accumulated path cost does not meet the specified routing metric constraints, node S initiates the P2P route discovery. As shown in Figure 3a, node S defines the location-constrained forwarding zone for P2P route discovery as the rectangular region that includes the current estimated location states of the node S and node D. The four corners of the forwarding zone are computed as follows:  
\[ \begin{split}\mathbf{B}_{fund}&=\left[x_{fund}^{lb} \quad x_{fund}^{ub}\right]\times\left[y_{fund}^{lb}\quad y_{fund}^{ub}\right]\\ &=\left[min(x_{S}^{lb},x_{D}^{lb})\quad max(x_{S}^{ub},x_{D}^{ub}) \right]\times\left[min(y_{S}^{lb},y_{D}^{lb})\quad max(y_{S}^{ub},y_{D}^{ub}) \right]\end{split}  \tag{11}\]  
where \(lb,ub,S\), and \(D\) denote the lower bound, upper bound, source, and destination, respectively. Node S then piggybacks the determined location-constrained forwarding zone \(\mathbf{B}_{fund}\) on a P2P-DIO message and disseminates the P2P-DIO message to one-hop neighbor nodes. Upon receiving P2P-DIO, an intermediate node I joins P2P route discovery only if it belongs to the location-constrained forwarding zone (i.e., \(\mathbf{B}_{I}\cap\mathbf{B}_{fund}\
eq\varnothing\) ). If there is an intersection area between the estimated region of node itself and the location-constrained forwarding zone, node I decides that it is inside the location-constrained forwarding zone and forwards a P2P-DIO. If not, node I simply discards P2P-DIO. Figure 2: The flowchart of the proposed algorithm. Figure 3b shows that nodes \(N_{1}\) and \(N_{2}\) satisfy the condition and participate in the route discovery procedure. On the other hand, node \(N_{3}\) discards the P2P-DIO as \(\textbf{B}_{N_{3}}\cap\textbf{B}_{fund}=\varnothing\). Once the node D receives P2P-DIO, the P2P path is established as the upward route to the temporary root (node S). Node D sends P2P-DRO, which contains the discovered route and stop flag set by 1 back to node S so that no more P2P-DIO messages are forwarded for this temporary DAG. Upon receiving P2P-DRO, intermediate nodes stop sending P2P-DIO periodically and send P2P-DRO to their neighbors. When P2P-DRO reaches node S, node S then sends a data packet along the discovered route. In this way, only a part of the nodes participate in the route discovery procedure, and hence network-wide flooding rarely occurs. The size of the location-constrained forwarding zone is determined depending on the distance between the source and destination nodes. A shorter distance between node S and node D results in a smaller size of forwarding zone, and a fewer number of nodes join the P2P route discovery procedure.