A Comprehensive Evaluation of RPL under Mobility Kevin Lee, Raghuram Sudhaakar, Jianxia Ning, Lillian Dai, Sateesh Addepalli, Flavio Bonomi * Paper accepted by Consumer Communications And Networking Conference (CCNC 2012) Special Session Information Dissemination in Vehicular Networks * Extended paper submitted to Focus Issue on Vehicular Ad-hoc Networks in International Journal of Vehicular Technology in February 2012

Introduction Traffic offloading from 3G to WiFi AP to save money Multihop-to-infrastructure WiFi access reduces the number of AP deployments Intuitively, tree structure is formed with root of the tree at the AP Consider Routing Protocol for Low power and lossy networks (RPL) * Wireless service providers such as AT&T and Verizon have changed from a fixed monthly fee to a tiered or per bit pricing structure for data usage. As free WiFi networks become more accessible from vehicles, users will have a strong economic incentive to opportunistically offload data traffic from 3G and 4G links to free WiFi links. * Since WiFi deployment is not prevalent, enabling vehicles to access roadside WiFi through other vehicles (multihop-to-infrastructure) allow more vehicles to take advantage of data offloading.

What About Other Ad Hoc Routing? Reactive routing protocols are susceptible to route breaks Link state routing protocols unscalable because of flooding global topology information

RPL Operation DIO 1 DIO DIO DIO DIO DIO 2 1 DIO 1 2 DIO DIO DIO DIO DIO 3 2 2 DIO 2 DIO 3 3 3 3 Directed Acyclic Graph (DAG) Information Option (DIO) messages are broadcast to build the tree; includes a node’s rank (its level), ETX, etc. ETX probe is sent periodically to probe neighboring ETX

RPL Summary RPL forms a Destination Oriented Directed Acyclic Graph (DODAG), with the root of the tree being the AP DODAG minimizes the cost to the root per Objective Function; used ETX here Directed Acyclic Graph (DAG) Information Option (DIO) messages are broadcast to build the tree; includes a node’s rank (its level), ETX, etc. A node selects a parent based on the received DIO msgs and calculates its rank Destination Advertisement Option (DAO) msg sent periodically to notify parent about routes to downward nodes A node selects a parent based on the received DIO msgs and calculates its rank: Let’s say a node receives 3 DIO’s from three neighbors. It will consider neighbors whose rank is less than its own. Based on its link etx to each node and their respective ETX to the root, the node can establish ETX using either of the two nodes. The node can then determine who its parent is.

RPL Fine Tuning for VANETs Timer fine-tuning: Immediate ETX probing for a new neighbor Immediate broadcasting DIO msg upon new parent election Immediate broadcasting DAO msg upon new parent election Loop avoidance and detection: Stamp DIO msg with parent’s ID; If a node that receives the DIO msg is equal to the parent’s ID in the DIO msg, it’ll discard the DIO msg

Loop Avoidance and Detection

Qualnet Evaluation Setup AP 2500m Car 7 Car 3 Car 2 Car 1 25mph, 45mph, 65mph 10-node caravan traversing a 5000m-long road Inter-node distance is maintained at 250m Traces of 25mph, 45mph, and 65mph Two-way CBR traffic are sent from any car of the caravan 5000m Packet delivery ratio (PDR) and overhead are obtained with varied DIO msg period and ETX probe period Lillian Dai, Sateesh, Raghu, Kevin Lee

Rank Change & Duration wrt Time and Speed Lead car (Car 1) establishes conn @ 199s/79s @ 25mph/65mph Rank duration is 22s for 25mph and 9s for 65mph 25mph 65mph

ETX Probe Period on PDR The packet delivery ratio decreases as ETX probe interval increases -- delayed response to changing topology

ETX Probe Period on Overhead As the probe period increases, the number of ETX probes decreases PDR does not degrade as much for ETX probe period 1s, 3s, and 5s => topology change is on the level of 5s granularity => An adaptive timer approach for ETX probe recommended

DIO Msg Period and Speed on PDR PDR drops as DIO msg period increases PDR drops as speed increases for a given DIO msg period Diminishing return with decreasing DIO msg period suggests trickle

DIO Msg Period and Speed on Overhead The lower the DIO msg period, the higher the DIO msg overhead DIO msg overhead increases with increasing speed

Modified RPL Improvement PDR before and after changes The improvement is about 20%

Conclusion Studied the impact of DIO msg period and ETX probe period on PDR and overhead => Show the value and advantage of trickle timer Fine tune RPL and improve its performance in vehicular networks by 20% Future work: More complex sim. scenario Consider speed and direction in forming a tree Consider trickle timer for ETX probe period