1. 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

2. 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.

3. 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

4. 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

5. 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.

6. 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

7. Loop Avoidance and Detection

8. Qualnet Evaluation SetupAP
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

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

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

11. 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

12. 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

13. 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

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

15. 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