ExOR: Opportunistic Multi-Hop Routing for Wireless Networks

S. Biswas, R. Morris, "ExOR: Opportunistic Multi-Hop Routing for Wireless Networks," ACM SIGCOMM Conference, (August 2005).

One line summary: This paper describes a wireless routing protocol called ExOR that is inspired by cooperative diversity schemes and that achieves significant improvement in throughput over traditional routing protocols.

Summary

This paper describes a wireless routing and MAC protocol called ExOR. In contrast to traditional routing protocols that choose a sequence of nodes along a path to route through and then forward packets through each node on that path in order, ExOR is inspired by cooperative diversity routing schemes, and broadcasts each packet to a list of candidate forwarders from which the best node to receive the packets forwards them in turn. The authors demonstrate that ExOR substantial increase in throughput over traditional routing protocols. One reason for this is that each transmission in ExOR may have a more chances of being received and forwarded. Another reason is that it takes advantage of transmissions that go unexpectedly far or that fall short. Four key design challenges for ExOR were (1) nodes must agree on which subset of them received each packet, (2) the closest node to the destination to receive a packet should be the one to forward it, (3) there is a penalty to using too many nodes as forwarders since the cost of agreement go up, and (4) simultaneous transmissions must be avoided to prevent collisions.

Briefly, the way ExOR works is as follows. The source batches packets destined to the same host and broadcasts the batch. Each packet contains a batch map that indicates the highest priority node to have received a packet. Each packet also contains a forwarder list of nodes that is in order of increasing cost of delivering a packet from that node to the destination. The cost metric used is similar to ETX. A node that receives a packet checks the forwarder list for itself. It checks the batch map in the packet and uses it to replace its local batch map if it indicates a higher priority node has received it. Nodes forward all remaining packets that have not been yet received by the destination and the nodes do this in the order in which they appear in the forwarder list. Nodes estimate the time they need to wait before sending or use a default value if they have no basis for guessing. Once the last node in the forwarder list has transmitted, the source starts the process again by broadcasting all packets not yet received by any node. The destination sends copies of its batch map to propagate information back to the sender about which packets were received. Nodes only continue transmitting until they receive indication that 90% of the batch has been received; the rest is then forwarded using traditional routing.

ExOR was evaluated on Roofnet. The authors measure the throughput obtained when transmitting 1MB using traditional routing and ExOR between each of 65 pairs of nodes. They compare the 25 highest and 25 lowest throughput pairs (with respect to traditional routing). ExOR outperforms traditional routing even over just one hop. They also show that ExOR had to retransmit packets on average half as many times as the traditional routing protocol did. They examine the performance of ExOR using various batch sizes and conclude the optimal size is likely between 10 and 100 packets. They use a simulator to study the effects of shared interference on the performance of ExOR and conclude that it only slightly hurts ExOR’s performance. Lastly they show that the throughput of ExOR exhibits less variance than that of traditional routing.

Critique

I liked the ExOR protocol because I thought it was unusual and clever and it did get much better throughput than traditional routing. I thought it was especially interesting that it got better throughput even over one hop. I also liked how in the evaluation section, the authors broke their results down to the 25 highest and lowest throughput pairs because it was informative and a nice way to think about the results.

All that said, here are some things I didn’t like about this paper. The authors assume that the reception at different nodes is independent and that there is a gradual falloff in delivery probability with distance, and the performance of ExOR in part depends upon these assumptions, but it seems these could be looked at more closely. I thought their simulation might have been too oversimplified to the extent that I’m not convinced it really gave much useful information. Another thing is that it seems like the nodes have to maintain a lot of state (delivery probability for all pairs, unless I’m misunderstanding something). Also, again as in some other papers from these authors, they ran their evaluations while other traffic was running over their Roofnet testbed. This just seems bad to me because I thought scientists were supposed to control as many variables as possible so that their experiments are repeatable. Another thing regarding their evaluations is that they had the traditional routing protocol send the entire file to the next hop before the next node starts sending. They claim this is more fair, and that may indeed be the case, but since I don’t think this is what traditional routing normally does it seems like they probably should have tried it both ways to verify that the modified approach indeed does give traditional routing better performance.

Overall though I liked this paper and I think it should stay in the syllabus.