1. ExOR: Opportunistic Multi-Hop Routing for Wireless Networks Sigcomm 2005 Sanjit Biswas and Robert Morris MIT Computer Science and Artificial Intelligence Laboratory Presented by Sungwon Yang 2009.05.12

2. What is ExOR?  Extremely Opportunistic Routing  Routing in multi-hop wireless networks  Cross-Layer Protocol: Routing + MAC  Aims to increase the throughput of large unicast transfers  Based on cooperative diversity routing

3. Motivation  Traditional routing protocols were designed for wired networks  Identify a route, forward over links  These protocols don’t take into account underlying wireless dynamics at MAC and PHY layer packet src AB dst C

4. Motivation  Radio is not wired  Every packet is broadcast  Reception is probabilistic 1234561 23635 1 42345612456 src AB dst C

5. Basic concept of ExOR  exploiting probabilistic broadcast  Decide who forwards after reception  Goal: only closest receiver should forward packet src A B dst C packet

6. Why ExOR might increase throughput (1)  Best traditional route over 50% hops: 3( 1 / 0.5 ) = 6 tx  Throughput  1 / # transmissions  ExOR exploits lucky long receptions  Assumes probability falls off gradually with distance srcdstN1N2N3N4 75% 50% N5 25%

7. Why ExOR might increase throughput (2)  Traditional routing: 1 / 0.25 + 1 = 5 tx  ExOR: 1 / (1 – (1 – 0.25) 4 ) + 1 = 2.5 transmissions  Assumes independent losses N1 srcdst N2 N3 N4 25% 100%

8. ExOR Design Challenges  How to determine which nodes have received a packet?  Agreement amongst the nodes which received each packet  What node (of the receivers) should forward a packet?  Need for a metric which decides the node which is closest to the destination  Minimize communication cost of coordination  Not too many nodes should be potential forwarders  Minimize collisions

9. ExOR Mechanism: Source’s Behavior  Collects enough packets of the same destination to form a batch  ExOR operates on batches of packets for efficiency  Source gathers batch of packets to same destination  Selects a set of nodes to be candidate forwarders, and includes the prioritized list in the header of every packet  Potential forwarders are prioritized by estimated cost to destination (by sender)  ETX (Expected Transmission Count)  Forwarding in order of priority

10. What is ETX (1)  Expected Transmission Count  Proposed by the MIT AI Lab in MobiCom 2003  Predict the number of transmission(including retransmission)  Designed for finding the high-throughput path in DSDV & DSR routing protocols  Using periodical probe packets

11. What is ETX (2)  Forward list: ECDBA  Broadcast in this order

12. ExOR Mechanism: Intermediate nodes’ Behavior (1)  Q: How can a node know whether it is one of the forwarders or not?  A: Check the forwarder list in the header of the received packet  If the node finds itself in the list, buffer the packet and keep state of this batch  If no, discard the packet

13. ExOR Mechanism: Intermediate nodes’ Behavior (2)  Q: How can a node know whether the packet it receives has also been received by a node with higher priority or not?  A: ExOR uses “Batch Map”  Acts as a gossip mechanism to carry reception information-- from high priority nodes to low  Included in every transmission so that node’s local batch maps will converge  Low priority node unlikely to forward a packet received by high-priority node

14. ExOR Mechanism: Intermediate nodes’ Behavior (3)  Q: How can a node know when it should send packets?  A: ExOR uses “Forwarding Timer”  Initially set long-enough  A node adjusts the timer when it hear other nodes’ packets  “Transmission Tracker” keeps track of the remaining number of packets needed to be sent

15. ExOR Mechanism: Destination’s Behavior  Actually destination is the last intermediate node and has the highest priority.  After the finish of src’s transmission. Destination sends out packets only including the batch map, to inform other nodes about the packets it has received  Upon >90% of batch reception in batch map, packet is not forwarded further -- finish using traditional mechanisms

16. Evaluation  Does ExOR increase throughput?  When/why does it work well? 1 kilometer Roofnet: 38 nodes ExOR implemented on Linux with 802.11b 65 node pairs randomly chosen 1.0MByte file transfer 1 Mbit/s 802.11 bit rate 1 KByte packets 9 iterations Traditional RoutingExOR 802.11 unicast with link- level retransmissions 802.11 broadcasts 100 packet batch size

17. Results (1)  Median Throughput  240 Kbits/sec for ExOR  121 Kbits/sec for Traditional Throughput (Kbits/sec) 1.0 0.8 0.6 0.4 0.2 0 0200400600800 Cumulative Fraction of Node Pairs ExOR Traditional

18. Results (2)  25 Highest throughput pairs Node Pair Throughput (Kbits/sec) 0 200 400 600 800 1000 ExOR Traditional Routing 1 Traditional Hop 1.14x 2 Traditional Hops 1.7x 3 Traditional Hops 2.3x

19. Results (3)  25 Lowest throughput pairs Node Pair 4 Traditional Hops 3.3x Longer Routes Throughput (Kbits/sec) 0 200 400 600 800 1000 ExOR Traditional Routing

20. Results (4)  ExOR moves packets farther Fraction of Transmissions 0 0.1 0.2 0.6 ExOR Traditional Routing 01002003004005006007008009001000 Distance (meters) 25% of ExOR transmissions 58% of Traditional Routing transmissions

21. Conclusion & Secret Sauce  Exploits radio properties, instead of hiding them  Benefit from long and lossy link  Also work well on one-hop link  ExOR achieves 2x throughput improvement  Real implementation and experiments  Clearly-defined primary goal  Achieve high throughput in large unicast transfer

22. Thank you