Wireless Ad Hoc Network Routing Protocols

1. Wireless Ad Hoc Network Routing Protocols CSE 802.11 Maya Rodrig

2. Ad hoc networking • Infrastructureless networking – mobile nodes dynamically establish routing among themselves to form their own network on the fly. • Mobile nodes operate as routers • Mobile nodes participate in an ad hoc routing protocol

3. Why not reuse existing protocols? • Highly dynamic interconnection topology • LS generates loads of link status change msgs • DV suffers from out-of-date state or generates loads of triggered updates • Heavy computational burden on mobile nodes • Wireless medium differs in important ways from wired media

4. The Protocols • DSDV, TORA, DSR, AODV • Proactive vs. reactive (on-demand)

5. Destination-Sequenced Distance Vector (DSDV) • Preserve the simplicity of RIP while avoiding the routing loop problem • Hop-by-hop distance vector • Routing table contains entries for every reachable node • Each route is tagged with a sequence number originated by destination (even numbers) • Routing info is transmitted by broadcast • Updates are transmitted periodically and when there is a significant topology change

6. DSDV cont. • Route R is more favorable than R’ if R has a greater sequence number or if the two routes have equal sequence numbers but R has a lower metric (hop count) • Broken links are indicated by “” metric and the sequence number of destination is incremented to odd number before broadcast

7. No count to infinity

8. Temporally-Ordered Routing Algorithm (TORA) • Based on a “link-reversal” algorithm • Node broadcasts a QUERY packet which propagates to destination or to node having a route to the destination • Recipient of the QUERY broadcasts an UPDATE packet listing its height with respect to the destination • Each node that receives the UPDATE sets its height to be greater than the height of the neighbor from which the UPDATE came  creates a series of directed links from the QUERY originator to the node initiating the UPDATE

9. TORA cont. • When a node discovers a route is no longer valid, it adjusts its height so that it is a local maximum and transmits an UPDATE • When a network partition is detected, a node generates a CLEAR packet to reset routing state and remove invalid routes

10. Dynamic Source Routing (DSR) • Packet headers contain the route the packet must follow • Route Discovery: • Source node S broadcasts Route Request packet that is forwarded through the network • Destination node D or another node that knows a route to D answers with a Route Reply • Route Maintenance: • When the network topology has changed s.t. the route to D can no longer be used, a Route Error packet is sent to S • S can try another route to D from its cache or invoke Route Discovery again • Network interfaces in promiscuous mode  nodes cache overheard route information

11. DSR Example

12. Ad Hoc On-Demand Distance Vector (AODV) • Combination of DSR (on demand) and DSDV (hop-by-hop routing, sequ nums) • Node S broadcasts a Route Request message for destination D, including the last known sequence number for D • Node with a route to D generates a Route Reply with its sequence number for D • Nodes that forward Route Request store reverse route back to S; nodes that forward Route Reply store forward route to D

13. AODV cont. • No HELLO messages from neighbor indicate link is down • Nodes that recently forwarded packets using the failed link are notified via an UNSOLICITED ROUTE REPLY with infinite metric for the destination  reinitiate Route Discovery

14. Simulation Environment • Model attenuation of radio waves between antennas • Link layer implements 802.11 standard MAC protocol DCF • Broadcast packets sent only when virtual and physical carrier sense indicate the medium is clear (no RTS/CTS and no ACKs)

15. Methodology • Network simulation • 50 wireless nodes moving in 1500m*300m flat space • Over 200 different scenarios • Movement model • “Random waypoint” model (pause times: 0, 30, 60, 120, 300, 600, 900 seconds) • Avg speed 10 meters/second • Communication model • Sending rates: 1, 4, 8 packets/second • 10, 20, 30 CBR sources • Packet size of 64 bytes

16. Metrics • Packet delivery ratio- ratio between num packets originated by sources and num packets received at their destination • Routing overhead- num routing packets transmitted during the simulation • Path optimality- difference between the num hops a packet took to reach its destination and the length of the shortest path

17. Packet Delivery Ratio • DSR and AODV deliver over 95% of data packet • TORA does well with 20 sources • DSDV fails to converge at pause time < 300

18. Routing Overhead • TORA, DSR, AODV are on demand • DSDV is largely periodic • DSR limits overhead of Route Requests through caching

19. Path Optimality • Internal mechanism knows the length of the shortest path between all nodes at any time • DSDV and DSR use routes close to optimal • AODV and TORA have a tail

20. Another Protocol: Greedy Perimeter Stateless Routing (GPSR) • Geography to achieve scalability in wireless routing protocols • Assume bidirectional radio reachability • Assume a location registration and lookup service that maps node addresses to locations • Position of a packet’s destination and positions of candidate next hops sufficient to make correct decisions

21. Greedy Forwarding • Beaconing algorithm provides all nodes with their neighbor’s positions • Packets are marked with their destinations’ locations • A forwarding node makes a locally optimal greedy choice: next hop is the neighbor geographically closest to the destination Problem: topologies in which the only route to the destination requires temporarily moving farther in geometric distance from the destination

22. Planar Perimeters • Right-hand rule : when arriving at node x from node y, the next edge traversed is the next one sequentially counterclock-wise about x from edge (x,y)  navigating around the void • Construct planarized graphs to eliminate crossing links from the network without partitioning the network

23. GPSR versus DSR Routing Overhead Packet Delivery Success Rate

24. Comparison cont. Path Length Network Diameter

25. Choosing Routes • Shortest path is not a good metric  choose routes with less capacity than best existing paths • Minimum hop-count routes include links with high loss ratios  retransmissions consume bandwidth

26. Link Behavior in Experimental Networks • Link quality distribution is spread out • 30% of link pairs are unusable • Best 40% of link pairs deliver 90% of their packets • 30% link pairs have asymmetric delivery rate • Delivery rates sometimes change very quickly (averaging not applicable) • No good correlation between delivery rate and radio’s signal strength We need practical estimates for link quality and ways to combine link metrics into path metrics

27. Expected Transmission Count (ETX) • Find paths with fewest expected number of transmissions required to deliver a packet to its destination • Use per-link measurements of delivery ratios in both directions • Modified DSDV and DSR • ETX outperforms minimum hop-count • ETX incurs more overhead due to loss-ratio probes

28. … • Early protocols assume cooperating nodes that are willing to forward packets for others • The role of power in routing protocols