Wireless Ad hoc networks Routing

1. Wireless Ad hoc networks Routing

2. Proposed ad hoc Routing Approaches • Conventional wired-type schemes (global routing, proactive): • Distance Vector; Link State • Proactivead hoc routing: • OLSR, TBRPF • On- Demand, reactive routing: • DSR (Source routing), MSR, BSR • AODV (Backward learning)

3. Wireless multihop routing challenges • mobility • need to scale to large numbers (100’s to 1000's) • need to support multimedia applications (QoS) • unreliable radio channel (fading, external interference, mobility, etc) • limited bandwidth • limited power

4. Conventional wired routing limitations • Distance Vector (eg, Bellman-Ford, BGP): • Tables grow linearly with # nodes • routing control O/H linearly increasing with network size • convergence problems (count to infinity); potential loops (mobility?) • Link State (eg, OSPF): • link update flooding O/H caused by network size and frequent topology changes CONVENTIONAL ROUTING DOES NOT SCALE TO SIZE AND MOBILITY DV LS Intra-AS RIP OSPF Inter-AS BGP

5. Proactive ad hoc schemes– OLSR and TBRPF • Link State explodes because of Link State update overhead • Question: how can we reduce the O/H? • Answer: Link State with “Topology reduction” • (1) if the network is “dense”, use fewer forwarding nodes • (2) if the network is dense, advertise only a subset of the links • Two leading IETF Link State schemes enhance scalability in large scale networks: • OLSR : Optimal Link State Routing • TBRPF: Topology Broadcast Reverse Path Routing

6. 24 retransmissions to diffuse a message up to 3 hops Retransmission node LSR (Link State Routing) • In LSR protocol a lot of control msg unnecessary duplicated

7. 11 retransmission to diffuse a message up to 3 hops Retransmission node OLSR (Optimal Link State Routing) • In OLSR only a subset of neighbors (MPR-Multipoint Relay Selectors) retransmit control messages: • Reduce size of control message; • Minimize flooding

8. OLSR Overview • RFC 3626, October 2003 • In LSR protocol a lot of control messages unnecessarily duplicated • In OLSR only a subset of neighbors (MPR-Multipoint Relay Selectors) retransmit control messages • Reduce flooding overhead • Adapted for dense network • OLSR retains all the advantages of LSR: • stable; • Does not depend upon any central entity; • Tolerates loss of control messages; • Supports nodes mobility

9. On-Demand Routing Protocols • Routes are established “on demand” as requested by the source • Only the active routes are maintained by each node • Channel/Memory overhead is minimized • Two leading methods for route discovery: source routing and backward learning (similar to LAN interconnection routing)

10. Existing On-Demand Protocols • Dynamic Source Routing (DSR) -- CMU • Multipath Source Routing (MSR) – TJU • Backup Source Routing (BSR) – UofO+TJU • Ad-hoc On-demand Distance Vector (AODV) • Associativity-Based Routing (ABR) • Temporarily Ordered Routing Algorithm (TORA) • Zone Routing Protocol (ZRP) • Location assisted routing (LAR, DREAM) • Signal Stability Based Adaptive Routing (SSA) • On Demand Multicast Routing Protocol (ODMRP) – UCLA

11. Dynamic Source Routing (DSR) • RFC 4728 – February 2007 • Forwarding: source route driven instead of hop-by-hop route table driven • Mobility ? • No periodic routing update message is sent • The first path discovered is selected as the route • Two main phases • Route Discovery • Route Maintenance

12. DSR - Route Discovery • To establish a route, the source floods a Route Request message with a unique request ID • The Route Request packet “picks up” the node ID numbers • Route Reply message containing path information is sent back to the source either by • the destination, or • intermediate nodes that have a route to the destination • Each node maintains a Route Cache which records routes it has learned and overheard over time

13. DSR - Route Maintenance • Route maintenance performed only while route is in use • Monitors the validity of existing routes by passively listening to acknowledgments of data packets transmitted to neighboring nodes • When problem detected, send Route Error packet to original sender to perform new route discovery

14. MSR - Multipath Source Routing • Direct Descendant of DSR • On-demand + Source Routing + Multipath • Probing-based adaptive load balancing among multiple paths • Motivation of MSR • Efficiently using the network resource • Alleviate the oscillation in adaptive single path routing • Fast re-routing • Reducing computing & storage requirement • Exploiting computing power of host instead of link capacity

15. Distributing Traffic among Multiple Paths • Quantities: A heuristic equation • Probing-based adaptive control • Decoupling between transport layer and network layer: SRPing • Cost effective • Scheduling: Packet Weighted Round Robin • TCP out-of-order (re-sequencing) problem

16. Distributing Traffic among Multiple Paths • Heuristic equation • Rationale: Autonomous system, homogeneous assumption, bandwidth-delay product constant where , is the delay of route with index i, is the maximum delay of all the routes to the same destination, R is a factor to control the switching frequency between routes. U is a bound value to insure that should not to be too large.

17. MSR Summary • Reduce network congestion • Improve throughput, delay, mobility, fault tolerance (CBR & FTP) • Acceptable routing overhead? • Little more than that of DSR • Route discovery • Route maintenance • Probing (unicast) add little O/H • Good candidate for QoS support • QoS-MSR, reliable-MSR • Acceptable packet out-of-order level ?

18. Backup Source Routing (BSR) • Establish and maintain backup routes that can be utilized after the primary path breaks • Define a new routing metric - route reliability, and use it to provide the basis for the backup path selection • Reduce the frequency of route discovery flooding, which is a major overhead in on-demand protocols • Can improve the performance significantly in more challenging situations of high mobility

19. Simulation Methodology • ns – Wireless extensions by CMU • Adopt methods used in [Broch98, Johnson99] • Two major files: • Movement pattern file • Communication pattern file • 50 mobile hosts placed randomly within a 1500m×300m area • 20 connections • Different traffic types: CBR & FTP • Two set of simulations: Max speed 20m/s & 1m/s

20. Performance Evaluation • MSR vs. DSR vs. BSR • Performance Metrics • Packet delivery ratio • Data throughput • End-to-end delay • Packet drop probability • Queue size

21. Simulation Results with UDP Traffic -- Packet delivery ratio for 20 sources 8

22. Simulation Results – CBR • End-to-end throughput

23. Simulation Results with UDP Traffic -- Average end-to-end delay for 20 sources 11

24. Simulation Results - CBR • Packets dropped at each node

25. Previous Work on Using Multiple Paths • Alternate use (primary and backup) • It works OK for CBR traffic (BSR, Bypass - DSR, Node Disjoint M-path AODV, etc) • TCP does not get much benefit. Backup path is used only after timeout; not efficient in mobility/errors.? • Concurrent use (ie, packet scattering) • MSR • TCP does well in a static, error free net with long paths (up to 50% improvement) • With mobility & errors, TCP suffers out-of-order problems because of RTT difference on the two paths

26. “TCP Performance on multiple paths in ad hoc nets..” Liaw et al ICC 2004 Static net, no errors, opt W: max improvement 50%; typical improvement between 8% and 18%

27. Multiple Path TCP with Packet Replicas • TCP data packet duplication on multiple paths • May introduce less O/H than repeated end to end retransmissions • Improve end-to-end route robustness when single route is not stable: • Replicate packet on multiple paths • Combat random, non correlated link losses • Combat path breakage

28. Variable Loss Rate[ 0.05;0.1;0.15;0.2] Total Throughput(bits/s) Mobility(m/s) Original TCP Multipath TCP