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Ad hoc routing

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An ad-hoc routing protocol is a convention, or standard, that controls how nodes decide which way to route packets between computing devices in a mobile ad hoc network . …

An ad-hoc routing protocol is a convention, or standard, that controls how nodes decide which way to route packets between computing devices in a mobile ad hoc network .

In ad-hoc networks, nodes are not familiar with the topology of their networks; instead, they have to discover it.The basic idea is that a new node may announce its presence and should listen for announcements broadcast by its neighbors.Each node learns about nodes nearby and how to reach them, and may announce that it, too, can reach them

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  • 1. Mobile Networks Module IRouting in mobile ad hoc networks Prof. JP Hubaux http://mobnet.epfl.ch 1
  • 2. The classical solution for mobile networks 2nd generation (GSM, IS-41,…) and 3rd generation (UMTS,…) deployed soon Huge, expensive fixed infrastructure License for a share of the spectrum Operational responsibility: network operators (telcos, ISPs) 2
  • 3. The new paradigm: ad hoc networks Terminal and node merge Everything is potentially mobile Initial applications: communication in the battlefield (Packet Radio Networks, in the 70’s) The network is self-organized when it is run by the users themselves Similar trend at the application layer: peer-to-peer 3 (e.g., Napster  Gnutella)
  • 4. Application examples of ad hoc networks Sensor networks Hybrid cellular / ad hoc networks (multi-hop cellular networks) Cars Assisted driving (adaptive cruise control,…) Collision avoidance Optimization of traffic flows … Crisis networks (e.g., rescue operations after major disaster) Military networks 4
  • 5. Upper bound for the throughput of ad hoc networks If we have: - n identical randomly located nodes - each capable of transmitting W bits/s Then the throughput  ( n) obtainable by each node for a randomly chosen destination is  W   (n)     n log n     Ref: P. Gupta, P. Kumar, The Capacity of Wireless Networks IEEE Transactions on Information Theory, March 2000 5
  • 6. Intuition behind the upper boundN nodes (users) O(N) transmissions from left to right over O( N) transmission links mean O( 1 ) capacity per attempted transmission NO(N) users O(N) users Ways to improve scalability: Cut set ~ N • Directional antennas • Locality of the traffic • Hybrid system 6
  • 7. Routing in ad hoc networks Peculiarities Node mobility High rate of link failure  Traditional routing approaches are not well suited Assumptions Multihop communication Symmetric links (in most cases) Omnidirectional antennas (in most cases) All nodes have equal capabilities and responsibilities Figures of merit Latency of route discovery Overhead (bandwidth, energy, processing power) Security Current status of research: Many, many proposals Optimal solution depends on deployment scenario: mobility patterns, radio model, traffic characteristics,… 7
  • 8. Brief reminder : Link-state protocols Example: OSPF May consume a lot of resources to update the routes Techniques to alleviate the problem : limit the propagation of information Does not seem to be well suited to cope with mobility 8
  • 9. Distance vector routing (1/2) B A B C D Distance vector 3A 0 1 5  1 1 A DB 1 0 1 3 5 7C 5 1 0 7 CD  3 7 0 (1 row stored in each node) Distance 1 0 1 3 vector of B Take the min + Distance from A to B = 2 1 2 4 Cost to dest. via B 0 1 2,B 4,B 9
  • 10. Distance vector routing (2/2) Even if the updates are asynchronous, the routing tables converge The algorithm is often called Bellman-Ford Problem: undesirable behaviour when links go up and down (e.g., count to infinity problem) 10
  • 11. Routing protocols for wireless ad hoc networksMobile ad hoc networks Sensor networks Response time, Energy bandwidth Proactive Reactive protocols protocols Dynamic Optimized Link- Ad Hoc On-DemandDestination-Sequenced Source Geography- Cluster-based State Routing Distance-VectorDistance-Vector (DSDV) Routing based routing (or hierarchical) (OLSR) (AODV) (DSR) routing Geodesic packet 11 forwarding
  • 12. Dynamic source routing (DSR) Reactive routing protocol 2 phases, operating both on demand: Route discovery • Used only when source S attempts to to send a packet to destination D • Based on flooding of Route Requests (RREQ) Route maintenance • makes S able to detect, while using a source route to D, if it can no longer use its route (because a link along that route no longer works) 12
  • 13. DSR: Route discovery (1) F K HQ A S E G D P J B M R I L C N 13
  • 14. DSR: Route discovery (2) F K HQ A S E G D P (S) J B M R I L C N 14
  • 15. DSR: Route discovery (3) (S,A) K F HQ A (S,E) S E G D P J B M R I L C N 15
  • 16. DSR: Route discovery (4) F K HQ A S E G (S,E,G) D P J B M R I L C (S,B,C) N 16
  • 17. DSR: Route discovery (5) (S,A,F,H) F K HQ A S E G (S,E,G,J)D P J B M R I L C N 17
  • 18. DSR: Route discovery (6) F K H (S,A,F,H,K)Q A S E G D P J B M R I L C N 18
  • 19. DSR: Route discovery (7) F K HQ A S E G D P J (S,A,F,H,K,P) B M R I L C N 19
  • 20. DSR: Route discovery (8) F K HQ A S E G D P J RREP(S,E,G,J,D) B M R I L C N 20
  • 21. DSR: Route Discovery (9) Route reply by reversing the route (as illustrated) works only if all the links along the route are bidirectional If unidirectional links are allowed, then RREP may need a route discovery from D to S Note: IEEE 802.11 assumes that links are bidirectional 21
  • 22. DSR: Data delivery F K HQ A DATA(S,E,G,J,D) S E G D P J B M R I L C N 22
  • 23. DSR: Route maintenance (1) F K HQ A DATA(S,E,G,J,D) E P X S G D J B M R I L C N 23
  • 24. DSR: Route maintenance (2) F K HQ A RERR(G-J) E P X S G D J B M R I L C N When receiving the Route Error message (RERR), S removes the broken link from its cache. It then tries another route stored in its cache; if none, 24 it initializes a new route discovery
  • 25. DSR: Optimization of route discovery: route caching Principle: each node caches a new route it learns by any means Examples When node S finds route (S, E, G, J, D) to D, it also learns route (S, E, G) to node G In the same way, node E learns the route to D Same phenomenon when transmitting route replies Moreover, routes can be overheard by nodes in the neighbourhood However, route caching has its downside: stale caches can severely hamper the performance of the network 25
  • 26. DSR: Strengths Routes are set up and maintained only between nodes who need to communicate Route caching can further reduce the effort of route discovery A single route discovery may provide several routes to the destination 26
  • 27. DSR: Weaknesses Route requests tend to flood the network and generally reach all the nodes of the network Because of source routing, the packet header size grows with the route lengh Risk of many collisions between route requests by neighboring nodes  need for random delays before forwarding RREQ Similar problem for the RREP (Route Reply storm problem), in case links are not bidirectionalNote: Location-aided routing may help reducing the number of useless control messages 27
  • 28. Ad Hoc On-Demand Distance Vector Routing (AODV) As it is based on source routing, DSR includes source routes in data packet headers Large packet headers in DSR  risk of poor performance if the number of hops is high AODV uses a route discovery mechanism similar to DSR, but it maintains routing tables at the nodes AODV ages the routes and maintains a hop count AODV assumes that all links are bi-directional 28
  • 29. AODV : Route discovery (1) F K HQ A S E G D P J B M R I L C N 29
  • 30. AODV : Route discovery (2) F K HQ A S E G D P J B M R I L C N : Route Request (RREQ) Note: if one of the intermediate nodes (e.g., A) 30 knows a route to D, it responds immediately to S
  • 31. AODV : Route discovery (3) F K HQ A S E G D P J B M R I L C N : represents a link on the reverse path 31
  • 32. AODV : Route discovery (4) F K HQ A S E G D P J B M R I L C N 32
  • 33. AODV : Route discovery (5) F K HQ A S E G D P J B M R I L C N 33
  • 34. AODV : Route discovery (6) F K HQ A S E G D P J B M R I L C N 34
  • 35. AODV : Route discovery (7) F K HQ A S E G D P J B M R I L C N 35
  • 36. AODV : Route reply and setup of the forward path F K HQ A S E G D P J B M R I L C N : Link over which the RREP is transmitted 36 : Forward path
  • 37. Route reply in AODV In case it knows a path more recent than the one previously known to sender S, an intermediate node may also send a route reply (RREP) The freshness of a path is assessed by means of destination sequence numbers Both reverse and forward paths are purged at the expiration of appropriately chosen timeout intervals 37
  • 38. AODV : Data delivery F K HQ A Data S E G D P J B M R I L C NThe route is not included in the packet header 38
  • 39. AODV : Route maintenance (1) F K HQ A Data E P S G X J D B M R I L C N 39
  • 40. AODV : Route maintenance (2) F K HQ A RERR(G-J) E P S G X J D B M R I L C N When receiving the Route Error message (RERR), S removes the broken link from its cache. It then initializes a new route discovery. 40
  • 41. AODV: Destination sequence numbers If the destination responds to RREP, it places its current sequence number in the packet If an intermediate node responds, it places its record of the destination’s sequence number in the packet Purpose of sequence numbers: Avoid using stale information about routes Avoid loops (no source routing!) 41
  • 42. AODV : Avoiding the usage of stale routing1. S … tables A D S A … 2. DSN(D) = 5 DSN(D) = 5 B B DSN(D) = 8 : Forward path D3. S A … 4. S A … RREQ DSN(D) = 5 RREP DSN(D) = 5 B B DSN(D) = 8 DSN(D) = 8 42 D D
  • 43. AODV : Avoiding loops A B S X D C : Forward path• Assume there is a route between A and D; link S-D breaks; assume A is not aware of this, e.g. because RERR sent by S is lost• Assume now S wants to send to D. It performs a RREQ, which can be received by A via path S-C-A• Node A will reply since it knows a route to D via node B• This would result in a loop (S-C-A-B-S)• The presence of sequence numbers will let S discover that the routing information from A is outdated• Principle: when S discovers that link S-D is broken, it increments its local value of DSN(D). In this way, the new local value will be greater than the one stored by A. 43
  • 44. AODV (unicast) : Conclusion Nodes maintain routing information only for routes that are in active use Unused routes expire even when the topology does not change Each node maintains at most one next-hop per destination Many comparisons with DSR (via simulation) have been performed  no clear conclusion so far 44
  • 45. Geodesic Packet Forwarding L. Blazevic, S. Giordano, J.-Y. Le Boudec (IP4) AP -geographical anchor point AGPF (Anchored Geodesic Packet Forwarding) - source routing with anchors AP1 B A AP2S D X TLR area of X S has anchored path {AP1,AP2} from S: packets are forwarded in direction of AP1 from A: packets are forwarded in direction of AP2 from B: packets are forwarded in direction of D’s position 45 from X : use of Terminode Local Routing (TLR)
  • 46. Other (Swiss) proposals Last Encounter Routing  H. Dubois-Ferrière, M. Grossglauser, M. Vetterli (EPFL, IP1 & 7) Principle: Nodes exchange information about their previous encounters No explicit location service, no transmission overhead to to update the state Ongoing work: prediction, based on declared mobility Face routing F. Kuhn, R. Wattenhofer, A. Zollinger (ETHZ, IP9) Principle: exploit the geometric properties of the connectivity graph Worst-case optimal 46
  • 47. Conclusion on routing DSR and AODV are the mainstream proposals Both have been extensively studied (by simulation) No clear superiority of one wrt the other Scalability is still an open issue Other very promising proposals 47
  • 48. References Ch. Perkins: Ad Hoc Networking, Addison Wesley, 2001 www.mics.org: IP1, IP4, IP7 and IP9 48