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Mobile Ad-hoc Networks

Mobile Ad-hoc Networks. Dr. Muazzam A. Khan. The manet problem. Mobile Random and perhaps constantly changing Ad-hoc Not engineered Networks Elastic data applications which use networks to communicate. Examples of such networks. Sensor networks Automotive networks

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Mobile Ad-hoc Networks

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  1. Mobile Ad-hoc Networks Dr. Muazzam A. Khan

  2. The manet problem • Mobile Random and perhaps constantly changing • Ad-hoc Not engineered • Networks Elastic data applications which use networks to communicate

  3. Examples of such networks • Sensor networks • Automotive networks • Military applications

  4. Networks deployed in random distribution Low power Delivering sensor data to a central site for some purpose Sensor networks

  5. “Smart cars” and “smart roads” Onboard systems “talk” to the “road”: Map obstacles and delays Obtain maps Inform the road of its actions Traffic networks

  6. Combat regiment in the field Perhaps 4000-8000 objects in constant unpredictable motion… Intercommunication of forces Proximity, function, plan of battle Special issues Low probability of detection Random association and topology Military applications

  7. Basis of routing: • Bellman-Ford-Moore proof In any graph there exists a spanning tree Ford (1956), Bellman (1958), Moore (1958) • Spanning tree: A set of arcs which visits every node in a graph exactly once

  8. Proof by construction • Select one node in graph • Place all arcs connected to that node in set of “available choices” • Place all other nodes and arcs in set of “unavailable choices” • Repeatedly select an arc from “available choices” If it leads to an “unavailable” node, Add that node and arc to the graph, Add all other arcs connecting to the new node to “available choices” Otherwise, discard the arc • All nodes get included in the graph Some arcs may be left over

  9. Issues in routing • Selection of arcs (metrics and objectives) • Characteristics of various interfaces • Convergence • Algorithms

  10. Selection of arcs (metrics) • Available bandwidth (traffic engineering) • Link speed (shortest path) Often measured as duration of a bit, or duration of an operation • Hop count or other administrative distance Might be power or S/N ratio for radio • General policy (BGP)

  11. Characteristics of various interfaces • LAN: Many to many • Frame Relay/ATM/MPLS: 1:many circuits • Serial Lines: 1:1 • Radio: 1:many with inconsistent sets Often unidirectional S/N ratio and distance affect connectivity

  12. Convergence goals • Ensuring routing consistency Algorithms vary by type of properties required • Managing inconsistency The backbone is said to never actually converge Manet networks probably don’t either

  13. Common algorithms • Exploration Network is explored to find possible routes Of which one is selected Example: IEEE 802.5 Source Routing • Bellman-Ford algorithm does this in a distributed manner Called "Distance Vector" these days Generates routes to a node from somewhere else • Shortest Path First (Ford, CACM, 1968) does this in a database Djikstra applied to networks, 1972 Generates routes from a node to somewhere else

  14. Specific issues in manet networks • Dynamic Connections Constant change (motion) Random interconnection • Radio Characteristics Unidirectional Links Varying S/N Ratio Overlapping connectivity

  15. Overview of Proposals • Reactive (on-demand) Protocols Overview of DSR Overview of AODV • Proactive (pre-calculated) Protocols Overview of TBRPF Overview of OLSR

  16. Dynamic Source Routing “IEEE 802.5 Source Routing” in wireless networks

  17. A route between two nodes is found by sending an Route Request Route Request builds a source route on every path through the network First Route Request to arrive is accepted; target responds on that path and tells initiator what the source route is Source route is used on subsequent data traffic Route Discovery K H I J E F G B C D Detected Source Route: (A, B, F, J, K) A

  18. Link reliability • Passing source routes cached To know how to send routing error messages • Network layer acknowledgements watched To ensure delivery To detect failed routing nodes • After several retransmissions, Neighboring routing node declared dead Route Error sent to senders using the node Packets flushed

  19. When routing node/routing node link changes, existing source routes no longer work routing nodes respond to source routes with a Route Error, triggering a new route discovery routing nodes may attempt to change source route and re-forward on a cached route Route Maintenance K H I J E F G B C D Route Error A

  20. Status of testing • Simulations and some operational testing • Testing shows some documented features in use of route cache don’t work properly • Ongoing research, seeking experimental status

  21. Ad-hoc On-Demand Distance Vector

  22. Target networks • Targets networks in which Routing churn is high enough that maintaining routes is unproductive, and Can absorb a network wide broadcast rate • Modified on-demand Distance Vector routing

  23. Messages • Route Request: “I need a route” • Route Response: “Route advertisement” • Route Error: “Withdraw route” • Periodic route response to neighbors acts as “hello”, installing and refreshing route

  24. A route between two nodes is found by sending an Route Request to a locality Initial locality small, grows with failure After that, a little larger than the locality target last found in Route Response sent By target if necessary By neighboring routing node if possible to “join” existing route Network stores the route Route Discovery C j k l g h i e d f A B

  25. Routes expire if not refreshed routing nodes log recent downstream users of a route When routes expire or are flushed, downstream users are notified to flush New route request triggered Route Errors C j k l g h i e d f A B

  26. Status of testing • Simulations and some operational testing • Not perfect, but few major problems • Ongoing research, seeking experimental status

  27. Topology Broadcast Based on Reverse-Path Forwarding

  28. Target networks • Targets networks in which Routing churn is consistent with pre-calculated routes, and Capable of supporting fairly intelligent device • Modified Shortest Path First routing Routing by “need to know”

  29. Messages • “Hello” Neighbor discovery and relationship maintenance • Topology Update Distributes subset of interconnectivity information

  30. Therefore every routing node effectively calculates all routes from all nodes for which it has information Topology information distributed to the set of systems that are deemed to need it. You need information about a routing node if your routing would use it… Topology Update Distribution

  31. Status of testing • Simulations and some operational testing Boeing and SRI actively promoting Not all algorithms publicly documented • Not perfect Needs operational experience to debug • Ongoing research, may seek experimental status

  32. Optimized Link State Routing

  33. Target networks • Targets networks in which Routing churn is consistent with pre-calculated routes, and Capable of supporting fairly intelligent device • Modified Shortest Path First routing Level 1/Level 2 routing distinction

  34. Messages • “Hello” Neighbor discovery and relationship maintenance • Topology Update Distributes subset of interconnectivity information

  35. Neighbor relationships • Each device emits a periodic “Hello” Advertise itself to its neighbors Determine who else is there Select some systems to act as Multi-Point Relays

  36. Multi-Point Relays • Passes Topology Information Acts as router between hosts, in wired parlance Minimizes information retransmission Forms a routing backbone

  37. MPRs form routing backbone Other nodes act as “hosts” As devices move Topological relationships change Routes change Backbone shape and composition changes Structure of an OLSR Network

  38. Status of testing • Simulations and some operational testing Inria in active research Not all algorithms publicly documented • Not perfect Needs operational experience to debug • Ongoing research, may seek experimental status

  39. Status of research

  40. Open problems • Behavior of applications in manet networks TCP likely to not work well if loss leads to congestion avoidance ECN (RFC 3168) allows separation of the concepts Various other research proposals

  41. Open problems in DSR • Route Maintenance as specified results in problems in routing Man-in-the-middle issues • Requires protocol number from IANA Small number space We don’t give those to just anybody

  42. Open problems in TBRPF • High computational overhead • High messaging overhead

  43. Open problems in OLSR • Selection and dynamic minimization of MPR set • Operational testing of protocol

  44. Open problems in AODV • Protocol needs operational experience to discover further issues

  45. Directions for manet working group • Probably will take several protocols to “experimental” status RFC is an archive, not a standard • Likely to standardize One reactive protocol One proactive protocol

  46. Mobile Ad-hoc Networks

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