A Framework for Reliable Routing in Mobile Ad Hoc Networks

1. A Framework for Reliable Routing in Mobile Ad Hoc Networks Zhenqiang Ye Srikanth V. Krishnamurthy Satish K. Tripathi

2. Reliable Routing • Routing for nodes communications • Nodes are vulnerable to failure • Battery drain • Compromise • One solution - multipath routing • Edge-disjoint path • Node-disjoint path

3. Outline • AODV • AODVM • A Framework For Reliable Routing

4. Ad Hoc Distance Vector Routing Protocol (AODV) • Source node floods RREQ message - if needs a route to a destination • Sends back RREP back to the source along the path by: • Destination, or • Intermediate node that knows a route to the destination • Intermediate node re-broadcast the RREQ only if • It does not know a route to the dest., and • It has not forwarded this RREQ

5. AODV (2) • Once a link fails, a RERR is sent to the src by the node that detects the link failure • Problem: some Possible node-disjoint paths to the dest may not be traced during query. • Reason: discarding duplicate RREQ

6. RREQ propagation in AODV

7. AODV-Multipath (AODVM) • An intermediate node keeps a RREQ table • Source • Destination • Neighbor who transmitted the RREQ • Hops to source • Expiration timer

8. AODVM – Destination Node • When dest. Receives the 1st RREQ • Update its sequence number • Generate an RREP which contains an additional field –‘last_hop_ID’ • Send back RREP along the same path but in the reverse direction. • Same for duplicate copies of RREQ

9. AODVM - Intermediate Node • When an intermediate node receives an RREP • Delete the entry of this neighbor from RREQ table • Add a routing entry to its routing table • Forward the RREP to a neighbor • which is in its RREQ table • whose path to the source is the shortest • Delete the entry of this neighbor from RREQ table • Other nodes delete the entry of this node from their RREQ tables

10. AODVM - Intermediate Node (2) • Sends back a RDER to its neighbor when its RREQ table is empty ( cannot forward a RREP) • The node who receives a RDER will try to forward RREP to another neighbor

11. AODVM – RRCM by Source • Source will send a RRCM in the first data packet sent on the corresponding route • An RRCM contains information of the route • Hop count • First hop relay • Last hop realy

12. AODVM – Loop Prevention • Sequence numbers: seq(i, j) - Node i’s latest sequence number known to node j • When source node initiates an RREQ, it increases seq(src, src) and seq(dst, src) by 1. • RREQ contains these two numbers denoted by seq(src, RREQ) and seq(dst, RREQ). • Destination will compute a new seq number when receives a RREQ: seq(dst, dst) = MAX{ seq(dst, RREQ), seq(dst, dst) } + 1 • Destination generates an RREQ that contains seq(dst, RREP) = seq(dst, dst)

13. AODVM – 2 Lemmas • Using AODVM, if a route is found , there is no loop in this route. • Using AODVM, if two routes are discovered, these two routes are node-disjoint.

14. AODVM – 2 Observations • The number of node-disjoint paths from a source to a destination is dependent on the node density in the ad hoc network. • As the distance between a source and its destination is increased, one could find no more than a very limited number of paths between them, even at moderate node densities (average node degree is 6.7). • Reason: sparse regions in the network act as bottlenecks.

15. Reliable Path • Reliable node • Power capable • Secure (e.g. in a tank) • Expensive to be deployed • Reliable segment • the number of node-disjoint paths that can be found between the end nodes of the segment is at least equal in number to k, or, • the segment entirety consists of reliable nodes. • Reliable path - consists of reliable segments

16. Reliable Path

17. Objective • Identify positions for the R-nodes such that the probability of the existence of a reliable path (given a value of k) between any two given nodes is high. • Achieved by modified min-cut algorithm

18. Min-cut Problem • Let G(V,E) be an undirected weighted graph which is connected. A cut in G is a partition of the vertices V into two nonempty sets S and ~S. • The value of a cut is the sum of the weights of the edges crossing the cut. • The min-cut is the cut(s) with the minimum cut value of all the possible cuts.

19. Contraction Algorithm • randomly choose an edge (x, y) in G and replace vertices x and y by a new vertex z; • for each v that is not in {x, y}, the weight of the new edge (v, z) is the sum of the weights of edge (v, x) and edge (v, y); the rest of the graph remains unchanged. • The contraction procedure is repeated until there are only two nodes and one edge left. • The cut value is then the weight of the edge that connects these two nodes. • If repeat the contraction algorithm O(n^2logn) times, the min-cut can be found with a reasonable probability.

20. Modified Min-cut Algorithm • Assume each node can obtain a partial topology view of the network (within k hops from itself). • Compute the min-cut of the partial graph of each node. • Remove the node and the edges incident on itself from the partial graph • Run the min-cut algorithm with modification: • The outermost links are contracted first, and the links that are closest to the node are contracted last.

22. The Distributed R-node Deployment Strategy • Each node periodically broadcasts a HELLO message to its neighbors within k hops. • In a HELLO message of a R-node, a flag indicates its motion status: • Static: position has been determined • Dynamic: in the process of determining where to move • Each normal node constructs 2 local topology graphs: with and without static R-nodes.

23. The Distributed R-node Deployment Strategy (cont’) • the weight of a direct link between two static reliable nodes is set to k. All the other links have weight of one. • A node periodically calculates it min-cut value and size of min-cut set and sends them out by HELLO. • A R-node moves to the proximity of the normal node that has the minimum min-cut value in its k-hop neighborhood. • If the min-cut values of two nodes are the same, R-node will move to the proximity of the node with a larger min-cut set.

24. Modifications to AODVM • A reliability flag is included in RREP • The flag is set to RELIABLE only this RREP passes through a R-node, otherwise, it is set to NORMAL. • If an intermediate R-node cannot find a next hop R-node to forward this RREP • it will split the RREP packet into multiple RREP packets equal in count to the number of neighbors specified in its RREQ table. • All of these RREP packets are marked NORMAL and then forwarded to the different neighbors.

26. Thank You!