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In the route discovery phase, each node on the route promises that the next node is not a misbehaving node based on its observation. Therefore, all detected misbehaving nodes can be excluded from the route. However, it is impossible to detect all misbehaving nodes in the network due to various reasons. Furthermore, some nodes such as low power nodes, or rapidly moving nodes are more likely to make the route unstable. In order to find the most reliable route in all discovered multiple routes, a sender node can take advantage of local reputation to evaluate all available routes and find the optimal route to the destination.

In this solution, packet forwarding ratio is used to evaluate node's local reputation, so all intermediate nodes' local reputation information can be used to calculate the path's packet delivery ratio. Here, it means if a sender node transmits a packet, what is the probability that this packet can reach the destination node along the expected route (retransmission is not considered). And this is the most important requirement of secure data forwarding in mobile ad hoc networks. If a path has higher packet delivery ratio, it means this path is more reliable.

Figure 27. Path quality evaluation

In figure 27, if a sender node A wants to send a packet to the destination node E, and A has an available route including three intermediate nodes B, C and D. Due to misbehaving nodes have been excluded from the route, we can suppose that all these three nodes are normal nodes. Local reputation LRA(B) means when A sends the packet to B, what is the probability

could be regarded as the probability that the packet reaches C through B. Therefore, if all intermediate nodes’ local reputations are considered together, it presents the probability that the packet reaches the destination node E.

All intermediate nodes’ local reputations should be used to calculate the path quality. It is calculated with the formula

QoP = LRS(N1) * LRN1 (N2) * … * LRNi(Ni+1) * … * LRNn-1 (Nn) (6)

in which S represents the sender node, and Ni, represents an intermediate node on this path.

According to QoP (quality of path) of each available path, the sender node can discover the optimal route.

If QoP of a path is 0.95, it means if the sender node sends a packet along this path, the packet will reach the destination at the probability of 95 percent without retransmission. Of course, higher a path’s QoP is, better performance this path can provide, such as lower packet delivery delay and higher throughput due to fewer packets are lost and less retransmission occurs.

In [65], it showed that the minimum acceptable value for T (same as QoP here) should not fall below about 0.6 including misbehaving nodes as well as loss due to uncontrollable events, such as congestion or jammed links. This value is for connection-less UDP, and T must be much higher for TCP connection.

In [64], a quantitative estimation is performed to evaluate the performance of mobile ad hoc networks. And one observation is that if “p” (same as local reputation or packet forwarding ratio in this report) is high, even small increase of “p” can cause significant benefit on the network’s reachability. And in one example, it increases the network’s absolute reachability by about 400%.

In this solution, all intermediate nodes on available routes are well-behaving nodes and their p values are at least higher than the threshold which should be high enough to identify misbehaving nodes. So there are big differences among the values of QoP of all available paths. For example, we suppose node S has two discovered routes to node D, and each of them contains 4 intermediate nodes. If packet forwarding ratio of each intermediate node on one path is 90%, then the QoP of this path will be only 0.66. And if packet forwarding ratio of each intermediate node on another path is 98%, the QoP of that path will be 0.92. We can find that there is large performance difference between these two paths. And the optimal path could be the most reliable route for data forwarding and increase the network performance greatly.

The optimal route selection also needs to take hop count into account. Several rules could be used for selection of the optimal route.

1. Hop count is the first criterion followed by path quality. 2. Path quality is the first criterion followed by hop count.

It is obvious that if only path quality is considered, the path with highest path quality will be chosen which means this path will have the best performance. However, it is unrealistic to neglect hop count which is the only metric used by almost all routing protocols. And the energy constrain is also directly related to hop count. More intermediate nodes one path has, more energy is required to deliver a packet on this path.

If we suppose that the hop count of a path from A and B is n, and its path quality (packet delivery ratio) is p. If A sends a packet to B, the packet will be forwarded n times on this path. And the probability of the packet loss on this path is 1-p. If the packet is lost on the way, node A will have to retransmit this packet (TCP) or does not perform any reaction (UDP). For TCP traffic, a retransmitted packet will still need n times forwarding on this path and with the loss probability of 1-p. It can be calculated that in theory after

p

n

times, the packet can be guaranteed to reach its destination. Therefore, there is another rule for optimal route selection.

3. The path with the highest value of

p

n

Which rule to choose depends on concrete scenario. For example, for UDP, the second rule may be a good choice. However, the third rule may be a nice one for TCP.