*3.6.1. The resistance concept*

266 Wireless Sensor Networks – Technology and Protocols

second hand information, SHI.

hand information, FHI.

forwarding.

a mechanism must be:

FHI: The direct observation of the neighbor's behavior, this will be referred to as first

SHI: The opinion of other nodes regarding the neighbor of concern. This will be called

 Neutral Behavior Periods (NBP): these are the periods during which a neighbor is observed doing nothing. That is, a neighbor does not receive anything to be tested for

Each node in the system continuously and periodically updates the risk values of its

neighbors based on the information collected during these update periods .

Node i monitors node j for the duration of the update period, Tupdate.

Update the old risk value, ri,j,old using the new calculated ri,j,FHI to get ri,j.

When node j is observed by i for n consecutive update periods to be idle in its behavior, node i will give node j a chance to be more trusted by reducing its current risk value. A node is considered to be in idle behavior if it does not perform any routing operation. The reduction procedure follows exactly the same methodology explained in rating based on FHI when ri,j,FHI=0. The only difference here is that in the case of neutral behavior the update is done after we observe such behavior during n consecutive update periods whereas it is done immediately after an update period in the case of ri,j,FHI=0. The choice of n is a design parameter that depends on how much a network is tolerable against attacks. High values of

A detailed discussion and analysis of the CRATER approach, simulation setup,

Reputation systems are very complicated systems to evaluate or compare. This is because each system has its own components' implementation methods, like monitoring strategy, rating approach and response mechanism. All these components affect the efficiency of the reputation system individually as well as a complete system. Therefore, it is important to come up with a simple mechanism that can evaluate and analyze a reputation system. Such

 Independent of the reputation system: This means that the inputs of the formulae or equations used in this mechanism should not use the specific parameters that determine

how the individual component of the reputation system is working.

**3.6. Reputation systems-independent scale for trust on routing (RESISTOR)** 

The general algorithm that a node i follows to rate its neighbor j is:

Update ri,j if neutral behavior periods are realized.

n mean that we are not willing to forgive malicious nodes quickly.

performance measures, and simulation results, can be found in [85].

At the end of each update period, do the following:

Calculate ri,j,FHI using the new FHI.

Calculate the ri,j,SHI using the SHI.

Update ri,j using the ri,j,SHI

RESISTOR is an evaluation procedure that is used to evaluate the performance of reputation systems that are designed to provide trust aware routing. The basic idea behind RESISTOR is to utilize some of the objectives of a reputation system in an analytical way to evaluate the performance of the system.

Any reputation system that is concerned with trustworthy routing has two main objectives:


Having these two objectives, we introduce the resistance metric. We define, generally, the resistance between node i and a malicious node j in the direction from i to j; RESi,j, as a ratio of the risk value ri,j to the number of packets that flow from node i to j, i.e. Pi,j.

Please notice here that the concept of resistance is only associated with malicious nodes. Thus, if ri,j is high, the resistance value will be high, reflecting that the reputation system is performing well since we are "resisting" a malicious node. Similarly, if Pi,j is small, the resistance value gets high, inferring that the reputation system is performing well, too. This is because we expect to pass few packets to a malicious node, ideally zero packets.

The resistance concept is analogous to the resistance phenomenon in electric circuits. We can think of the risk value of a malicious node j as seen by i as the voltage difference between j and i and the packet flow from i to j as the current flow. The resistance, then, increases as the voltage, ri,j increases and the current Pi,j decreases similar to Ohm's law; R=V/I. Following this analogy, we have:

$$RES\_{i,j} = \frac{r\_{i,j}}{P\_{i,j}} \tag{2}$$

Thus, a good reputation system must provide high resistance. A perfect reputation system should provide an infinite resistance since Pi,j=0. A detailed discussion and evaluation of CRATER using RESISTOR approach, simulation setup, and simulation results, can be found in [85].
