**4.2. The enhanced protocol: GETAR**

## *4.2.1. Basic idea*

GETAR is a geographic and energy aware routing protocol that has the additional feature of trust awareness. The trust awareness is achieved by the rating functionality of a running reputation system that will feed the routing protocol with the trust metric that will be the risk values, ri,j. The risk value ri,j , as discussed earlier, is a quantity that reflects, to some extent, the expectation that a node j will not forward the packet received from node i, assuming non forwarding attack.

The risk value metric, along with distance and energy metrics, are used to compute the learned cost function for each neighbor. The concerned node, then, makes the routing decision by selecting the neighbor of lower cost as in normal GEAR.

As we can see, GETAR is a modification extension to the GEAR protocol to account for some security issues. In GEAR, the choice of a next hop router to the desired destination is made locally by each node based on the learned cost function obtained using equations (3) and (4).

It should be clear that the main idea behind this cost function in GEAR is to provide a tunable preference to the distance or energy consumption as routing metrics based on the value of α. It is important to notice that the two metrics are considered to be routing resources for the node as well as the network. The new contribution in GETAR is to add in the cost function the risk value as the trust metric to account for trust awareness and which is also considered to be a routing resource.

To illustrate the idea, let's make an analogy between energy and trust. From the energy perspective, a node will prefer to select the next hop that has the least consumed energy level according to GEAR. This local decision and selection is the best effort that the node can do to cooperate in the routing operation and simultaneously conserve the total network energy. Similarly, from the security perspective, a node will prefer to select the next hop that is least risky among others in neighborhood. Such a selection will guarantee the safest decision that the node can do to cooperate in packet delivery. However, the node here tries to maintain trust as a resource.

### *4.2.2. Forwarding in GETAR*

270 Wireless Sensor Networks – Technology and Protocols

neighbor to C.

Now, h(C,R) > h(B,R)

C (see the figure 3).

**Figure 3.** Escaping void regions in GEAR

assuming non forwarding attack.

*4.2.1. Basic idea* 

**4.2. The enhanced protocol: GETAR** 

**Void Regions Problem :** If a node wants to forward a packet and it finds out that the learned costs of all its neighbors are greater than its own learned cost, the node should select itself. However, the node's transmission range does not cover the destination. In this case

S selects a next hop, C, that is in a void region, i.e. h(C,T) < h(Ni,T) where Ni is a

C forwards the packet to a node, call it B, based on a predefined ordering, e.g. node ID.

Later, when node S wants to transmit a new packet to T, it will forward it to B instead of

GETAR is a geographic and energy aware routing protocol that has the additional feature of trust awareness. The trust awareness is achieved by the rating functionality of a running reputation system that will feed the routing protocol with the trust metric that will be the risk values, ri,j. The risk value ri,j , as discussed earlier, is a quantity that reflects, to some extent, the expectation that a node j will not forward the packet received from node i,

The risk value metric, along with distance and energy metrics, are used to compute the learned cost function for each neighbor. The concerned node, then, makes the routing

As we can see, GETAR is a modification extension to the GEAR protocol to account for some security issues. In GEAR, the choice of a next hop router to the desired destination is made locally by each node based on the learned cost function obtained using equations (3) and (4). It should be clear that the main idea behind this cost function in GEAR is to provide a tunable preference to the distance or energy consumption as routing metrics based on the value of α. It is important to notice that the two metrics are considered to be routing resources for the node as well as the network. The new contribution in GETAR is to add in

decision by selecting the neighbor of lower cost as in normal GEAR.

the node is said to be in a void region. GEAR escapes this void region as follows: Assume that a source node S wants to transmit a packet to a destination T.

Then it updates its cost function h(C,T) to be h(C,T)=h(B,T)+c(C,B).

GETAR forwards the packets and makes routing decision following the same procedure in GEAR. However, the major difference is in calculating the estimated cost function that is used to learn the cost to different destinations. In GETAR, the estimated cost function that a node i evaluates for every neighbor j is given by:

$$t(j,R) = \beta r(j) + (1-\beta)[c(j,R)]\tag{5}$$

where t(j,R) is the *trust-aware* cost of using the node j by node i as a router to the center of R. r(j) is the *risk function* that evaluates the risk value of using j as a router. is a tunable parameter to prefer trust as opposed to other resources.

Using equation (3), we can rewrite equation (5) as:

$$t(j,R) = \beta r(j) + (1-\beta)[\alpha d(j,R) + (1-\alpha)e(j)]\tag{6}$$

If we are concerned about trust more than other resources, β should be close to 1. When β equals 1, the trust-aware cost will consider only the trust part of equation (6) and the next hop will be the most trusted one. Setting β to zero, however, turns the protocol to pure GEAR without any security considerations from the routing protocol perspective.

### *4.2.3. The risk function r(.)*

There can be several ways to represent the risk function evaluated by a node i for using node j as a router. In this work, however, the risk function r(.) is nothing but the risk value ri,j. Thus equation (6) is rewritten as:

$$t(j,R) = \beta(r\_{i,j}) + (1-\beta)[\alpha d(j,R) + (1-\alpha)e(j)]\tag{7}$$

### *4.2.4. Dissemination and voids in GETAR*

In GETAR, the routing operation involves only packet forwarding phase and does not implement dissemination. This is because in the dissemination phase in GEAR, the packets

are intended to be forwarded to all nodes in the target region. However, when we consider trust awareness, a misbehaving node should not be given a chance to have the packet since it will not forward the packet. Thus, GETAR continues to forward packets based on the routing decisions made by the learned cost function.

Regarding the problem of void regions, there is no change in the escaping operation proposed by GEAR. The only difference in GETAR is that the reason of being in a void region can be also related to the existence of misbehaving nodes in the proximity of the node of interest.
