*2.9.3. SeFER - Secure, flexible, and efficient routing protocol*

SeFER (Secure, flexible, and efficient routing protocol for sensor networks)[67] is based on random key pre-distribution mechanism. This mechanism aims to provide an easy way for managing the keys in WSN without using public key cryptography. The protocol assumes non symmetric communication architecture in which a tree of sensor nodes delivers information to a controller according to an inquiry sent into the network. Two nodes may communicate indirectly, but securely over a multiple hop path where each pair of nodes on this path shares a common key. The protocol provides the methods for nodes to securely share their keys and communicate directly so that the efficiency of communication is increased.

In fact, all previously mentioned protocols are crypto based solutions. They can successfully fight against attacks in which an intruder falsifies his identity to be a relay for the source such as sybil attack. However, other attacks like selective forwarding, blackhole and HELLO flooding are still possible especially when the attack is performed by an insider node or a node compromised by an intruder. Moreover, any misbehavior due to selfishness or faulty operational nodes cannot be prevented or even detected.

### *2.9.4. Watchdog and pathrater*

Two extensions to the Dynamic Source Routing (DSR) protocol to mitigate the effects of routing misbehavior in ad-hoc networks were proposed in [6,7], namely the Watchdog and the Pathrater. The watchdog is the monitoring part that is designed to be responsible for detecting only non forwarding misbehavior. This is accomplished by overhearing the transmission of the next node. The node thus is assumed to be in a continuous promiscuous mode. When the attack is detected, the observing node informs the source of the concerned path. In this approach, each node maintains a buffer of recently sent packets; in case the packet is not forwarded on within a certain timeout or the overheard packet is different than the one stored in the buffer, the watchdog increments a failure counter for the node responsible for forwarding the packet. If the counter exceeds a certain threshold, the node is considered as misbehaving and the source is notified.

The pathrater is the component used for reputation. Ratings are kept about every node in the network based on its routing activity and they are updated periodically. Nodes select routes with the highest average node rating. Thus, nodes can avoid misbehaving nodes in their routes as a response. The pathrater combines knowledge of misbehaving nodes with link reliability data to select the route most likely to be reliable. Specifically, each node maintains a rating for every other node it knows about in the network and calculates a path metric by averaging the node ratings in the path, enabling thus the selection of the shortest path in case reliability information is unavailable. Negative path values indicate the existence of one or more misbehaving nodes in the path. If a node is marked as misbehaving due to temporary malfunction or incorrect accusation, a second-chance mechanism is considered, by slowly increasing the ratings of nodes that have negative values or by setting them to a non-negative value after a long-timeout. However, misbehaving nodes can still transmit their packets as there is no punishment mechanism adopted here. Moreover, no second hand information propagation view is considered which limits the cooperativeness among nodes.
