# Counteracting Selfish Nodes Using Reputation Based System in Mobile Ad Hoc Networks
## 2 Background and Literature Review
### Credit Based Schemes  
Cooperative nodes in credit-based schemes are rewarded in terms of virtual currency [34]. For packet forwarding activity, the source or destination node must pay virtual currency to intermediate nodes. Following are examples of credit-based schemes. Nuglets [28] was the leading perception used to prevent the selfish behavior of nodes in MANETs. The source or destination nodes pay in terms of virtual currency, called nuglets, to the intermediate nodes for their packet forwarding services. For charging the nodes for packet forwarding services, two models are used: packet purse model (PPM) and packet trade model (PTM). In PPM, the source node must pay the number of nuglets to the intermediate nodes for their packet forwarding services. The sender must provide a sufficient number of nuglets with the packets to the intermediate nodes as service charges for their forwarding activity. The issue with this model is that, if more or fewer nuglets are loaded than required, extra nuglets will be lost and if less, then the packet may not reach the destination. Another problem with this model is that if link failures or technical faults occur in the middle of the transaction, then the nuglets charged by some nodes will be wasted [18]. On the other hand, in the packet trade model, every forwarding node in the route purchases the packets from the preceding one and sells the packet to the next node along the path; finally, the destination node purchases the packet. The author in [19] provides the credit-counter concept to enhance PTM. A node counter will be decreased upon sending their own packets while it will increase on forwarding packets for other nodes in the network. To persist in the network, a node must balance its sending and forwarding activities. In Sprite [29], the credit administration relies on a central authority known as credit clearance service (CCS). For charging packet forwarding activity, a node submits a receipt for credit to the CCS. The CCS provides credit to the node after confirmation of the receipt from the next node. The CCS defines the rate for sending and forwarding the packets; thus, the sender loses the credit while intermediate nodes receive credit if CCS obtains satisfactory verification from the next node. Some other credit-based schemes are presented in [35; 36; 37; 38]. Credit-based schemes provide a flexible way for nodes to manage credit, but these schemes did not receive wide acceptance because of the following shortcomings [39].  
* Increased Cost: to secure credit management, these schemes require tamper-proof hardware for every node in the network, which may increase cost;
* Lack of Fairness: there is an unequal chance available to nodes for earning credit. Nodes at the middle of the network have a high chance of earning credit while nodes located at the boundaries have less or no chance to obtain credit;
* High Overhead: payment give-and-take may increase overhead.  
In the existing schemes, reputation is a measure of cooperation computed by each node for others. This does not indicate anything about the services consumed by nodes. Therefore, such schemes have limited performance against selfish and malicious nodes. In particular, nodes that do not participate in the route establishment process (i.e., nodes drop route request and route reply messages) make use of network resources to their own advantage [40]. Moreover, nodes that locate at positions where they are always treated as source nodes freely enjoy the benefits of other nodes, but without contributing the same back to the network [41; 42]. Similarly, detecting and isolating partial droppers is also challenging since these nodes take advantage of the reputation threshold [43]. The selfish and malicious nodes discussed in the cases above act as free riders, where they usually consume network services without providing any services back to the network.