## 2 Related Work  
Trust-based routing is a scheme that discovers and maintains trusted paths by using a measure of trustworthiness derived from trust evaluation. In this section, we review existing trust-based routing methods that can be utilized in MANETs and WSNs. AOTDV [7] is a trust-based multipath routing method that extends AOMDV [8], which maintains multiple paths during the route discovery phase. AOMDV is one of the representative multi-path routing protocols and proposes a method for creating loop-free multi-paths. However, it is vulnerable to attacks by malicious nodes because it focuses on delivering data without considering the trustworthiness factor. AOTDV has been further proposed to solve these problems. In AOTDV, a source node can create multiple loop-free paths to a destination during the route discovery process. A destination will respond with at most k shortest paths as candidates that satisfy the trust requirements of data packets. Each node has trust evaluation results composed of trust values and a hop count. After the trust path has been created, the source node allocates paths that satisfy the trustworthiness required to generate mission-critical data packets and then generates the data. As an intelligent agent, each node evaluates its neighbors' behaviors and selects the shortest trusted path to forward packets. However, this scheme cause problem like bottlenecks because it can allocate only a single path for transmitting mission-critical data even though multiple paths are kept for an alternative when the allocated path becomes failed to use. In addition, AOTDV considers trustworthiness and hop count factors as a routing metric, so it is difficult to guarantee the QoS requirements of mission-critical data or the survivability of sensor devices. TQR [9] proposed the view of trust and QoS metrics evaluation to create a trust-based QoS model. They introduce the definition of trust and QoS parameters estimation into a classic routing to enhance the security of networks. The proposed trust model obtained the degree of trust between nodes from the direct and indirect trust. In addition, due to the NP-completeness of the multi-QoS constraints problem, they only consider the link delay as the QoS constraint requirement for establishing trusted routing. They measured the expected transmission count (ETX), trustworthiness, transmission delay, and propagation delay in calculating the metric. They focus on exploring a feasible way to estimate the available link delay requirement by considering link quality and incorporating a trust-aware scheme into the route discovery procedure to enhance the security of the network. However, the delay cumulated in the queue was not regarded, and only a single path was used for data transmission, making trustworthy communication challenging. TQR considers trustworthiness and QoS factors as routing metrics, but it is difficult to derive accurate values without learning historical data. Moreover, the survivability of the sensor device can be reduced because the energy factor is not taken into account. CENTERA [10] uses the powerful gateway to collect trust values from individual nodes and evaluate the best paths after isolating \"malicious\" nodes. Gateway establishes a global view of the network and evaluates three quality metrics: maliciousness, cooperation, andcompatibility. The gateway estimates the battery of every node-based and evaluates several metrics for every node. Gateway periodically collects trust values from the individual nodes about the number of packets sent through neighbors. Gateway utilizes a method to disseminate updated routing information to all the network nodes such that each node knows its uplink nodes to forward their packets to the gateway and its next-hop downlink node forwards its own packets through it. However, CENTERA considers only the packet forwarding ratio to perform the trust evaluation of the individual nodes.