## 8 Related Work  
Research on the sensor network routing has been carried out for nearly a decade. [PERSON] [1] introduce a clustering algorithm for WSNs, called LEACH. In LEACH, sensors are organized into clusters. Each cluster has one cluster head (CH) which collects and aggregates information from its members (non-CH sensors in the same cluster) and passes on information to the base station (BS). However, LEACH has a number of shortcomings. LEACH assumes every node can directly reach the base station by transmitting with sufficiently high power. However, one-hop transmission directly to the base station is not feasible in large-scale WSNs due to the resource-limitations of sensors. On the other hand, LEACH is vulnerable from several attacks including HELLO flood, selective forwarding, and Sybil attacks [16]. Recent works have taken into account of both performance and security such as SecRout [2], SEEM [3], SeRINS [4], and TTSR [17]. SecRout [2] guarantees that sinks receive correct queries resulted from the sensor network. Only a high efficient symmetric cryptography is used in SecRout. SEEM [3] uses a principle similar to the Client/Server software architecture. The BS performs the route discovery and maintenance as well as route selection. Instead of a single path, the BS periodically selects a new path from multi-paths based on the current energy level of nodes along each path. SeRINS [4] focuses on detecting and isolating compromised nodes. The major shortcoming of those approaches is that they assume the BS is stationary and all sensor nodes know the BS's location. This assumption makes them fail to work in case the BS (sink) is mobile. TTSR [17] is a secure and efficient routing protocol for heterogeneous sensor networks (HSNs) which takes advantage of powerful high-end sensors (H-sensors) in an HSN. TTSR possesses a similar sink mobility problem with SecRout, SEEM, and  
Figure 16: Success ratio vs. sink speed. SeRINS. On the other hand, TTSR is only suitable for HSNs with sufficient powerful sensor nodes, not large-scale homogeneous WSNs. Besides, relying only on some particular nodes makes them prone to deplete there energy sooner or later. Using fixed coordinators makes attackers easy to choose 'right' nodes to compromise. There are a number of routing protocols aiming to support mobile sinks in WSNs [5][6][7][17][18]. Declarative Routing Protocol [7], and Directed Diffusion (DD) [18] suggest that each mobile sink needs to continuously propagate its location information throughout the sensor field, so that all sensor nodes get updated with the direction of sending future data reports. However, frequent location update from multiple sinks leads to both increased collisions in wireless transmissions and rapid power consumption of the sensor's limited battery supply. SAFE [5] uses flooding that is geographically limited to forward the query to nodes along the direction of the source. Considering the large number of nodes in WSNs, the network-wide flooding may introduce considerable traffic. SEAD [6] considers the distance and the packet traffic rate among nodes to create a near-optimal dissemination. SEAD strikes a balance between end-to-end delay and power consumption that favors power saving over delay minimization. It is therefore only suitable for applications with a less strict delay requirement. TTDD [17] exploits local flood within a local cell of a virtual grid which sources build proactively. However, it does not optimize the path from the source to the sinks. When a source communicates with a sink, the restriction of grid structure may multiply the length of a straight-line path by \(\sqrt{2}\). Also, TTDD frequently renews the entire path to the sinks. It therefore increases energy consumption and connection loss ratio. Those multi-hop routing protocols face many security problems from spoofed, altered, or replayed routing information [9][16], selective forwarding [16], sinkhole [16], Sybil [10], wormhole [11], and HELLO flood (unidirectional link) attacks [16].