## 2 Related Work  
Various routing protocols have been proposed for MWSNs. TTDD (a Two-Tier Data Dissemination) [15] initially builds a rectangular grid structure which divides the network into cells with several dissemination nodes that are used to relay the query and data to/from the proper source. Whenever sinks request data, they query the network by local flooding within the cell and the query packets are relayed to the source nodes through dissemination nodes. A data path from the source to the sink is then established using the reverse of the path taken by the data request. However, TTDD suffers from the high overhead of constructing a separate grid for each source node. Unlike TTDD, Grid-Based Energy-Efficient Routing From Multiple Sources to Multiple Mobile Sinks (GBEER) [16] constructs a single combined rectangular grid structure for all possible sources to eliminate the high overhead of constructing separate grids for each source. However, the frequent grid change due to solving the hotspots introduces extra traffic on numerous nodes residing between the crossing points. In place of a rectangular grid, HPDD (Hexagonal Path Data Dissemination) [17] utilizes a common grid structure composed of hexagons. Which can provide shorter data and sink advertisement routes, but it suffers from the same hotspot problem. SEAD (Scalable Energy-efficient Asynchronous Dissemination) [18] constructs a minimum-cost weighted Steiner tree for the mobile sink by considering the distance and the packet traffic rate among nodes to save communication energy. Like TTDD, separate trees are constructed for each source; thus, the overhead of establishing such trees is very high. Ring Routing [19] designs an easily accessible ring structure to mitigate the anticipated hotspot problem with low overhead and minimize the data reporting delays. However, its questionable scalability is not good. Besides, the overhead of the initial ring construction for large or sparse networks will be high. In agent-based schemes, one or more agents, which take on the role of representatives for the sources or the sink, are selected to relay the traffic between sources and the sink. These protocolsusually utilize infrequent flooding to advertise the location of the agents. IAR (Intelligent Agent-Based Routing Protocol) [20] provides efficient data delivery to mobile sinks in large scale wireless sensor networks with sink mobility. IAR reduces the packet loss and signal overhead, meanwhile improves degraded route called triangular routing problem. However, as the hotspot, the agent (or IR) is chosen only depending on the distance to the sink without considering its remaining energy. Thus, once a node with low remaining energy is chosen to be the agent, it may die quickly, which leads to break the existing path to the sink. DHA (Data Dissemination Protocol Based on Home Agent and Access Node) [21] employs Home Agent and Access Node to design a simple protocol. Similar to IAR, the load on the home agent is immense, and changing the home agent requires global flooding. To decrease the frequency of global floods, SinkTrail [7] constructs and utilizes a logical coordinate space, which does not require position aware sensors and enables simple data dissemination by logical greedy geographic routing. However, the sink's mobility is assumed to be nomadic and limits the applicability of the protocol. VGE-MGP [22] is another efficient mobile sink based routing without the physical geographic information. Recently, nature-inspired algorithms are applied to solve the problem of the mobile based protocol. SIMPLE (swarm intelligence based protocol) [23] is based on a swarm agent that integrates the remaining energy of nodes into the route selection mechanism and maximizes the network's lifetime by evenly balancing the remaining energy across nodes and minimizing the protocol overhead. The protocol is robust and scales well with both the network size and in the presence of multiple sinks. The movement of sinks may lead to the breakage of the existing routes.