## 2 Related Works and Motivation  
### The Overview of Directed Diffusion  
DD [12] is a data-centric routing protocol based on the distributed mode. In DD, the propagation and aggregation of both interest and sensing data is determined by localized message exchanges between neighbor nodes. The next hop was only decided as a next route for multi-hop communication instead of whole route path. The operation of DD is divided into the two periods of interest propagation and the data aggregation. In the first period, a sink and sensor nodes collect the routing information by propagating an interest for transmitting their data. When a sensor application requests particular sensing data, a sink creates an interest specifying the target data and then disseminates the interest over WSNs. The interest represents the application's request including attribute-value pairs that indicate target sensing data. When the interest arrives at each node, the node checks whether the request matches its own sensing data or not. At the same time, the node sets up a gradient toward the direction to the sink node and forwards the interest to its neighbors. After setting up gradients, the route can be determined at each node with its value of gradient using the routing algorithm of DD. The second period is started after interests are propagated across the network. In the second period, sensors are triggered to start sending the data and a sensor application begins collecting data. The target sensing data are aggregated to a sink based on the route with the value of gradients. Figure 1(a) shows the process of interest flooding into the whole network. Figure 1(b) shows the step for setting up the gradient at each node through interest propagation period from the sink node to a source node. The data transmission state along the selected route is shown in Figure 1(c). There are several versions of DD. The original, Two Phase Pull (TPP) offers efficient communication between sources and sinks. TPP consists of two phases of interactions which can provide data collection service using several control packets: interest, exploratory data, and positive/negative reinforcement. The first phase of TPP is composed of the interest flooding and exploratory data to find source nodes and to set up gradient at all nodes. The initial data from the source node is marked as exploratory data and is sent again to all neighbors which has matching gradients. The second phase of TPP is a step to reinforce the route and to send data through the reinforced route. The positive and negative reinforcement packets are used to increase or decrease the gradient on a certain route. Especially, the negative reinforcement suppresses loops or duplicated routes that may arise due to the network changes. After the route was reinforced, sensing data is sent through the reinforced route to a sink. The control packets of TPP for recovering network changes enable TPP to provide robust communication in WSNs. However, since too many interest and exploratory packets flooded to the networks, the energy of each sensor node tended to be depleted rapidly in TPP. In order to overcome the problems of TPP, One Phase Pull (OPP) was proposed to eliminate explicit exploratory data and reinforcement packets. OPP has only one phase to provide collecting sensing data using an interest packet only compared to TPP. OPP used the interest only to set up the gradient and sent the data directly on the selected route without additional control packets. This simple feature of OPP is a significant benefit because it can minimize the flooding of TPP. When an interest arrives at a source node, it does not mark its first data as exploratory data. The source node sends data only on the route with preferred gradient. The preferred gradient is determined by the neighbor who was the first to send the matching interest. Therefore, in OPP, the lowest latency route is implicitly reinforced without requiring additional reinforcement packets.