## 2 Related Work
To overcome the weakness of flooding, restricted flooding techniques are developed, such as the Facilitating Match-Making Service (FMMS) and Geography-based Content Location Protocol (GCLP) [5,15]. In FMMS, a service provider advertises the service in a cross-shaped trajectory along the network, as shown in Figure 1.  
Service provider P sends a service advertisement packet in four directions, and the packet is forwarded until it reaches the boundary of the network forming the publish trajectory. A user C who wants to use the service also propagates the query packet in four directions, and the query packet is forwarded until it reaches the boundary of the network packet forming subscribe trajectory similar to advertisement trajectory. The nodes that belong to both trajectories (publish trajectory and subscribe trajectory) reply to user C with the service descriptions. GCLP reduces the query forwarding overhead by stopping the propagation of a query packet when the subscribe trajectory crosses the publish trajectory. FMMS and GCLP reduce the flooding overhead by limiting the flooding to four trajectories; however, both protocols are unable to provide scalability because they do not consider the query amounts, such that the trajectory always has the same size. Figure 1: Service location in FMMS. The distance-sensitive service discovery problem in wireless sensor and actor networks was formalized, and a novel localized algorithm, iMesh was proposed in [38]. Unlike existing solutions, iMesh uses no global computation and generates constant per-node storage load. In iMesh, new service providers (_i.e._, actors) publish their location information in four directions, updating an information mesh such as GCLP. Information propagation for relatively remote services is restricted by a blocking rule, which also updates the mesh structure. Based on an extension rule, nodes along mesh edges may further advertise newly arrived relatively near service by backward distance-limited transmissions, replacing previously closer service location. The final information mesh is a planar structure constituted by the information propagation paths. It stores locations of all the service providers and serves as service directory. Service consumers (_i.e._, sensors) conduct a lookup process restricted within their home mesh cells to discover nearby services. We analytically study the properties of iMesh including construction cost and distance sensitivity over a static network model. A service discovery protocol based on hierarchical grid architecture in an ad hoc network which enhanced the GCLP was proposed in [39]. The geographical area was divided into a 2D logical hierarchical grid and the information of available services was registered to a specific location along a predefined trajectory. To enhance resource availability and effective discovery of GCLP, each grid cell selects a directory to cache available services. This work utilizes the transmitting trajectory to improve the efficiency of registration and discovery. First, the service provider registers a service along the proposed register trajectory. The requestor then discovers the service along the discovery trajectory to acquire the service information.