## 4 Mobile Peer-to-Peer Architectures
### Architecture
#### 4.2.1 Layered  
_Layered_ architectures are a straightforward approach for adapting traditional P2P concepts to MP2P. Here, the MANET underlay and the P2P overlay are strictly separated; no cross-layer information is harnessed to optimize the system. As the routing and maintenance overhead of traditional P2P  
Figure 6: MP2P architectures.  
architectures would exceed the strongly limited resources of the MANET underlay, several techniques to reduce the maintenance overhead were proposed for _layered_ approaches.  
In order to reduce overhead on mobile nodes with limited resources, the Mobile Stealth DHT architecture proposed in [12] distinguishes between mobile and static nodes. Static nodes are preferred for data storage and routing, while mobile nodes only have to manage the traffic generated by their own requests. This way, most overhead is shifted to the static nodes which are assumed to have stronger resources than mobile nodes. Further, the overhead generated by object relocation as a result of the mobility of nodes (mobility related churn) is avoided. This results in a reduced number of sent messages and an increased availability of objects.  
To influence the routing maintenance phase directly, [PERSON] _et al._ propose the Bamboo DHT as P2P overlay of an MP2P network [13]. The Bamboo routing algorithm is similar to Pastry but differs in the routing maintenance. While Pastry updates the routing tables whenever the topology changes, Bamboo maintains the routing tables at specific time intervals. By increasing these intervals, routing related traffic is reduced.  
Several approaches for MP2P networks harness location awareness of nodes to minimize routing overhead. Whenever node-IDs are generated based on the geographical position of the nodes, the routing algorithm may benefit from this information. The PeerNet [14] architecture splits the ID-space in several equal zones and assigns each virtual zone to a geographical area. Virtual residences are introduced as a geographical area where a node may be found with the highest probability. A proxy is required for each virtual residence to provide information about the nodes that reside in this virtual area. Each mobile node stores a mobility profile at the proxy of the virtual residence when leaving this zone. The mobility profile provides information based on which the current geographical position of the node can be estimated. As PeerNet is combined with the geographical MANET routing algorithm PILOT [14], overhead may be reduced as no flooding based routing algorithm is required. Another example for geographical routing is the location aware Bamboo DHT presented by [PERSON] _et al._[15]. This approach uses a landmarking system to assign node-IDs to nodes. Landmarking architectures split the ID-space in equal sized groups and assign a landmark-ID to a specific node within this group. This node broadcasts the landmarking signal. Each node within a landmarking zone forwards the landmarking signal. A node that receives a landmarking message of a foreign landmark group compares the distance to the foreign landmark node with the distance to its own landmark node. If the new landmark node is closer, the node changes the node-ID and joins the closer landmark zone. As each landmark zone assigns node-IDs with a specific prefix, nodes with matching prefixes are physically close to each other. Due to this, a location aware routing is combined with a reduction of the overhead of the routing table maintenance phase by the Bamboo DHT. A major advantage of a location aware routing is the reduced overhead as the number of underlay hops is reduced.