A Survey on Security in Mobile Peer-to-Peer Architectures—Overlay-Based vs. Underlay-Based Approaches <s> Architecture <s> The advances in wireless networking and the consequent emergence of new applications that wireless networks increasingly support inevitably leads to low capability mobile nodes connecting to peer-to-peer networks. However, the characteristics of mobile nodes and limitations of access point coverage often cause mobile nodes to lose connectivity, which can cause many mobile nodes to simultaneously leave the network. Continuous departures and joins due to the mobility of nodes leads to mobility churn, which can often degrade the performance of the underlying peer-to-peer network significantly. In this paper, we use simulations to demonstrate that the stealth distributed hash table (stealth DHT) algorithm is ideally suited for networks with mobile nodes. By avoiding storing state in unreliable nodes, a stealth DHT prevents mobile nodes from being used by other nodes to provide services. Consequently, stealth DHTs eliminates the mobility churn effect and significantly reduce the amount of overhead as compared to a generic DHT. <s> BIB001 </s> A Survey on Security in Mobile Peer-to-Peer Architectures—Overlay-Based vs. Underlay-Based Approaches <s> Architecture <s> Ad-hoc networks are an emerging technology with immense potential. Providing support for large-scale service and application deployment in these networks, however is crucial to make them a viable alternative. The lack of infrastructure, coupled with the time-varying characteristics of ad-hoc networks, brings about new challenges to the design and deployment of applications on a large-scale. This paper addresses these challenges and presents PeerNet, a unified, overlay-based service architecture to support large-scale service and application deployment in MANETs. We discuss the main functionalities of PeerNet, describe the algorithms for resource registration and discovery, and present PILOT, a novel power-aware, location-driven traffic forwarding algorithm to enable node interaction. We conclude the paper by comparing PILOT to LAR and AODV for a network of mobile nodes. <s> BIB002 </s> A Survey on Security in Mobile Peer-to-Peer Architectures—Overlay-Based vs. Underlay-Based Approaches <s> Architecture <s> Internet connected wireless multi-hop networks are an interesting alternative for providing broadband wireless access. In order for the network to be transparent, the same services need to be available as in standard infrastructure wireless deployments. However, there is a significant challenge in providing services such as authentication, name resolution, VoIP over multi-hop mesh networks as dedicated servers implementing those services might be not available. Therefore, deploying overlay networks in the mesh to decentralize those services and move towards a Peer-to-Peer paradigm is an interesting approach. However, the multi-hop nature of wireless mesh networks and the restrictions in resource availability might cause problems when deploying overlay networks on top of such environments. In this paper, we investigate the overhead and trade-offs when deploying a structured overlay solution such as the Bamboo DHT over wireless multi-hop mesh networks. We provide various simulation results characterizing the overhead of management and control traffic and give recommendations for performance improvement. <s> BIB003
While underlays based on (mostly) wired and infrastructure-based systems such as the Internet provide sufficient bandwidth for DHTs even when routing tables have to be maintained constantly due to the changing topology, resources of MANETs as underlay are more limited. Further, as the topology changes more often due to the mobility of nodes in MP2P networks, routing tables have to be maintained more frequently. Due to this, more control traffic is generated in an MP2P network compared to a traditional infrastructure-based P2P network but less bandwidth is available. Therefore, MP2P architectures have to reduce the overhead generated by the routing algorithm and the routing table maintenance. Recently, several approaches were proposed to combine MANETs and P2P networks. The resulting MP2P architectures can be distinguished by the implementation of the P2P algorithm into layered, integrated and cross-layered approaches as shown in Figure 6 . 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 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 BIB001 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, Castro et al. propose the Bamboo DHT as P2P overlay of an MP2P network BIB003 . 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 BIB002 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 BIB002 , 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 Millar et al.  . 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.