## 3 Foundations of MANETs
### Architectures  
Several routing algorithms for MANETs were proposed in the last years. These may be distinguished by their routing characteristics. For this paper, we differentiate between _proactive_, _reactive_, _hybrid_, and _geographical_ algorithms. In _proactive_ approaches, routing tables are updated periodically and, thus, fresh routes to all nodes in the network are readily available (if the network is not partitioned). Due to this, no routing delay occurs before (and during) data transmission. On the other hand, overhead is generated as routing tables must be maintained regularly due to node mobility. The _proactive_ Optimized Link State Routing Protocol (OLSR) [9], as an often referenced example, utilizes a link state algorithm for routing but also reduces  
Figure 2: Challenges in Peer-to-Peer networks. overhead by selecting Multipoint Relays (MPR). Every node has to determine its MPRs by identifying the smallest set of direct neighbors required to cover every second hop neighbor as shown in Figure 3. In order to reduce the routing overhead, link state messages are sent to the MPRs only. _Reactive_ algorithms, in contrast to _proactive_ approaches, start routing for a specific destination only on demand, not unless a route is required for data transmission. As the topology in MANETs may change constantly due to node mobility, routing overhead may be significantly reduced when a route discovery is initiated only when a route is required. On the downside, when data has to be transmitted to a node for which no entry in the routing table exists, a route has to be established before the first packet can be sent. The Ad hoc On-Demand Distance Vector (AODV) [10] routing protocol is a well-known and often referenced _reactive_ protocol for MANETs. Here, when a route is required, the source node has to broadcast a Route Request message (RREQ). Each node in transmission range has to check whether it is the destination of this request or whether a valid route to the destination is stored in the routing table. If so, the node sends a Route Reply message (RREP) to the source. Otherwise the RREQ is re-broadcasted (as shown in Figure 4(a)). A node drops duplicates of RREQs to prevent loops. A route is established when the source node receives the RREP. _Hybrid_ protocols combine proactive and reactive approaches. While routing to nodes close to the source is done in a _proactive_ way, routing to nodes that are further away is performed _reactively_. This way, routing overhead is reduced and a communication with nodes close to the source is not affected by routing delay. The Zone Routing Protocol (ZRP) [11] is a well known _hybrid_ protocol. Routes to nodes within a specific local zone are maintained _proactively_ while routes to nodes further away are established on demand. When the destination is not within the local zone, a request is sent to the nodes at the border of this zone. Those nodes check their own local zone for the requested node and forward the request to their own border nodes again if the node could not be found within their zone. This proceeds until the destination node is found. When the network nodes are aware of their _geographical_ position, routing can be optimized by harnessing this information. The geographical position of the destination node can be used to limit the dissemination of routing messages to a specific area. This results in a reduced routing overhead as routing messages are not broadcasted in the whole network. For example, the Location-Aided Routing (LAR) determines an expected zone around the last known position of a destination node. Based on the expected zone, a request zone is defined around the position of the source node and the expected zone. Route requests are broadcasted in this request zone only and, due to this, routing overhead is reduced (as  
Figure 3: MPRs selected by an OLSR node. shown in Figure 4(b)).