geographic routing
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GEOGRAPHIC ROUTING Namitha T.N Lecturer in Computer Science and Engineering JECC [email protected] Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network, electronic data networks (such as the Internet), and transportation (transport) networks…In some networks, routing is complicated by the fact that no single entity is responsible for selecting paths: instead, multiple entities are involved in selecting paths or even parts of a single path…A routing protocol is a protocol that specifies how routers communicate with each other to disseminate information that allows them to select routes between any two nodes on a network. In internetworking, the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path. Routing is often confused with bridging, which performs a similar function. The principal difference between the two is that bridging occurs at a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where the software component is more important. And because routing occurs at a higher level, it can perform more complex analysis to determine the optimal path for the packet. A routing table is a set or rules, viewed in a tabular format and this used to define the routes of the data packets. All the network devices, which have IP, enabled functionality such as routers and switches use the routing tables. Routing table stores the information and configurations of every router in the IP enabled network. A routing table contains the information necessary to transmit the packets toward its destination. When a packet is received, the network devices matches the information contained in the packets and the information in the routing tables and then it defines the shortest possible route for the transmission of the packets towards its destination. Each packet contains the information of its origin and destination and the routing table contains the following information. • Destination: The IP address of the packet’s final destination (next hop). Next hop: The IP address to which the packet is forwarded • Metric: It assigns the cost to each route so that most-effective paths can be picked up. • Routes: It includes directly attached direct subnets, indirect subnets, that are not directly
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connected to the device but it can be accesses through one ore more hops • Interface: The outgoing network interface the device should use when forwarding the packet to its final destination. Geographic routing (also called georouting or position-based routing) is a routing principle that relies on geographic position information. It is mainly proposed for wireless networks and based on the idea that the source sends a message to the geographic location of the destination instead of using the network address. The idea of using position information for routing was first proposed in the 1980s in the area of packet radio networks and interconnection networks .Geographic routing requires that each node can determine its own location and that the source is aware of the location of the destination. With this information a message can be routed to the destination without knowledge of the network topology or a prior route discovery. Geographic routing protocol use the location information to progressively forward packets through the physical space toward the destination location with intermediate next hop routing decisions based on selecting the neighbor that has the closest distance compass setting or some other measure of forward progress toward the destination. This process termed geographic forwarding. Geographic routing (or position-based routing) uses location information for packet delivery in multihop wireless networks Neighbors locally exchange location information obtained through GPS (Global Positioning System) or other location determination techniques. Since nodes locally select next hop nodes did not base on this neighborhood information and the destination location, route establishment nor is perdestination state required in geographic routing. As large-scale sensor networks become more feasible, properties such as stateless nature and low maintenance overhead make geographic routing increasingly more attractive. Also, location-based services such as geocasting can be best realized using geographic routing. The most popular strategy for geographic routing is simply for warding data packets to the neighbor geographically closest to the destination. Although this greedy method is effective in many cases, packets may get routed to where no neighbor is closer to the destination than the current node. Many recovery schemes have been proposed to route around such voids for guaranteed packet delivery as long as a path exists. These techniques typically exploit planar sub graphs (i.e., Gabriel graph, Relative Neighborhood graph), and packets traverse faces on such graphs using the well-known right-hand rule. Most geographic routing protocols use one-hop information, but generalization to two-hop neighborhood is also possible.
ADVANTAGES OF GEOGRAPHIC ROUTING:
Geographic routing protocol offers a number of advantages over conventional adhoc routing strategies. Geographic forwarding does not require maintenance of routing tables or route construction prior to or during the forwarding process. The forwarding process also allows a packet to adapt to changes in the topology by selecting the next best choice if an intermediate node used by previous packets becomes unavailable. Without the need for route construction these approaches do not require table maintenance other than immediate neighbors nor dissemination of topology information 2
PROBLEMS ASSOCIATED WITH GEOGRAPHIC ROUTING:
The complexity and overhead required for a distributed location database service is a disadvantage of geographic routing. The dependence of geographic forwarding on the physical network topology means that obstacles such as a building or the lack of radio coverage may result in voids in the physical network topology. These voids may inhibit forward progress of packets local minima where there are no neighbors available that are closer to the destination resulting in the failure of the forwarding strategy. GEOGRPHIC ROUTING PROTOCOLS A routing protocol is a protocol that specifies how routers communicate with each other to disseminate information that allows them to select routes between any two nodes on a network. Typically, each router has a priori knowledge only of its directly attached networks. A routing protocol shares this information successively, first among immediate neighbors and then throughout the entire network. This way routers can gain knowledge of the network topology at large. Geographic routing protocol based primarily on a forwarding strategy and secondarily in a recovery strategy.There are various approaches, such as single-path, multi-path and flooding-based strategies. Most single-path strategies rely on two techniques: greedy forwarding and face routing. Greedy forwarding tries to bring the message closer to the destination in each step using only local information. Thus, each node forwards the message to the neighbor that is most suitable from a local point of view. The most suitable neighbor can be the one who minimizes the distance to the destination in each step (Greedy). Alternatively, one can consider another notion of progress, namely the projected distance on the source-destination-line (MFR, NFP), or the minimum angle between neighbor and destination (Compass Routing). Not all of these strategies are loop-free, i.e. a message can circulate among nodes in a certain constellation. It is known that the basic greedy strategy and MFR are loop free, while NFP and Compass Routing are not. The main types of routing protocols associated with geographic routing are • • • • • • • • • • • • • Random progress method Most forward with fixed radius R Nearest with forward progress NFP Greedy forwarding Location aided routing LAR Distance routing effect algorithm for mobility DREAM Compass Routing Geographic Distance Routing GEDIR Depth first search with Dominant sets GPSR GFG Sooner back Geographic routing algorithm GRA Terminode Routing
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Terminode Local Routing Scalable Location Update Based Routing Protocol SLURP Adaptive Face Routing Beaconles Routing Exponential Age search EASE
Random progress method Forwards packets to a random neighbor from among those that are closer to the destination. The random selection of the next hop was suggested to provide an even distribution of traffic load. Most forward with fixed radius R Forwards packets to the neighbor within a set radius of the current node that makes the most forward progress (or the least forward progress) along the line drawn from the current node to the destination. Progress is calculated as the cosine of the distance from the current node to the neighbor projected back onto the line from the current node to the destination... The nearest with forward Progress (NFP). This strategy forwards a packet to the closest neighbor in the forward direction. The current node then modifies it’s transmit power to suit the connection. This results in higher delivery rates due to reduce interference and contention at the cost of increased hop count. Greedy forwarding: Greedy forwarding which selects the next hop as the node closes to the destination. The greedy approach unlike MFR allows a packet to move to a node i.e. beyond the destination if that node is closer to the destination than the previous node. 5. Location-Aided Routing (LAR) and Distance Routing Effect Algorithm for Mobility (DREAM) focus on ad-hoc networking environments. LAR is an on-demand routing protocol that uses the last known position of the destination node and its velocity to limit the flooding of routing requests toward the destination. Flooding is limited to an area between the source and a circle, calculated around the destination, with its center at the last known position and a radius, which is determined by the nodes velocity. This improves the efficiency of the underlying on-demand protocol but still suffers the problem of scalability and latency associated with on-demand strategies. DREAM is based on the flooding of data without the prior establishment of a route. Messages are flooded into an area that is limited in a similar manner to that used in LAR. However, the use of directional flooding of data packets, as opposed to flooding of route requests in LAR, may still incur a significant bandwidth overhead. Compass Routing (DIR) a variation on forwarding based on the direction for routing decisions are proposed. In this strategy, packets are forwarded to the neighbor on the closest compass setting to the destination. DIR has the disadvantage that it is susceptible to looping and does not guarantee a delivery. To guarantee a delivery, face traversal of disjoint regions was proposed, where a packet is forwarded around one side of each face until the packet reaches the further edge of the face that intersects the line from the source to the destination. From this point the packet traverse the next face in a similar manner until the destination is reached. Geographic Distance Routing (GEDIR) adopts the Greedy strategy proposed by Finn, where packets are forwarded to the neighbor closest to the destination. To allow a packet to move through the local minima, GEDIR does not include the current node in the
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distance calculation and permits a packet to be sent in the reverse direction if no forward node is available. To prevent looping the packet is not permitted to be passed from the neighbor back to the previous node. This addresses single- hope looping, but packet may loop back via an alternate path. Two variations of GEDIR were proposed to address the problem. These include flooding at the local minimum (f-GEDIR) and maintaining 2-hop neighbor information to predict and avoid the local minima. F-GEDIR is found to be effective at the expense of increased control overhead, whereas 2-hop GEDIR was an improvement but still allows loops of 2+hops. A multipath version c-GEDIR is also proposed to add reliability. FACE-2 algorithm extracts a connected planar sub graph by converting the network topology to a Gabriel graph. FACE-2 has the advantage that it guarantees delivery in a connected static graph, although it does not provide the optimal path and also requires a unit graph with equal transmission radii for planar graph conversion. Greedy Forward Greedy (GFG) was proposed, which incorporates both GEDIR (Greedy forwarding) for routing and FACE-2 (planar graph traversal) to recover when Greedy forwarding encounters the local minima. Depth-First Search with Dominant Sets uses greedy forwarding. When route failure occurs, the packet backtracks to the previous node, which forwards the packet to the next closest neighbor to the destination. To reduce the number of nodes involved in routing and thus reduce the number of hopes involved in route determination, routing is restricted to the dominant set until the destination is known to the current node. GPSR is a packet-switched routing protocol implementation of GFG using Greedy forwarding and planar graph traversal. Nodes are only required to maintain 1-hop neighbor location information that is exchanged using periodic beacons. Packets are first transmitted with a mode flag set to Greedy. When the local minima is reached, the flag is changed to the perimeter mode, and a face traversal algorithm is used until Greedy forwarding can be resumed. Packets are not permitted to traverse an edge previously traversed to ensure that the packet does not loop. GFG-sooner-back (GFG-s) algorithm, Data et al proposed improvements to reduce the hop counts in GFG. First, FACE-2 is modified by introducing 2-hop neighbor information to determine if there is a closer node to the destination, which will allow the packet to exit the FACE mode earlier than in the previous FACE-2. Second, GFG-s was proposed to use a shortcut procedure involving 2-hop neighbor information to check for a shorter path than that provided by the immediate neighbor. The number of hops is further reduced by using the dominant set to minimize the nodes involved in route determination. Geographic Routing Algorithm (GRA) initially uses a Greedy forwarding strategy to forward packets to the neighbor closest to the destination. When local minima are reached, a depth-first search route discovery process is initiated to find a path to the destination. Nodes cache the routing information and progressively build up routing tables from the discovery procedures. These tables are then used in place of geographic forwarding when the cached route information is available. Terminode Routing focuses on large-scale networks and uses a hybrid approach to routing. Terminode Local Routing (TLR) maintains distance vector routing tables within a set radius of a node. Terminode Remote Routing (TRR) uses a set of anchor points or waypoints to route packets through the network. Anchors are established through a discovery procedure in conjunction with cached anchors from “friend” nodes that are
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considered reliable. After the anchor points for a destination have been discovered, the list of anchor point vectors is inserted into the packet header, and the packet is forwarded progressively though the list by using geographic forwarding. When a node is reached, which has a distance vector entry for the destination, the local information is used to complete the route. Scalable Location Update Based Routing Protocol SLURP incorporates location management which divides a geographical area into rectangular regions called home regions. Each node in the network maintains a location table that maps the node ID to the corresponding home area ID for all other nodes in the network. SLURP forward packets to the centre of Homeregion by using MFR without backward progression. Adaptive Face Routing (AFR) is based upon the face traversal of a planar graph. To optimize routing, AFR incorporates FACE and Bounded Face Routing (BFR) which places a bound on the face traversal defined by an ellipse with the foci at the source and destination. The size of the bound is initially estimated, and then, if BFR fails and the packet returns to the source, the bound is doubled and the BFR process is repeated. Beaconless Routing (BLR) does not assume that nodes have information regarding neighboring nodes and eliminate beaconing, which is typically used to maintain adjacency tables in other geographic routing protocols in the basic routing mode, a node broadcasts a packet with its location and the destination location. Only nodes that determine that they are within a 60-degree sector from the previous node to the destination consider forwarding the packet. Each of these nodes delays transmission, depending on the progress that the node makes toward the destination. When the node closest node transmits after the minimum delay, the other candidates detect the transmission, are thereby informed that the packet has been forwarded successfully, and drop the packet. Exponential Age Search (EASE) and Greedy EASE (GREASE) use node mobility to disseminate node location information based on the time and location of the last encounter with each of the other nodes in the network. EASE uses Last Encounter Routing (LER), in which surrounding nodes are searched in an increasing radius until a node is found, whose last encounter with the destination is less than or equal to half the time of the current nodes last encounter with the destination. This nodes location is then used as a waypoint for routing (although no specific routing strategy is specified). In GREASE, if a node is encountered with a more recent estimate of the destination location than the waypoint, then that location becomes the new waypoint.
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References http://www.webopedia.com/TERM/r/routing.htmlh http://www.cs.ucla.edu/classes/fall03/cs218/paper/p96rao.pdfttp://en.wikipedia.org/wiki/Geographic_routing http://www.cs.umd.edu/users/slee/pubs/nadv-mobihoc05.pdf http://ieeexplore.ieee.org/Xplore/ http://enl.usc.edu/papers/cache/cldp_nsdi05.pdf
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connected to the device but it can be accesses through one ore more hops • Interface: The outgoing network interface the device should use when forwarding the packet to its final destination. Geographic routing (also called georouting or position-based routing) is a routing principle that relies on geographic position information. It is mainly proposed for wireless networks and based on the idea that the source sends a message to the geographic location of the destination instead of using the network address. The idea of using position information for routing was first proposed in the 1980s in the area of packet radio networks and interconnection networks .Geographic routing requires that each node can determine its own location and that the source is aware of the location of the destination. With this information a message can be routed to the destination without knowledge of the network topology or a prior route discovery. Geographic routing protocol use the location information to progressively forward packets through the physical space toward the destination location with intermediate next hop routing decisions based on selecting the neighbor that has the closest distance compass setting or some other measure of forward progress toward the destination. This process termed geographic forwarding. Geographic routing (or position-based routing) uses location information for packet delivery in multihop wireless networks Neighbors locally exchange location information obtained through GPS (Global Positioning System) or other location determination techniques. Since nodes locally select next hop nodes did not base on this neighborhood information and the destination location, route establishment nor is perdestination state required in geographic routing. As large-scale sensor networks become more feasible, properties such as stateless nature and low maintenance overhead make geographic routing increasingly more attractive. Also, location-based services such as geocasting can be best realized using geographic routing. The most popular strategy for geographic routing is simply for warding data packets to the neighbor geographically closest to the destination. Although this greedy method is effective in many cases, packets may get routed to where no neighbor is closer to the destination than the current node. Many recovery schemes have been proposed to route around such voids for guaranteed packet delivery as long as a path exists. These techniques typically exploit planar sub graphs (i.e., Gabriel graph, Relative Neighborhood graph), and packets traverse faces on such graphs using the well-known right-hand rule. Most geographic routing protocols use one-hop information, but generalization to two-hop neighborhood is also possible.
ADVANTAGES OF GEOGRAPHIC ROUTING:
Geographic routing protocol offers a number of advantages over conventional adhoc routing strategies. Geographic forwarding does not require maintenance of routing tables or route construction prior to or during the forwarding process. The forwarding process also allows a packet to adapt to changes in the topology by selecting the next best choice if an intermediate node used by previous packets becomes unavailable. Without the need for route construction these approaches do not require table maintenance other than immediate neighbors nor dissemination of topology information 2
PROBLEMS ASSOCIATED WITH GEOGRAPHIC ROUTING:
The complexity and overhead required for a distributed location database service is a disadvantage of geographic routing. The dependence of geographic forwarding on the physical network topology means that obstacles such as a building or the lack of radio coverage may result in voids in the physical network topology. These voids may inhibit forward progress of packets local minima where there are no neighbors available that are closer to the destination resulting in the failure of the forwarding strategy. GEOGRPHIC ROUTING PROTOCOLS A routing protocol is a protocol that specifies how routers communicate with each other to disseminate information that allows them to select routes between any two nodes on a network. Typically, each router has a priori knowledge only of its directly attached networks. A routing protocol shares this information successively, first among immediate neighbors and then throughout the entire network. This way routers can gain knowledge of the network topology at large. Geographic routing protocol based primarily on a forwarding strategy and secondarily in a recovery strategy.There are various approaches, such as single-path, multi-path and flooding-based strategies. Most single-path strategies rely on two techniques: greedy forwarding and face routing. Greedy forwarding tries to bring the message closer to the destination in each step using only local information. Thus, each node forwards the message to the neighbor that is most suitable from a local point of view. The most suitable neighbor can be the one who minimizes the distance to the destination in each step (Greedy). Alternatively, one can consider another notion of progress, namely the projected distance on the source-destination-line (MFR, NFP), or the minimum angle between neighbor and destination (Compass Routing). Not all of these strategies are loop-free, i.e. a message can circulate among nodes in a certain constellation. It is known that the basic greedy strategy and MFR are loop free, while NFP and Compass Routing are not. The main types of routing protocols associated with geographic routing are • • • • • • • • • • • • • Random progress method Most forward with fixed radius R Nearest with forward progress NFP Greedy forwarding Location aided routing LAR Distance routing effect algorithm for mobility DREAM Compass Routing Geographic Distance Routing GEDIR Depth first search with Dominant sets GPSR GFG Sooner back Geographic routing algorithm GRA Terminode Routing
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• • • • •
Terminode Local Routing Scalable Location Update Based Routing Protocol SLURP Adaptive Face Routing Beaconles Routing Exponential Age search EASE
Random progress method Forwards packets to a random neighbor from among those that are closer to the destination. The random selection of the next hop was suggested to provide an even distribution of traffic load. Most forward with fixed radius R Forwards packets to the neighbor within a set radius of the current node that makes the most forward progress (or the least forward progress) along the line drawn from the current node to the destination. Progress is calculated as the cosine of the distance from the current node to the neighbor projected back onto the line from the current node to the destination... The nearest with forward Progress (NFP). This strategy forwards a packet to the closest neighbor in the forward direction. The current node then modifies it’s transmit power to suit the connection. This results in higher delivery rates due to reduce interference and contention at the cost of increased hop count. Greedy forwarding: Greedy forwarding which selects the next hop as the node closes to the destination. The greedy approach unlike MFR allows a packet to move to a node i.e. beyond the destination if that node is closer to the destination than the previous node. 5. Location-Aided Routing (LAR) and Distance Routing Effect Algorithm for Mobility (DREAM) focus on ad-hoc networking environments. LAR is an on-demand routing protocol that uses the last known position of the destination node and its velocity to limit the flooding of routing requests toward the destination. Flooding is limited to an area between the source and a circle, calculated around the destination, with its center at the last known position and a radius, which is determined by the nodes velocity. This improves the efficiency of the underlying on-demand protocol but still suffers the problem of scalability and latency associated with on-demand strategies. DREAM is based on the flooding of data without the prior establishment of a route. Messages are flooded into an area that is limited in a similar manner to that used in LAR. However, the use of directional flooding of data packets, as opposed to flooding of route requests in LAR, may still incur a significant bandwidth overhead. Compass Routing (DIR) a variation on forwarding based on the direction for routing decisions are proposed. In this strategy, packets are forwarded to the neighbor on the closest compass setting to the destination. DIR has the disadvantage that it is susceptible to looping and does not guarantee a delivery. To guarantee a delivery, face traversal of disjoint regions was proposed, where a packet is forwarded around one side of each face until the packet reaches the further edge of the face that intersects the line from the source to the destination. From this point the packet traverse the next face in a similar manner until the destination is reached. Geographic Distance Routing (GEDIR) adopts the Greedy strategy proposed by Finn, where packets are forwarded to the neighbor closest to the destination. To allow a packet to move through the local minima, GEDIR does not include the current node in the
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distance calculation and permits a packet to be sent in the reverse direction if no forward node is available. To prevent looping the packet is not permitted to be passed from the neighbor back to the previous node. This addresses single- hope looping, but packet may loop back via an alternate path. Two variations of GEDIR were proposed to address the problem. These include flooding at the local minimum (f-GEDIR) and maintaining 2-hop neighbor information to predict and avoid the local minima. F-GEDIR is found to be effective at the expense of increased control overhead, whereas 2-hop GEDIR was an improvement but still allows loops of 2+hops. A multipath version c-GEDIR is also proposed to add reliability. FACE-2 algorithm extracts a connected planar sub graph by converting the network topology to a Gabriel graph. FACE-2 has the advantage that it guarantees delivery in a connected static graph, although it does not provide the optimal path and also requires a unit graph with equal transmission radii for planar graph conversion. Greedy Forward Greedy (GFG) was proposed, which incorporates both GEDIR (Greedy forwarding) for routing and FACE-2 (planar graph traversal) to recover when Greedy forwarding encounters the local minima. Depth-First Search with Dominant Sets uses greedy forwarding. When route failure occurs, the packet backtracks to the previous node, which forwards the packet to the next closest neighbor to the destination. To reduce the number of nodes involved in routing and thus reduce the number of hopes involved in route determination, routing is restricted to the dominant set until the destination is known to the current node. GPSR is a packet-switched routing protocol implementation of GFG using Greedy forwarding and planar graph traversal. Nodes are only required to maintain 1-hop neighbor location information that is exchanged using periodic beacons. Packets are first transmitted with a mode flag set to Greedy. When the local minima is reached, the flag is changed to the perimeter mode, and a face traversal algorithm is used until Greedy forwarding can be resumed. Packets are not permitted to traverse an edge previously traversed to ensure that the packet does not loop. GFG-sooner-back (GFG-s) algorithm, Data et al proposed improvements to reduce the hop counts in GFG. First, FACE-2 is modified by introducing 2-hop neighbor information to determine if there is a closer node to the destination, which will allow the packet to exit the FACE mode earlier than in the previous FACE-2. Second, GFG-s was proposed to use a shortcut procedure involving 2-hop neighbor information to check for a shorter path than that provided by the immediate neighbor. The number of hops is further reduced by using the dominant set to minimize the nodes involved in route determination. Geographic Routing Algorithm (GRA) initially uses a Greedy forwarding strategy to forward packets to the neighbor closest to the destination. When local minima are reached, a depth-first search route discovery process is initiated to find a path to the destination. Nodes cache the routing information and progressively build up routing tables from the discovery procedures. These tables are then used in place of geographic forwarding when the cached route information is available. Terminode Routing focuses on large-scale networks and uses a hybrid approach to routing. Terminode Local Routing (TLR) maintains distance vector routing tables within a set radius of a node. Terminode Remote Routing (TRR) uses a set of anchor points or waypoints to route packets through the network. Anchors are established through a discovery procedure in conjunction with cached anchors from “friend” nodes that are
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considered reliable. After the anchor points for a destination have been discovered, the list of anchor point vectors is inserted into the packet header, and the packet is forwarded progressively though the list by using geographic forwarding. When a node is reached, which has a distance vector entry for the destination, the local information is used to complete the route. Scalable Location Update Based Routing Protocol SLURP incorporates location management which divides a geographical area into rectangular regions called home regions. Each node in the network maintains a location table that maps the node ID to the corresponding home area ID for all other nodes in the network. SLURP forward packets to the centre of Homeregion by using MFR without backward progression. Adaptive Face Routing (AFR) is based upon the face traversal of a planar graph. To optimize routing, AFR incorporates FACE and Bounded Face Routing (BFR) which places a bound on the face traversal defined by an ellipse with the foci at the source and destination. The size of the bound is initially estimated, and then, if BFR fails and the packet returns to the source, the bound is doubled and the BFR process is repeated. Beaconless Routing (BLR) does not assume that nodes have information regarding neighboring nodes and eliminate beaconing, which is typically used to maintain adjacency tables in other geographic routing protocols in the basic routing mode, a node broadcasts a packet with its location and the destination location. Only nodes that determine that they are within a 60-degree sector from the previous node to the destination consider forwarding the packet. Each of these nodes delays transmission, depending on the progress that the node makes toward the destination. When the node closest node transmits after the minimum delay, the other candidates detect the transmission, are thereby informed that the packet has been forwarded successfully, and drop the packet. Exponential Age Search (EASE) and Greedy EASE (GREASE) use node mobility to disseminate node location information based on the time and location of the last encounter with each of the other nodes in the network. EASE uses Last Encounter Routing (LER), in which surrounding nodes are searched in an increasing radius until a node is found, whose last encounter with the destination is less than or equal to half the time of the current nodes last encounter with the destination. This nodes location is then used as a waypoint for routing (although no specific routing strategy is specified). In GREASE, if a node is encountered with a more recent estimate of the destination location than the waypoint, then that location becomes the new waypoint.
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References http://www.webopedia.com/TERM/r/routing.htmlh http://www.cs.ucla.edu/classes/fall03/cs218/paper/p96rao.pdfttp://en.wikipedia.org/wiki/Geographic_routing http://www.cs.umd.edu/users/slee/pubs/nadv-mobihoc05.pdf http://ieeexplore.ieee.org/Xplore/ http://enl.usc.edu/papers/cache/cldp_nsdi05.pdf
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