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Smart Computing Review, vol. 5, no. 3, June 2015 135 Smart Computing Review The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey Komal Zaman1, Muhammad Shafiq2, Jin-Ghoo Choi2, and Muddesar Iqbal1 1 Faculty of Computing and Information Technology, University of Gujrat / Gujrat, Pakistan / [email protected], [email protected] 2 Department of Information and Mobile Communication Engineering, Yeungnam University / South Korea / [email protected], [email protected] *Corresponding Author: Jin-Ghoo Choi Received March 9, 2015; Revised April 23, 2015; Accepted May 18, 2015; Published June 30, 2015 Abstract: In mobile ad hoc networks, nodes autonomously exchange their messages each other without centralized control, possibly under highly dynamic topology. The objective of routing protocol in mobile ad hoc networks is to establish a well-organized route between transmitting and receiving nodes to send their messages with minimum control overhead and bandwidth use. In early 1990’s, various protocols had been designed for mobile ad hoc networks, which can be classified as Proactive (or Table-driven), Reactive (or On-demand), and Hybrid protocols. Nowadays, mobile ad hoc networks are facing many challenges like mobile ubiquity, scalability, security, energy constraint, quality of service support, multicast, bandwidth limitations, etc., which makes the routing much harder. The major facets of routing protocols in mobile ad hoc networks are the identification of best available route, coordination with neighboring nodes for route maintenance, route reliability enhancement, overhead reduction, and network efficiency maximization. This article overviews various routing protocols in mobile ad hoc networks while comparing their respective functionalities. Keywords: mobile ad hoc network, table driven protocol, on-demand routing protocol, hybrid protocol Introduction M obile ad hoc networks (MANETs) can be established and deployed haphazardly on short notice. In MANETs, the mobile nodes are connected with each other by wireless links. There is a need of wireless connections for people and automobiles that want to connect to the Internet, irrespective of established communication infrastructure. MANETs This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2012R1A1A1005972). DOI: 10.6029/smartce.2015.03.002 136 Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey have some general characteristics such as a dynamic topology, energy constraints, limited bandwidth, and security vulnerabilities. A dynamic topology refers to the ability of mobile nodes to move freely at random speeds, which causes constant changes in network topology. Unlike wired networks, energy constraints arise in mobile nodes, which is a major issue caused by the unavailability of a permanent energy source. The continuous usage of batteries due to the portable nature of mobile nodes leads to unreliable communications. Due to this problem, energy conservation is a demanding feature in ad hoc networks. Wireless networks also use less bandwidth than wired networks due to the higher demand for the wireless spectrum and the nature of the wireless medium, which causes interference, noise generation, fading effects, and collision. Similarly, wireless networks, especially MANETs, are more prone to security vulnerabilities than that of wired networks. For instance, a single denial-of-service attack (DOS) or a simple jamming attack might disrupt the functionality of the entire network. Therefore, in addition to creating routes, a trivial routing protocol must also take measures against security threats (e.g., eavesdropping, spoofing, and DOS attacks). Routing is a process between nodes to establish an up-to-date and efficient path for information exchange. The major concern is to discover and maintain the paths among mobile nodes, while maintaining unidirectional links in a dynamic topology, particularly in infrastructure-less networks where all nodes behave like a specific router for itself. Usually, route efficiency is measured by various attributes such as the number of hops, control traffic, and security. Communication between an exclusive sender and receiver takes place through identical routing protocols, as discussed in following sections, to handle the underlined concerns in MANETs. Classification of Routing Protocols in MANETs Routing protocols are usually classified according to their properties. In general, routing protocols fall into three different categories: Unicast routing, Multicast routing, and Broadcast routing. Unicast routing represents one-to-one communication and is the largest class of routing protocol in ad hoc networks. The multicast routing protocol deals with sending a data stream to multiple destinations. Therefore, such protocols are much more useful in group communication and save on bandwidth better than multiple unicast protocols. Evidently, broadcast routing refers to sending information by a sender to its one-hop-neighbor in ad hoc networks. However, this paper covers routing protocol categories based on the type of temporal information they use. The protocols that they use to explore the paths and maintain them are called Table-driven protocols, On-demand protocols, and Hybrid routing protocols, and are shown in Fig. 1. In this paper we investigate each of them individually. Proactive or Table-Driven Protocols Proactive or table-driven protocols establish up-to-date routing information of the overall network topology by maintaining routing tables. Thereby, each node proactively exchanges their tables before sending a communication request to collect and store the routing information. If any situation arises due to node mobility, the rest of the nodes are required to update their topological records. The significant drawbacks of this table-driven approach are the control overhead that regularly updates the entries in routing tables, and the problem of keeping the temporal information based on past evaluations. Also, after the description of the representative chosen table-driven protocols, they are all summarized at a glance in Table 1. ■ Destination-sequenced Distance Vector The Destination-sequenced Distance Vector (DSDV) [1] was the first purpose protocol developed for MANETs, and it introduces modifications to the distributed Bellman-Ford (BDF) algorithm. These modifications imply a sequence number in the routing table to guarantee a loop-free mechanism. It uses the algorithm to find the single, shortest path to its intended destination. It maintains a routing table at all nodes, which has a record of the attributes of the next hop, number of hops to the destination, the sequence number, and other details. However, the routing table entries cause overhead, which travels throughout the network at the cost of throughput performance. Two types of update packets, called Full Dump Packets and Incremental Packets were used to reduce the overhead. The first packet was designed to carry the entire information of the route topology, while the next incremental packet was only designed to carry the information about topological changes. The incremental packets are updated more frequently than full dump packet which degrades the protocol performance for a large and dense environment. Smart Computing Review, vol. 5, no. 3, June 2015 137 ■ Wireless Routing Protocol The Wireless Routing Protocol (WRP) [2] is an enhanced version of DSDV and also relates to the algorithm of an overall pathfinding class [3]. It also maintains routing information in a table. However, unlike DSDV, it uses various kinds of routing tables (i.e., Distance table, Link cost table, Routing table, and Message retransmission list) to execute pathfinding and maintenance. Under a highly-scalable network, WRP leads to the maximization of the memory overhead, which introduces an additional burden on member nodes. As a consequence, the overall throughput performance decreases, and convergence time increases, which leads to energy starvation for member nodes in MANETs. Moreover, WRP also exchanges hello messages among neighbor nodes to keep the messages alive. When no such previous packet transmission exists, each node needs to be activated every time, which leads to a great amount of bandwidth and power consumption. ■ Global State Routing The Global State Routing (GSR) [4] protocol also falls into the category of proactive routing and works on the basis of a link state algorithm. This protocol limits the update packets among intermediate nodes by making some improvements to the traditional link state algorithm. In GSR, all the nodes manage the latest received messages in a link state table. These messages come from adjacent nodes, and thereby information is periodically shared among one-hop neighbors only, which reduces the control overhead throughout the network. However, flaws still exist in updating messages, as these messages are comparatively large in number. Particularly when the network size is large, the size of update messages will also increase, which has the potential to degrade throughput performance. ■ Fisheye State Routing The Fisheye State Routing (FSR) [5] protocol is the descendant of the global state routing protocol, which is also based on the link state foundation updating mechanism. Unlike GSR, this protocol distributes update messages at a greater speed to its adjacent nodes, and reduces the update message size by avoiding broadcasting on remote nodes that are not in a fisheye scope. Using the fisheye technique reduces control overhead by disseminating topology information in which information is updated for every destination on the basis of the source distance in the network. The node stores the Link State, and the node broadcast periodically updates messages to its neighbors and updates closer node broadcasts more frequently. ■ Optimized Link State Routing Optimized link state routing (OLSR) [6] is a variation of table driven protocol to be work in link state algorithm style. It is a point to point routing protocol that performs hop to hop routing in which all the nodes route a packet on the basis of most recent information about topology knowledge and maintenance in network. Thereby all nodes interchange their link state messages periodically. It uses multipoint relay nodes (MPR) scheme for the sake of reduction in the size of control messages alternatively in addition to reduces the number of nodes which are used for rebroadcasting at every route update. In MPR scheme, a set of adjacent nodes known as MRP set of that node are selected by every node for resending the update packets. Hence, a node which is not the part of that set then it can only read and process all the packets but unable to resend. Thus, basic purpose of MRP nodes is to reduce the network flooding. ■ Distance Routing Effect Algorithm for Mobility The Distance Routing Effect Algorithm for Mobility (DREAM) [7] is a table-driven protocol that effectively uses the Global Positioning System (GPS) to synchronize the member nodes in a network. The DREAM protocol keeps geographical coordinate information in purpose-built tables called location tables. The exchange of a location information table makes DREAM more scalable, because it consumes significantly less bandwidth than exchanging complete tables of the link state and distance vector protocols. It also does not require stationary nodes to send update messages, which further reduces routing overhead and improves network efficiency. 138 Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey Figure 1. Taxonomy of routing protocols in MANETs ■ Source-Tree Adaptive Routing The Source-tree Adaptive Routing (STAR) protocol [8] is another type of table-driven routing protocol category based on the link state algorithm. In STAR, a source tree is maintained by all routers. In the source tree the ideal route information to reach any intended destination is stored. It is very useful to reduce a great amount of routing overhead in the entire network by supporting the least overhead routing approach (LORA) while exchanging routing information. If needed, STAR also supports the Optimum Routing Approach (ORA). It uses ORA to eliminate the periodic update mechanism present in the link state algorithm by distributing update information according to pre-defined conditions or events. As STAR stores route information in a routing table, it experiences low latency and also consumes less bandwidth for routing updates, which makes it more suitable for highly-scalable networks. It, however, comes at the cost of increased processing and memory overhead that varies based on the complexity of the maintenance of network graphs by all the nodes, which may quickly be exchanged with adjacent nodes to continuously report to different source trees. Smart Computing Review, vol. 5, no. 3, June 2015 139 ■ Multimedia Support In Mobile Wireless Networks The Multimedia Support in Mobile Wireless Networks (MMWN) protocol [9] is a kind of proactive routing protocol where a clustering hierarchy is used to maintain a network route in which all clusters have two kinds of mobile nodes: Endpoints and Switches. In MNWN, each cluster stores its entire routing table in a dynamically-distributed database (DDD). However, location management uses a specific node as location manager (LM), which is responsible for updating and finding locations. Therefore, effectively using DDD and LM reduces the routing overhead. In a highly-dense topology, due to its hierarchical structure, location management information moves to the location management tree very deeply, which makes finding and updating of locations more complex. Moreover, the change in the hierarchical cluster membership of the location manager generates implementation problems, because it affects the tree’s hierarchical management. ■ Cluster head Gateway Switch Routing Protocol The Cluster-head Gateway Switch Routing Protocol (CGSR) [10] is a variation of table-driven routing that effectively handles scalability issues in ad hoc networks. It implies hierarchical routing. In CGSR, nodes proactively form clusters by partitioning a complete network. To build clusters, an algorithm known as least cluster head change (LCC) is used. In CGSR, two kinds of mobile nodes are used to manage different responsibilities, called cluster heads and gateway nodes. Cluster heads manage all the nodes and their communication within the boundary of a cluster called intra-communication, whereas gateway nodes are responsible for the connection of two or more clusters for inter-cluster communication. The data traffic passes from the cluster head to the gateway and from the gateway to the cluster head, thus it reaches from the source to its intended destination. As this is a kind of distance vector protocol, it maintains two tables for each node. Therefore, its routing table records one entry to its cluster head and another for all the clusters in the network. Hence, CGSR effectively controls the population of route entries in routing tables, particularly in a highly-dense environment. To route a packet, the source looks to the cluster member table to find its destination’s cluster head, to find the next hop, and finally consults the distance vector routing table to reach its destination. Moreover, CGSR reduces the routing table’s broadcast packet size by making one entry for all nodes in an identical cluster. However, there is the downside of maintaining the structure of clusters for highly-mobile networks. ■ Hierarchical State Routing The Hierarchical State Routing (HSR) protocol [11] also falls into the table-driven protocol category, and is based on the link state algorithm, which maintains a map of a hierarchical topology. In this hierarchical topology, all clusters hold three kinds of nodes: cluster heads, gateways, and internal nodes. The internal nodes are all of the nodes in all clusters that do not have a special role. In HSR, all nodes carry a unique identity (i.e., MAC address) and a hierarchical identity (HID) for path setup. The source uses HID to route packets towards a destination by proposing cluster nodes in a logical way instead of a geological way. In logical clustering, the logical link information at the lower level is interchanged by logical nodes, and the information regarding the link state is broadcast down to the lowest level. Finally, the lowest level physical node maintains the hierarchical topology of the network. The resultant advantage is less overhead due to a limited number of broadcasts on the entire network. However, it requires relatively more overhead for cluster formation and maintenance. ■ Direction Forward Routing The Direction Forward Routing (DFR) protocol [12] aims to overcome the old entries problem (i.e., outdated records) in the routing table, known as the “stale” routing table entry problem. As in the DSDV and AODV protocols, the shortest path is broadcast to the destination when a packet is sent to a predecessor node. If a predecessor moves to another location, the entries of its routing table become invalid, which makes predecessor-based forwarding fail. Alternatively, a geo-coordinatesystem-equipped ad hoc network resolves this problem in a way that whenever an update happens, the node records its geographical location along with the direction of arrival. In DFR, if a predecessor forwarding failure happens, the data packet is sent to the most promising adjacent node available in the record. In DFR, the routing updates are used to learn directions, unlike in geo-routing, which learns about directions from the destination coordinates. The major advantage of DFR is that it does not require destination coordinates. Consequently, messages are not sent to dead ends, because there is no need of “perimeter re-routing” [13] in its operations. Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey 140 ■ Topology Broadcast Reverse Path Forwarding The Topology Broadcast Reverse Path Forwarding (TBRPF) protocol [14] is a proactive routing protocol based on the link state algorithm. This protocol performs hop-by-hop routing and uses a reverse path forwarding mechanism to route the distribution of updated packets in the reverse direction with a spanning tree protocol. These spanning trees contain the shortest hop path from the nodes that lead to the update source. In its operation, a source tree is calculated at every node that offers all accessible destination paths. It also uses multiple periodic hello messages at every node to report its status to its neighbors. These hello messages only report the change in neighboring nodes’ statuses and are very useful to reduce the bandwidth overhead. ■ Intra-zone Routing Protocol The Intra-zone Routing Protocol (IARP) [15] is also a variation of a table-driven protocol with a limited scope and support when compared to existing global routing protocols. The radius of the routing zone is used to define the IARP’s range, which is basically a hop distance, and is delivered by IARP route updates. The immediate availability of routes to destinations avoids latency and traffic overhead while finding a route. The requirement of the global route to find a remote destination requires more efficient bandwidth, which is broadcast and directed towards the routing zone edges. IARP’s routing zone provides better and real-time route maintenance when a route is discovered. Within its routing zone, multiple hop routes are used to avoid the failure of a link. Table 1. Comparison of proactive routing protocols Protocol Structure Method Updates Advantage Disadvantage DSDV Flat Broadcast Hybrid Loop free mechanism High overhead WRP Flat Broadcast Periodical Loop free mechanism Memory overhead GSR Flat Broadcast Periodical Localized update High memory overhead FSR Hierarchical Broadcast Periodical Reduces control overhead OLSR Flat Broadcast Periodical DREAM Flat Broadcast Mobility Based STAR Hierarchical Broadcast Conditional Reduces control overhead MMWN Hierarchical Broadcast Conditional Reduces control overhead CGSR Hierarchical Broadcast Periodical Reduces control overhead HSR Hierarchical Broadcast Hybrid Low control overhead Location management needed DFR Flat Broadcast Periodical Overcome the old entries problem Selection of the most forward node to the destination along with direction may cause a loop TBRPF Flat Broadcast Periodical/ Differential Low control overhead High memory overhead IARP Flat Broadcast Periodical AWDS Flat Broadcast Periodical LANMAR Hierarchical Broadcast Periodical Reduces control overhead and contention Reduces memory overhead and control overhead Avoid the traffic overhead and link failure Support layer three protocols with fast convergence Reduces control overhead High memory overhead and less accuracy Two-hop neighbor knowledge required GPS is require Increases memory overhead as well as processing overhead. Management of mobility and maintenance of different clusters required. Cluster information and maintenance required Limited scope Scalable in local area network Limited scope ■ Landmark Ad Hoc Routing The Landmark Ad Hoc Routing (LANMAR) protocol [16] is actually a combination of the fisheye state routing and landmark routing protocols. LANMAR is designed for the development of fixed wireless area networks. In LANMAR, a landmark is a router and its adjacent routers contain routing entries corresponding to this landmark router. In its operation, a network is divided into many logical subnets along with selected and predefined landmarks. All the nodes within one Smart Computing Review, vol. 5, no. 3, June 2015 141 subnet are connected with FSR and move as a group. Exchanging the distance vector provides a route to a landmark and finally to corresponding subnets. Due to the extension of FSR, there is an advantage of group mobility in LANMAR, which summarizes the routes of group members to a single route towards a landmark. ■ Ad-hoc Wireless Distribution Service The Ad Hoc Wireless Distribution Service (AWDS) protocol [17] is based on a link state routing protocol similar to the OLSR protocol. It uses a unique MAC address in the wireless local area network (WLAN) card instead of an IP address for fast convergence. It creates a virtual network interface that can be used by the kernel, as in typical local area network interface cards. Therefore, it supports all kinds of layer-three protocols such as IPv4, IPv6, DHCP, and IPX. Reactive or On Demand Protocols Reactive protocols are also known as on-demand routing protocols, in which routing information is maintained by the member nodes only when communication exists on run time. If a node wants to communicate with another node, a demand mode route is found and a connection is established between source and destination to send/receive packets. A route request is sent out to discover a route. There are two types of on-demand protocols. The first is source routing, in which a complete source is carried by all data packets towards a destination. All data packets have some information in their headers that help the intermediate node to forward these packets. The second is hop-by-hop routing, in which all data packets keep the addresses of the next hop and the destination to establish a route. Hence, routing tables are used by intermediate nodes for information to reach the destination. Unlike table-driven protocols, a significant advantage of reactive protocols is their suitability for ad hoc networks and no periodic updates, which consequently utilizes less bandwidth. The following represents the representative on-demand protocols, and an at-a-glance comparison can be found in Table 2. The following represents the major protocols in reactive routing. ■ Ad Hoc on-demand Distance Vector The Ad Hoc On-demand Distance Vector (AODV) protocol [18] is based on the DSDV and DSR algorithms, in which the mechanism of route discovery and route maintenance are taken from DSR, whereas periodic update packets and point-topoint routing sequence number are taken from the DSDV protocol. In AODV, the nodes that are not available on an active path do not need to maintain routes towards the destination node. It implies different messages for path discovery and its maintenance such as “Route Request,” “Route Replies,” and “Route Errors.” It uses the UDP/IP protocol mechanism to receive these messages. In AODV, next hop information is stored on the basis of the flow to transmit data packets by both nodes, i.e., intermediate nodes and source nodes. It uses the destination sequence number of the destination node to set up the route, while the highest sequence number is selected to find a new route. The source forwards the route request, and a node with a fresh route is selected. A route reply is sent back to the source. This algorithm introduces many advantages such as adaptability on highly-dynamic networks, low overhead, detection of fresh routes towards the destination, and a very small delay in the connection setup. ■ Dynamic Source Routing The Dynamic Source Routing (DSR) protocol [19] is a reactive routing protocol with source-based routing that is especially designed for multi-hop ad hoc networks. It eliminates messages to reduce bandwidth consumption. A route cache concept is used by nodes containing source routes. Whenever a new route is required, the entries in the cache are updated dynamically. It also uses dynamic source routing to discover routes and for maintenance. In the route discovery mechanism, a control message for a route request (RREQ) is forwarded to the neighboring nodes that are available in the cache. The intermediate nodes rebroadcast this RREQ message further. Finally, a node sends a route reply (RREP) to the source. This RREP is stored in the cache for future use. In case of a link failure, a route error (RERR) message is sent to the source, and the source node deletes that route from its cache. If any other route exists as an alternative to the deleted route in the cache, then it will be replaced. Otherwise it will re-initiate the route discovery mechanism. It uses a flooding technique called light-weight mobile routing (LMR) to find the routes. There is no need for a route discovery mechanism because every node has multiple routes to its destination, which increases its reliability by allowing nodes to select the next available route. Eventually extra delays are avoided. Moreover, every node just maintains its one hop neighbor information, which further 142 Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey reduces storage overhead. However, sometimes LMR introduces illegal routes that cause extra delays in finding the right one. This protocol also increases network connectivity in a highly-dynamic environment, which may cause nodes to consume too much power and enter sleep mode. ■ Routing On-demand Acyclic Multi-path The Routing On-demand Acyclic Multi-path (ROAM) protocol [20] is a reactive routing protocol that uses a diffusing computation operation in which intermodal coordination is used with acyclic sub graphs. This acyclic sub graph is created by the routers involved. ROAM has some advantages, namely no “Search to Infinity” problem, which affects many ondemand protocols. It ends this problem by stopping a multiple flood search when the needed destination is no longer available. It also saves significant bandwidth, because all routers maintain entries for the destination and keep the entries table up to date. ■ Temporally Ordered Routing Algorithm The Temporally Ordered Routing Algorithm (TORA) [21] protocol is a distributed routing protocol designed to determine routes on demand. It reduces communication overhead and quickly establishes paths towards the destination. This protocol also uses LMR repair techniques in which an LMR query reply mechanism is similar to a directed acyclic graph (DAG). TORA performs three simple functions: route creation, route maintenance, and route erasure. Route creation makes a DAG route towards the destination node using height as a metric. After the creation of a DAG route, links are assigned to neighboring nodes on the basis of a comparative height metric. During route maintenance, during mobility, if a DAG breaks down, it reestablishes a DAG route towards the destination in a similar fashion. Route erasure involves erasing invalid routes by flooding a clear packet (CLR) throughout the network. This protocol has the ability to select the most convenient route instead of the shortest path, which minimizes traffic overhead. One disadvantage is that sometimes TORA introduces an invalid temporary route. ■ Associativity-based Routing The Associativity-based Routing (ABR) [22] protocol uses a reactive approach and introduces a new metric for node route selection, called degree of association stability, in which beacons are used by nodes to indicate their presence, and also to update routes. Upon receiving the beacon, the associativity tick of the receiving nodes increases along with the beaconing node. For any specific node, the highest value of the associativity tick shows its comparatively static nature. The associativity tick is reset when moving any node from one neighborhood to another. ABR has some drawbacks, for example, determining the degree of associativity for any link requires periodic beaconing lead nodes to become active all the time, which increases power consumption. It lacks a route cache or multiple routes towards a destination, which leads to the unavailability of an alternate route in the case of an alternative route demand, whenever the route discovery mechanism becomes essential. ■ Signal Stability Adaptive The Signal Stability Adaptive (SSA) protocol [23] is basically a descendent of the ABR protocol. Instead of using an associativity tick to select the route, this protocol uses location stability and signal strength. In SSA, there is less need to reconstruct the route, because the routes are not selected on the basis of the shortest path. The major drawback of SSA is it categorizes a link as a table or a week on the basis of beacon count. Hence, a potential shorter route, which can be discovered by the intermediate node, does not get considered by intermediate nodes, which causes long delays for route generation. In the case of link failure, no effort is made for route repair as in the DSR and AODV protocols. Another disadvantage is that this protocol introduces some additional delays when reconstruction happens on a source node. ■ Location-aided Routing The Location-aided Routing (LAR) [24] protocol is based on the algorithm of flooding, as is used in the DSR protocol, but it reduces network flooding by using the location information of a specific node. To gather location information, every node has to carry a GPS. Since the LAR protocol calculates the estimated zone of a particular node with location information, it might give a rough estimation about a particular node while mobile. Two different schemes are used by its operation. The first scheme computes the zone that defines the boundary of route request packets to reach the destination, Smart Computing Review, vol. 5, no. 3, June 2015 143 whereas the second scheme keeps the destination coordinates in the packets of the route request. These route request packets travel in the direction of the shortest route from one hop to another. This protocol consumes less bandwidth and limits control overhead. The main limitation is the GPS requirement at every node. ■ Ant-colony-based Routing Algorithm The Ant-colony-based Routing Algorithm (ARA) [25] is an on-demand routing protocol that effectively reduces the routing overhead in the network by copying ant food search behavior. This protocol uses a route discovery approach similar to the RREQ method in the DSR protocol in which a Forwarding ANT (FANT) is distributed throughout the network. In this scheme, each node calculates the shortest path and forwards the FANT to its adjacent nodes. When the destination is reached, it sends a Backward ANT (BANT) to the source. After receiving the BANT, a route is determined and the packet delivery from the source towards the destination begins. A data packet travels among all the nodes for the maintenance of all routes, which increases the pheromone (i.e., short lived path) value. Otherwise, this value reduces energetically until it expires. ARA has advantages of less overhead due to the small size of both FANT and BANT, but the drawback of a lack of scalability when the number of nodes increases. ■ Relative Distance Micro discovery Ad Hoc Routing The Relative Distance Micro-discovery Ad Hoc Routing (RDMAR) protocol [26] is an on-demand routing protocol variant that uses the relative distance micro discovery (RDMAR) procedure. This procedure computes the distance between source and destination nodes to minimize the routing overhead, and thus reduces all route request packets to a set number of hops. In case of link failure, route maintenance is used, which reduces bandwidth and battery power. Moreover, it does not require GPS for route estimation. However, there exists another limitation regarding the relative distance micro-discovery procedure that is only useful when the source and destination are tightly synchronized to talk and listen to each other. Otherwise this procedure behaves like flooding. ■ Flow Oriented Routing Protocol The Flow Oriented Routing Protocol (FORP) [27] is a variation of the reactive protocol, which selects and maintains its routes by applying a prediction-based scheme. It predicts the link expiration time (LET) for a link and estimates a route expiration time (RET) for a given route. On the basis of routing decisions, it ensures quality of service (QoS) for certain levels. Less control overhead is required for prediction. For mutual timing references among nodes in networks, it requires GPS, which adds to hardware complexity. If a source wants to send packets to a destination, it checks its own routing table first. If it has an unexpired path towards the destination, it directly sends the packet to its destination. If not, it initiates a request packet for a route that contains a flow identification number and a sequence number, along with the binary addresses of the source node and the destination node. A packet with a lower sequence number and flow identification number than the existing sequence number is discarded by an intermediate node. If a packet has equal sequence numbers, the intermediate node sends the route request message only if it is received from a path of a larger RET. Otherwise, the intermediate node adds the LET of the link from where it received the message and adds its address to broadcast the packet. Through this, the route request message reaches the destination node continuing the entire crossed path, along with its RET. In the case of a longer RET, a new path is used instead of the current one. When route expiry is determined by destination, it initiates a hand-off message and floods the entire network. On receiving the hand-off message, the source determines the best route and sends a set-up message along with a new selected route. ■ Cluster-based Routing Protocol The Cluster-based Routing Protocol (CBRP) [28] is a hierarchy-based protocol in which nodes are exclusively organized into clusters. Cluster heads are responsible for communication within a cluster as well as between clusters, which is the advantage of this protocol, because only cluster heads share routing information. This also leads to less overhead throughout the network. However, some issues regarding cluster establishment and maintenance exist. Cluster heads are also a single point of failure due to the heavy energy requirements, which might disrupt the entire network and lead to frequent maintenance operations in highly dynamic and scalable networks. Table 2. Comparison of reactive routing protocols Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey 144 Multiple routes Route maintenance by Protocol Structure Advantage Disadvantage AODV Flat No Route table Adjustable with the highly dynamic topologies Hello messages, problem of scalability, introduces large delays DSR Flat Yes Route cache Support multiple routes and overhearing promiscuous Scalability problems due to source routing and flooding, large delays LMR Flat Yes Route table Multiple routes Temporary routing loops ROAM Flat Yes Route table Remove the problem of searchto-infinity Huge control overhead in highly mobile networks TORA Flat Yes Route table Multiple routes Temporary routing loops ABR Flat No Route table Route stability Scalability problems SSA Flat No Route Table Stability of route Long delays, problem of scalability, failure of route and re-establishment LAR Flat Yes Route cache Localized discovery of route Source routing, flooding is prefer if no location information is available ARA Flat Yes Route table Small size of control packets, reduces overhead Use process of route discovery is based on flooding RDMAR Flat No Route table Localized discovery of route If no previous communication between nodes then flooding is used FORP Flat No Route Table A minimization mechanism implies for route failure Route discovery based on flooding CBRP Hierarchical No Route table at cluster Routing information is only exchange by cluster-heads Maintenance of cluster, temporary loops SSR Flat No Route table Fewer route reconstructions Potentially long delays, no route discovery mechanism ■ Signal Stability Routing The Signal Stability Routing (SSR) protocol [23] is an on-demand routing protocol in which a route is selected among nodes on the basis of node signal strength and location stability. Signal stability routing can be categorized further into two cooperative protocols, namely, a dynamic routing protocol (DRP) and a static routing protocol (SRP).The former maintains the routing table and signal stability table. It is also responsible for transmission reception and processing. The DRP sends the received packet to SRP after updating all the table entries. However, in the case of the latter, if a packet reaches the intended receiver, it passes the packet to the top of the stack; otherwise, SRP forwards the packet according to the destination in the routing table. If there is no destination in the routing table, the SSR initiates a route search process. There is less reconstruction of routes, because SSR prefers the stable and longer path than the shortest path. In case of link failure, an error message is forwarded to the source by intermediate nodes specifying the failed channel. Later, a route search mechanism is initiated by source to develop a fresh route towards a destination. An erase message is sent to the rest of the nodes to notify them about broken links. There exists a major drawback, which is that the intermediate nodes do not reply to route requests sent to a destination, which causes longer delays before discovering a route. Hybrid Routing Protocols Hybrid protocols involve the combined features of both proactive and reactive protocols, in which routes are initially initiated with some proactive forecasts, which then serve the demand reactively. This reduces delays in route discovery and also balances the cost of routing tables in an efficient and controlled manner [29]. Hybrid routing protocols are suitable for a highly-dense environment. Table 3 lists the significant hybrid protocols together for comparisons at a glance. Smart Computing Review, vol. 5, no. 3, June 2015 145 ■ Zone Routing Protocol The Zone Routing Protocol (ZRP) [30] has a routing zone that defines its range in terms of hops. In ZRP, all nodes maintain network connectivity proactively. In ZRP the nodes are divided into two zones, peripheral nodes and interior nodes. In this zone-based routing scheme, every node is associated with a zone, which is actually a collection of different nodes. In ZRP, the network performance can be disturbed due to zone size and the overlapping of zone regions. Big routing zones are preferred for slowly-moving nodes and high route demands, whereas small routing zones are used for fastmoving nodes that are fewer in number where the demand for routes is less. In ZRP, proactive protocols are used inside the zone, whereas reactive protocols are used between zones. ZRP further uses various protocols for a successful operation. It uses an intra-zone routing protocol (IARP) [15] to ensure a consistent update of routing tables for all the nodes within the boundary of a zone. Each zone stores information for all destinations. The inter-zone routing protocol (IERP) [31] mechanism is used when no destination is available inside the zone. Finally, the border cast resolution protocol (BRP) is used by border nodes to perform search operations on demand to deliver routing information to the nodes that reside outside the zone of a source node. ■ Zone-Based Hierarchical Link State The Zone-based Hierarchical Link State (ZHLS) protocol [32] is a variation of the hybrid protocol, in which the network is distributed into non-overlapping zones. Within the zone, all nodes are aware of related zones and connectivity information in the entire network. ZHLS works like link state protocols, where link state routing is accomplished in two levels, i.e., the node level and the global zone level. It does not require cluster heads due to its hierarchical structure. Only destination zone ID and node ID are used for route discovery and maintenance operations. ZHLS sends location requests to all zones to find the ID of a destination zone. However, it does not require any location search until the destination moves to another zone. There is a drawback in that all the nodes must maintain the static zone map. Table 3. Comparison of different hybrid routing protocols Protocol Structure Multiple routes Route maintained by Advantage Disadvantage ZRP Flat No Need tables for inter-zone and intra-zone communication Reduce retransmissions Overlapping zones exists ZHLS Hierarchical Yes Need tables for Inter-zone and Intra-zone communication Reduces control overhead and reduction of single point of failure. Static zone map required SLURP Hierarchical Yes Require cache for location and a node list Using home regions for location discovery Required the map of static zone DST Hierarchical Yes Route tables required Reduce retransmissions Root node DDR Hierarchical Yes Intra-zone and inter-zone table required Does not require any map and coordinator of zone Ideal neighbors may causes jams ■ Scalable Location Update Routing Protocol The Scalable Location Update Routing Protocol (SLURP) [33] is a hybrid approach in which non-overlapping zones are used to organize nodes in a network. To reduce the maintenance cost of routing information, this protocol eliminates global-route discovery by allocating a home region to all the nodes in the network. All the nodes have a unique home region, which is determined by using the function of static mapping such as “f(NodeID) → regionID”. Since the node ID for all nodes is constant, this function evaluates the same home region. In SLURP, all nodes unicast an update message to its home regions for zone maintenance within its respective zone. Once the zone update packet reaches the home region, this update packet is broadcast to all the nodes that exist in that region. Once the zone is found, the data transmission starts between the source and destination through a geographical forwarding mechanism called Most Forward with Fixed Radius (MFR). The major drawback of this protocol is the maintenance of the static zone map. Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey 146 ■ Distributed Spanning Trees based The Distributed Spanning Trees-based (DST) [34] protocol is a variation of a hybrid routing protocol in which the nodes are grouped together into a number of trees comprising route node and internal node. However, the nodes of a network in the DST protocol can also be categorized in one of three diverse states, depending on the types of their tasks. They are the Router, Merge, and Configure states. To determine route, DST proposes two diverse routing approaches: Hybrid TreeFlooding (HFT) and Distributed Spanning Tree Shuttling (DST). In the former approach, the control packets are disseminated to all neighbors in a manner similar to spanning trees control packet to attach its bridges. However, the latter approach is for the distribution of control packets between the source and destination, in which the control packets are rebroadcast along the edges of the tree. Whenever a control comes down to a leaf node, it is sent up the tree until it reaches the shuttle level, which is actually a higher level. After reaching the shuttle level, the control passes down the adjoining bridges. The main drawback of the DST protocol is the root node, which is a single point of failure. Another disadvantage of this protocol is the extra delays, which are introduced by packets due to holding time. Table 4. Comparison of main categories in routing protocols Parameters Proactive Reactive Hybrid Routing Philosophy Flat or Hierarchical Flat Hierarchical Routing Scheme Table-based On demand Combination of both Routing Overhead High Low Medium Scalability Level Low Not suitable for large network Designed for large network Latency Low due to routing tables High due to flooding Low inside the zone and outside the zone is same as in on demand protocols Availability of Routing Information Always available , stored in table Available when required Combination of both Periodic Updates Yes, whether the topology of the network changes Not needed as route available on demand Yes needed inside the zone Storage Capacity High due to routing table Generally low based on no. of paths Based on zone’s size, sometime high inside the zone as proactive routing protocol Mobility Supports Periodical updates Route Maintenance Combination of both Advantages Information always available. Latency is reduced in the network Route availability when required,low overhead , loop free Appropriate for huge networks , availability ofup to date information Disadvantages High overhead , flooding routing informationthroughout the network Latency is increased in the network Complexity increases ■ Distributed Dynamic Routing The Distributed Dynamic Routing (DDR) protocol [35] is another kind of hybrid routing protocol which is based on treebased routing in which periodic beacon messages are used for creation of tree. These beacon messages are only interchanged by adjacent nodes. In its operation, trees form a forest and then use a gateway node to connect to each other by using a unique zone ID of the forest. Since all nodes are related with one zone, there can be multiple non-overlapping zone in the network. There are five stages in the DDR protocol: preferred neighbor election, forest construction, intra-tree clustering, zone naming, and zone partitioning. The preferred neighbor election stage prefers a node with multiple neighbors for selection. The forest construction stage connects all the nodes to their preferred neighbor for the construction of a forest. The intra-tree clustering stage initiates the structure of a tree and creates routing tables for intra-zone communication. Inter-tree clustering involves determining connectivity with neighboring zones. The zone naming stage involves assigning a unique name to all zones. The zone partitioning stage partitions a network into a number of nonoverlapping zones. To find a stable route between the source and destination, a hybrid ad hoc routing protocol (HARP) [36] is used, which further uses the routing tables of intra-zone and inter-zone to work on top of DDR to create a stable route. This protocol has some advantages. DDR doesn’t depend on a static zone map for routing. It does not require a cluster head Smart Computing Review, vol. 5, no. 3, June 2015 147 for data management and transmission of control packets among the nodes and the zones. DDR introduces large delays due to its preference for neighboring nodes, because if a node is a preferred neighbor to many other nodes, many nodes may want to communicate with it, which causes long delays. The table 4 describes the comparison among proactive, reactive and hybrid routing protocols on the basis of their features. · · · · · · Inconsistent route No periodic updates Relatively suit able with scalability Does not guarantee shortest path Harvest energy Suitable to mobility Hybrid approach · · On demand approach · · · · · · · · Prone to mobility Hardware support required (e.g., GPS) Suitable for higher level of scalability Table driven approach Consistent route Periodic update Large overhead Prone to scalability Shortest path High memory required Consumes more energy Figure 2. The life cycle of routing problem in MANETs Routing Life Cycle in MANETs: The performance trade-off of the aforementioned classical protocols is based on the argument that they are most likely to be less suitable to meet the modern requirements of ad hoc wireless networks for many reasons. The foremost reason is the highly-dense medium due to the drastic traffic saturation of the Industrial Scientific and Medical (ISM) band, which leads to bandwidth limitations day by day. Hence, under such circumstances, huge control traffic cannot be afforded at the expense of high bandwidth consumption. Likewise, huge concerns associated with node mobility have arisen in modern scenarios compared to older networks, due to large support for mobile architecture and the rapid transition of technologies (i.e., 3G to 5G). Moreover, the energy harvesting protocols are more urgent under the light of huge technology shifts and the slow development of robust nodes with increased battery life. Therefore, a suitable protocol for the modern age should effectively organize the network nodes into small sets to handle scalability issues and resolve the single point of failure problem. For instance, the centralized approaches that map topological changes efficiently without affecting the transmission parameters by implementing back-up routes and fast convergence mechanisms for the quick set-up of route discovery and maintenance involving the least number of nodes. It must take care of QoS parameters for variant data traffic applications. It should also devise a loop-free mechanism to avoid stale routing, and control broadcasting information to save bandwidth power and cut extra memory requirements. Unfortunately, there is always a trade-off in the solution of a problem. As described in the previous section, three wellknown solutions exist for the mobile ad hoc routing problem. Fig. 2 illustrates the cycle of routing problems in MANETs. The table-driven approach can make the routing protocols simpler but cannot guarantee efficiency in large-scale networks due to high memory requirements and control overhead. Conversely, the on-demand protocols can compensate for issues related to the table-driven approaches. However, the on-demand approach suffers from inconsistent routes and relatively 148 Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey long paths for data traffic. So far, the best solution to the routing problem is to use the hybrid approach with the added benefit of both approaches. However, the hybrid approach is susceptible to mobility dynamics and hardware support requirements. Conclusion This article aimed to briefly explore the state-of-the-art routing protocols for mobile ad-hoc networks and focused on detailed and relative revisions of proactive, reactive, and hybrid routing protocols. Various routing protocols have been discussed and compared, and their pros and cons are summarized in tabular form. A performance tradeoff exists in various protocols due to their distinct features which makes it really hard to choose the best approach for diverse ad hoc network environments. Moreover, numerous challenges such as mobility, scalability, and bandwidth limitations also affect the performance of routing protocols in ad hoc networks. It is hard to choose a single protocol that is the best for all scenarios since each protocol has its own advantages and disadvantages, and is well-suited for a certain situation. We believe that our discussion will be beneficial for the researchers to understand the salient features of various routing protocols and their strengths and weaknesses. The researchers will be able to resolve many challenges on the routing protocols under-study in a better way. 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Bonnet, "DDR: distributed dynamic routing algorithm for mobile ad hoc networks," Proceedings of the 1st ACM international symposium on Mobile ad hoc networking & computing. IEEE Press, 2000. Article (CrossRef Link) N. Navid, C. Bonnet, N. Nikaein, "Harp-hybrid ad hoc routing protocol," Proceedings of international symposium on telecommunications (IST), 2001. Komal Zaman is the candidate for the Master Degree of Information Technology in University of Gujrat, Gujrat, Pakistan. She received the B.S. degree in Information Technology, University of the Punjab, Gujranwala, Pakistan, in 2014. She joined University of Gujrat, Gujrat, Pakistan as Associate Lecture, in 2014. Her research interest includes routing protocol design and analysis in mobile ad hoc networks. Choi et al.: The Life Cycle of Routing in Mobile Ad Hoc Networks: A Survey 150 Muhammad Shafiq is Ph.D. scholar in Department of Information and Mobile Communication Engineering, Yeungnam University, South Korea. He received M.S. degree in Computer Science, from UIIT. PMAS. Arid Agriculture University Rawalpindi, Pakistan, in 2010. He received master degree in Information Technology from University of the Punjab, Gujranwala, Pakistan, in 2006. He has been serving as Lecturer in Faculty of Computing and Information Technology, University of Gujrat, Gujrat, Pakistan, since 2010. He also served as visiting lecturer in Federal Urdu University, Islamabad, Pakistan, in 2009. His research interest span routing and medium access control design, in mobile and cognitive radio ad hoc networks. Jin-Ghoo Choi received the B.S., M.S., and Ph.D. degrees in electrical engineering and computer science, Seoul National University, Seoul, Korea, in 1998, 2000, and 2005, respectively. From 2006 to 2007, he worked with Samsung Electronics as a Senior Engineer. In 2009, he was with the Department of Electrical and Computer Engineering in the Ohio State University, USA, as a visiting scholar. He joined the Department of Information and Mobile Communication Engineering, Yeungnam University, South Korea, as a faculty member in 2010. His research interests include performance analysis of communication networks, packet scheduling in wireless networks, and wireless sensor networks. Muddesar Iqbal received Ph.D., degree in wireless mesh networks from Kingston University, U.K., in 2009. In 2010, he joined Faculty of Computing and Information Technology, University of Gujrat, Gujrat, Pakistan, as Associate Professor. Since 2012, he has been also serving as Director of the Office for Research Innovation and Commercialization (ORIC) in University of Gujrat, Gujrat, Pakistan. He won awards of appreciation from Association of Business Executive (ABE), U.K. and People’s Republic of China, in 2008. His research interests span the area of mobile ad hoc routing and admission control in wireless mesh networks. Copyrights © 2015 KAIS