Routing Protocols in MANETs

Chapter 4 Routing Protocols in MANETs 4.1 Introduction The main aim of any Ad Hoc network routing protocol is to meet the challenges of the dynamically changing topology and establish a correct and an efficient communication path between any two nodes with minimum routing overhead and bandwidth consumption. The design problem of such a routing protocol is not simple since an Ad Hoc environment introduces new challenges that are not present in fixed networks. A number of routing protocols have been proposed for this purpose like Ad Hoc On Demand Distance Vector (AODV), Dynamic Source Routing(DSR), Destination- Sequenced Distance Vector(DSDV) routing. 4.2 MANET Routing Protocols Classification There are different criteria for designing and classifying routing protocols for wireless Ad Hoc networks. For example, what routing information is exchanged; when and how the routing information is exchanged; when and how routes are computed; how many routes are computed for a single route discovery etc. a) Proactive vs. Reactive Routing: Proactive schemes determine the routes to various nodes in the network in advance, so that the Some part of this chapter appears in the Proceedings of International Conference on Open Source Computing INCOSC-08, 2008, pp: 139-144 50

route is already present whenever needed. Route Discovery overheads are large in such schemes as one has to discover all the routes. Examples of such schemes are the conventional routing schemes, Destination Sequenced Distance Vector (DSDV), Wireless Routing Protocol (WRP), Cluster-head Gateway Switch Routing Protocol (CGRP) etc. Reactive schemes determine the route when needed. Therefore they have smaller Route Discovery overheads. Examples of such schemes are Dynamic Source Routing (DSR), Ad Hoc On Demand Distance Vector Routing (AODV) etc. b) Single path vs. Multi path: There are several criteria for comparing single-path routing and multi-path routing in Ad Hoc networks. First, the overhead of route discovery in multi-path routing is much more than that of single-path routing. On the other hand, the frequency of route discovery is much less in a network which uses multi-path routing, since the system can still operate even if one or a few of the multiple paths between a source and a destination fail. Second, it is commonly believed that using multipath routing results in a higher throughput. 4.3 Working of existing MANET Routing Protocols 4.3.1 Destination Sequenced Distance Vector (DSDV) Routing Protocol DSDV protocol is based on classical Bellman-Ford routing algorithm designed for MANETs. Each node maintains a list of all destinations and number of hops to each destination. Each entry is marked with a sequence number. It uses full dump or incremental update to reduce network traffic generated by route updates. The broadcast of route updates is delayed by settling time. The only improvement made here is avoidance of routing loops in a mobile network of routers. With this improvement, routing information can always be readily available, regardless of whether the 51

source requires the information or not. DSDV solves the problem of routing loops and count to infinity [112] by associating each route entry with a sequence number indicating its freshness. In DSDV, a sequence number is linked to a destination node, and usually is originated by that node (the owner). The only case that a non-owner node updates a sequence number of a route is when it detects a link break on that route. An owner node always uses even numbers as sequence numbers, and a non-owner node always uses odd numbers. With the addition of sequence numbers, routes for the same destination are selected based on the following rules: i) a route with a newer sequence number is preferred; ii) in the case that two routes have a same sequence number, the one with a better cost metric is preferred [113]. The routing table in DSDV contains the following information: (i) All available destinations IP addresses (ii) Next hop IP address (iii) Number of hops to reach the destination and (iv) Sequence number assigned by the destination node The sequence number is used to distinguish stale routes from new ones and thus avoid the formation of loops. The stations periodically transmit their routing tables to their immediate neighbors. These updates carry information on which destinations are reachable from each mobile node and the number of hops required to reach them. An update is also sent when major topological changes occur. Hence the update packets are both time-driven and event-driven. The updates from a mobile node can contain either the entire routing table or just those entries for which there has been a metric change since the last update that was advertised. The former is known as a 'full dump' while the latter is known as an 'incremental update'. Full dumps are the best suited for an Ad Hoc network where the topology changes very frequently. Using incremental updates in such cases would lead to a large growth in the number of incremental packets that hog the network bandwidth. In the case of an Ad 52

Hoc network that has very few topology changes, the use of full dumps is very frequent. 4.3.2 Dynamic Source Routing protocol DSR [114] is a simple and a completely on demand routing protocol. It does not flood the network with periodic routing advertisements, link status sensing or neighbor detection packets. Hence the routing overhead is scaled to only those needed to react to a change in a route that is currently active. Two of the important mechanisms that DSR is composed of is: Route Discovery and Route Maintenance as shown in Figure 4.1. Figure 4.1: DSR Route Discovery In route discovery, it has two messages i.e., route request (RREQ) and route reply (RREP). When a node wishes to send a message to a specific destination, it broadcasts the RREQ packet in the network. The neighbor nodes in the broadcast range receive this RREQ message and add their own address and again rebroadcast it in the network. This RREQ message, if reached to the destination will have complete route to the 53

specific destination. In case the message did not reach to the destination then the node which received the RREQ packet, will look if previously a route is used for the specific destination or not. Each node maintains its route cache which is kept in the memory for the discovered route. The node will check its route cache for the desired destination before rebroadcasting the RREQ message. By maintaining the route cache at every node in the network, it reduces the memory overhead which is generated by the route discovery procedure. If a route is found in that node route cache then it will not rebroadcast the RREQ in the whole network. So it will forward the RREQ message to the destination node. The first message reached to the destination has full information about the route. That node will send a RREP packet to the sender having complete route information. The route maintenance uses two kinds of messages i.e., route error (RERR) and acknowledgement (ACK). The messages successfully received by the destination nodes send an acknowledgement ACK to the sender. The packets transmitted successfully to the next neighbor nodes gets acknowledgement. If there is some problem in the communication network a route error message denoted by RERR is transmitted to the sender, indicating that there is some problem in the transmission. In other words the source didn t get the ACK packet due to some problem. So the source gets the RERR packet in order to re-initiate a new route discovery. On receiving the RERR message the nodes remove the route entries. DSR is best suited for a mobile Ad Hoc environment where there are limited mobile nodes. It also performs well with nodes having very high mobility rates. DSR supports multiple routes to a destination. This offers the source the flexibility of choosing a suitable route for the purpose of load balancing and to obtain increased robustness. 54

4.3.3 Ad Hoc On-demand Distance Vector Routing Protocol AODV uses the route discovery and maintenance mechanism of DSR and the sequence number technique of the DSDV routing protocol. It does away with the maintenance of the routing table of the entire network as in the DSDV. AODV is considered to be a pure on-demand routing protocol, since nodes that are not in the selected path to a destination, do not participate in routing decisions or maintain any routes. AODV is a routing protocol, and it deals with route table management. Route table information must be kept even for short-lived routes, such as are created to temporarily store reverse paths towards nodes originating RREQs. AODV uses the following fields with each route table entry: a) Destination IP Address b) Destination Sequence Number c) Network Interface d) Hop Count (number of hops needed to reach destination) e) Next Hop f) Lifetime (expiration or deletion time of the route) Whenever a source S needs to communicate with destination D, it checks for an existing route to the destination. If the route is not present, it initiates a route discovery by broadcasting a RREQ (Route REQuest) packet to its neighbors. This RREQ packet is flooded onto the Ad Hoc network in a controlled manner, until it reaches the destination or until it reaches a node, which has the latest route to the destination. The route with the highest sequence number indicates the latest route. Each node receiving the RREQ message creates a reverse route to source. The destination/intermediate node sends back Route Reply (RREP) message, which includes number of hops in between and its sequence number. Each node receiving the RREP message creates a forward route to the destination. Thus, each node remembers only the next hop required to 55

reach any of the hosts, not the whole route. Each route entry has associated with it a timer, which indicates the time period for which the route is valid. Since the RREP is forwarded along the path the RREQ was received, AODV supports only symmetrical links unlike DSR. Maintenance of routes is done by generating and propagating a RREP within finite metric back to the source node by the upstream neighbor of the node, which has moved out of range. Such an RREP is called the link failure indication message. Upon receipt of such a message, the source node can re-initiate the route discovery if it still desires a route to the destination. Other than these messages, AODV also demands the exchange of periodic hello messages between mobile nodes to indicate the presence of other nodes and to maintain connectivity. The AODV RREQ message format is represented in the Figure 4.2. Hop Count RREQ ID Destination IP Address Destination Sequence Number Originator IP Address Originator Sequence Number Figure 4.2: AODV RREQ message format i. Hop Count: The number of hops from the Originator IP Address to the node handling the request. ii. RREQ ID:A sequence number uniquely identifying the particular RREQ when taken in conjunction with the originating node's IP address. iii. Destination IP Address: The IP address of the destination for which a route is desired. iv. Destination Sequence Number: The latest sequence number received in the past by the originator for any route towards the destination. v. Originator IP Address: The IP address of the node which originated the Route Request. 56

vi. Originator Sequence Number: The current sequence number to be used in the route entry pointing towards the originator of the route request. The AODV RREP message format is represented in the Figure 4.3. Hop Count Destination IP address Destination Sequence Number Originator IP address Lifetime Figure 4.3: RREP message format of AODV i. Hop Count: The number of hops from the Originator IP Address to the Destination IP Address. ii. Destination IP Address: The IP address of the destination for which a route is supplied. iii. Destination Sequence Number: The destination sequence number associated with the route. iv. Originator IP Address: The IP address of the node which originated the RREQ for which the route is supplied. v. Lifetime: The time in milliseconds for which nodes receive the RREP packet, considering the route to be valid. 4.4 Performance comparison and analysis of three important MANET Routing Protocols MANETs do not rely on extraneous hardware, which makes them ideal candidate for rescue and emergency operations. The unique feature of these protocols is their ability to trace route in spite of a dynamic topology. These protocols can be categorized into two main types: reactive and proactive. The nodes in an Ad Hoc network generally have limited battery power and, so, reactive routing protocols endeavor to save power by discovering routes only when they are essentially required. In contrast, 57

proactive routing protocols establish and maintain routes at all instants of time so as to avoid the latency that occurs during new route discoveries [115][116]. Mobility models define node s movement pattern in Ad Hoc networks. Since, MANETs are currently not deployed on a large scale and due to the inherent randomness of mobility models, research in evaluating the performance of routing protocols on various mobility models is simulation based [117].Therefore in most of the cases performance analysis is carried out using various popular simulators like NS-2. The performance of MANET using DSDV, AODV and DSR routing protocols are evaluated by using extensive simulation experiments. Two models, Traffic Generation Model (TGM) and Mobility Generation Model (MGM) are used in measuring the performance. 4.4.1 Models used for measuring the performance a) Traffic Generation Models (TGM) Manually giving traffic connections for a large number of nodes would be cumbersome. So random traffic connections of TCP (Transmission Control Protocol) and CBR (Constant Bit Rate) can be setup between mobile nodes using a traffic scenario generator script. The generator script is available under /indep-utils/cmuscen-gen directory of NS-2, and the file name is cbrgen.tcl. Using this script it is possible to generate random traffic connections between any number of nodes. It is required to define the following to generate random traffic connections [118][119]: i. The type of traffic connection (CBR or TCP ) ii. The number of nodes for which simulation is being done iii. A random seed value iv. Maximum number of connections v. Rate, whose inverse is used to compute the interval time between CBR packets. 58

CBR means constant bit rate in traffic supplied to the network. In CBR, data packets are sent with fixed size and fixed interval of time. Establishment phase of connection between nodes is not required, even the receiving node doesn t send any acknowledgement messages. Connection is unidirectional, i.e., from source to destination. TCP is a connection oriented and reliable transport protocol. To ensure reliable data transfer TCP uses acknowledgement, time out and retransmission. Acknowledgement indicates successful transmission of packets from source to destination. If an acknowledgement is not received during a certain period of time, which is called time out then TCP transmits the data again. b) Mobility Generation Models (MGM) Scenario file is used to store the initial position of the nodes and movement of nodes at different times, at different speeds etc. Since it is difficult to manually give initial position, movement of the nodes and their speed for each movement at different times, a random file generator can be used here. The node movement generator is available under /indep-utils/cmu-scen-gen/setdest/ directory of NS2. It is available under the name setdest, which is a.exe file. This file is run with certain arguments to create the scenario file. The arguments are: i. Number of nodes ii. Pause time iii. Maximum speed iv. Simulation time v. X-axis dimension vi. Y-axis dimension The following four metrics are used to compare the performances of three routing protocols: a) Packet delivery ratio (PDR): The fraction of packets sent by the application that are received by the receivers. PDR is computed using the following equation: 59

PDR = (4.1) b) Delay: End-to-end delay indicates how long it took for a packet to travel from the application layer of the source to the application layer of the destination. Average delay is computed using the following equation: (4.2) c) Throughput: The throughput is defined as the total amount of data a receiver receives from the sender divided by the time it takes for receiver to get the last packet. It is measured in terms of Kilobits per seconds (Kbps). d) Control Overhead: The control overhead is the ratio of total number of routing packets to the total number of data packets. The performance of DSDV, DSR and AODV has been analyzed by fixing simulation parameters as shown in the Table 4.1. Table 4.1: Simulation Parameters Simulation time 200 seconds Number of nodes 10,20,30,40,50,60,70,80,90,100 Map size 1000m X 1000m Speed 10 m/s Mobility Model Random Way Point Traffic type Single CBR flow per node Number of seeds 25 Packet size 512 bytes Propagation range 250m Packet rate 10 packets/sec Pause time 40 seconds 60

4.4.2 a) PDR vs. number of nodes at low mobility (Pause time =40 seconds) The Table 4.2 (a) and Figure 4.4 show the PDR comparison of the 3 routing protocols at low mobility. Table 4.2 (a): PDR versus no. of nodes at low mobility PDR in % No. of nodes DSDV DSR AODV 10 99.4 99.85 99.9 20 99.55 99.67 99.75 30 99 99.2 99.6 40 98.2 99 99.45 50 96 97.2 99.2 60 92 92 98.5 70 85 90.5 96 80 75 87.8 95 90 68 80 87 100 56 73 83 Figure 4.4: PDR versus no. of nodes 4.4.2 b) PDR vs. number of nodes at high mobility (Pause time =0 seconds) The Table 4.2 (b) shows the PDR comparison of the 3 routing protocols at high mobility Table 4.2 (b): PDR versus no. of nodes at high mobility PDR in % No. of nodes DSDV DSR AODV 10 45 46 54 20 44 45 52 30 42 43 50 40 41 42 48 50 38 40 45 60 30 35 42 70 25 28 38 80 18 20 30 90 15 18 26 100 8 10 25 61

Throughput (Kbps) 4.4.3 Delay vs. number of nodes Table 4.3 and Figure 4.5 show the End-to-End Delay for each protocol versus number of nodes. Table 4.3: Delay versus no. of nodes for different protocols Delay in milliseconds No. of DSDV DSR AODV nodes 10 40 60 65 20 50 73 82 30 68 82 88 40 75 95 102 50 90 105 116 60 104 117 125 70 118 138 145 80 130 150 162 90 145 168 184 100 158 184 194 4.4.4 Throughput vs. No. of nodes Figure 4.5: Delay versus no. of nodes Table 4.4 and Figure 4.6 show the Throughput for each protocol versus No. of nodes. Table 4.4: Throughput versus no. of nodes for different protocols Throughput in Kbps No. of DSDV DSR AODV nodes 10 180 200 195 20 172 195 193 30 184 199 205 40 173 210 202 50 163 208 215 60 158 212 218 70 162 216 210 80 164 225 202 90 172 218 198 100 160 222 196 Throughput vs. no. of nodes 250 200 150 100 50 0 No. of nodes DSR DSDV AODV Figure 4.6: Throughput versus no. of nodes 62

4.4.5 Control overhead vs. No. of nodes Table 4.5 shows the control overhead for each protocol versus No. of nodes at Pause time=20 seconds. Table 4.5: Control overhead versus no. of nodes for different protocols Control Overhead in % No. of DSR AODV DSDV nodes 10 2 2 2 20 2 5 5 30 3 6 10 40 5 8 12 60 8 12 20 80 10 15 25 100 11 17 32 4.4.6 Result of Comparison The performance of DSDV, DSR, AODV routing protocols is compared and analyzed using NS-2.34 simulator. The QoS metrics Average delay, Packet delivery ratio and Throughput are measured by varying the number of nodes from 10 to100. a) PDR comparison: It is observed that the PDR of AODV is better in increasing the number of nodes as compared to DSR and DSDV. For example, at 80 nodes, the PDR of AODV is 95%. The performance of DSDV is reduced, as the number of nodes is increased. For example, at 80 nodes, the PDR of DSDV is 75%. This is because of the packets dropped due to link breaks. The performance of DSR is better than DSDV, but it is lower than AODV. As the number of nodes is increasing (80-100), the performance of the network degrades in terms of QoS metrics due to more control overhead as shown in Table 4.5 and more delay. i.e., the data have to pass from many mobile nodes, which cause more delay. b) Delay comparison: It is observed that the delay of DSDV is better than DSR and AODV. DSDV clearly shows that it has less average 63

delay as shown in Table 4.3. For example, at 80 nodes, the delay of DSDV is 130ms. Whereas for DSR and AODV, delay is 150ms and 162ms respectively. Since DSDV proactively keeps the routes to destinations in its routing table, it does not have to initiate the route discovery process as frequently as DSR and AODV. c) Throughput comparison: The AODV and DSR protocols perform better at high density compared to DSDV. Table 4.4 shows the reduction in throughput of DSDV protocol. The reason behind this is that, in the case of DSDV the entire routing table is broadcasted causing more routing overhead. The simulation has also been carried out by taking less pause time. The pause time can be less than 40seconds. i.e., it can start from 0sec and end in any high values. At high mobility situations the original routing protocols AODV, DSDV and DSR do not perform well in terms of QoS metrics such as PDR as shown in Table 4.2 (b). Since AODV includes the best features of DSDV and DSR, the performance AODV is better than these two protocols. It borrows the destination sequence number feature from DSDV protocol to maintain up to date path to the destination and the on-demand feature from DSR to initiate the route discovery process only when there is a need. 4.5 Original AODV Route discovery algorithm The Figure 4.7 shows the flowchart for Route Discovery of original AODV. 64

Start Process and Send RREQ message YES Is route available in routing table? NO Forward the information Save information in Queue and initiate RREQ message Stop Figure 4.7: The flowchart for Route Discovery in AODV The best case time complexity of this algorithm is O(1) and the worst case is O(n), where n is the total number of neighboring nodes from source to destination of a network having N number of nodes. 4.6 Summary An Ad Hoc routing protocol is a convention or standard that controls how nodes come to agree which way to route packets between computing devices in Mobile Ad Hoc Networks (MANETs). A central challenge in the design of Ad Hoc networks is the development of dynamic routing protocols that can efficiently find routes between two communicating nodes. MANET routing protocols are broadly classified under the two headings: Proactive versus Reactive, and Single versus multi path. Proactive protocols require that nodes in a Mobile Ad Hoc network should keep track of routes to all possible destinations so that when a packet needs to be forwarded, the route is already known and can be used immediately. Any changes in topology are propagated through the network, so that all nodes know those changes in the topology. Examples include DSDV, WRP, GSR, FSR etc. On-demand protocols attempt to build routes only when desired by the source node so that the network topology 65

is detected as needed (on-demand). When a node wants to send packets to some destination but has no routes to the destination, it initiates a route discovery process within the network. Once a route is established, it is maintained by a route maintenance procedure until the destination becomes inaccessible or until the route is no longer needed. Examples include AODV, DSR, TORA etc. Proactive protocols have the advantage that new communications with arbitrary destinations experience minimal delay, but suffer the disadvantage of the additional control overhead to update routing information at all nodes. The performances of three important routing protocols DSDV, DSR, AODV are compared and analyzed using NS-2.34 simulator. The QoS metrics, Average delay and Packet delivery ratio by varying the number of nodes have been calculated. It is observed that the AODV routing protocol is better in performance as compared to DSR and DSDV. But the performance degrades as the number of nodes and mobility increases. 66