an enhanced link repair technique for...

International Journal of Computing and Information Technology3 (1) (2011): 1– 13

AN ENHANCED LINK REPAIR TECHNIQUE FORMAODV ROUTING PROTOCOL

Md. Saiful Azad, Md. Arafatur Rahman, and Farhat Anwar

Abstract

Ad-hoc On-demand distance vector (AODV) is a variant of classical distance vectorrouting algorithm. A distinguishing feature of AODV is the ability to provide unicast,multicast and broadcast communications. The multicast extension of AODV, also knownas Multicast Ad hoc On-Demand Distance Vector (MAODV) protocol is a tree-basedmulticast routing protocol which enables dynamic, self-starting, multi-hop routingbetween participating mobile nodes wishing to join or participate in a multicast groupwithin an ad hoc network. MAODV has minimal control overhead and route acquisitionlatency. However, the mobility of mobile nodes often causes link breakage in the tree-based protocols which sometime results in tree partitioning and poor performance. InMAODV, when a broken link is detected between two nodes on the multicast tree, thenode downstream to the break is responsible for initiating the repair. After link repair, it islikely that the previous distance between the nodes (nodes downstream to the linkbreakage) and the group leader will not remain same. We investigate and identify that theshortest path from the group leader to the nodes downstream to the node which initiateslink breakage is not ensured after link breakage. In this paper, a new technique is proposedwhich ensures shortest path from any node to the group leader after link repair. Simulationresults demonstrate significant improvement in performance metrics compared to standardMAODV.

Keyword: Multicast AODV, Link Breakage, downstream node, group leader, optimumpath.

1. INTRODUCTION

Mobile ad hoc networks (MANET) are characterized by lack of any fixed networksinfrastructure, low bandwidth, high packet loss, finite energy resources and frequent topologychanges. These characteristics are extremely different from wired networks. Applicationsthat support one-to-many or many-to-many communications can be benefited from the

Faculty of Engineering, Department of ECE, International Islamic University Malaysia (IIUM), Malaysia.E-mail: G0623131 @stud.iiu.edu.my, [email protected] and [email protected]

2 International Journal of Computing and Information Technology (IJCIT)

broadcasting nature of this type of wireless network. In order to facilitate communicationswithin a group of users, multicast routing protocols are used to discover routes betweennodes. Multicast has great impact since it overcomes the overheads of the unicast routingprotocol. Multicast routing protocols in ad hoc networks can be classified based on theglobal data structured (topology) used to maintain connectivity among group members; theyare tree-based and mesh-based. Representative of the tree-based approaches includes:MAODV (Multicast Ad-hoc On-Demand Distance Vector) protocol [1] [2], AMRoute(Ad-hoc Multicast Routing Protocol) [3], AMRIS (Ad-hoc Multicast Routing with Increasingid-numberS) [4]. Representative of the mesh-based approaches includes: On-demandMulticast Routing Protocols (ODMRP) [5], and Core-Assisted Mesh Protocol (CAMP) [6].

A tree-based multicast routing protocol establishes and maintains either a shared multicastrouting tree or multiple source-based multicast routing trees to deliver data packets fromsource(s) to a group of receivers. In contrast, mesh-based multicast routing protocols forwardpackets to destination via scoped flooding. Since the mesh-based protocols maintain multiplepaths to the receivers, failures of links and nodes are less disruptive for packet delivery thanin trees. However, redundancy (because of data packet duplication) results in higher overheadin mesh-based technique. Furthermore, under conditions of high traffic load and largenetworks scenario, mesh-based protocols congest the network faster which leads to thedrastic drop in the packet delivery ratio. Since Tree-based protocols use only one path, theyare more scalable than mesh based protocols.

MAODV protocol is the multicast extension of AODV which is used for multicasttraffic. To support multicasting, it builds a shared bi-directional multicast tree to connectgroup members. Since this tree has only one path from senders to receivers, it is enormouslyimportant to repair broken links quickly and efficiently. In this paper, an improved LinkRepair Technique is proposed for MAODV routing protocol.

The remainder of the paper is organized as follows. Related works are described andanalyzed in section II. Description of MAODV link repair technique is given in Section III.Section IV and V present motivation of the research and proposed new link breakagetechnique respectively. Simulation environment and simulation results are discussed in sectionVI. Finally, in section VII, conclusion is drawn.

2. RELATED WORKS

In recent years, several researchers performed their study on MAODV and proposed quitea lot of extensions. For instance, in research paper [7], authors extend performance ofAODV and MAODV by using link state prediction method. Link state prediction methodcan predict the exact link breakage time of an active link before the breakage actuallyoccurs. A new route can be constructed utilizing this technique before the old route becomesunavailable. This pro-active route maintenance technique reduces packet loss (between32% and 72%) with slight overhead increase (between 4% and 49%) for AODV andthroughput improves form 70% to 85% with overhead increase below 12% for MAODV.In [8], authors propose a new application called name directory, which can be categorized

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An Enhanced Link Repair Technique for MAODV Routing Protocol 3

as peer-to-peer application in MANET. Through that name service, which announces theinformation of neighbors, ad hoc users will be able to know who is reachable in the network.Multiple Tree Multicast Ad Hoc On-demand Distance Vector (MT-MAODV) routing protocol,an extension of MAODV protocol, is proposed in [9]. In this paper, authors attempt toimprove video multicast over ad hoc network by using multiple tree concept. Two optimallydisjoint trees are constructed employing a single routine. To distribute the video evenly andindependently between these disjoint trees, the Multiple Description Coding (MDC) schemeis used for video coding. Simulation results demonstrate the video quality improvement overthe conventional single tree based multicast AODV. Supporting Quality of Service (QoS) inad hoc network is an inherently complex and difficult issue due to bandwidth constraint anddynamic topology. Authors of paper [10], deal with this issue. Their solution of QoS multicastrouting problem is based on lower layer specifics and implemented on MAODV. In paper[11], a novel link enhancement mechanism is proposed to deal with mobility managementproblem in vehicular ad hoc networks. To enhance the link break prediction accuracy andcongestion occurrence two machine learning techniques, particle swarm optimization andfuzzy logic systems, are incorporated into the proposed schemes. This technique isimplemented in both AODV and multicast extension of AODV (MAODV) and experimentalresults supports the effectiveness and feasibility of the proposed schemes. Modified Shared-tree Multicast Routing Protocol (MSMRP) and modified shared-tree multicast routingprotocol extension (MSMRPx) [12], are based on MAODV. The primary intent behindMSMRP is to improve the end-to-end delay in the shared-tree method. It uses n-hop localring search to establish a new forwarding path and limit the flooding region. Authors alsopropose an extension by using the periodic route discovery message to improve the networkthroughput for the high mobility network.

All the extensions of MAODV discussed above do not consider the consequence ofbroken link on the performance. Nonetheless, a distributed wireless links repair technique isproposed in [13]. This is a unicast-type multihop local repair method to recover lost linksefficiently while increasing network reliability, minimizing the number of control messages,and reducing repair delay. For minimizing repair delay and increasing successful repair rate,the repair approach determines the optimal substitute node based on two repair orderswhich includes the nearest neighbor first (NHF) and the least hop count first (LHF). Accordingto the presented numerical results, the proposed multihop local repair approach outperformsother repair approaches in successful repair rate and control message overhead. However,the mechanism for link repair proposed here is more complex than that of MAODV, whichprovides a simple local repair mechanism that tries to find out an alternative route to a treeby broadcasting the RREQ message from the downstream nodes of the broken link. Thelink repair technique proposed in [13] is more suitable for wired networks than wireless.Because, unlike wired networks, wireless networks broadcast every packet by nature(whether this is a unicast or multicast or broadcast packet). Generally physical layer transmitspacket in the air using omni-directional antenna. All the neighbors of the sender receive thatpacket. Only the intended destination further processes that packet while others drop it.Moreover, this mechanism does not ensure the shortest path from all the nodes to the group


leader. In [14], another new Multicast Link Repair Strategy (MLRS) for MAODV is proposed.The core characteristic of MLRS is local repair which allows upstream node of a brokenlink to do the repair. The upstream node of a broken link will broadcast route requestpacket. A modified group hello message is used to deliver the information of hop count froma branch node to all the nodes on the multicast tree. A node that receives the group hellomessage can estimate the duration of local repair. However, this technique provides poorperformance when the scenario appears to be the one given in Fig. 1. Since there is onlyone link between X and Y; and that link is already broken, upstream node has no method torepair that link. The nodes down stream to the broken link will be isolated from the tree.They can not even form a multicast group. In conventional MAODV, the node downstreamto the link break initiates link repair. MAODV link repair technique is discussed more detailsin section “repairing link breakage”.

Fig. 1: Problem with Repairing Multicast Tree Branch by Upstream Node

3. REPAIRING LINK BREAKAGE

This section illustrates only the link repair technique of MAODV. Description of the protocolis out of the scope of this paper. Interested user can read paper [1], [2], and [16] to understandthe mechanisms of MAODV protocol.

A link breakage is determined by the nodes in the same way as described in [15]. Whena broken link is detected between two nodes on the multicast tree, they delete the link fromthe multicast routing table. The node downstream of the break is responsible for initiatingthe repair of the broken link. If nodes on both sides of the break initiate repairing, they mightend up with repairing the link through different intermediate nodes, thus forming a loop [16].Therefore, the downstream node broadcasts a RREQ with the join flag set and with multicastgroup leader extension included. The hop count field in multicast group leader extension isset equal to node's current distance from the multicast group leader, so that only the nodes


that are no further from the group leader can respond. This technique also avoids formationof loop by preventing nodes on the same side of the break from responding to the RREQ.The RREQ is broadcasted using an expanding ring search technique, thereby allowing for alocal repair and preventing the RREQ from broadcast across the entire network.

A node receiving the RREQ can only respond by unicasting a RREP, if it is a memberof the multicast tree, its record of the multicast group sequence number is at least as greatas that contained in the RREQ, and its hop count to the multicast group leader is less than orequal to the contained in the hop count field. Once the discovery period has ended and thedownstream node has chosen its next hop (path selection technique is given in [1], [2]), itactivates this entry in its Multicast Routing Table (MRT) by setting the activation flagassociated with that next hop. It then constructs a Multicast Activation (MACT) messageand unicasts this message to its selected next hop. After receiving the MACT message, anode activates the next hop entry for the sending node in its MRT. If this node is already amember of the multicast tree, the addition of the new branch to the tree is completed.Otherwise, it unicasts a MACT message to its next hop. This processing continues until anexisting member of the multicast tree is reached. Fig. 2 (a) and (b) illustrates the linkbreakage and multicast tree after the repair is completed.

(a) Link break (b) Repaired multicast tree

Fig. 2: Repair of Multicast Tree Branch

Once the repair is finished, it is likely that the node which initiated the repair is now adifferent distance from the group leader than it was before the break. If this is the case, itmust inform its downstream next hops of their new distance from the group leader. Thenode perform this task by broadcasting a MACT message with the update flag set, and theHop Count field set to its new distance from the group leader. When a downstream node onthe multicast tree receives the MACT message and determines that this packet arrivedfrom its upstream neighbor, it increments the hop count value contained in the MACTpacket and updates its distance to the group leader. If this node has one or more downstreamnext hops, it in turn sends this update message to them and this procedure continue tillreaching to the leaf nodes.


4. MOTIVATION

We shall use the notation Di,k to symbolize downstream nodes throughout the paper, wherei represents the level number and k represents node number. For example, the downstreamnode will be represented as D1,1 or D1, since there is only one downstream node and nosibling is possible. Multicast member 5 and 6 in Fig. 2, can be represented as D2,1 and D2,2respectively.

After repairing link breakage, it is possible that the node which initiated the repair, i.e.,D1 is now at a new distance from the group leader. Consequently, the current distance ofthe downstream nodes Di ≥ 2,k also change. For example, in Fig. 2, D1 was 2 hops far fromgroup leader before link breakage and after link repair; it is now 3 hops away from groupleader. Accordingly, nodes D2,1 and D2,2 both are now 4 hops far from group leader whilethe previous distance was 3 hops. In Fig. 3, after link repair, D1 is now 6 hops away fromgroup leader while the distance was 2 hops before link breakage. Node D2,1 becomesupstream node for D1 and the new distance for D2,2 is 7 hops.

Let's consider that the member nodes, Di = 2,k in Fig. 2 and Fig. 3 also have the capabilityto initiate route request after link breakage, then we may have different distance fromnodes to group leader, which is shown in Fig. 4. After route repair, the new generated treeswill look like Fig. 4 (a) and (b) instead of the trees in Fig. 2 (b) and Fig. 3 (b) correspondingly.From Fig. 4 (a) and (b), we obtain new distance from D2,2 is 3 hops (which is 4 in Fig. 2 (b))and from D2,2 is 6 hops (which is 7 in Fig. 3 (b). In both cases, new technique reduces onehop distance. If there are several labels of successors associated with D2,2 (Fig. 2 (b)) andD2,2 (Fig. 3 (b)), all of the packets to or from those successors need to travel additionaldistance.

(a) Link break (b) Repaired multicast tree

Fig. 3: Repair of Multicast Tree Branch

The scenarios discussed above lead us to the conclusion that the shortest path from thegroup leader to the nodes downstream to the node which initiates link breakage is notensured in MAODV. The longer the path a packet has to travel, the higher its chance of


getting damaged or lost due to collision and/or congestion [17]. Consequently, MAODVdemonstrates poor performance in terms of packet delivery ratio, throughput andEnd-to-End Delay where link breakage is frequent.

(a) (b)

Fig. 4: Repaired Multicast Tree Using New Technique

5. PROPOSED TECHNIQUE

As mentioned earlier in previous section, along with D1, if member nodes Di =2,k also attemptto find a route to group leader, it is possible to assure optimum path from all the downstreamnodes. However, if D1 and Di =2,k nodes initiate repair at the same time, they may form aloop. Hence, other nodes must allow D1 to initiate the repair first. If D1 discovers a pathwhich has equal hops like the earlier one before the link breakage, Di =2,k member nodes donot need to attempt to find a route. Di =2,k nodes will seek for a route if and only if thedistance after repair vary. Nonetheless, if all Di =2,k member nodes attempt to find optimumpath at identical time, the possibility of loop formation raises again. Therefore, it is justifiedto attempt route discovery one after another. We numbered each of the Di =2,k membernodes i.e., successors of D1, who are members of the tree. To perform this task, a newmessage, Successor Confirmation Message (SCM) and a new data structure, the successorpredecessor table, is used. The SCM message format which is given in Fig. 5 contains thefields described in Table 1.

Fig. 5: Successor Confirmation Message (SCM) Packet Format


Table 1Descriptions of the Fields of SCM Message

Field Description

Type 11

SUC Successor Flag; set when a predecessor sends a node no. to its successor.

Reserved Sent as 0; ignored on reception.

Node No. A number which identify each successor separately. Node no. 1 means,this is the first successor of its predecessor and so on.

Hop Count It is initialized to 1. After receiving SCM packet by successors, theydecrease this value and when hop count reach to zero, packet isdropped.

Multicast Group IP Address The IP address of the multicast group.

Destination IP Address The IP address of the destination node.

Source IP Address The IP address of the source node.

Sequence Number The sequence number of MACT message.

Successor-Predecessor table includes the following fields:

• Multicast Group IP Address

• IP Address

• Sequence Number

• Node No.

• SP flag (successor / predecessor flag)

• New/Old flag

In this table, all the information of a node's successors and predecessors are stored.Two flags are used here. The SP flag is used to identify whether the entry is about asuccessor (SP = 0) or about a predecessor (SP = 1). New/Old flag is used to identify newpredecessor (flag = 0) or old predecessor (flag =1). The New/Old flag is utilized only forpredecessors, not for successors. For instance, an entry {“m”, “n”, “o”, “p”, 1, 0} means,this is an entry of a new predecessor which is a member of "m" multicast group with “n” IPaddress and “o” sequence number. The table entry is maintained by a node which is “p”number successor of a node “n”.

We also need to modify hop count field of MACT massage. In earlier one, hop countfield is used only when update flag is set, otherwise sent as 0. However, in the new technique,hop count field is initialized to zero and the field is incremented each time a node forwardsthis packet.

Once the route discovery period has ended and the node has chosen its path to thegroup leader, it then creates and unicasts MACT message to its selected next hop. Whenevera node receives a MACT message, it checks hop count field of that message. If the value


in hop count field is 1 and there is no entry for this node in its successor predecessor table,it then creates one SCM packet, unicasts this packet and makes an entry in the table. Afterreceiving SCM packet from its predecessor, successor makes an entry in successorpredecessor table.

After route repair by D1, if the distance changes, it informs its downstream node usingMACT message, sets the upgrade flag, and sets hop count field equal to its distance fromthe group leader. Once receiving this message, Di =2,k member nodes compare the new hopcount with the earlier one. If they discover that the earlier distance has changed to a newdistance, Di =2,k member nodes initiate route discovery one after another according to theirnode number. Node, Di =2,1 will attempt to find an optimum node first while other nodes willwait. After expiring maximum discovery period for one node, Di =2,2 will attempt and so on.Once the discovery period has ended and subscribing node has chosen its next hop, itactives its route and informs its downstream nodes with the technique described in section“repairing link breakage”.

6. SIMULATION ENVIRONMENTSAll the simulations in this paper have been performed using QualNet version 4.5 [17], asoftware that provides scalable simulations of Wireless Networks, which is a commercialversion of GloMoSim [18]. In our simulation, we consider a network of 50 nodes(one source and one destination) that are placed randomly within a 1000m × 1000m areaand operating over 500 seconds simulated time. Multiple runs with different seed numbersare conducted for each scenario and collected data is averaged over those runs.

A two-ray propagation path loss model is used in our experiments with lognormalshadowing model. The transmission power of the routers is set constant at 20 dBm and thetransmission range of the routers is 250 meters. The data transmission rate is 2Mbits/s.At the physical layer 802.11b and at MAC layer MAC 802.11 is used. The node movementsin these experiments are modeled using the random waypoint mobility model [14] [15].

The traffic source is implemented using Multicast Constant Bit Rate (MCBR). Thepacket size without header is 512 bytes. The length of the queue at every node is 50 Kbyteswhere all the packets are scheduled on a first-in-first-out (FIFO) basis. The senders and thereceivers are chosen randomly among multicast members. A member joins the multicastsession at the beginning of the simulation and remains as a member throughout the simulation.In our simulation, initial 10 seconds is kept to perform this task. Once joining the multicastgroup, we let the source to transmit data for 500 seconds simulation time and remaining90 seconds is set to allow the last packets to be processed and routed to the destination.

Two different quantitative metrics are used to evaluate the performance enhancementof MAODV protocol. They are:(1) Packet Delivery Ratio (PDR): The ratio of the number of data packets received by

the receivers verses the number of data packets supposed to be received. This numberpresents the effectiveness of a protocol.

(2) Average End-to-end delay: End-to-end delay indicates how long it took for a packetto travel from the source to the destination.


7. RESULTS AND DISCUSSIONS

To study the performance of MAODV and MAODV with new route repair technique(MAODV-NRR), we performed a set of experiments over different network parameters,such as node mobility and traffic load.

7.1. Impact of Node Mobility

In this scenario, node mobility is increased from 0 m/s to 20 m/s. The sender transmitsmulticast traffic at a rate of 1 packet/sec. This scenario is important since node mobility isone of the key reasons of link breakage.

In case of Multicast PDR illustrated in Fig. 6 (a), it is observed that the performance ofboth MAODV is affected by the increasing mobility. Increased node mobility results inpacket loss due to higher link breakage. When the nodes were stationary, PDR of MAODVand MAODV-NRR were same. However, the performance of MAODV deteriorates morerapidly in higher mobility than MAODV-NRR, since it does not ensure optimum path fromeach node to group leader. If a packet travels longer path, its chance of getting damaged orlost due to collision and/or congestion is higher.

(a) Packet Delivery Ratio

(b) End-to-end Delay

Fig. 6: Performance of MAODV and MAODV-NRR with Respect to Nodes Mobility


The average end-to-end delay incurred by MAODV-NRR is the lowest in higher mobilitywhich is shown in Fig. 6 (b). End-to-end delay was same when the nodes were stationary,since there was less link breakage. Once the nodes mobility increased, end-to-end delay ofMAODV ascended more rapidly than MAODV_NRR, ad a packet needs to travel longerpath than the other one.

7.2. Impact of Traffic Load

In this scenario, we increased traffic load from 1 packet/sec to 50 packets/sec. To introducelink breakage, we used nodes mobility of 10 m/s. This scenario is also important to study theimpact of data traffic load when link breakage occurs.

Fig. 7 (a) illustrates the packet delivery ratio with various traffic loads. The performanceof both MAODV and MAODV-NRR are affected by the increasing network traffic. Sinceincreased network traffic results in packet loss due to higher collisions, buffer overflow andcongestion. For all kinds of traffic load, MAODV-NRR outperforms MAODV, since itensures optimum path from each node to the group leader.

(a)

(b)

Fig. 7: Performance of MAODV and MAODV-NRR with Respect to Multicast Traffic Load

Fig. 7 (b) illustrates end-to-end delay of both MAODV and MAODV-NRR. The averageend-to-end delay of MAODV-NRR is the lowest in almost all cases. Since MAODV-NRRensures shortest path to the group leader for all nodes, a packet needs to travel less paththan MAODV after link breakage.


8. CONCLUSION

Tree-based multicast routing protocols like MAODV has only one path from senders toreceivers to transmit and receive multicast traffic. That's why, it is enormously important torepair broken links quickly and efficiently. In this paper, we proposed an improved LinkRepair Technique for MAODV routing protocol. We investigate and identify that the shortestpath from the group leader to the nodes downstream to the node which initiates link breakageis not ensured after link repair. This technique ensures optimum path from all the nodes tothe group leader. Simulation results performed over different network parameters demonstrateimprovement compared to MAODV.

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