A Review of Gateway Load Balancing Strategies in Integrated Internet-MANET

Rafi-U-Zaman, Khaleel-Ur-Rahman Khan Computer Science and Engineering Department

Muffakham Jah College of Engg. And Tech. Hyderabad, India.

rafi.u.zaman@mjcollege.ac.in, khaleelrkhan@mjcollege.ac.in

A. Venugopal Reddy Computer Science and Engineering Department

University College of Engg., Osmania University Hyderabad, India.

avgreddy@osmania.ac.in

Abstract— Interconnection of wired Internet and wireless mobile ad hoc networks, called Integrated Internet-MANET enables communication between wired and wireless peers. Many strategies exist which provide Internet connectivity to the mobile ad hoc network nodes through one or more Internet gateways. Mobile nodes need to be registered with an Internet gateway in order to communicate across the Internet. A key issue in such strategies is to distribute the Internet bound traffic across the Internet gateways evenly. The problem of Internet gateway load balancing has been addressed by researchers in interesting ways. In this paper, we present a review of the gateway load balancing solutions under common parameters. In the second part of the paper, a new gateway load balancing strategy based on a load balanced routing protocol called Modified AODV is proposed and is simulated using the ns-2 simulator.

Keywords- Gateway Load Balancing, Modified AODV, Integrated Internet-MANET, Wired Peer to Wirelees Peer

I. INTRODUCTION Wireless mobile ad hoc network (MANET) allows

wireless mobile users to communicate with each other in the absence of communication infrastructure. Each mobile host in the ad hoc network also serves as a router. A review of routing protocols for ad hoc networks is in [1]. Mobile IP [2] allows a wireless mobile node to obtain single-hop Internet connectivity. Many solutions which have been proposed to integrate mobile ad hoc networks with the Internet use Mobile IP. A review of such strategies is in [3]. Some solutions use the Foreign Agent of Mobile IP as an Internet gateway. Other solutions employ intermediate mobile gateways between the Foreign Agents and mobile nodes. These intermediate mobile nodes are themselves selected mobile nodes within the ad hoc network. All mobile nodes within the ad hoc network must be registered with an Internet gateway in order to obtain Internet connectivity. The registration process can be sub-divided into three phases: gateway discovery, gateway selection and Registration. Thus, detection and selection of Internet gateway by the mobile nodes is a very important function in the integration of MANET and the Internet [4]. The registration of mobile nodes with the Internet gateway can be done in one of three ways. The Internet Gateways periodically flood the ad hoc network with Agent advertisements. The mobile nodes respond to these Agent advertisements by requesting registration. This approach is called the proactive approach of registration. In the absence of Agent advertisements, mobile nodes can 978-1-4244-4793-0/09/$25.00 © 2009 IEEE

send out Agent solicitation messages, to obtain Internet gateway information. The Internet Gateways reply by sending Agent advertisement messages to the mobile node which solicited Agent information. In reply to this, the mobile node sends a registration request. This approach is called the reactive approach of registration. Some protocols use a combination of both proactive and reactive approaches, which are termed hybrid approaches.

Internet Gateways are the entry points to the Internet for mobile nodes in the MANET. There might be a scenario where many mobile nodes are registered with one gateway, and very few are registered with another gateway. We can say that the previous gateway is heavily loaded and the latter is lightly loaded. The problem is to provide a load balancing solution. Very few strategies have been proposed in the literature to address the Internet gateway load balancing problem. In this paper, a brief overview of these gateway load balancing strategies is presented. The main issues under consideration are the architecture of the strategy, the ad hoc routing protocol, the Internet gateway discovery mechanism and finally, the gateway load balancing algorithm. By studying the gateway load balancing strategies, it has been observed that they make use of traditional routing protocol like AODV and DSDV. None of the strategies makes use of a load balanced routing protocol.

In order to study the impact of a load balanced routing protocol in the load balancing of the Internet Gateway, we have used an existing load balanced routing protocol called Modified AODV [10] and used it in the Integrated Internet-MANET. The proposed strategy was simulated using the ns-2 simulator and the results compared with those of the existing strategy. It is observed that the proposed strategy gives better packet delivery ratio when compared to the existing strategy, while suffering a worse end-to-end delay and incurring a higher routing load.

The rest of the paper is organized as follows. Section II presents briefly, five Internet gateway load balancing strategies. Section III compares the solutions presented in Section II. Section IV presents a new gateway load balancing strategy based on a load balanced routing protocol. Section V presents the simulation results and Section VI concludes the paper.

II. INTERNET GATEWAY LOAD BALANCING STRATEGIES

A. Zhou et al’s Strategy: In this strategy [5], the network architecture consists of

Mobile IP Foreign Agents (FA), Dynamic Gateways (GW) one hop away from FA, and Mobile Nodes (MN). Mobile

nodes use Mobile IP to communicate with the Internet and use DSDV to communicate within the MANET. The Dynamic gateway is used to solve the problem of load balancing. There can be more than one Dynamic gateway at any given point of time. Dynamic gateways are also MANET nodes. Registration is at two levels. The first level is the registration of GW with FA, which is a proactive process and the other is the registration of MANET nodes with GW, which is reactive. The FA and GW agent advertisements contain important information like their current queue length (QL) and number of nodes currently registered (NN). After the gateway discovery process, comes the selection of a gateway. The selection of an FA by a dynamic gateway and the selection of a dynamic gateway by a MANET node for registration depend on the load balancing algorithm. The load balancing mechanism works as follows:

GWExp= DS*n% + NN*m% + QL*k% FAExp = DS*n% + NN*m% + QL*k%

Where, GWExp and FAExp are the weighted values of dynamic gateway and FA respectively. DS- Euclidean distance between two nodes NN-No. Of nodes registered QL-Queue length of data packets And n, m and k are the respective weights given to DS, NN and QL and n + m + k = 100% The gateway selection is based on the following criteria: Selected GW=Min {GWExpj} Selected FA=Min {FAExpj} Where, j is the no. of advertisements that a node receives.

The load balancing mechanism is combined with the gateway selection process. The presence of dynamic gateways ensures broader coverage of the ad hoc network, but at the same time, it increases the complexity of the registration process.

B. Trivino-Cabrera et al’s Strategy: In the strategy proposed by Trivino-Cabrera et al [6],

the network architecture consists of Access Router (AR), one Default gateway (DG), Candidate gateway (CG) and MANET nodes. One node within the coverage area of the access router acts as a default gateway whereas other nodes within the coverage areas are called candidate gateways. At one instant of time, one default and none or multiple CG may coexist. AODV is used for routing within the ad hoc network. Access router broadcasts router advertisement (RA) messages which are received by the Default gateway. The default gateway broadcasts the modified router advertisement (MRA). Mobile nodes send modified router solicitation (MRS) (if they want internet connectivity). The mobile nodes register with the Default gateway in order to avail Internet connectivity. In the existing proposal, the default gateway may become a bottleneck node. A modification which incorporates load balancing is proposed wherein mobile nodes can be served by candidate gateways if the default gateway is overloaded. This proposal is as follows: If a mobile node wants internet connection, it sends MRS which, will also be received by Candidate Gateways. All the gateways have to regularly compute the bandwidth that they are consuming. This is done using the following formula

X(i) = α X(i) + β BC(i)

Where X(i) and BC(i) are the estimates of the bandwidth and the byte count of the routed packets for the ith window, while α and β are two coefficients describing the relative importance of the previous and present traffic measurements. All the gateways, DG and CG compute their respective load. The DG inserts its bandwidth estimator in its MRA packets. Therefore, all the nodes (including CG) which receive the MRA will know the current load of the DG. When a CG receives a MRS, it compares its own load with that of the DG. If its load is lesser than the DG’s, it responds to the MRS by sending its own MRA. Otherwise, it just forwards the MRS to the DG. In this strategy, new DGs are dynamically created as and when the current DG becomes overloaded. The load balancing mechanism is part of the gateway discovery process. The drawback of this strategy is that the problem of single point of failure is not completely eliminated.

C. Hsu et al’s Strategy: In Hsu et al‘s strategy, the network architecture consists of two tiers. At one tier, mobile nodes which form the MANET are served by DHCP servers, which are mobile devices and also act as gateways. This forms the lower layer. This layer runs the DSDV routing protocol. All the MANET nodes access the internet through this gateway node. The DHCP server is also responsible for providing the MANET nodes with valid IP addresses. MANETS may or may not be having a DHCP server. MANETS with DHCP servers are called sub-MANETS whereas MANETS without a DHCP server are called disconnected components. The high-tier consists of cellular infrastructure which may be heterogeneous. The DHCP servers are connected to the base stations of the cellular network. Each gateway is connected to a single base station and handovers by gateways are not supported. The DHCP server acts a gateway for the MANET nodes. Hence, registration of MANET nodes with the gateway is through the DHCP protocol. The gateways have the potential to become a single point of failure. Therefore a load-balancing solution is proposed. Mobile hosts choose their gateway based on minimum-load–index routing (MLI). MLI establishes service range boundaries of gateways by taking into account the load indices of gateways and traffic loads of hosts into account. A gateway ‘g’ periodically calculates its load index (Lg), which is the ratio of the traffic load of its high-tier interface and the maximum bandwidth of its high-tier interface. ‘g’ periodically broadcasts agent advertisements which also contain Lg. Whenever a host x hears advertisements from any gateway g, it does the following: If x is not connected to any gateway, it chooses g as its gateway, records g’s current load index and rebroadcasts the advertisement. If g is already x’s serving gateway, x updates g’s index and rebroadcasts the advertisement. If g is different from x’s currently serving gateway g1, then x will change its gateway from g to g1 if the following condition holds true. x will not rebroadcast the agent advertisement :

Lg + ( Tx / Cg ) + ∆l <= Lg1 – ( Tx / Cg1 ) Where Tx is the traffic index of x, Cg is the total capacity of the high-tier interface of g, and ∆l is a

predefined gateway switching threshold. In the above process, the service range of a gateway will expand or contract dynamically, based on its current traffic load. Therefore, we observe that the load balancing mechanism is part of the gateway discovery and selection mechanism. The drawback of this strategy is that handoffs are not supported between gateways.

D. Shin et al’s Strategy: In this strategy [8], the traffic in internet–connected ad

hoc networks is classified into internal traffic and external traffic. Both source and destination of packets are located in the ad hoc network in case of internal traffic. One of source or destination of packets is located in the internet and the other in the ad hoc network in case of external traffic. An external traffic session is further classified into incoming traffic and outgoing traffic. Therefore a mobile node maintains two separate default Internet gateways (IG) for incoming and outgoing traffic. Normally, an Internet gateway with the shortest hop distance from a mobile node will be the default Internet gateway for the mobile node. External traffic causes congestion on the Internet gateways. Mobile nodes can re-register with other less congested IG s in order to ease congestion. The registration of mobile nodes with Internet gateways is integrated with the load balancing scheme. The load balancing scheme is divided into two parts: One for outgoing traffic and the other for incoming traffic. A reactive ad hoc routing protocol is assumed for routing with in the ad hoc network. When a mobile node wants to find the route to a CN which is not in the MANET, it initiates a route request, which is ultimately received by the Internet Gateways. On receiving a RREQ the IG is supposed to reply with a proxy route reply message (P-RREP). The load balancing strategy is integrated in the delay caused while sending the P-RREP. The IG will delay sending the P-RREP according to its current load .The delay will be directly proportional to the current load of an IG. The mobile node which initiates the RREQ will receive the P-RREP of the IG which is lightly loaded. To receive data form the Internet, a mobile node has to be registered with an IG. Reactive registration scheme is used. In this scheme mobile node desiring internet connectivity will initiate an Agent Solicitation. In reply, the IG will send Solicited Agent Advertisement (S-AA). The load balancing strategy is integrated in the delay of the S-AA. The more loaded an IG, the more it will delay sending S-AA.

To measure the load of an IG, the average packet queue size (Qavg) is used .The load of IGi at time T is written as:

L(IGi,T) = * (Qavg(T)/Qmax) + (1-) * L(IGi,T-∆T) Where 0 <= L(IGi,T) <=1, 0<=<=1: averaging coefficient, ∆T: Measurement period, Qmax: maximum size of the packet queue, Qavg(T): average packet queue size from time (T-∆T) to T.

Thus, the gateway load balancing mechanism is built into the gateway discovery process.

E. Le-Trung et al’s Strategy: In this strategy [9], the complete network is divided

into MANET domains. Each MANET domain is served by an Internet gateway (IGW). Each IGW, say IGWj manages a network topology (lj,wj), which can overlap with other

network topologies, where l and w represent the length and breadth of the topology respectively. In order to access the Internet, a mobile node has to get registered with an Internet gateway. An Internet gateway periodically broadcasts Agent Advertisement messages. Therefore, the registration mechanism is proactive. These advertisements contain information about the current load of the gateway, based on which the mobile node makes a decision about the selection of the gateway. Any proactive routing protocol can be used for routing within the ad hoc network.

The load balancing metric consists of three components. A mobile node’s decision about registering with an Internet gateway is dependent on this metric. This metric consists of three components. The first component is the shortest distance (in terms of hop-count) between the MANET node and the selected IGW. The second component is the inter-MANET traffic load via each IGW, which is represented as the number of registered MANET nodes sending/receiving traffic to/from internet via that IGW. The third component is the intra-MANET traffic load within the network topology managed by each IGW.

Information about the three components is attached to the agent advertisement in the form of (Lj,Wj,Nreg(j),Nj), where Lj,Wj represents the first component ,Nreg(j) is the no. of registered MANETS node with the IGWj and represents the second component, Nj is the no. of nodes present in the network topology managed by IGWj and represent the third component. Lj,Wj is static information and every IG has information about its values. The IG also knows the number of nodes registered with it. The number of nodes present in the MANET is determined by looking into the routing table. The hybrid load metric can be represented as:

W(i,j) = 1.D(i,j)+ 2.LBinternet(j)+ 3.LBMANET(i,j) Where, 1+2+3=1, D(i,j) is the minimum distance in terms of hop count between node i and IGWj, LB internet (j) is the no. of nodes accessing the internet through the IGWj, LB MANET (i,j) is the intra-MANET traffic load in the network topology managed by IGWj.

The load balancing strategy is part of the gateway selection process.

III. A FRAMEWORK FOR COMPARING THE LOAD BALANCING STRATEGIES

In this section, we present the comparison of the load balancing strategies based on common parameters. In Table I, Network Architecture of the load balancing strategy can be either Two-Tier or Three-Tier. The MANET Routing Protocol is the protocol used for routing packets within the ad hoc network. Gateway discovery is the gateway discovery mechanism.

TABLE I. GATEWAY DISCOVERY PARAMETERS

Load Balancing Strategy

Network Architecture

MANET Routing Protocol

Gateway Discovery

Zhou et al Three-Tier DSDV Proactive + Reactive

Trivino-Cabrera et al Three-Tier AODV Proactive

Hsu et al Two-Tier DSDV Reactive

Shin et al Two-Tier Reactive Reactive Le-Trung et al Two-Tier Proactive Proactive

In Table II, the gateway load balancing mechanism can be a part of the gateway discovery or gateway selection process. The parameters on which the gateway load balancing Strategy depends are the load balancing Parameters.

TABLE II. LOAD BALANCING CRITERIA

Load Balancing Strategy

Load Balancing Mechanism

Load Balancing Parameters

Zhou et al Gateway selection DS, NN, QL Trivino-

Cabrera et al Gateway Discovery Bandwidth

Hsu et al Gateway Discovery and selection

Traffic Load, Bandwidth

Shin et al Gateway Discovery Packet Queue Size

Le-Trung et al Gateway selection Distance, Traffic Load

In the next section, we propose a new gateway load

balancing strategy based on a load balanced routing protocol.

IV. GATEWAY LOAD BALANCING IN INTEGRATED INTERNET-MANET USING MODIFIED-AODV

In this section, we propose a new gateway load balancing strategy based on a load balanced version of AODV routing protocol called Modified-AODV [10]. The network consists of a set of Internet Gateways (IGW) providing Internet connectivity to mobile nodes which form a mobile ad hoc network. The IGW is the critical node through which mobile nodes can connect to the outside world. Correspondent Node (CN) is the wired node with which mobile nodes interact. Even though IGW are connected to the wired Internet, they are also considered part of the ad hoc network. Therefore, IGW runs two protocols at the network layer, IP for wired network and Modified-AODV for the wireless ad hoc network.

Figure 1. Network Architecture of the gateway load balancing strategy

The protocol stacks of different nodes are given below:

Figure 2. Protocol Stacks of different nodes in Proposed Strategy

A. Modified AODV Routing Protocol AODV routing protocol uses an on-demand route

approach for finding routes. A route is established only when it is required by a source node for transmitting data packets. Therefore, it is a reactive routing protocol. It employs destination sequence numbers to identify the most recent path. In AODV the source node and the intermediate nodes store the next hop information corresponding to each flow for data packet transmission. The source floods the Route Request packet in the network when the route is not available for the desired destination. A Route Request carries the source identifier, the destination Identifier, the source sequence number, the destination sequence number, the broadcast identifier, and the time to live field. When an intermediate node receives a Route Request, it either forwards it or prepares a Route Reply if it has a valid route to the destination. All intermediate nodes having valid routes to the destination, or the destination node itself, are allowed to send Route Reply packet to the source. When a node receives a Route Reply packet, information about the previous node from which the packet was received is also stored in order to forward the data packet to the next node as the next hop towards the destination. In the case when more than one Route Reply is received in response to a Route Request, a single path has to be selected. In the original implementation of AODV, this selection is based on hop count. In Modified-AODV, a modified version of AODV is presented wherein Aggregate Interface Queue Length (AIQL) is used as path selection criteria instead of hop count. In this mechanism, when an intermediate node receives a Route Request, it adds its interface queue length to the Route Request and forwards it. This aggregation of queue lengths of all the nodes lying on a path is called the Aggregate Interface Queue Length. This process is repeated until the Route Request reaches the destination. The destination or intermediate node makes a selection of the best route based on the AIQL and sends a Route Reply back to the source. In summary, instead of hop count, AIQL is maintained in the routing table and is used as the route selection criteria.

B. Working of theProposed Gateway Load Balancing Strategy based on Modified AODV

The proposed strategy is based on the Modified-AODV routing protocol. The Modified-AODV routing protocol is based on the Aggregate Interface Queue Length (AIQL) rather than the hop count. In this strategy, Internet connectivity to the mobile nodes is provided using the hybrid registration mechanism. In hybrid registration mechanism, proactive method will be followed for a certain range of the ad hoc network. Beyond this range, reactive gateway discovery method will be followed.

In the proactive method, the Internet Gateway (IGW) periodically broadcasts agent advertisements into the ad hoc network. This advertisement contains important information like the address of the gateway, the time to live of this advertisement, and its current interface queue length (AIQL), and hop length (HL) field which is incremented on every hop. When a mobile node receives this agent advertisement if it has information about the IGW, then it updates the value of the AIQL field other wise, it adds the information about IGW to its routing table. After processing the agent advertisement, the mobile node adds its interface queue length to AIQL field, and increments the hop length and forwards the modified agent advertisement to its neighbors. When the mobile node desires Internet connectivity, it unicasts a registration request to the IGW and on receiving the registration reply, the registration process is complete. Now, the data to be sent to an external wired node is forwarded to the IGW which in turn uses IP mechanisms to forward the data to the intended destination. This is the proactive registration mechanism. In the ad hoc network, the mobile nodes communicate among themselves using the Modified-AODV routing protocol. When a mobile node is outside the range of the proactive registration mechanism, that is, the agent advertisements broadcast by the IGW do not reach it, a reactive registration mechanism is followed. The mobile node broadcasts agent solicitation message to its neighbors. If any intermediate node has information about IGW, it sends it to the requesting mobile node. In this way, the proactive and reactive registration mechanisms are combined in the hybrid registration mechanism.

Hamidian et al [11] simulated architecture similar to the one in figure 1. Their work is basically a comparative study of the performance of the three Gateway discovery mechanisms in the integration of mobile ad hoc networks and the Internet, namely proactive, reactive and hybrid gateway discovery mechanisms. An extended version of AODV, called AODV+ is used for routing. This strategy does not implement any load balancing mechanism. The proposed gateway load balancing strategy is simulated and compared with the results obtained for Hamidian et al’s integration strategy.

V. SIMULATION OF THE PROPOSED GATEWAY LOAD BALANCING STRATEGY BASED ON MODIFIED AODV A simulation of the proposed gateway load balancing

strategy was carried out to determine the effectiveness of its performance when compared to the existing Hamidian

strategy. The network simulator used was ns-2.31 [12]. The simulations were carried out, for varying Packet Sending Rates. The performance of the proposed Strategy is analyzed with respect to the following performance metrics: Packet Delivery Ratio: It is defined as the percentage of the number of packets received to the total number of packets sent. End-to-End Delay: This is the average overall delay for a packet to traverse from a source node to a destination node. Normalized Routing Load: It is the number of routing control packets per data packet delivered at the destination.

The simulation parameters common to all the simulations are given in the following table.

TABLE III. SIMULATION PARAMETERS OF PROPOSED STRATEGY

Parameter Name Value

Number of mobile nodes 25

Number of source nodes 5

Number of IGW 3

Number of Correspondent Nodes 3

Topology Size 1200 X 500m

Transmission Range 250m

Traffic Type Constant Bit Rate

Mobile Node Speed 20 m/sec

Packet Size 512 bytes

Pause Time 5 sec

Mobility model Random Waypoint Model

Carrier Sensing Range 500m

Simulation time 900sec

Packet Sending Rate 5 – 40 Packets/Sec

Interface Queue Length 50 packets

Advertisement Interval 5 sec

Advertisement Zone 3 hops

A. Simulation Results and Discussion Figures 3 to 5 show the comparison of the proposed

gateway load balancing strategy with the Hamidian integration strategy for varying packet sending rates for different metrics. The packet sending rate was varied from 5 to 40 packets /sec. Figures 3 and 4 shows the comparison of end to end delay and normalized routing load of Hamidian strategy and the proposed strategy. We can observe that the end to end delay and routing overhead of the proposed strategy are comparatively higher than that of Hamidian. This is due to the fact that the operation of the proposed strategy is a bit complex

when compared to the Hamidian strategy. The main advantage of the gateway load balancing strategy is evident in figure 5, which compares the packet delivery ratio of the two strategies. It can be observed that the packet delivery ratio of the proposed strategy is significantly better than that of Hamidian strategy due to lesser number of thrown away packets.

Figure 3. End to End Delay as a function of Packet Sending Rate

Figure 4. Normalized Routing Load as a function of Packet Sending

Rate

Figure 5. Packet Delivery Ratio as a function of Packet Sending Rate

VI. CONCLUSION We make a few noteworthy observations regarding the

load balancing strategies presented in this paper. The most important one is regarding the gateway load balancing mechanism which can be part of the gateway discovery process, gateway selection process or both. When the load balancing mechanism is part of the gateway discovery process, the load balancing mechanism is entrusted to the

Internet gateway as in [6] and [8]. These gateway load balancing mechanisms can be termed as proactive gateway load balancing mechanisms. When the load balancing mechanism is part of the gateway selection process, the load balancing mechanism is entrusted to the mobile node as in [5] and [9]. These gateway load balancing mechanisms can be termed as reactive gateway load balancing mechanisms. The load balancing mechanism can be a part of both gateway discovery and gateway selection as in [7]. Such load balancing mechanism can be termed as hybrid gateway load balancing mechanism.

In the second part of the paper, a new gateway load balancing strategy based on Modified-AODV routing protocol was described. Its performance was compared with the existing Hamidian strategy based on three metrics End to End Delay, Normalized Routing Load and Packet Delivery Ratio. It is observed that the proposed strategy offers enhancements in packet delivery ratio while incurring a higher routing load and end to end delay. According to our classification the proposed gateway load balancing strategy follows a reactive gateway load balancing mechanism.

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