Traffic Load Distribution in Ad Hoc Networks through ... - CiteSeerX

Traffic Load Distribution in Ad Hoc Networks through Mobile Internet Gateways A. Triviño-Cabrera, E. Casilari, D. Bartolomé, A.Ariza Departamento de Tecnología Electrónica, University of Málaga 29071 Málaga, Spain {atc, ecasilari}@uma.es

Abstract. Ad hoc networks were initially conceived as stand-alone entities without connection to any external infrastructure. A possible solution to give access to ad hoc terminals to the Internet trough conventional WLAN networks is described in the mobile multi-gateway support. This scheme proposes to utilize a mobile node within the transmission range of the WLAN access point as a connecting gateway between the MANET and the Internet. As the gateway can move, its functionality can be transferred to another node present in the cover area of the access point acting as a router. In any case, all the traffic flowing to/from Internet passes through the same node, which can impose a bottleneck in the system. In this paper, a modification is suggested to distribute the Internet traffic among different gateways.

KEYWORDS: Hybrid MANET, ad hoc networks, Gateway, load balancing

1 Introduction The original conception of Mobile Ad hoc NETworks (MANETs) implied a generic scenario in which a number of mobile devices could intercommunicate without the use of any pre-existent infrastructure. However, in most practical applications of ad hoc networks a connection to the Internet is required. Therefore, a crucial aspect in the development of these networks is the incorporation of new mechanisms that give access to the Internet in multi-hop wireless networks. The integration with the conventional Internet requires to incorporate in the MANET architecture an Access Router (AR) behaving as an Internet Access Point, that is, an interface between the wireless medium and the wired network. However the introduction of a router is not sufficient as a technique for connecting MANET terminals must address two basic problems. Firstly, the mobile nodes require a global IPv6 address in order to be accessible from external networks. To guarantee the Internet routing hierarchy, an entity must be responsible for providing the necessary information to enable nodes to get an appropriate IPv6 address. This functionality could be associated to a dedicated entity, as in DHCP, so that a special server assigns the IPv6 global address to all the terminals within the domain under its control [1]. Although this strategy has been employed in different works, it presents a major drawback: its demands for an additional equipment (a centralized entity) that restricts the scenarios where MANET could operate, violating the self-configuring philosophy of MANETs [2] [3] [4] [5]. An alternative to suppress this equipment is based on the address auto-configuration. In this proposal the Access Router periodically sends Router Advertisements (RA messages) containing a suitable IPv6 address prefix [6]. In order to generate a global IPv6 address, all the terminals requiring a connection to Internet concatenate a random value, for example the EUI-64 MAC identifier, to this received prefix. The reception of RA messages is also necessary for the nodes to detect the presence of the access router. So, a node perceiving changes in the RA messages will assume to be moved to another domain. In that case, the node triggers the corresponding procedure to carry on the on-going Internet connections. In the IPv6 context, the Mobile IPv6 technology specifies the procedures to communicate with the Internet if changes in the point of attachment take place [7]. However, the RA messages are formatted according to the Neighbor Discovery protocol (NDP). NDP is adequate to provide connectivity to nodes of an infrastructure WLAN as the transmissions of NDP packets are restricted [6] to one hop (they should not be forwarded). In any case,

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the network must not presume that the access router is a member of the MANET. Thus, it will be not capable of routing packets to any node in the MANET. In other words, in multi-hop wireless LAN, where mobile nodes could be several hops away from the Access Router, the one-hop restriction cannot be admitted. To avoid this limitation, an extra element has to be included in the MANET: the gateway. A gateway is a MANET node in charge of broadcasting the information from the access point by diffusing an altered version of the RA packets: the so called Modified Router Advertisements (MRA messages), which are not one hop restricted [8]. The gateway will perform the ad hoc routing functionalities that are not present in the Access Router. In most solutions that have been proposed to integrate MANETs into the Internet, the Gateway is defined as a dedicated entity that must be present in those scenarios where ad hoc networks are going to be deployed [8] [9] [10]. Under this consideration, the software of the gateway must be directly loaded in the router or, in other case, it should be located in a fixed (candidate) node within the router transmission area. This assumption does not follow the MANET principle, i.e. to operate wherever and whenever without any special hierarchy among the nodes. To overcome this restriction, mobile multi-gateway support could be applied [11]. In this solution, gateways’ functions are transferred to a MANET node that is one hop away from the Access Router. As the gateway is now enabled to be a mobile node, the draft in [11] proposes a strategy to switch the role of gateway between the different nodes that receive the RA messages. Nevertheless, as all both outgoing and incoming Internet traffic will be traversing the gateway, this node can become a bottleneck for the network performance. In this paper we propose a simple modification in the election of the gateway to distribute the traffic among the different candidate nodes as a function of their traffic load. The rest of this paper is organized as follows. In Section 2, we summarize the principal characteristics and drawbacks of some solutions that integrate MANET into the Internet. The mobile multi-gateway support is commented in Section 3 while Section 4 comments some previous studies on traffic load balancing. Section 5 describes our specific proposal for a proper load balancing in hybrid ad hoc networks. Section 6 presents the results of different simulations aimed at evaluating the benefits of our proposed algorithm. Finally Section 7 summarizes the main conclusions of the paper.

2 Hybrid ad hoc networks In the IPv6 context, one of the first proposals to grant Internet connectivity to MANETs was an Internet draft by R. Wakikawa et al. [8]. According to this mechanism, often called global connectivity, a fixed Gateway is attached to the Access Router to distribute globally routable addresses to the ad hoc nodes. Although the scheme to detect gateways was initially evaluated under proactive and reactive methods, a hybrid method for this mechanism has also been studied by other authors [12]. In this line, M. Ghassemian et al. analysed the addressing issues related to the global connectivity mechanism under scenarios with multiple Gateways interconnected by the use of Mobility Anchor Points [13]. In [9], C. Jelger et al. investigated a scenario where gateways are responsible for auto-configuration of global addresses by sending periodic messages that inform nodes about the prefixes that the gateway serves. As diverse gateways can offer service to the same ad hoc network, the nodes may receive multiple autoconfiguration messages from different gateways. The proposed criterion to select the gateway is that the mobile nodes can have a continuous path to the Gateway, intending that all the nodes composing the path share the same prefix. In [14], S. Ruffino and P. Stupar also studied the coexistence of multiple fixed gateways to integrate MANETs into the Internet. As in most of previous cases, gateways periodically announce their prefixes through special messages so that the nodes can generate the appropriate IPv6 addresses. In this proposal, nodes may retain different IPv6 addresses (one for every detected Gateway), so they can dynamically change the global address that they utilize for their Internet connections if, for example, a gateway ceases to be operative. In all these studies, Gateways are conceived as non-mobile nodes which are purposely located in the vicinity of Access Routers (or even installed in the routers themselves) to enable the MANET to perform hybrid communications (Internet plus internal connections). The existence of this fixed infrastructure requires to prepare the scenarios where multi-hop hybrid ad hoc networks are going to operate by the introduction of the corresponding gateways. One of the first mechanisms that contemplated mobility in the gateways was published by H. Ammari and H. El-Rewini [15]. Under this strategy several nodes in the

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MANET are allowed to behave as Gateways. Although they can move, their movements are restricted to the coverage area of the access router. In this case, the distinction of mobile nodes between enabled and not enabled gateways also deteriorate the intrinsic flexibility of MANET communications. The node differentiation and the restriction to movements are avoided with the policy of mobile multiple gateway support [11]. In this solution, mobile gateways can leave the coverage area of the access router. Therefore, a distributed algorithm is required for a dynamic gateway selection. In the next section this mechanism is explained in depth.

3. Mobile Multi-Gateway Support As it has been remarked, this mechanism was created to simplify the restrictions associated to heterogeneous environments where multi-hop ad hoc networks may coexist with fixed networks. By the transfer of Gateway functionalities to a MANET node, the introduction of a dedicated Gateway is suppressed. Therefore, the number of situations and applications where MANETs can perform increases. The resulting architecture could be useful in quite different scenarios where a rapid, robust and auto-configurable connection to Internet is required. Examples of this could be emergency scenarios (e.g.: natural disasters, terrorist attacks,…) where the ad hoc nodes (in this case the medical equipments) could retransmit via Internet to a hospital the patients’ biosignals received from other nodes. It would be also appropriate for automotive scenarios where a vehicle temporally connects to a petrol station or to an UMTS access and it performs the Gateway functionalities for some other vehicles in the surrounding area. In both situations the gateways have a complete freedom of movements. The method for selecting the node which will operate as the Gateway as well as the treatment associated to its mobility are the main concerns of the Mobile Multi-Gateway Support. Among all the devices that are in the coverage area of the Access Router, one of them is temporally selected to behave as the Internet Gateway. This node is called the Default Gateway (DG) whereas the rest of the terminals sited in this transmission range are referred as the Candidate Gateways (CG). So, at a certain instant of time, one DG and none or multiple CG may coexist. All the Gateways (Default or Candidate) hear the RA messages generated by the Access Router but only the DG propagates this information to the network. For this purpose, the DG periodically sends unsolicited MRA (Modified Router Advertisement) messages, which can be forwarded through multiple hops through the ad hoc networks. MRA messages can be typically implemented by adapting NDP Router Advertisement packets as well as by modifying the messages of route announcement in the ad hoc routing protocol. For example, [8] proposes a slight change in the format of AODV Route Replay (RREP) messages to broadcast the identity of the DG. By the reception of a MRA message, a mobile device can build up its global IPv6 address and update the corresponding route to the DG. When a terminal generates traffic to an external (Internet) host, it will forward the packets to the DG using this route. If the node does not know which the DG is (e.g.: because the MRA packet was lost), it can initiate a procedure to determine a route to the Internet. This procedure to discover the path to the DG is performed by broadcasting a special message to the network. As in the case of the MRA, this message can be put into operation by altering the typical Router Solicitation which is defined by NDP, generating the so-called Modified Router Solicitation (MRS) messages. Similarly, it could be implemented [8] with a modified version of the Route Request (RREQ) packets of AODV. In any case, as soon as one of these broadcasted packets reaches the DG, an unicast MRA message is backpropagated to the soliciting node. As it will be described in next section, this method could be also utilized to detect the presence of alternative routes to Internet through the use of CGs. On the other side, when the Access Router receives any packet whose destination is a MANET component, it will forward it to the DG, which will perform the conventional ad hoc routing to the destination ad hoc node. The information conveyed in the MRA messages is considered to be valid during a certain time interval (modified advertisement interval). The MRA packets inform about this interval (which must be longer than the period between two consecutive RAs) by means of a lifetime field. Before this period (plus a ‘guard’ time) expires, the mobile nodes expect to receive a new MRA message from the DG. While the DG resides in the coverage area of the Access Router, the nodes in the MANET will periodically receive its broadcasted MRA messages. On the contrary, if the DG stops receiving RA packets from the Access Router, it will assume that it has escaped from its transmission area and will stop sending MRA messages in order to cease

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its gateway functionalities. As no MRA will be received before the expiration of the corresponding modified advertisement interval, all the network nodes (including CGs) will consider there is no active DG. At that moment, the CGs should launch the gateway selection procedure. The interchange of messages under this scheme for providing Internet connectivity has been depicted in Figure 1. External (Internet) Node

Internet

Ad hoc network

4) MRA CG 3) MRS 1) RA

Acce ss Router (AR)

DG 2) MRA

1) One-hop broadcasted RA from the AR 2) Multi-hop broadcasted MRA from the DG 3) Multi-hop broadcasted MRS from the ad hoc nodes 4) Multi-hop unicast MRA from the CGs

Figure 1. Messages for gateway configuration in a hybrid network under Mobile multiple gateway support

This procedure is a distributed algorithm that respects the MANET nature as it does not need any centralized entity to select the future DG. The mechanism consists of triggering a random internal timer. Every CG verifies if any MRA message is detected during this random. From the reception of a MRA it would infer the existence of a new configured DG. In other case, the CG will automatically self-configure itself as the new DG by broadcasting its own MRA message immediately. The Internet draft does not contemplate the case of receiving multiple MRAs from different CGs (e.g.: because a MRA packet loss occurs or two CGs keep waiting during very similar random intervals). In [17] we propose a simple election based on the IPv6 address: nodes (including recent self-configured DG) will give priority to the DG with a lower IPv6 address.

4. Related works on load balancing The issue of load balancing is not new in the area of MANETs. In this sense, most studies in the literature have focused on evaluating the benefits of distributing the traffic from the ad hoc nodes by means of multipath routing. Due to the wireless nature of transmissions in ad hoc networking, the main problem when using multi-path routing is that alternate routes between two terminals can interfere even if they traverse a completely disjoint set of routing nodes. The study in [18] analytically confirmed that the possible advantages of multipath are strongly related to the number of simultaneous utilized paths (with an optimum in 3) and the average path length. In [19], Pearlman et al. showed that the improvements of multipath routing are insignificant for single-channel networks because of the coupling between the paths. Similarly, in [20] Ganjali et al. demonstrated that a multipath policy does not necessarily imply a smoother traffic distribution within the network. These authors suggest that the benefits of multipath could be achieved only if the alternative routes push the traffic from the “center” of the network, that is to say, where the node (and interference) density is higher. This could be put into practice if nodes implement location based routing. Unfortunately, this routing demands the knowledge of the physical position of the nodes, which requires a special hardware (e.g.: GPS or other positioning system) in each node. In this line, Jones et al. [21] formulate an algorithm that minimizes the interference ‘correlation’ between the paths basing on the network connectivity. However, the algorithm requires a perfect and updated knowledge of the network topology in the nodes, which is only possible if a link-state routing protocol is utilized (not being the case of the reactive protocols). Moreover, multi-path techniques are not natively supported in some popular ad hoc routing protocols such as AODV.

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Apart from these studies on multi-path routing, other works base load balancing strategies on slight modifications in the treatment of Route Request (RREQ) Messages of reactive routing protocols. So, authors in [22] suggest that overloaded nodes should not propagate RREQ messages while the study in [23] proposes that all the nodes retransmitting a RREQ packet should add information to the message concerning their queue occupancy. As a result, the destination node, after receiving different RREQs from different routes, it will backpropagate the Route Reply choosing the route with less queue occupancy.

5. A proposal for load balancing in hybrid networks In spite of the aforementioned studies, no much research efforts have been devoted to load balancing for the particular case of hybrid networks. In this type of networks, the problem of traffic concentration is specially critical as all the Internet flows (which could correspond to a high percentage of the global traffic in the network for most practical applications) is routed to the same node (the DG). In [21], for example, it is suggested that network efficiency could be dramatically improved by means of load balancing in the scenario where a stub multi-hop network connects to Internet through different gateways. Aiming at minimizing the congestion around the default gateway, we propose to distribute the Internet traffic among the different CGs basing on the traffic load. In this new architecture, all the nodes initiating an Internet connection will send a Router Solicitation (by emitting a MRS message) independently if they know the existence of the DG. As CGs are allowed to respond (in a unicast way) to MRS messages, nodes will be able to select a particular CG for their packets to Internet. The election of the gateway node to utilize is performed at the nodes depending on the overall traffic that the gateways are supporting. For this purpose, all the gateways have to regularly compute the bandwidth they are consuming. This bandwidth should be updated in an internal variable computed for observation windows of a fixed duration. Although different traffic estimators could be employed, we initially consider a classic first order autoregressive filter: X (i ) = α ⋅ X (i − 1) + β ⋅ BC (i ) (1) being X(i) and BC(i) the estimator of the bandwidth and the byte count of the routed packets for the i-th time window, respectively, while α and β are two coefficients describing the relative importance of the previous and the present traffic measurements. Consequently, the computation in the nodes is basically limited to a simple byte count of the packets they are transmitting/receiving to/from the AR. When a candidate gateway has to respond to a MRS message from a node, it will include information on this measurement adding a field in the MRA packet (in case of using modified NDP messages, the measurement should be incorporated in the options field [6]). As the DG will include its own updated bandwidth estimator in its broadcasted MRA packets, all the ad hoc nodes (including the CGs) will know at any time the load that the DG is supporting. The CGs that receive a MRS from a node willing to connect to the Internet will compare their traffic load with that of the DG. Thus, they will only send a MRA answer in case of being less saturated that the DG, which will reduce the overhead provoked by MRA messages. Nodes that have sent MRS will wait for a short time interval to receive the MRAs from the gateways. In case of receiving several answers, the node will elect the gateway with the lowest value of the estimated bandwidth. In order to avoid sudden and spurious re-distribution of traffic within the network, nodes will keep using the selected gateway until the route breaks. So nodes will start the gateway solicitation just when they have no configured gateway or when the route to the most recent configured gateway is lost. As the behaviour of the DG is respected and its MRA are periodically broadcasted, nodes that do not implement this algorithm could maintain their connections to the Internet through the DG. Thus, this strategy for load balancing could be gradually incorporated to the nodes. On the other hand, a problem is derived from the utilization of distinct gateways in both uplink and downlink communications. If no special routing information is provided within the packets, all the ad hoc nodes that are routing packets to Internet will utilize the route to the default gateway. Similarly, all the incoming traffic will be diverted by the Access Router to the DG. So, every ad hoc terminal should inform both the ad hoc network and its correspondent Internet node about the Candidate Gateway to employ. This action could be accomplished by the inclusion of a proxy routing header in the packets [24]. This header allows a source node to list one or more intermediate nodes to be “visited” by the packet on the way to its destination. Under this procedure, the ad hoc node could complete this header by specifying the CG and the Access Router addresses as intermediate nodes. When the correspondent node receives the packets with a proxy routing header built by the ad hoc node, it will check

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the integrity and authenticity of the source address and, immediately, it will start sending its packets to the ad hoc node with a proxy routing header that will follow the reverse route of the received header.

6. Simulations and Results In order to evaluate the performance of the proposed mechanism, a software module to include mobile multigateway support was developed and integrated into the Network Simulator 2 (ns-2 [25]). Ns-2 is by far the most extended simulation software in the studies on ad hoc networking. Similarly, the proposed algorithm for load balancing between the gateways was incorporated to the software. Simulations considered a typical 1500 m x 300 m bi-dimensional space, placing the access router in the center of the area. The ad hoc network was formed by 50 simulated mobile nodes. Two opposite scenarios have been evaluated: one with no pause time (for which the nodes are continuously in motion) and a static scenario (without mobility). In the first scenario, which could represent a typical application in which the mobility of the nodes is determined by human actions (e.g.: laptops during a conference), the movements were based on the popular Modified Random WayPoint model [26]. For our experiments, we considered diverse types of mobility changing the constant velocity of the nodes from 1 m/s (habitual speed of a walking man) to 20 m/s (case of a scenario with motorized vehicles). In the second scenario, which could correspond to a network of fixed sensors, as there is no mobility, the most determining parameter is the node distribution. Up to 10 different random topologies were simulated to estimate the mean network performance. Due to its popularity, AODV was chosen as the ad hoc protocol. As it respects to the traffic pattern, 10 Constant Bit Rate (CBR) data connections were considered. The connections were designed to simultaneously generate both uplink and downlink traffic between 10 randomly chosen nodes and a generic exterior (Internet) host. For the simulations, all the traffic is transmitted between the mobile node and the Access Router. So, the analysis just computed the packets losses and delays which took place in the hops within the ad hoc network (Internet losses and delays are not modeled). All the simulation parameters are summarized in Table 1.

Simulation Area No. of Mobile nodes Mobility pattern (Mobile scenario) Initial Node distribution (Fixed and mobile scenarios) Traffic pattern (uplink /downlink) Simulation Time Transmission Range Number of runs/point Ad hoc protocol

Table 1 Simulation parameters 1500 m x 300 m 50 Constant speed (Ranging from 1 to 20 m/s) Uniform

10 CBR sources. Rate = 8 packets/s Packet size= 512 Bytes 5000 s 250 m 3 AODV (Link Layer Detection enabled Local Repair disabled) Link Level Layer 802.11 Internal Node Queue 64 packets RA Advertisement interval 2s MRA interval 4s Parameters for traffic Measurements Coefficients for the filter: α=0, β=1 (α=0 as in the gateways CBR and constantly emitting sources are employed). Observation Window: 10 s The performance of the network is measured by means of the following metrics: -PDR (Packet Delivery Ratio), estimated as the ratio between the packets received at the destination and the packets generated by the sources. -Mean End-to-End (E2E) packet delay.

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-Normalized overhead, defined as the percentage of control packets with respect to the received data packets. Each hop of any control packets is computed as a new control packet. Figures 2-4 show these estimated metrics for the case of the first scenario. Figures 2 and 3 (for PDR and delay respectively) clearly indicate that load balancing techniques are appropriate for situations of lowmedium mobility (with node velocities under 10 m/s) although it augments the protocol overhead, especially due to the broadcast of router solicitation (MRS). For low velocities, load balancing can even reduce in a 50% the packet delay just at the cost of a very slight increase in the overhead (see Figure 4). On the contrary, in situations of very high mobility (with typical velocities of motored vehicles, greater than 10 m/s), the behaviour of load balancing clearly underperforms that of the case in which all the traffic is routed through the same node (the DG). This could be explained by the fact that, as the velocity grows, the route duration rapidly decays. In that case, the process of gateway re-configuration (through broadcasted MRS messages) that the load balancing requires after a route break may become highly inefficient. Packet delivery ratio

End-to-End delay

95

350 Without Load Balancing With Load Balancing

90

End-to-End delay (ms)

300

PDR (%)

85

80

75

Without Load Balancing With Load Balancing 200

150

70

65

250

0

2

4

6

8 10 12 Node speed (m/s)

14

16

18

100

20

Figure 2. Packet delivery ratio as a function of the node velocity

0

2

4

6

8 10 12 Node speed (m/s)

14

16

18

20

Figure 3. End-to-end packet delay as a function of the node velocity

Protocol Overhead 4 3.8

Table 2. Results for the static scenario (Values are the mean of the estimated metrics for ten different simulated topologies of node distribution)

3.6 Without Load Balancing With Load Balancing

Overhead (%)

3.4 3.2 3 2.8

PDR e2e delay Overhead

2.6 2.4

Without load balancing 78.18 % 1214.6 ms 2.94%

With load balancing 87.02 % 354.79 ms 2.93%

2.2 2

0

2

4

6

8 10 12 Node speed (m/s)

14

16

18

20

Figure 4. Normalized Overhead as a function of the node velocity

Effectively, in the case without load balancing, a node can recover from the loss of the route to the DG through the reception of the periodical unsolicited MRA from the DG. In the scheme with balancing, every link break implies the broadcasting of new MRS solicitations. In addition, in this scenario of high mobility it is also more probable that a node selects a CG which is eventually traversing the coverage area of the AR at an elevated speed. As soon as this node escapes from the transmissions of the AR, the process for gateway solicitation is triggered. On the other hand, the case of a static scenario can be regarded as the optimal situation for the utilization of load balancing. Table 2 condenses the mean results of the performance metrics for ten arbitrary distributions of the nodes within the simulation area. In practically all the tested topologies, the routing with load

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balancing outperformed the architecture that just employs the DG for supporting the connections to the Internet. The mean statistics (those indicated in Table 2) clearly show a dramatic improvement of the PDR and, specially, the packet end-to-end delay for a practically identical overhead. This implies that, for the case of a sensor network (which could be an evident example of this type of static ad hoc network), the benefits of load balancing could be achieved with a negligible increase of the overhead traffic and consequently, with no extra consumption of the battery power in the nodes (a critical aspect in the management of a sensor network).

7. Conclusions This paper has evaluated the benefits of load balancing in scenarios of hybrid ad hoc networks in which the MANET nodes access to the Internet by means of mobile multi-gateway support. Different simulations in both static and dynamic scenarios show that the use of load balancing notably improves the network performance (measured in terms of Packet Delivery Ratio and End to End packet delay) at the cost of a moderated increase in the protocol overhead. Only in an environment where the nodes exhibit high mobility, the load balancing technique is showed to be more inefficient than the simple utilisation of a single (default) gateway. For a node wishing to communicate with an external Internet terminal, the proposed architecture suggests a criterion to select the mobile gateway that will provide access to a conventional infrastructure WLAN Access Point. In particular, the election is based on the traffic load that the gateways are retransmitting. The implementation of the proposed method is very simple as long as it just requires that the gateway nodes include information about their supported traffic in the advertisement messages that they emit to inform about their status. Further studies should contemplate the possibility of Variable Bit Rate sources as well as reactive traffic sources (such as TCP data sources).

Acknowledgments This work was partially supported with public funds by the Spanish National Project No.TIC2003-07953C02-01.

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