Seamless Integration of MANETs into IP-based Access Networks

Transcription

1 11ª Conferência sobre Redes de Computadores 1 Seamless Integration of MANETs into IP-based Access Networks A. Triviño-Cabrera Abstract Access Routers inform about some configuration parameters (such as those related to the construction of temporary IP addresses) to the devices that are inside their coverage area so that the devices can access to external networks. However, the Neighbor Discovery Protocol, whose specifications describe this functionality, prevents messages from being propagated so this technology cannot be straightforwardly applied into multihop wireless networks. In order to overcome this shortcoming, an Internet Gateway is used. Opportunistic gateways, which are mobile nodes configured to act as gateways, offer a flexible and reliable solution to guarantee access to IPbased access network in MANET. Although some mechanisms to integrate MANET into the Internet suggest the use of opportunistic gateway, none have specified how the Access Router becomes aware of which node is acting as the Internet Gateway. This paper focuses on this concern by developing a simple and successful scheme to use opportunistic gateways in Internet-connected MANETs. By using conventional Neighbor Discovery messages, a seamless and transparent integration that does not alter conventional Access Router s behavior is achieved. Index Terms Integration, MANET, Multihop wireless network, Neighbor Discovery Protocol, Opportunistic Gateway. T I. INTRODUCTION HE use of a wireless access to connect to the Internet is a key issue for ubiquitous computing paradigm. Under some specific geographical constraints, the development of a wireless infrastructure that guarantees the mobile nodes access within a particular area is economically unfeasible. In these cases, wireless connection can benefit from Mobile Ad Hoc Network (MANET) technology to reduce the costs of the deployments. Specifically, the use of multihop communication can easily eliminate dead zones and it also virtually extends the coverage area of the Access Routers or GGSN (Gateway GPRS Support Node) in UMTS (Universal Mobile Telecommunication Systems) [1]. However, the use of MANETs in this context prompts several technological challenges that need to be previously solved. Concerning the architecture, the infrastructured network provides an Access Router (or a GGSN in UMTS), to which mobile devices connect to access Internet hosts. In IPv6, Access Routers are responsible for informing about some configuration parameters to those nodes that are inside their coverage area [2]. With this information, mobile devices become aware of the technique to configure their global IP addresses which will be valid in the context where they are temporarily. When a stateless configuration mechanism is applied, the nodes also received the prefix information from which they can construct their own IP addresses [3]. Therefore, the information that Access Routers emit is vital to guarantee the mobility of wireless devices that dynamically connect to the available points of attachment and which must acquire valid IP addresses in each temporary scenario. However, conventional Access Routers does not provide this information to all nodes in the MANET as their messages are restricted to one hop. This significant drawback is due to the fact that this functionality is described in the Neighbor Discovery Protocol (NDP) [2]. This protocol prevents all the messages from being propagated as they are emitted with link local addresses [4]. Consequently, the nodes outside the coverage of the Access Routers will not know how to acquire a valid and temporary IP address nor even how to route the packets to the Access Router so they can be received by external hosts. Another significant limitation of Access Routers is related to routing. Since conventional Access Routers do not implement any ad hoc routing protocol [5], when they receive packets whose destination corresponds to a MANET node which is located outside their coverage area, they cannot forward the packets because they do not know any route to the destination and they cannot even discover the corresponding routes. These two shortcomings are overcome by installing an Internet Gateway which complements the Access Router (AR). Firstly, it generates new messages which contain the information emitted by the AR. However, these messages can be propagated in the MANET so all the nodes in the network become aware of the configuration parameters. On the other hand, the Internet Gateway also provides the routing procedures that are commonly absent in conventional Access Routers. With this new element, when the Access Router receives a packet whose destination is a node in the MANET, the packet is forwarded to the Internet Gateway which conveniently routes the packet in the MANET. This forwarding could be explicitly performed, i.e., the Access Router is configured so that all packets destined to the MANET nodes are sent to the Internet Gateway which is in charge of routing them. By this technique, the conventional behavior of the Access Router is modified in order to include information related to the Internet

2 11ª Conferência sobre Redes de Computadores 2 Gateway in its routing tables. This necessary adaptation could be easily accomplished in scenarios where dedicated and fixed Gateways are permanently attached to the Access Routers. However, the formation of mobile ad hoc network could spontaneously occur in unplanned scenarios where fixed Gateways are not present. In these cases, the use of an opportunistic or occasional Gateway provides a feasible solution for Internet connectivity in MANET. Opportunistic gateways are mobile nodes which configure themselves to operate as the Internet Gateway on behalf of the rest of the nodes in the MANET. As opportunistic gateways must work with Access Router to route the packets in the MANET, they must be directly connected. As any node holding these conditions could operate as the Internet Gateway, the use of opportunistic gateways provides more flexibility and robustness for the Internet connection of MANET. The use of opportunistic gateways is appropriate for coverage area extensions in cellular networks where mobile nodes belonging to the same ad hoc network communicate among them without any previously deployed network while the communication with external host demands a MANET node acting as the Gateway that is neither provided nor controlled by the telecommunication operator. Moreover, these gateways could be also suitable for automotive scenarios where a vehicle temporally connects to a petrol station or to an UMTS access while performing the Gateway functionalities on behalf of some other vehicles [6]. To make this strategy succeed, the configured opportunistic gateway should not be private from its freedom to move anywhere, even to some areas where it is not reachable by the AR. With this freedom, the gateway functionalities are expected to be transferred among the nodes in the MANET. So, in these cases, temporary opportunistic gateways are dynamically configured. Despite of the benefits that the use of opportunist gateway produces, so far there is not any specification about how the opportunistic gateway should be employed by the Access Router. Specifically, an important issue should be defined: how does the Access Router know which node is playing the role of the Internet Gateway (especially, when these functionalities are dynamically transferred in the MANET)? An automatic mechanism is necessary for this setting. Aiming at not altering the usual Access Router functionalities, the use of opportunistic gateways should not be based on forcing the Access Router to analyze the messages generated by the Gateway in order to extract the Internet gateway information, such as its IP address. Therefore, a transparent, seamless and automatic cooperation between the Access Router and the Internet Gateway is preferred. In this paper, a new simple and successful scheme which holds these requirements is proposed. The structure of this paper is as follows. Section II presents the related work about how opportunistic gateways are proposed to be used. Section III describes the proposed scheme for opportunistic gateway. The proposal is evaluated by analyzing its behavior in the possible situations where opportunistic gateways could be employed. This evaluation is shown in Section IV. Section V shows the cost of the proposal in terms of number of messages injected into the network. Finally, Section VI draws the main conclusions derived from this work. II. RELATED WORK IP-based access networks may offer an AR to which mobile devices connect. For the integration of mobile devices in a MANET into IP-based access networks, there is a great variety of proposed supports. All of them are supported by a Gateway that complements the Access Router so that it can cope with multihop wireless communication. Figure 1 shows this interconnection. INTERNET MANET Fig. 1. Scheme of an Internet-Connected MANET. AR Gateway The Gateway characteristics as mobility, implementation details or their availability to work with some other Gateways differentiate these proposals. About the Gateway s implementation, two opposite approaches can be basically observed. Firstly, the Internet Gateway could be considered as a dedicated device which is specifically pre-installed in those environments where a MANET is expected to be connected to the Internet [7] [8] [9]. In this sense, the dedicated Internet Gateway has two interfaces: one to connect to the ad hoc network and another to communicate with the Access Router. Furthermore, the Internet Gateway could be also integrated into the Access Router. As mobile ad hoc networks could be spontaneously formed in scenarios where no specific ad hoc equipment has been previously set up, some works [10] [11] [12] propose the use of MANET nodes that configure themselves to operate as

3 11ª Conferência sobre Redes de Computadores 3 Internet Gateways on behalf of the rest of ad hoc devices. These nodes are referenced as opportunistic or occasional gateways. In [10], gateways are restricted to stay in the coverage area of the AR while in [11] gateways are completely free to move. The proposal in [11] is one of the documents which specify the behavior of opportunistic gateways. Specifically, it detects two techniques that are necessary when working with opportunistic gateways. These techniques are: A. Selection of the Opportunistic Gateway Among all nodes with direct access to AR, one of them will assume the role of the Internet Gateway. The method to select the node should be based on a distributed algorithm which does not need any centralized equipment. Therefore, the nodes in the MANET will coordinate to select one Internet Gateway. For instance, the proposal in [11] suggests the use of internal timers set with random values. When the timer expires, the node check if there is a Gateway configured. If not, the node will configure itself as the new Gateway and it will immediately announce its new acquired state to the MANET. Additionally, the mechanism specified in [11] also proposes that the nodes placed in the coverage area of the AR that does not work as Gateway could route packets to the AR in order to balance traffic. B. Transfer of the Gateway Functionalities When the node acting as Gateways escapes from the coverage area of the AR, it cannot operate as gateway any longer. Under these circumstances, a mechanism by which the nodes detect the absence of the gateway is necessary so the nodes could trigger the procedures to configure a new gateway [13]. III. PROPOSAL FOR THE USE OF OPPORTUNISTIC GATEWAYS The AR must know which node is acting as the Gateway in order to forward the packets to be routed in the MANET. So far there is not any specification about how the AR configures itself to use opportunistic gateways. In this sense, the identification of the opportunistic gateways by analyzing the specific messages that the gateways generate could be considered a valid strategy. However, this functionality is not included in conventional AR so specific ARs would be demanded. In order to overcome this significant limitation, our proposal forces opportunistic gateways to configure themselves as address resolution proxy on behalf of the other MANET nodes [2]. This is a reasonable requisite as NDP is already incorporated in MANET nodes. As a benefit, the AR behavior is unaltered with this proposal. The configuration of opportunistic gateways as address resolution proxy reveals itself when the AR proceeds to discover the link-layer address associated to an IPv6 address to which the AR must send a packet. The discovery could be triggered because the AR does not know the destination link layer address or because it has detected that the one it knew has become invalid. This functionality, which replaces IPv4 ARP (Address Resolution Protocol) in IPv6 context, works as follows [2]. When the AR receives a packet whose destination is a node in the MANET, it studies if the destination is in its coverage area. With this purpose, the AR first checks if it knows the link layer address of the destination. For this operation, it queries its Neighbor Cache which binds IP addresses to link layer addresses. In case there is not any information about the intended destination in this structure, the AR proceeds to determine destination link layer address. Consequently, it generates a Neighbor Solicitation message. This is a one-hop message so it cannot be forwarded. Then, only the nodes in the coverage area of the AR will receive this solicitation. On receipt, the opportunistic gateway, configured as address resolution proxy, will reply with a Proxy Advertisement message. With this message, the opportunistic gateway is expressing its willingness to receive packets intended to other nodes and then, it will forward the packets to the corresponding destination. Occasionally, when the final destination receives the solicitation generated by the AR, it will reply with a Neighbor Advertisement message. In these cases, the AR receives two messages for a solicitation: the Proxy Advertisement and the Neighbor Advertisement. NDP specifications gives priority to Neighbor Advertisement messages so their information prevails against the data contained in the Proxy Advertisement independently on which messages was received first. When NDP was specified, this coincidence was taken into account so the authors include the override bit by which the messages generated by the final destination priories. Figure 1 depicts the procedure by which link-layer address are determined in the proposed scheme. In this diagram, the inclusion of multicast messages is represented by incorporating the destination or the origin of the messages [14]. IV. ANALYSIS OF THE PROPOSAL: STUDYING THE CASES As a first step, the correctness of the proposal is evaluated by analyzing the possible conditions on which the AR is when it needs to retransmit a packet whose destination is a MANET node. The identified states on which the AR could be are: A. The Neighbor Cache binds the destination IP address to its corresponding link layer address. The destination is inside the coverage area of the AR. The entry in the Neighbor Cache is considered valid and, consequently, it can be correctly used. Thus, the AR sends the packet to the final destination. B. The Neighbor Cache binds the destination IP address to the gateway s link layer address. The gateway is inside the coverage area of the AR. The AR will use the information contained in the Neighbor Cache to retransmit the packet to the gateway. Then, the gateway will route the packet to the final destination if it keeps valid routing data. Otherwise, the gateway will trigger the ad

4 11ª Conferência sobre Redes de Computadores 4 hoc routing procedures to learn a new path to the final destination. C. The Neighbor Cache binds the destination IP address to its corresponding link layer address. This information is stale because the destination is outside the coverage area of the AR. When the AR notices that this information is invalid (this could be perceived by any OSI layer), the procedures associated to the address resolution are executed. Therefore, the AR emits a Neighbor Solicitation message demanding for the link layer address of the IP address that is set in the destination field of the packet. D. The Neighbor Cache binds the destination IP address to the gateway s link layer address. This information is stale because the node that was acting as the gateway is not providing this service any longer, that is, a new gateway has already been chosen. In this case, the AR will proceed to emit the packet using the previous gateway s link layer (which is stored in the Neighbor Cache). If the previous gateway is still inside the coverage area of the AR, it will receive the packet and it can retransmit it to the destination as it is a MANET node. On the other hand, if the previous gateway is unable to receive the packet, any OSI layer will notify the NDP module that the packet has not been received. The AR will be demanded to initiate the address resolution procedures. E. The Neighbor Cache does not contain any data about the link layer address of the destination node. The AR will execute the address resolution procedures, as indicated in the NDP specifications. In the states C, D and E, the AR initiates the address resolution procedures. Thus, it emits a Neighbor Solicitation message demanding the link layer address of the IP address set in the destination field. The message will be responded by a Proxy Advertisement (generated by the gateway) and it can also prompt the response of the final destination if it is in the coverage of the AR. Therefore, the AR can receive three types of response depending on the timing of the potential receivers (the gateway and the final destination). These responses are: 1. The destination is not in the coverage area of the Access Router. In this case, the AR receives just one message: the message generated by the gateway. The proxy advertisement is analyzed by the AR and it binds the destination IP address to gateway link layer address. 2. The destination is inside the coverage area of the Access Router and its Neighbor Advertisement message reaches the AR after the Proxy Advertisement Under these circumstances, the AR firstly associates the destination IP address to the gateway link layer address as the Proxy Advertisement message indicates. Thus, the Neighbor Cache is conveniently updated but setting that the information has low priority. When the Neighbor Advertisement (generated by the final destination) is received, the AR updates its Neighbor Cache. The Neighbor Cache has already information for that destination but as it is marked with low priority. Therefore, the data contained in the Neighbor Advertisement is preferred. The packets will be transmitted directly to the final destination. 3. The destination is inside the coverage area of the Access Router and its Neighbor Advertisement message reaches the AR before the Proxy Advertisemen.t With the Neighbor Advertisement, the AR updates its Neighbor Cache. Later, the Proxy Advertisement generated by the gateway is received by the AR. As all Proxy Advertisements, the data contained in them are marked with low priority. Therefore, the Neighbor Advertisement information prevails and the Proxy Advertisement is silently discarded [2]. It is important to note that the situations explained above follows conventional NDP procedures. By configuring the opportunistic gateway as Address Resolution proxy, the AR communicates with the destination directly when it is in its coverage area. In another case, the AR sends the packets to the opportunistic gateway. With this procedure, the bottleneck in the link AR-Gateway is also lightened. V. SOLUTION COSTS As a second approach, the proposal is evaluated by means of a set of simulations supported by a simplified model of a MANET in MATLAB [15]. 50 mobiles nodes are uniformly placed in a scenario of 1500x300 m 2. The AR is set at the center of the simulation area. In this model, a geometric transmission is used so that two nodes are assumed to be connected when the distance separating them does not exceed the transmission range. In the simulations, this parameter is set to 250 m. On the other hand, the effect of interferences and collisions are captured by the parameter named probability of loss. The probability of loss indicates the probability that a message generated by the NDP does not reach its intended destination. In our experiments, the probability of loss ranges from 0, 0.1 and 0.2. The nodes move according to the Random WayPoint model. The speed of the nodes varies from a not-null minimum speed (1 m/s), as recommended in [16], to a maximum speed. The maximum speed of the nodes is varied (from 1 m/s to 10 m/s) in order to analyze the proposal under different mobility conditions. For every possible maximum speed, 10 runs are executed and the mean value of the results is used for the representations. The pause time is fixed to 0 s. Concerning the data traffic, the Access Router receives a data packet to a MANET node every second. The destination of the packets is fixed for every simulation. One simulation lasts 1000 seconds. The gateways are selected following the mobile multi-

5 11ª Conferência sobre Redes de Computadores 5 Number of messages gateway support. The gateway functionalities are transferred among the MANET nodes in an optimized way as in [13]. In the first set of experiments, the correctness of the proposal was verified. In this sense, for the different mobility conditions and the different probability of loss, it was checked that the AR could always discover the link layer address of the MANET node destination (bind to the opportunistic gateway or to the node itself). In scenarios with probability of loss, the AR may be obliged to send the Neighbor Solicitation message multiple times. In the second set of experiments, the costs of the proposal are quantified. Towards this goal, we have employed the number of messages that the proposal produces. The Proxy Advertisement messages, the Neighbor Advertisements and the Neighbor Solicitations are counted for this purpose. Figure 2 shows this metric for different mobility conditions and different probability of loss loss probability = 0 loss probability = 0.1 loss probability = Maximum Speed (m/s) Fig. 2. Number of messages provoked by the proposal. As can be observed, increasing the speed of the nodes leads to an increment in the number of changes in the Neighbor Cache. The updates of this structure is achieved by means of NDP messages so the number of messages that the proposal injects in the network also increments. On the other hand, the probability of loss also impact on this metric. Including a not-null probability of loss implies that the information contained in the Neighbor Cache is valid for a shorter period than in idealistic propagation conditions. However, by retransmitting some messages, the proposal is able to make the AR work with dynamic gateways. Considering the number of messages per change in the NeighborCache, it was checked that the mean value of is figure was lower than 3 for idealistic conditions. When a probability of loss is applied, the number of messages necessary to operate increases. The repetition of messages is responsible for this increment. VI. CONCLUSIONS Connection of MANET to IP-based access networks gains reliability and flexibility when opportunistic gateways are used. Opportunistic gateways are nodes in the MANET that configure themselves as gateway on behalf of other nodes. Despite of the benefits associated to the use of opportunistic gateways, there is not any specification about how the AR becomes aware of which node is playing the role of gateway. The automatic use of opportunistic gateways is especially vital in scenarios where the gateway functionalities are frequently transferred among the MANET nodes. This paper proposes a new and simple scheme by which opportunistic gateways are used in a seamless and transparent way in Internet-connected MANETs. The analysis of the possible situations where the proposal is executed demonstrates its correctness. By means of simulations, the costs associated to the use of the proposal are quantified. ACKNOWLEDGMENT This work has been partially supported by Project No. TEC C REFERENCES [1] Kaaranen, H., Ahtiainen, A., Laitinen, L., Naghian and S., Niemi, V UMTS Networks: Architecture, Mobility and Services. Wiley Press. [2] Narten, T., Nordmark, E., Simpson, W. and Soliman, H., Neighbor Discovery for IP version 6 (IPv6). IETF RFC [3] Thomson, S., Narten and Jinmei, T IPv6 Stateless Address Autoconfiguration. IETF RFC [4] Hinden, R. and Deering, S IP Version 6 Addressing Architecture. IETF RFC [5] Royer, E. M. and Toh, C-K A Review of Current Routing Protocols for Ad Hoc Mobile Wireless Networks. 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6 11ª Conferência sobre Redes de Computadores 6 [15] [16] Yoon, J., Liu, M., Noble, B.: Random WayPoint considered harmful. In: 22nd Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM), pp , San Francisco (2003)