Dynamic address autoconfiguration in hybrid ad hoc networks

Emilio Ancillotti, Raffaele Bruno, Marco Conti, Antonio Pinizzotto
2009 Pervasive and Mobile Computing  
The use of ad hoc networking technologies is emerging as a viable and cost-effective solution to extend the range of traditional wireless local area networks (WLANs). In these networks, mobile client traffic reaches the access points through multi-hop wireless paths that are established by using an ad hoc routing protocol. However, several technical challenges have to be faced in order to construct such an extended WLAN. For instance, traditional autoconfiguration protocols commonly used in
more » ... astructure-based WLANs, such as DHCP or Zeroconf, are not directly applicable in multi-hop wireless networks. To address this problem, in this paper we propose extensions to DHCP to enable the dynamic allocation of globally routable IPv4 addresses to mobile stations in hybrid ad hoc networks, which transparently integrate conventional wired technologies with wireless ad hoc networking technologies. Some of the attractive features of our solution are its ability to cope with node mobility, the introduction of negligible protocol overheads, and the use of legacy DHCP servers. We have implemented a prototype of our scheme, and tested its functionalities considering various topology layouts, network loads and mobility conditions. The experimental results show that our solution ensures short address configuration delays and low protocol overheads. 2 E. Ancillotti et al. / Pervasive and Mobile Computing ( ) - nearby access points, availability of a limited number of orthogonal non-interfering frequency channels, as well as cost and management overheads, limit the effectiveness of this alternative solution. To overcome the limitations of the above-discussed approaches, several authors have recently advocated a new architecture for WLANs, which integrates ad hoc networking technologies in the network infrastructure [2] [3] [4] [5] . Traditionally, mobile ad hoc networks (MANETs) are conceived as an isolated collection of mobile nodes connected together over a wireless medium, which self-organize into an autonomous multi-hop wireless network [6] . However, it is now recognized that the ad hoc networking paradigm can also be applied to infrastructure-based wireless networks, building a hybrid ad hoc network, and providing a flexible, robust and cost-effective increase of network coverage. Specifically, we envisage an extended WLAN in which static and mobile clients transparently communicate using traditional wired technologies or ad hoc networking technologies. Thus, the client traffic can be forwarded to the access points through multi-hop wireless paths established by using an ad hoc routing protocol [5] . It is important to underline that other classes of hybrid ad hoc networks have emerged from this vision, such as: Multi-hop Cellular Networks (MCN), which combine the features of cellular systems and ad hoc networks [7] , and mesh networks, which employ a multi-hop wireless backbone to provide Internet access to mobile users [8] . Several technical challenges have to be faced in order to construct such a hybrid ad hoc network because the characteristics of the ad hoc networking (e.g., multi-hop relaying, lack of a centralized administration, etc.) differ significantly from the conventional IP architecture. For instance, the address autoconfiguration protocols commonly used in infrastructure WLANs, such as the Dynamic Host Configuration Protocol (DHCP) [9] or the Zeroconf protocol [10], are not directly applicable in multi-hop wireless networks. However, a mobile device cannot participate in unicast communications until it has been assigned a free IP address and the corresponding subnet mask. It is evident that preconfiguration is impractical in mobile environments, as well as a violation of the self-organizing paradigm. Thus, an address autoconfiguration protocol is crucial to allow the dynamic and automatic allocation of unique IP addresses to mobile clients. To tackle this problem we propose extensions to DHCP to enable the automatic allocation of globally routable IPv4 addresses to mobile stations in the envisaged extended WLAN. 1 Important features of our proposed solution are the following: (i) it is a fully distributed and automatic scheme that does not maintain state information in the already configured nodes, (ii) it does not assume that the address allocation space is known a priori by the new nodes, (iii) it does not require changes of the legacy DHCP-server implementation, (iv) no DHCP servers are deployed in the ad hoc component of the extended WLAN (see Section 3 for a detailed description of the network architecture), (v) it is designed to efficiently cope with node mobility, and (vi) it generates negligible and controlled protocol overheads. Note that DHCP is usually considered not applicable to MANETs since, in case the DHCP server is running on a mobile node, the DHCP server might not be permanently reachable by all nodes. However, our solution is not affected by this problem, since new nodes communicate directly with the DHCP servers deployed on the wired part of the extended WLAN by exploiting the relay capabilities of already configured nodes. In principle, it may be argued that any other autoconfiguration protocol proposed for ad hoc networks might be also employed to assign a unique network-layer identifier to mobile stations in the envisaged extended WLAN. However, autoconfiguration protocols for MANETs are generally designed to select an identifier with a scope limited to the ad hoc network [12] . This approach is reasonable for stand-alone MANETs, which are not connected to external networks, but it introduces additional complexities once we permit the interconnection between ad hoc networks and the Internet. Specifically, if private IP addresses are used within the MANET, a network address translator (NAT) has to be implemented on each gateway to enable IP communications. Then, the NAT-based gateway translates the source private IP address of outgoing traffic with a globally valid IP address, which is routable on the Internet. However, recent studies have clearly demonstrated that NAT-based gateways are very inefficient when multi-homing (i.e., more than one gateway in the same MANET) is allowed and the network topology is highly dynamic [5, 13, 14] . On the contrary, in our previous paper [5] we have shown that the use of globally routable IP addresses in the ad hoc network permits to implement very efficient gateways that support transparent IP communications, even in highly mobile conditions. These observations motivate our efforts to use DHCP for assigning globally valid IP addresses also to ad hoc nodes. Note that an alternative approach to configure globally routable IP address would be to use a hardware-based addressing. In other words, a global network prefix may be assigned a priori to the ad hoc network, and the IP address is then completed using the node's unique hardware interface identifier. However, this approach requires additional features that are only available in IPv6. In addition, it is not always true that network interfaces have globally unique addresses, but violations of this assumption are possible. To verify if our scheme guarantees satisfactory configuration delays and an acceptable efficiency in terms of protocol overheads, we have implemented a fully operational prototype and we have tested its functionalities, taking into consideration various topology layouts, network loads and mobility conditions. Our experimental results show that: (i) even if the new client is several hops far from the DHCP server, and asymptotic TCP flows saturate the wireless links, the configuration delays are acceptable, and (ii) the protocol overheads are negligible even if node mobility interferes with the operations of the autoconfiguration protocol. The remaining of this paper is organized as follows. Section 2 outlines the related work on address autoconfiguration protocols for MANETs. In Section 3 we define the architecture of an extended WLAN. Section 4 briefly reviews the DHCP specification. The basic idea of the proposed solution is presented in Section 5, while the protocol details are described in Section 6. Section 7 presents the experimental evaluation, and Section 8 concludes the paper with final remarks.
doi:10.1016/j.pmcj.2008.09.008 fatcat:qacyabyelrdxheda3cmoxbsmae