Mobile Internet Router for Multihop Ad hoc Networks Stefan Aust, Daniel Proetel, Carmelita Görg University of Bremen Department of Communication Networks Otto-Hahn-Allee, NW1, 28359 Bremen, Germany email: {aust|daf|cg}@comnets.uni-bremen.de
Abstract Future wireless ad hoc networks will use mobile routers to provide Internet connectivity to mobile ad hoc users. A mobile router also allows mobility of an entire ad hoc network, e.g. in plains, trains, cars, and other vehicles, where mobile users may use an Internet access within an ad hoc network domain. Mobile Internet access is also required for various mobile networks such as Personal Area Networks (PAN) and Body Area Networks (BAN). This paper describes the realization of a Mobile Internet Router and experimentally demonstrate the mobility of an entire multihop ad hoc network. The Mobile Internet Router must support so-called vertical handoffs for seamless mobility of an entire ad hoc network which roams between different access technologies. In this study, the concept design of a Mobile Internet Router is presented that uses Mobile IP for mobility support and connects a multihop ad hoc network (AODV) with wireless LAN (IEEE 802.11b) and a cellular system (GPRS). This paper examines the design of the proposed Mobile Internet Router and how it improves the routing performance of mobile ad hoc networking.
Keywords Mobile IP, ad hoc, vertical handoff, hotspots, AODV, WLAN, GPRS, PAN, BAN, NAT.
1. Introduction There is a demand of non infrastructure IP domains in the future to exchange data between mobile users. The interaction between mobile users can be provided by ad hoc protocols which allows IP connectivity in a limited domain without Internet access. The IETF specifies various ad hoc protocols where AODV (Ad hoc On-Demand Distance Vector) is one of the most appropriate canditate for ad hoc routing [1]. Nevertheless, there will be scenarios in the future where ad hoc users want an Internet access via rotuers to be online. For this case the entire ad hoc network can be attached via one router to the Internet where the router is part of the ad hoc domain. In scenairos where the ad hoc network is stationary located such as in trains, plains and busses, the router must have a wireless link to the Internet. Therefore, the router must implement mobility features to provide roaming features, so the entire ad hoc network will always be connected to the Internet via one mobile router. The router must implement some mobility functions, e.g. Mobile IP, which is an appropriate canditdate for mobility support in heterogeneous wireless networks [2], [3]. The mobile router is located inside the ad hoc network and allows Internet connectivity to every mobile ad hoc node. For instance, the mobile router can be placed inside a train where it provides Internet access to an ad hoc domain within a cabin. Mobile users who have mobile devices such as PDAs or tablet PCs may take advantage of this possibility to use the Internet.
Cornel Pampu Siemens AG Information & Communication Mobile Networks Siemensdamm 62, 13623 Berlin, Germany email:
[email protected] The Mobile Internet Router is the required entity to provide mobile Internet access to an entire ad hoc network. Therefore, the mobile ad hoc network must have a router to connect the Internet and it has to support Mobile IP features to provide mobility to the entire ad hoc network. In this study, the Mobile Internet Router is the interface between the network and an ad hoc network. The router implements Mobile IP features and acts as a Mobile Node which roams between heterogeneous access networks such as WLAN and GPRS. This roaming capability is required for the mobile router to be always connected either via WLAN in so-called hotspots or GPRS. WLAN access is required to be online when high data bandwidth is available as in hotspots. GPRS connectivity is required when an ad hoc network has to provide high mobility as in vehicles with high velocity. Therefore, GPRS offers high mobility and always on connectivity, but with low bandwidth. This can be complemented by WLAN access with high data bandwidth when the velocity of the vehicle is low. In the following the Mobile IP features and ad hoc capabilities are described in order to outline the concept design of the Mobile Internet Router which provides the mobility of an entire ad hoc network.
2. Mobile Internet Access with Mobile IP The Internet protocol was originally designed without any mobility support. The routing mechanisms of IP assume that a node (host or router) is attached to the Internet by the point indicated by its IP address. Should a mobile node change its point of attachment and move to a new location incompatible with its IP address, it would be unable to send or receive data traffic. Mobile IP (MIP) is an extension of the basic protocol design for mobile node support. MIP introduces three new network entities, namely the Home Agent (HA), Foreign Agent (FA), and Mobile Node (MN). Alternative MIP configurations, e.g. MIPv6, may omit the FA. Every mobile node is permanently allocated an IP address in its home network, which in turn is required to host a HA. When roaming, a mobile node acquires an additional temporary IP address (care-of address) associated with its current point of attachment. As a mobile node moves, it is required to register its care-of address with the HA, hence establishing a binding. The basic functionality of MIP resembles the post office forwarding service. For every registered Mobile Node the HA acts as a proxy in the home network, intercepts all incoming traffic and redirects it to the Mobile Node’s most recently registered care-of address [4]. IP was originally designed to interconnect heterogeneous wire lined networks. This integrating property of IP is inherited by its MIP extension that focuses on mobile environments. For IP and MIP the individual properties of the underlying network access technology that delivers the IP traffic is transparent. As such, the problem of integrating heterogeneous bearers is reduced into a matter of Internet routing.
3. Ad hoc Networks A mobile ad hoc network (MANET) is an autonomous, infrastructure less system of mobile nodes (simultaneously hosts and routers) connected by wireless links. The mobile nodes are free to move randomly and organise themselves arbitrarily. Thus, the topology of the network may change rapidly and unpredictably. Such a network may operate in a standalone fashion, or may be connected to the larger Internet. In the latter case, a mobile node that has been permanently allocated an IP address from within its home network will expect to be able to establish and maintain communications as if it where in its home network. Due to the inability of conventional routing protocols to adhere to ad hoc conditions several ad hoc routing protocols have been developed, like AODV (Ad hoc On-Demand Distance Vector), DSR (Dynamic Source Routing), DSDV (Destination Sequenced Distance Vector), and TORA (Temporally Ordered Routing Algorithm). Ad hoc protocols are divided in two main groups, namely • proactive ad hoc protocols • reactive ad hoc protocols Proactive (also known as table-driven) are routing protocols that attempt to maintain up-to-date routing information from each node to all other nodes in the network. These protocols require each node to maintain one or more tables to store routing information, and they respond to maintain a consistent network view. Reactive (also known as on-demand) routing protocols tend to create routes only when desired by the source node. This is achieved by initiating a route discovery process within the network. Once a route has been established, it is maintained by a route maintenance procedure until either the destination becomes inaccessible along every path from the source or until the route is no longer desired. The following paragraph contains the description how Mobile IP and the AODV ad hoc protocol have to be interconnected to realize a Mobile Internet Router.
4. Mobile Internet Router for Mobile Mutlihop Ad hoc Networks The purpose of this study is to experimentally demonstrate the mobility of an entire multihop ad hoc network. For this, the topology illustrated in figure 1 was constructed. The topology consists of a wireless connection between WLAN access point and the Mobile Internet Router and a GPRS connection. Thus, the Mobile Internet Router may switch between these two wireless access networks that depends on the coverage of the WLAN. If there is no WLAN coverage available the Mobile GPRS
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Figure 1: Mobile Internet Router for mobile ad hoc networks (system overview)
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Figure 2: Mobile Internet Router The Mobile Internet Router consists of a Mobile IP mobile node and uses a Mobile IP infrastructure. Furthermore, the Mobile Internet Router uses an AODV ad hoc implementation and supports gateway discovery. At last the Mobile Internet Router also contains generic router features to act as an interface between Mobile IP infrastructure/mobile node and the ad hoc network. The router supports routing tables and routing entries to provide routing information to MIP and AODV. 4.1. Internet connectivity via Mobile Node The Mobile Internet Router uses a Mobile IP infrastructure to provide Internet connectivity to the mobile ad hoc network. The Mobile IP infrastructure and the setup for the Mobile Internet Router are depicted in figure 3. In figure 3 the Mobile IP infrastructure is connected to the Mobile Internet Router where a Mobile Node is installed
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Internet Router forces a Mobile IP handoff to the GPRS network, and vice versa. In combination with the ad hoc network the Mobile Internet Router supports seamless mobility to this ad hoc network and provides mobile Internet connectivity. Actually most of the mobile operators use private IP addresses for mobile GPRS clients which are not routable for Mobile IP [5]. To solve such NAT (Network Address Translation) problems the UDP tunnelling is used, which is described in [6]. For the ad hoc network the AODV implementation [7] is used which provides multihop ad hoc communication. To provide seamless and mobile Internet connectivity to mobile ad hoc networks the Mobile Internet Router consists of three modules: • Mobile Node/Mobile IP for mobile Internet access • Router features/Interface between Mobile IP and ad hoc protocol • AODV ad hoc protocol integration The three modules of the Mobile Internet Router are depicted in figure 2 and allow mobile Internet access of an entire ad hoc network.
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Figure 3: Mobile Internet Router – Mobile IP infrastructure
which provides mobility capabilities in terms of roaming between different bearer access networks. The Mobile IP infrastructure consists of a Home Agent (HA), Correspondent Node (CN), Foreign Agent (FA), and the Mobile Node (MN). The Home Agent manages the bindings that are sent from the Mobile Node to transmit the current point of attachment. After the Mobile Node changes its point of attachment the Mobile Node sends a binding update to the home Agent. Hence, the Home Agent manages the current binding the Mobile Node is reachable from the Internet after it changes the point of attachment. There are two different Foreign Agents depicted in figure 3. There is one Foreign Agent which is connected directly to the Internet and to a WLAN (IEEE 802.11b) access point. This Foreign Agent is required to receive the agent advertisements when the Mobile Node moves from its home network to a foreign network. The second Foreign Agent manages the required tunnel mechanisms between the Home Agent and the Foreign Agent that is necessary to establish a communication via GPRS networks. This allows the Mobile Internet Router to receive Mobile IP agent advertisements when the GPRS link is established and the Mobile Node moves between GPRS and WLAN coverage.
the Uppsala University (AODV-UU) is used [7]. The AODV-UU implementation allows gateway discovery that is required for Internet connectivity for ad hoc nodes. The ad hoc nodes start a gateway discovery and the gateway is used by the ad hoc nodes as a default gateway to the Internet. In figure 5 the complete setup of the Mobile Internet Router is presented. The figure shows the Mobile Internet Router in combination with the additional Foreign Agent to estabilsh tunneling capabilites via the GPRS network. Furthermore, the Mobile Internet Router is connected to four solid state PCs and an aditional PDA (Compaq iPAQ) is available which uses Linux OS and contains the AODV implementation.
5. Results At first, the routing performance of the ad hoc network is evaluated. The ad hoc network consists of five ad hoc nodes which uses the AODV-UU implementation from Uppsala University. The performance evaluation is divided into two different measurements. The first measurement is based on an increasing number of ad hoc nodes and starts with two ad hoc nodes and ends with five ad hoc nodes. The second ad hoc RTT eines ICMP-Datenpakets mit 1400 Byte Nutzlast Mittelwert aus 128 Messungen mit Zeitsyncronisation 60
4.2. Internet Connectivity via Ad Hoc Network The Mobile Internet Router also provides ad hoc connectivity. Therefore, it implements the AODV ad hoc routing protocol. Figure 4 shows the ad hoc integration that is used by the Mobile Internet Router.
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Figure 4: Mobile Internet Router – ad hoc integration In figure 4 an ad hoc network that consists of four different ad hoc nodes is depicted. The ad hoc nodes use the AODV ad hoc protocol for infrastructure less connection between the nodes. To provide Internet connectivity to the ad hoc network the nodes may use the Mobile Internet Router as a default gateway to the Internet. For AODV support the implementation from
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Figure 6: RTT of ICMP packets (1-4 hops) measurement is based on a decreasing number of ad hoc nodes, starts with five ad hoc nodes and ends with two ad hoc nodes. The experiments were performed in a lab environment at the University of Bremen. In the lab all ad hoc nodes were located on the same desk and connectivity was controlled using iptables for MAC filtering. This setup allows to simulate mobility of ad hoc nodes by instantaneously changing RTT eines ICMP-Datenpakets mit 1400 Byte Nutzlast Mittelwert aus 128 Messungen bei besserer Zeitsyncronisation 300
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Figure 8: GPRS/WLAN handoff for one single ad hoc node (active route timeout, 3 seconds) connectivity between ad hoc nodes. For the measurements ICMP packets are used with a packet size of 1400 bytes. In figure 6 the RTT (Round Trip Time) of an increasing number of ad hoc nodes can be observed. The first hop between two ad hoc networks has an RTT of 8 ms. Every 60 seconds the number of ad hoc nodes increases by one node. It can be observed that the RTT is increasingly high (peak) after the number of nodes increases. This is due to the fact of route discovery using the incoming node for the ad hoc communication. In figure 7 the RTT of a decreasing number of ad hoc nodes is shown. The four hops between five ad hoc nodes have the RTT of 30 ms. Every 60 seconds the number of ad hoc nodes decreases by one node. It can be observed that the peak after the number of nodes decreases is not so as high as the peak that can be observed by an increasing number of nodes. To investigate the routing of the Mobile Internet Router the setup for the measurement was modified and consists of the RTT eines Ping Paketes bei Übergabe WLAN GPRS Nutzlast 1400 Byte und Active Route Timeout 6 Sekunden 100000
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Figure 9: GPRS/WLAN handoff for one single ad hoc node (active route timeout, 6 seconds)
Mobile Internet Router and on ad hoc node. In figure 8 the RTT of one single ad hoc node is measured while frequent Mobile IP handoffs between GPRS and WLAN occur. This figure shows the results of handoffs between WLAN and GPRS every 60 seconds. The figure shows three handoffs of the Mobile Internet Router between WLAN (low RTT, 20 ms) and GPRS (high RTT, 3 sec.). In figure 8 it can be observed that route timeouts are the reason for high RTT. The parameter for Active Route Timeout is defined in the AODV RFC 3561 to 3 seconds which was used for this measurement setup. In figure 9 the RTT is presented which is measured based on an active route timeout of 6 seconds. The figure 9 shows that an Active Route Timeout more than 3 seconds may be more suitable for mobile ad hoc Internet connectivity and also suitable for the proposed Mobile Internet Router in this study.
6. Conclusion This paper presented the realization of a Mobile Internet Router which connects an entire ad hoc network to the Internet using Mobile IP. The Mobile Internet Router supports socalled vertical handoffs for seamless mobility of an entire ad hoc network which roams between different access technologies. This study described the concept design of a Mobile Internet Router which connects an entire AODV ad hoc network with IEEE 802.11b and GPRS for global mobility. This paper examined the design of the proposed Mobile Internet Router and evaluated the performance of the Mobile Internet Router for multihop ad hoc networks.
7. Acknowledgements This work was done within the framework of the ‘IPonAir’ project which is partly funded by the German Ministry of Education and Research (BMB+F). http://www.iponair.de
8. References [1] Charles E. Perkins, Elizabeth M. Belding-Royer, and Samir R. Das. Ad hoc On-Demand Distance Vector (AODV) Routing, RFC 3561, July 2003. [2] Charles E. Perkins, IP Mobility Support for IPv4, RFC 3344, IETF, August 2002. [3] Charles E. Perkins, Mobile IP, Design Principles and Practices. Wireless Communications Series. Addison-Wesley, 1997. [4] J. Solomon, Mobile IP: The Internet Unplugged. Prenticehall, 1998. [5] L. Phifer, The Trouble with NAT, The Internet Protocol Journal, Volume 3, Number 4, December 2000. [6] S. Aust, D. Proetel, A. Könsgen, C. Pampu, C Görg, Design Issues of Mobile IP Handoffs between GPRS Networks and WLAN Systems, Wireless Personal Multimedia Communications, October 2002. [7] Erik Nordstrom and Henrik Lundgren. AODV-UU Implementation from Uppsala University.
