Abstract
Mobile IP, which provides global fixed IP addresses to mobile hosts, is the current means to facilitate seamless roaming in wireless environment. As for a real communication environment, a mobile host may access the network via a number of communication devices. Unfortunately, roaming over a number of communication devices is not taken into account in mobile IP. In this paper, we propose an adaptive communication system based on mobile IP to solve this problem. Our proposed system is mainly to achieve seamless roaming service for a mobile host with multiple communication devices. The selection of communication devices can be based on environment constrain or based on user requirement. A prototype system is implemented and evaluated. Experimental results show that our system works well and very few overhead is introduced in mobile hosts.
Keywords : Wireless network, Mobile computing, seamless roaming.
1. Introduction
Recent advances in semiconductor and wireless communication technologies have made it possible to support computing activities over a mobile environment. A critical issue involving in mobile computing is to provide mobile hosts a dis-interrupt communication service while the user changes the mobile host’s point of attachment to the network. Mobile IP, defined by Internet Engineering Task Force (IETF), is the only current means to offer seamless roaming over the Internet [1][2][9]. It solves the roaming problems in network layer and targets to achieve application transparency and the possibility of seamless roaming. Mobile IP composes of three related functions, i.e. agent discovery, registration and tunneling, to achieve single IP address roaming for mobile hosts over Internet. The procedures of mobile IP are the discovery of mobile hosts, the registration of mobile hosts, and the packet delivery and routing to mobile hosts. Unfortunately, mobile IP only solves the roaming problem for the single point of attachment of a mobile host. In other words, mobile IP only considers a mobile host with a single communication device. For a real communication environment, a mobile host may have more than one communication device and the communication device may be changed due to two possible reasons. The first reason comes from that the mobile host may be forced to change its communication medium owing to environment constrains. For example, a mobile host may connect to Internet by wireless LAN in some place but it can access Internet by Ethernet in some other places without wireless LAN support. The second reason comes from that users may want to change the communication medium according to their requirements if there are several communication mediums available. The requirement can be specified by access cost, bandwidth, or a set of quality of service (QoS) parameters. Therefore, providing seamless roaming service for a mobile host with multiple communication devices is required intensively.
Previous studies have investigated seamless roaming technologies such as routing policies and the performance improvement of delivering packets over wireless/Internet interconnected network [7][10][11]. Some efforts are made to solve the triangular routing problem in mobile IP [3][4]. They tired to eliminate the home agent route while tunneling the packets to mobile hosts. To improve the TCP/IP performance over wireless network, Baker and Badrinath suggested using split model to isolate the troublesome introduced by wireless link characteristics [8]. IP packet caching involving IP-layer caching mechanism between wired and wireless link is another approach to isolate the different characteristics between wired and wireless links. TACO proposed by Hansen et al. is mainly to combine the direct and split transmission model for mobile host [6]. Based on TACO system, the transmission model will adapt for the current attached network of the mobile host. Although, previous research has explored issues involving seamless roam over wireless network, the roaming for one mobile host with multiple communication devices is lack of study. In this paper, we investigate the seamless roaming services over wireless and Internet interconnected network. We focus on providing adaptive and on-demand selection of communication mediums for a mobile host with multiple communication devices. We elaborate an adaptive communication system based on mobile IP to support seamless roaming over multiple communication devices.
The rest of the paper is organized as follows. Section 2 depicts the system environment for mixed-mode wired/wireless interconnected network. Section 3 presents our proposed adaptive communication system. Section 4 illustrates our implementations and experimental results. Finally, we conclude this paper in Section 5.
2. System Environment
Figure 1 depicts a typical environment for a wireless/Internet interconnected network. A mobile host accesses Internet via wireless network in its home network. On behalf of mobile IP, while the mobile host moves to foreign network via the same communication medium, it can be still located by using the same IP address. For a practical environment, it is desired to have roaming via different access mediums on the Internet. In Figure 1, we show that the mobile host may move to a place where only wired network is available, it is necessary for mobile host to access the Internet by using the same IP address via the wired network. Therefore, the first objective of our study is to provide the roaming services over different access mediums, which adapt for the environment constrains. For another scenario, the mobile host may move to a place, which more than one communication medium is available. In this situation, the user may want to choose one of the communication mediums to access network based on their requirements. The requirement can be specified by connection cost, network bandwidth or some other QoS parameters. For example, the user transfer a big file from Internet, he/she may choose wired network as communication medium for sake of high network bandwidth. On the other hand, if the user only requires a low network bandwidth such as E-mail, he/she may choose wireless network as the communication medium owing to the convenient access. Therefore, the second objective of this study is to provide on-demand selection of communication devices for seamless roaming. The demand can be specified by some pre-defined policies such as maximal bandwidth, minimal cost or minimal jitters. However, mobile IP can not support these features. In this paper, we study this problem and propose a novel communication system based on mobile IP to facilitate the seamless roaming over different communication mediums.
Figure 1 Environment of wireless/Internet interconnected network.
3. Design of an Adaptive Communication System
Our proposed adaptive communication system can be divided into a number of issues. We present the design philosophies in the following sections.
3.1 Adaptive communication system
Our system has several design goals. The first one is that a mobile host publishes a global fixed IP address to Internet hosts, and the IP address can support roaming over Internet and multiple devices. The second one is to offer the application transparency about the current active communication device. In other words, applications need not be modified and will not notice about the current communication device. The third one is that mobile hosts can change the communication devices dynamically according to the environment constrain. The last one is that users can specify the policies on selecting their communication devices if more than one communication medium is available. In order to change the point of attachment to the network via different communication devices dynamically, the network protocol stack of mobile hosts should be modified. We insert our proposed system in between network and medium access control layers. The adaptive communication system works as a network layer protocol. Figure 2 shows the network protocol stack after inserting our adaptive communication system.
Figure 2 Network protocol stack in mobile hosts
3.2 Virtual IP address and packet receiving/transmission
To facilitate roaming over a number of communication devices, we assign a shared IP address, called virtual IP, to all devices. In order words, each device has two IP addresses, one unique IP address and one shared IP address. Suppose we have N different devices, N+1 IP addresses are required to employ our scheme. Mobile IP can apply to both kinds of IP addresses, but the virtual IP is used to provide roaming over all network devices of a mobile host. Figure 3 shows the operations in adaptive communication system in more detail.
Figure 3 Receiving and dispatching of IP datagram on multiple devices
We will assign each device one IP address, and assign a virtual address to all of them. The packets to the unique IP address will be received and transmitted normally. As for the packets to the virtual IP, the adaptive communication layer will collect them and forward to upper layer, i.e. IP layer. As for transmission of the virtual IP packets, the adaptive communication layer will send to only one device according to the setting.
Basically, adaptive communication system performs two major functions, i.e. the incoming and outgoing packet dispatching. In Figure 3, suppose we have two different communication mediums such as ethernet and wireless LAN. We assign each device a unique IP address, and assign another IP address, i.e. virtual IP, to both of the devices. Packets to the unique IP will be received and delivered normally through the specified communication device.
3.3 Registration and selection of active communication device
We apply mobile IP to the virtual IP. Therefore, if the mobile host moves to a new network, the mobile IP will receive agent advertisement packets from agents. If only one device receives the agent advertisement packets, the mobile IP will perform the registration procedure. Then, the communication device is set as the active communication device. If adaptive communication system detects that more than one device can receive agent advertisement packets, it will prompt user to choose a proper device as the active one. Otherwise, the proposed adaptive communication system will determine a device based on predefined policy. The policy can be specified by maximal bandwidth oriented, minimal cost oriented, and etc.. Once the adaptive communication system determines the active communication devices and accomplishes the registration process, the virtual IP will receive packets via the active communication device.
3.4 MAC to IP mapping, update and MAC replacement in agent
The registration procedure is not only to sending mobile host information to agents but also to update MAC address to virtual IP address mapping maintaining in the agents. After the proper communication device is active, the adaptive communication system will receive and send the packet from/to the devices. Since our system locates in layer 3, i.e. network layer, our system need not do the frame fragmentation and reassembling. It is simple to forward packets and receive packets. If the user changes the communication device on-the-fly, the adaptive communication system will send the registration request again to the agent by using the new MAC address. Then, the MAC address to virtual IP mapping will enforce to update. After the MAC/IP mapping is updated, the packets will receive via the new active adapter. Our system does not process packet lost detection and re-transmission functions. All functions will forward to IP layer. Note that our system will guarantee only one device with virtual IP is active to send/receive packets. Moreover, the other IP addresses assigned still can support roaming if mobile IP is activated, but they have roaming service over a single device.
In order to support roaming over different communication devices, mobile IP is extended with some functions. Note that we do not insert new messages or protocol into mobile IP, we just point out the modification on the implementation of mobile IP to support such services. According to design of mobile IP, proxy ARP should perform in agents to response the current MAC address of the mobile hosts. If the mobile hosts move out from the home network, the home agent should reply the ARP with its MAC address. Then, the packets are tunneled to the foreign agents. According to our design, agents need to maintain a MAC to IP address mapping. While the foreign agent receives the tunneled packets, it re-packs the packets and forwards them to the mobile hosts. It is very important to notice that we will check the current MAC to virtual IP mapping while re-packing. If we find that the current MAC has been replaced, the MAC address in packets will be modified with the new MAC address. In that way, we can ensure that packets will be delivered to the right devices in the mobile host.
Another modifications of mobile IP are the response to agent advertisement packets, the time to send registration packets and the actions in agents while mobile hosts do the registration. The agent advertisement is a broadcast packet in the network so that every connected devices will receive the packets. However, only one device will reply the packets. The device is selected by our adaptive communication system either by user requirement or due to environment constrains. Once the agent receives the registration packets, it updates its MAC/IP mapping table. Other operations remain the same in mobile IP.
4. Implementation and Experimental Results
The experimental environment is depicted in Figure 4. Ethernet and wireless LAN are adopted as communication mediums. Detail software and hardware configurations are listed in Table 1. Our implementation is based on the mobile IP on Linux which was developed by [13]. Besides extending mobile IP to support multi-devices roaming, we also port the mobile IP program from Linux to Windows NT. We choose Windows NT as the platform for mobile host. The reason is that we are going to implement our adaptive communication system to Windows CE 2.1 which will be available around the end of the year. According to the announcement by Microsoft, the network structure of Windows CE 2.1 will follow that in Windows NT NDIS [12]. Therefore, we implement our system as an intermediate driver and the driver can be easily ported to Windows CE in the coming future. Figure 5 shows packet flows and operations in the network protocol stack in a mobile host. The intermediate driver works in the network layer and it is in charge of dispatching packets to the correct device. To evaluate the overhead introduced by our system, a benchmark program is developed to examine the overhead. The benchmark program is developed by UDP. Table 2 shows the experimental results. Results show that the overhead is quite low and around 3% overhead will be introduced if the driver is installed. We also develop a user-level control panel to configure the adaptive communication system in a very convenient way. The program can support the change of setting and the change of communication devices. Figures 6, 7, and 8 show the snapshoot of the adaptive communication system.
Porting Mobile IP ver1.00 [13] from Linux to Windows NT 4.0, we face some problems. First, the system services provided by the two Operating Systems are quite different. For example, “ioctl” in Linux is easily way to manipulate with underlying devices in Linux, but it is not true in Windows NT. Second, we can access the ARP cache table and routing table in Linux. However, it is not quite easy to achieve that in Windows NT. Therefore, we implement the modification and update of ARP cache table and default gateway information in our intermediate driver.
Figure 4 Experimental Environment
Figure 5 Implementation of our adaptive communication system based on Windows NT NDIS architecture.
Table 1 Experimental environment configurations
Items
Configurations
Operating system in mobile hosts
Windows NT 4.0 service packet 3.
Operating system in router and mobile IP agents
Linux 2.0.33.
Mobile IP
Dept. of Computer Science State University of New York Binghamton version 1.0[13].
Wireless LAN
Lucent WaveLAN.
Ethernet
NE-2000.
Table 2 Processing overhead introduced by the proposed adaptive communication system
Ethernet (IEEE 802.3)
IP datagram receiving (without adaptive communication system)
5.509 Mb/sec
IP datagram receiving (with adaptive communication system)
5.308 Mb/sec
Percentage of receiving overhead (%)
3.65
IP datagram transmission (without adaptive communication system)
8.776 Mb/sec
IP datagram transmission (with adaptive communication system)
8.424 Mb/sec
Percentage of transmission overhead (%)
4.001
Figure 6 Snapshoot of the active device controller (I)
Figure 7 Snapshoot of the active device controller (II)
Figure 8 Snapshoot of the active device controller (III)
5. Conclusions
In this paper, we investigated the problems of seamless roaming for a mobile host with multiple communication devices. We elaborated an adaptive communication system based on mobile IP to support roaming over different communication mediums. According to our design, the selection of communication devices can be either based on physical environment constrains or based on user specified selection policies. A prototype system is implemented to examine the correctness and practicability of our design. Experimental results show that less than 3% system overhead is introduced to employ our system.
References
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