Overlay Network for Transparent Mobility on WLAN
As wireless LAN (WLAN) becomes an integral component of an enterprise network, the ability to ensure continuous network connectivity as mobile nodes (MN) move around becomes critically important, especially in light of the emerging trend of running voice over IP applications on WLAN. On the other hand, there is a wide variety of WLAN-capable portable devices deployed within an enterprise such as laptops, PDAs, cellular phones, etc. How to support seamless host mobility across a large number and variety of mobile devices is a major challenge for IT staff.
Most production-mode wireless LANs operate in the infrastructure mode. In this mode, MNs must be associated with an access point (AP) before they can access the network. When a MN moves from the domain of one AP to that of another AP, it needs to disassociate with the first AP and then associate itself with the second AP. In IEEE 802.11 standard, the WLAN interface card performs this link-layer handoff in a way transparent to the systems software.
If the two APs that are involved in a link-layer handoff belong to the same IP subnet, network connections on the MN can proceed as before without invoking network-layer handoff. If the two APs that are involved in a link-layer handoff belong to different IP subnets, a full-scale network-layer handoff mechanism such as Mobile IP is required. Despite mature standard and stable prototype implementation, Mobile IP has not taken off as expected for the following reasons. First, the deployment of mobile agents is relatively cumbersome. Given the wide variety of mobile devices, such as PDAs, cell phones, and portable computers, it is not an easy task to support Mobile IP on all these different platforms. Second, the handoff latency of existing Mobile IP implementations is too high to be acceptable for interactive real-time communication applications such as Voice over IP. The application-visible handoff latency of the best Mobile IP implementation that is specifically optimized for WLAN is around 100 msec, about 20 msec of which is due to WLAN hardware link layer handoff latency. The handoff detection, authentication and tunnel setup accounts 80 msec latency overhead on LAN environment. In contrast, Voice over IP applications require a handoff latency of less than 50 msec.
This project proposes an Ethernet over IP overlay network architecture that can support low-latency network-layer handoff in WLAN-enhanced enterprise networks without requiring any additional software on the MNs. That is, when the proposed wireless overlay network is deployed on a WLAN-equipped building, a visitor with an arbitrary WLAN-capable PDA or portable computer can roam through the building and enjoy seamless network-layer handoff without installing any additional software. Because the proposed WON architecture can accommodate APs that belong to multiple IP subnets, it can completely replace Mobile IP in micro-mobility environments.
This project addressed the following issues of the overlay network:
l A proxy ARP and broadcast filtering module is developed for each AP to prevent broadcast storms on the overlay network.
l A shortcut virtual link mechanism is developed to eliminate triangle routing.
l A link-layer routing update mechanism is developed to facilitate link-layer routing state update after a handoff.
The proposed architecture exhibits several crucial advantages.
l First, being a layer-2 solution, it is completely transparent to the MNs and the remote nodes they are communicating with. Therefore no modifications are required at end hosts or layer-3 routers. This makes it a very practical solution to the enterprise WLAN mobility problem since it is quite easy and inexpensive to deploy in existing enterprise networks.
l Second, the handoff latency visible to user applications can be as small as the link-layer handoff latency that WLAN hardware supports directly. There is no network-layer handoff at all.
l Third, by channeling all WLAN traffic through a central point, it is possible to apply existing enterprise security solutions such as firewalls, intrusion detection systems, VPNs, etc., to WLAN traffic directly.
l Fanglu Guo and Tzi-cker Chiueh, ``Device-Transparent Network-Layer Fast Handoff for Intra-Domain Mobility,''
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