An Inter-domain Routing Protocol for Multi-homedWireless Mesh NetworksYair Amir, Claudiu Danilov, Raluca Mus˘aloiu-Elefteri, Nilo Rivera{yairamir, claudiu, ralucam, nrivera}@dsn.jhu.eduDepartment of Computer ScienceJohns Hopkins UniversityTechnical Report CNDS-2006-1 - July 2006http://www.dsn.jhu.eduAbstract— This paper presents a routing protocol for multi-homed wireless mesh networks that provide uninterrupted con-nectivity and fast handoff. Our approach integrates wirelessand wired connectivity, using multicast groups to coordinatedecisions and seamlessly transfer connections between severalInternet gateways as mobile clients move between access points.The protocol optimizes the use of the wireless medium by short-cutting wireless hops through wired connections, paying a verylow overhead during handoffs. The paper demonstrates thatinter-domain handoffs occur instantaneously, with virtually noloss or delay, for both TCP and UDP connections.I. INTRODUCTIONWireless mesh networks extend the connectivity range ofmobile devices by using multiple access points, some of themconnected to the Internet, to create a mesh topology andforward packets over multiple wireless hops. Mobile clientscan freely roam within the area covered by the mesh accesspoints and maintain their connectivity at all times.As the size of a wireless mesh network increases, the num-ber of Internet connected access points (Internet gateways)needs to increase to disperse traffic and avoid congestion. Inpractice, Internet gateways will reside at different locationsand will often be connected to different network domains.We refer to such mesh networks as multi-homed. In this typeof networks, a mobile client is served by a nearby access pointthat forwards data packets (potentially over multiple wirelesshops) to its closest Internet gateway.Multi-homing poses a challenge in providing continuousconnectivity to mobile clients that may move between theareas covered by different access points. Those access pointswill often have different Internet gateways closest to them.When such a transition (handoff ) occurs, we would like tomaintain all previously opened connections, and transfer themto the new Internet gateway as quickly as possible, withoutany involvement from the mobile device.This paper presents a simple and elegant architecture thatsupports seamless routing in multi-homed wireless mesh net-works. The routing protocol integrates wired and wirelesscommunication and optimizes performance of the hybrid rout-ing, in our case by minimizing the usage of wireless transmis-sions. The handoff between Internet gateways is completelytransparent to the mobile devices, which have no indicationof when, or whether a handoff takes place at all, and is fastenough for real-time applications, such as VoIP, where anyinterruption in connectivity can have an adverse impact onthe service quality.As a mobile device moves, a handoff between access pointsis required at two levels. An intra-domain handoff transfersthe connectivity between access points to which the mobiledevice is directly connected. At a higher level, an inter-domainhandoff between Internet gateways is required when the clientconnects to an access point that is closer to a different Internetgateway. While solutions exist for intra-domain handoff [1],[2], [3], we believe that currently there are no efficient andtransparent protocols for inter-domain handoff in multi-homedwireless mesh networks. The routing protocol presented inthis paper addresses the inter-domain handoff, providingfast handoff, and seamless connectivity as mobile devicesmove between Internet gateways located in different networkdomains.In our approach, new connections always use the closestInternet gateway at the time of their creation, while existingconnections are forwarded through the wired infrastructure tothe Internet gateway where they were originally initiated. Asthe handoff process requires routing agreement and transfer-ring connections between the involved Internet gateways, ourprotocol guarantees that packets are routed correctly, at alltimes.We implemented our protocol, extending our SMesh system[1] to support optimized hybrid wireless-wired routing andfast inter-domain handoff. SMesh is a seamless wireless meshnetwork that provides intra-domain handoff with real-timeperformance. Fast intra-domain handoff in SMesh is achievedby using multicast groups joined by access points in thevicinity of each client. These multicast groups are used forlocal coordination between access points that can potentiallyserve the mobile device, and for forwarding data packets tothe client through multiple paths in periods of instability,until a designated access point is elected, thus providinguninterrupted service.We believe that the combination of the inter-domain routingprotocol presented in this paper, and the intra-domain connec-tivity provided by SMesh, realizes the first complete multi-homed wireless mesh network that is transparent to mobiledevices, and provides a fast intra- and inter-domain handoffsuitable for real-time applications such as VoIP. The system iscurrently deployed over three buildings at the Johns HopkinsUniversity campus, is used by several students and facultyon a daily basis, and the software is freely available to thecommunity.The main contributions of this paper are:• A simple and practical architecture that integrates seam-lessly wired and wireless connectivity in multi-homedwireless mesh networks.• A hybrid routing protocol for wireless mesh commu-nication that optimizes routes as mobile devices movebetween Internet gateways.• A fast inter-domain handoff protocol for multi-homedwireless mesh networks that supports real-time applica-tions such as VoIP.The rest of the paper is organized as follows: Section IIpresents related work. In Section III we describe the ar-chitecture of our multi-homed mesh network approach, andin Section IV we present the real-time handoff protocolbetween the wired Internet gateways. Experimental results arepresented in Section V, and Section VI concludes the paper.II. RELATED WORKWe propose a mechanism for multi-homed wireless meshnetworks that optimizes routing and provide fast inter-domainhandoff between Internet connected access points potentiallyon different networks. As such, our work relates to mobility,wireless mesh networks and wireless handoff. Good surveysaddressing some of these areas were overviewed by Akyildizet al. in [4] and [5]. Note