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Wireless mesh networks (WMNs) have recently emerged to be a cost-effective solution to support large-scale wireless Internet access. Handoff management plays an important role in WMNs in delivering Quality of Service to mobile users. In this paper, the handoff support in Internet-based infrastructure WMNs is investigated. Previous solutions on handoff management in infrastructure WMNs mainly focus on intragateway mobility (e.g., single gateway is assumed in IEEE 802.11s WMNs). Furthermore, new handoff issues involved in intergateway mobility in WMNs (e.g., the network-layer handoff detection issue) have not been properly addressed. In this paper, a new architectural design, IMeX, is proposed to facilitate intergateway handoff management in Internet-based infrastructure WMNs. The proposed IMeX architecture is designed to specifically address the special handoff challenge in Internet-based WMNs. It can facilitate parallel executions of handoffs from multilayers, in conjunction with a data caching mechanism which guarantees minimum packet loss during handoffs. The required number and optimal placement of special mesh routers that form the IMeX architecture are modeled as a set covering problem which is solved based on a greedy algorithm. A comprehensive simulation study shows that the IMeX architecture enables fast handoffs and reestablishment of session communications in the intergateway mobility environment. Performance results demonstrate that the proposed IMeX architecture, with both the parallel handoff execution and data caching mechanism, outperforms the conventional WMN handoff schemes in terms of reducing handoff delays caused in multilayers and improving end-to-end performance of applications in WMNs.