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Performance of TCP can be severely degraded in mobile IP-based wireless networks where packet losses not related to network congestion occur frequently during inter-subnetwork handoffs by user mobility. To solve such a problem in the networks using Mobile IP, the packet buffering method at a base station recovers those packets dropped during handoff by forwarding the buffered packets at the old base station to the mobile users. But, when the mobile user moves to a congested base station in a new foreign subnetwork, those buffered packets forwarded by the old base station are dropped and TCP transmission performance of a mobile user degrades severely. In this paper, a PBM (packet buffering marker) at a base station is proposed to prevent burst losses of out-of-profile packets during handoffs for DiffServ Assured Services. From simulation results, it is shown that PBM scheme improves TCP throughputs of an Assured Service flow during handoffs.