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Application-layer multicast (ALM) has grown tremendously in recent years, making the distribution of bulk data such as streaming video economically feasible for small companies and even individuals. The efficiency of an ALM network depends on its data distribution overlay, which is constructed based on metrics such as round-trip time (RTT) measurement between peers. However Internet measurement experiments revealed that RTT is far from an accurate estimator of bandwidth availability and as such, may lead to sub-optimal performance in the constructed ALM overlays. This paper tackles this problem by developing a new in-band bandwidth probing tool which can estimate the amount of achievable bandwidth available in the target network path so that excess data traffic can be diverted from the congested path without causing new congestion in the target path. Moreover, the probing tool does not incur any bandwidth overhead as it piggybacks on the existing data flow. Simulation results show that multioverlay ALM networks constructed based on achievable bandwidth consistently out-performs RTT-based and residual bandwidth-based approaches in terms of data delivery ratio and video playback continuity. Moreover, the proposed bandwidth probing tool can be implemented entirely within the application and thus can be readily incorporated into existing ALM protocols.