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On-demand and live multimedia streaming applications (such as Internet TV) are well known to utilize a significant amount of bandwidth from media streaming servers, especially as the number of participating peers in the streaming session scales up. To scale to higher bit rates of media streams and larger numbers of participating peers, overlay tree or mesh topologies are typically constructed, such that peers utilize their available upload capacities to alleviate the excessive bandwidth demands on stream servers. Previous works rely on random selections of upstream peers, without optimizing the topologies towards maximized utilization of peer upload bandwidth, and as a result, minimized bandwidth costs on streaming servers. We propose Outreach, a distributed algorithm to construct overlay topologies among participating peers in streaming sessions. The design objective of Outreach is to optimize the quality of overlay topologies towards scalability, with respect to the number of participating peers in the session. To be scalable, Outreach seeks to maximize the utilization of available upload bandwidth on each participating peer, and consequently minimize the total bandwidth costs on streaming servers. With analysis, we show that Outreach constructs topologies such that peers can fully utilize their upload capacities, and present a practical distributed algorithm. With simulation-based comparison studies, we show that Outreach effectively achieves its goals in a high-churn peer-to-peer network with an assortment of peer uplink capacities and link delays.