A fundamental problem in peer-to-peer streaming is how to select peers with desired media data so that the best possible streaming quality can be maintained. In this paper, we propose an outbound bandwidth based streaming model in which peers are layered according to their offered outbound bandwidth and are permitted to request data of peers from upper layers peers only. Based on the layered approach, a media data assignment algorithm for the subset of media data is presented to select qualified sending peers to ensure that they will be received before their scheduled playback time. We also present two resolutions for request conflicts, which arise when there are more than one peer simultaneously requesting data from the same sending peer that can?t afford outbound bandwidth for all requests. We evaluated the proposed streaming model through simulations. Experimental results show that streaming quality of the proposed streaming model is excellent and the properties of scalability as well as robustness are obtained even in a highly dynamic environment where peers join and leave frequently.