In this paper we present OPSS, an Overlay Peer-to-peer Streaming Simulator designed to simulate a large scale (i.e. in the order of 100K nodes) peer-to-peer streaming systems. OPSS is able to simulate a fair (i.e.“TCP-like”) sharing of the uplink and downlink bandwidth among different connections, and it guarantees extensibility by allowing the implementation of different peer-to-peer streaming algorithms as separate modules. Therefore it allows to simulate the behavior of arbitrary tree-based or mesh-based approaches. In particular, we implemented two trivial tree-based and mesh-based approaches for which we could easily find an analytic model of chunk distribution delay. The results of the model have been compared with the simulation output, showing an excellent fit. Source code of OPSS is available under the GPL license1 Copyright 2007 ICST.