Testing peer-to-peer (P2P) systems is difficult because of the high numbers of nodes which can be heterogeneous and volatile. A test case may be composed of several ordered actions that may be executed on different nodes. To ensure action ordering and the correct behavior of the test case, a synchronization mechanism is required. In this paper, we propose a synchronization algorithm for executing test case actions in P2P systems. The main goal of the algorithm is to progressively dispatch the actions of a test case to a set of nodes and ensure that all nodes completed the execution of an action before dispatching the next one. We validated our synchronization algorithm through implementation and experimentation on an open-source P2P system. The experimentation shows how the algorithm was able to detect implementation problems on the P2P system.