Peer-to-peer applications such as multiplayer online games are characterized by considering group communication among geographically distributed peers. In such environments, causal ordering is an essential property for consistent exchange of information among peers. Although several works are oriented to ensure message causal order, most of them are not suitable for hierarchical overlay networks. In this paper, we propose an efficient causal protocol oriented to be used in a hierarchical overlay network. In our protocol the overhead timestamp per message is based on the number of messages with immediate dependency relation. By using the information about network architecture and representing message dependencies on a bit level, proposed protocol ensures causal message ordering without enforcing super peers order to all of the peers in a group. The protocol has been simulated and the results show that it presents lower overhead than currently existing causal protocols.