Many new applications like groupware systems, news and file distribution or audio/video systems are based on multicast as a prerequisite for scalability. Many of these applications need the multicast support to be reliable, which is realized in a scalable way by tree based multicast transport protocols, where the receivers are organized in a ACK tree. Tree based approaches raise the problem of setting up and maintaining the ACK tree, which is usually done by variations of the expanding ring search (ERS) approach. In this paper we present an alternative approach for creating ACK trees which is based on the concept of a distributed token repository service. Simulation results show that our approach leads to a lower message overhead than ERS and results in better shaped ACK trees.