Many new applications like groupware systems, news and file distribution or audio and video systems are based on multicast as a prerequisite for scalability. Many of these applications need a reliable multicast support, which is realized in a scalable way by tree based multicast transport protocols, where the receivers are organized in a so-called 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 building up ACK trees that is based on the concept of a distributed token repository service. Our analysis and simulations show that our approach leads to a significantly lower message overhead compared to ERS and results in better shaped ACK trees, which has a positive effect on reliability and delay.