To avoid the well-known implosion problem, the majority of reliable multicast protocols use hierarchical structures to pass acknowledgment messages back to the sender. In most cases, a technique called expanding ring search (ERS) is used to construct the acknowledgment tree. In this paper we analyse ERS by simulation studies. ERS is a simple and fault tolerant approach, but our simulation results show that it has disadvantages like great reliance on the multicast routing protocol and poor scalability. If the background load exceeds a critical level, the message overhead rises exponentially. In this paper, we will present an alternative approach based on a so-called token repository service. The token repository service stores a token for each successor, a node in an ACK tree can accept. To find a node to connect to, the searching node just retrieves a token from the token repository. We describe how such a service can be implemented in a way that it can handle a large number of multicast groups. Our simulation results show that the token repository service is scalable, independent of the multicast routing protocol and builds well-shaped ACK trees, causing message delays that are comparable to ERS or even lower.