Many reliable multicast protocols use so-called ACK-trees to avoid the
well-known acknowledgment implosion problem in case of large multicast groups.
For constructing ACK trees, usually expanding ring search techniques are
applied. Our simulation results show that those techniques have scalability
problems itself. In this paper, we propose a novel approach for building ACK
trees, the token repository service (TRS). The basic concept of our approach is
a token, which represents the right to connect to a certain node in the
corresponding ACK tree. For each node in the ACK tree TRS stores a token for
each potential successor this node can accept. A node that wants to join a
group requests TRS for an appropriate token. The TRS protocol described in this
paper provides for height-balanced ACK trees. Our simulation results show that
the created height-balanced ACK trees have significant benefits. They reduce
round trip delay and optimize reliability in case of node failures. Moreover,
compared to expanding ring search, TRS results in a much lower message
overhead.