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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.
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