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A great variety of todays networked applications require a reliable
multicast service. A number of the proposed reliable multicast
protocols use a positive acknowledgment scheme, which returns ACKs
to the sender to confirm correct delivery. To avoid the well-known
implosion problem in the case of large receiver groups, often a
tree-based approach is used, i.e., receivers are organized in a tree
and ACK messages are passed along the edges of this so-called ACK
tree. For building up this tree variations of the Expanding Ring
Search (ERS) scheme have been proposed. However, our simulations
show that ERS scales poorly. In this paper, we propose an
alternative scheme for building up ACK trees. This scheme is based
on a so-called token repository service, where a token represents
the right to connect to a certain node in the corresponding ACK
tree. Nodes that want to join a group just request a token for this
group from the (distributed) token repository service. Our
simulations show that our scheme causes a much lower message
overhead than ERS. Moreover, the quality of the resulting ACK trees
in terms of delay and reliability is in many cases higher if
generated with our scheme
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