In the acyclic peer-to-peer architecture, servers communicate with each other symmetrically as peers, adopting a protocol that allows a bidirectional flow of subscriptions, advertisements, and notifications. Hence we use an undirected graph to represent the topology of this architecture. (As always, the external clients of the service use the standard client/server protocol described in Sec- tion 3.) The configuration of the connections among servers in this architecture is restricted so that the topology forms an acyclic undirected graph. Figure 6 shows an acyclic peer-to-peer architecture of servers. The communication be- tween servers is represented by thick undirected lines, while the communica- tion between clients and servers is represented by.
