  We examine the problem of rendezvous, i.e., having multiple mobile agents
gather in a single node of the network. Unlike previous studies, we need to
achieve rendezvous in presence of a very powerful adversary, a malicious agent
that moves through the network and tries to block the honest agents and
prevents them from gathering. The malicious agent is assumed to be arbitrarily
fast, has full knowledge of the network and it cannot be exterminated by the
honest agents. On the other hand, the honest agents are assumed to be quite
weak: They are asynchronous and anonymous, they have only finite memory, they
have no prior knowledge of the network and they can communicate with the other
agents only when they meet at a node. Can the honest agents achieve rendezvous
starting from an arbitrary configuration in spite of the malicious agent? We
present some necessary conditions for solving rendezvous in spite of the
malicious agent in arbitrary networks. We then focus on the ring and mesh
topologies and provide algorithms to solve rendezvous. For ring networks, our
algorithms solve rendezvous in all feasible instances of the problem, while we
show that rendezvous is impossible for an even number of agents in unoriented
rings. For the oriented mesh networks, we prove that the problem can be solved
when the honest agents initially form a connected configuration without holes
if and only if they can see which are the occupied nodes within a two-hops
distance. To the best of our knowledge, this is the first attempt to study such
a powerful and mobile fault model, in the context of mobile agents. Our model
lies between the more powerful but static fault model of black holes (which can
even destroy the agents), and the less powerful but mobile fault model of
Byzantine agents (which can only imitate the honest agents but can neither harm
nor stop them).
