A Fault-Tolerant Distributed Directory Service for Mobile Agents

Two major approaches have been proposed to route messages transparently to mobile agents. Either messages may be sent to a fixed “home agent”, which forwards them to mobile agents, or messages may follow forwarding pointers left by mobile agents. A home agent appears as a single point of failure: when it exhibits a failure, it becomes impossible to track mobile agents or to route messages to them. Similarly, the failure of a node containing a forwarding pointer may prevent the delivery of messages routed through that node; however, the distributed nature of the forwarding pointer approach offers opportunities to introduce redundancy in order to tolerate failures. In the context of a communication layer for mobile agents, we define a directory service as the component that tracks mobile agents’ positions; the information it provides may be used to route messages to mobile agents. We have designed a fault tolerant distributed directory service for mobile agents, which extends the forwarding pointer approach, by introducing redundancy in the information about agents’ locations. The correctness of the algorithm is established by proving two properties. The safety property ensures that the location returned by the directory service is the one occupied by the agent; the liveness property guarantees that location information gets updated as agents migrate. We have formalised the algorithm and derived a fully mechanical proof of its correctness using the proof assistant Coq; the complete source code of the proof is made available from the WWW.