We discuss how mobile agents, moving in the Internet from node to node, can communicate with each other by forwarding messages in a robust way. As a solution, we present the general idea of a class of communication networks called the multi-path message forwarding networks (MMFNs), which can transmit the messages to mobile agents at the current location in a robust way based on multiple paths between the nodes and the target. The networks are formally defined in terms of graph theory, and the dynamic nature of the networks (i.e., how they evolve) is represented by a logical system, named Ln, consisting of six inference rules. It is shown that the system is sound in the sense that it generates only MMFNs. A computational interpretation of Ln is also discussed.