  Communication in multi-agent reinforcement learning (MARL) has been proven to
effectively promote cooperation among agents recently. Since communication in
real-world scenarios is vulnerable to noises and adversarial attacks, it is
crucial to develop robust communicative MARL technique. However, existing
research in this domain has predominantly focused on passive defense
strategies, where agents receive all messages equally, making it hard to
balance performance and robustness. We propose an active defense strategy,
where agents automatically reduce the impact of potentially harmful messages on
the final decision. There are two challenges to implement this strategy, that
are defining unreliable messages and adjusting the unreliable messages' impact
on the final decision properly. To address them, we design an Active Defense
Multi-Agent Communication framework (ADMAC), which estimates the reliability of
received messages and adjusts their impact on the final decision accordingly
with the help of a decomposable decision structure. The superiority of ADMAC
over existing methods is validated by experiments in three
communication-critical tasks under four types of attacks.
