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Robustness to Interference

Relying on communication protocols involves the danger that an opponent could actively interfere by mimicking an agent's obsolete messages: since there is a single communication channel, opponents can hear and mimic messages intended for teammates. However, the <Encoded-stamp> field guards against such an attempt. As a test, we played a communicating team (team C) against a team that periodically repeats past opponent messages (team D). Team C set the <Encoded-stamp> field to <Uniform-number tex2html_wrap_inline1626 (send-time + 37). Thus teammates could determine send-time by inverting the same calculation (known to all through the locker-room agreement). Messages received more than a second after the send-time were disregarded. The one-second leeway accounts for the fact that teammates may have slightly different notions of the current global time.

In our experiment, agents from team D sent a total of 73 false messages over the course of a 5-minute game. Not knowing team C's locker-room agreement, they were unable to adjust the <Encoded-stamp> field appropriately. The number of team C agents hearing a false message ranged from 0 to 11, averaging 3.6. In all cases, each of the team C agents hearing the false message correctly ignored it. Only one message truly from a team C player was incorrectly ignored by team C players, due to the sending agent's internal clock temporarily diverging from the correct value by more than a second. Although it didn't happen in the experiment, it is also theoretically possible that an agent from team D could mimic a message within a second of the time that it was originally sent, thus causing it to be indistinguishable from valid messages. However, in this case, the content of the message is presumably still appropriate and consequently unlikely to confuse team C.

Peter Stone
Thu Dec 17 15:26:44 EST 1998