The FDDI MAC meets self-stabilization

A self-stabilizing protocol is one that can continue to operate correctly after arbitrary corruption of protocol state variables and the injection of arbitrary messages. Self-stabilizing protocols are extremely robust and can be simpler than other protocols (because they use a uniform mechanism to deal with a large number of possible faults). We demonstrate these advantages by modifying the Fiber Distributed Data Interface Media Access Control (FDDI MAC), a widely used token-ring protocol, to make it self-stabilizing. Our modifications are simple: the major step is to add a token rotation counter and a phase bit that are carried in all messages. Not only is our modified protocol more robust (for example, our protocol recovers from multiple tokens in less than 5.75 ms, while the existing MAC might never recover), but it also recovers from lost tokens more quickly (0-0.36 ms versus 2.5-4.1 ms). We describe the modest changes made to the existing MAC state machine.