A ring purger for the FDDI token ring

The fiber distributed data interface (FDDI) is a 100 Mb/s token ring which includes extensive reliability and robustness mechanisms to provide fault detection, isolation, monitoring, and recovery functions. The authors examine two well-known problems on token rings and their potential impact on FDDI. The two problems are no-owner frame (NOF) and duplicate tokens. FDDI effectively resolves these problems only when either the NOF or the duplicate token collides into a transmitting station. The impact of NOFs and duplicate tokens during the period from the occurrence of the NOF and the duplicate token until they are removed by a transmitting station can be severe. This paper presents a ring purger that transparently and continuously removes the NOFs and detects and removes duplicate tokens. A ring purger is a designated station on a ring that performs its normal functions as a station and, in addition, performs the removal of NOFs and duplicate tokens. A distributed election algorithm is used to elect a single station on a given ring to be the ring purger. The authors also present performance and robustness considerations used in the design of the ring purger algorithms. They study the performance effect of continuously purging the ring and show that the effect is negligible even in the worst case