Cooperative Token-Ring Scheduling for Input-Queued Switches

We present a novel distributed scheduling paradigm for Internet routers with input-queued (IQ) switches, called cooperative token ring (CTR), that provides significant performance improvement over existing scheduling schemes with comparable complexity. Many classical schedulers for IQ switches employ round-robin arbiters, which can be viewed as noncooperative token rings, where a separate token is used to resolve contention for each shared resource (e. g., an output port), and each input port acquires a token oblivious of the state of other input ports. Although classical round-robin scheduling schemes achieve fairness and high throughput for uniform traffic, under nonuniform traffic, the performance degrades significantly. We show that by using a simple cooperative mechanism between the otherwise noncooperative arbiters, the performance can be significantly improved. The CTR scheduler dynamically adapts to nonuniform traffic patterns and achieves essentially 100 percent throughput. In addition, our proposed CTR scheduling paradigm can amortize the arbitration time over multiple time slots such that tokens are exchanged only on an as-needed basis. The proposed cooperative mechanism is conceptually simple and is supported by experimental results. To provide adequate support for rate guarantees in IQ switches, we present a weighted CTR, a simple hierarchical scheduling mechanism. Finally, we analyze the hardware complexity introduced by the cooperative mechanism and describe an optimal hardware implementation for an N x N switch with a time complexity of Theta(logN) and a circuit size of Theta(N logN) per port.