Performance of i-SLIP scheduling with large round-trip latency

As packet switches continue to grow in physical size, the round-trip latency between line cards and switch core has increased significantly. We study the impact of a large round-trip latency on the performance of input-queued packet switches. We find that the implementation of the control-path protocol between virtual output queues and scheduler is crucial for performance. We show that performance degrades sharply under non-uniform traffic with a stateless protocol. We address this problem with a stateful protocol in which the scheduler maintains the state of the virtual output queues. With this protocol, the throughput performance is no longer affected by the round-trip latency, although the minimum packet delay is twice that of switches with a buffered core.