Combining ECN and RTT for Datacenter Transport

Datacenter transports should provide low average and tail flow completion times (FCT) to achieve desired application performance. While most prior datacenter transports take either ECN or RTT as congestion signal, this paper makes a case that both signals are indispensable: ECN, as a per-hop signal, is more effective to prevent packet loss; while RTT, as an end-to-end signal, controls end-to-end queueing delay better. As persistent low flow completion times imply low queueing delay and near zero packet loss, we introduce EAR, a new datacenter transport that hears and reacts to both ECN and RTT. Our preliminary results show that: 1) compared to delay-based DCTCP, EAR achieves up to 91% lower packet losses and 93% fewer timeouts; 2) compared to ECN-based DCTCP, EAR reduces RTT by up to 32% for cross-rack traffic in a 4-level fattree. As a result, EAR delivers persistent low average and tail completion times under various scenarios in large scale simulations.