DCQCN: definition · Chip Field Guide

Beginner

A set of rules network cards follow to slow down when switches say they’re crowded and speed up again when they’re not.

Novice

Data Center Quantized Congestion Notification, the common congestion-control scheme for RoCEv2. Switches mark packets with ECN; the receiving NIC sends a congestion notification packet back; the sending NIC cuts its rate, then raises it again in steps.

Expert

Zhu et al. 2015. CP: ECN marking via RED; NP: at most one CNP per flow per 50 µs; RP: $R_{\mathrm{C}} \leftarrow R_{\mathrm{C}}(1 - \alpha/2)$ , $\alpha \leftarrow (1 - g)\alpha + g$ on CNP, $\alpha$ decays without CNPs, then fast recovery toward $R_{\mathrm{T}}$ and additive increase. Rate-based, NIC-resident, and sensitive to its many parameters.