Necessary and Sufficient Condition for Triggering ECN Before PFC in Shared Memory Switches

Flow control in a data center network (DCN) prevents packet losses by pausing transmissions from upstream switches, whereas congestion control prevents network overload by regulating traffic sources. For two widely deployed flow and congestion control algorithms, namely PFC and DCQCN, we derive a necessary and sufficient condition that ensures that congestion control mechanisms in a shared-memory switch are triggered before flow control. The condition creates an imbalance of buffer requirements at the ingress and egress of a switch. For fair queuing and priority scheduling at the egress, we present traffic scenarios that saturate the condition. The lack of such traffic scenarios for FIFO scheduling suggests that choosing appropriate schedulers may help reducing minimal buffer requirements.