Congestion Avoidance in AI Fabric – Part II: Priority Flow Control (PFC)
Priority Flow Control (PFC) is a mechanism designed to prevent packet loss during network congestion by pausing traffic selectively based on priority levels. While the original IEEE 802.1Qbb standard operates at Layer 2, using the Priority Code Point (PCP) field in Ethernet headers, AI Fabrics rely on Layer 3 forwarding, where traditional Layer 2-based PFC is no longer applicable. To extend lossless behavior across routed (Layer 3) networks, DSCP-based PFC is used.
In DSCP-based PFC, the Differentiated Services Code Point (DSCP) field in the IP header identifies the traffic class or priority. Switches map specific DSCP values to internal traffic classes and queues. If congestion occurs on an ingress interface and a particular priority queue fills beyond a threshold, the switch can send a PFC pause frame back to the sender switch, instructing it to temporarily stop sending traffic of that class—just as in Layer 2 PFC, but now triggered based on Layer 3 classifications.
This behavior differs from Explicit Congestion Notification (ECN), which operates at Layer 3 as well but signals congestion by marking packets instead of stopping traffic. ECN acts on the egress port, informing the receiver to notify the sender to reduce the transmission rate over Continue reading