DCQCN

Data Center Quantized Congestion Notification (DCQCN) is an end-to-end congestion control algorithm designed specifically for RoCEv2. It combines Explicit Congestion Notification (ECN) and PFC to support end-to-end lossless Ethernet. While PFC prevents packet loss at the link level, it cannot prevent congestion spreading across the network - DCQCN addresses this gap.

Switches use ECN to mark packets as they encounter congestion along their way (Congestion Point (CP)).

The receiving adapter sees the marking (Notification Point) and notifies the sender. The sending adapter (Reaction Point) responds by slowing the traffic.

The mechanism works as follows:

• Congestion Point (CP): Switches mark packets using ECN when congestion is detected.
• Notification Point: The receiving adapter detects ECN marks and signals the sender.
• Reaction Point: The sending adapter reduce its transmission rate in response.

The switch configuration for PFC and ECN thresholds must balance the requirements as follows:

• Ensure the PFC does not trigger early, which would suppress ECN marking.
• Ensure the PFC does not trigger late, which could result in packet drops.