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Implementing Differentiated QCN in Congestion Management: Traffic Flows and Policing Techniques

This document discusses the implementation of Differentiated Quantized Congestion Notification (QCN) for managing traffic flows to storage. Two flows are mapped to the same Class of Service (CoS) with distinct Quality of Service (QoS) profiles. It analyzes congestion points and the role of policing in managing these flows. A comprehensive exploration is provided on setting thresholds for congestion queuing based on packet colors and configurations through VLAN tagging or DSCP marking. Key considerations regarding algorithm convergence and adherence to standards are also discussed.

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Implementing Differentiated QCN in Congestion Management: Traffic Flows and Policing Techniques

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  1. Differentiated QCN Marina Lipshteyn

  2. The Fabric Reaction Point 1 Congestion Point Flow 1 Flow 2 Reaction Point 2 Typical Use Case There are two “flows” of traffic to storage. Both flows are mapped to the same CoS / Traffic Priority in the egress. For example: the mapping is done by VLAN tag priority.

  3. Flows QoS profiles • Flow 1 has QoS profile: CIR 100Mb/s PIR 1Gb/s • Flow 2 has QoS profile: CIR 500Mb/s PIR 1Gb/s Both flows are injecting 1Gb/s

  4. The Fabric Reaction Point 1 Congestion Point Flow 1 Flow 2 Reaction Point 2 Policing Assume some of flow1 packets are colored yellow by a policer Flow2 packets are colored green by a policer Possibly only Reaction Point2 will get a CNM !

  5. Differentiated QCN at the Congestion Point At the Congestion Point define two thresholds for the queue: • The first threshold: When number of bytes in the queue exceeds it, send CNM ONLY IF the probed packet is YELLOW. • The second threshold: When number of bytes in the queue exceeds it, send CNM for any probed probed packet

  6. How to configure such a threshold? • Option 1: Classify through VLAN tag • Option 2: Use DSCP marking • Option 3: Change flow id ( for example one bit can represent the color)

  7. My thoughts and questions • Does that work? Will QCN algorithm converge? • Is this behavior according to the standard? • Does it violate the standard?

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