OcNOS supports Priority-based Flow Control (PFC) to pause frames using defined times for each of the eight priority classes. This prevents congestion and improves transmission performance by letting the transmitter adjust its data flow according to the receiver’s processing capacity.

Also supports Explicit Congestion Notification (ECN), which provides end-to-end congestion signaling between ECN-enabled senders and receivers in TCP/IP networks. Instead of dropping packets, ECN marks them to indicate congestion, prompting the sender to temporarily reduce its transmission rate until congestion clears. This reduces both packet loss and delay. ECN is defined in RFC 3168.

This functionality enables:

This functionality is applicable to the chips Tomahawk 2 (TH2) series platforms, Tomahawk3 (TH3) platforms, Tomahawk4 (TH4) platforms, Tomahawk5 (TH5) platforms, Trident3 (TR3) platforms and Trident4 (TR4) platforms.

Improved Congestion Management – Prevents buffer overflows and packet drops by dynamically controlling traffic flow.

Per-Priority Traffic Control – Ensures that critical traffic classes (e.g., storage or real-time applications) are not impacted by congestion in other classes.

Reduced Packet Loss – Uses packet marking instead of dropping to signal congestion, minimizing retransmissions.

Higher Throughput Efficiency – Link utilization can be optimized via adjusting transmission rates based on real-time network conditions.

PFC monitoring data requires a working PFC configuration and active PFC traffic. Similarly, ECN monitoring data requires a working ECN configuration and active ECN traffic.

The configuration procedure outlines the steps required to enable ECN and PFC Support for Lossless TCP/IP Transport on the L2 networks, ensuring the network can handle high-priority, lossless AI/ML traffic.

The section includes the following configuration and validations:

The topology uses a Switch1 with an ingress interface cd2/1 (connected to a node which generates traffic) and an egress interface xe66 (connected to destination node which receives the traffic). Congestion is induced on the egress interface xe66 using shapers within QoS policy maps.

The following topology shows PFC pause frame monitoring on the ingress and egress interfaces of Switch 1.

 

The following topology shows ECN Marked packets monitoring on the ingress and egress interfaces of Switch 1.

The following steps configure monitoring of PFC and ECN packets transmitted and received on an interface when monitoring is enabled.

Verify the traffic rates on interfaces.

High ingress rate on cd2/1 (~4324.81 Mbps), while egress xe66 is capped at ~1000.08 Mbps due to the shaper. This confirms congestion on xe66 without PFC pause (ingress rate is high).

 

Verify packet and byte counters for traffic passing through queues defined in applied policy maps.

For Interface xe66, queue q3 shows a very large number of dropped packets (294034145 -> 307021247 ) alongside significant total packets (902926872 ) and total bytes (115574639616) successfully transmitted. As ECN marking via random-detect is enabled on this queue, although some actual tail drops might occur if buffers completely fill, as q3 is not configured as lossless here and PFC is disabled.

Verify the count of packets marked with ECN CE (Congestion Experienced) code point on a per-interface basis.

Interface xe66 shows a large and increasing number of ECN marked packets (47941117 -> 50871013), directly confirming ECN marking is active due to the egress congestion.

These log messages are generated due to the monitor ecn command on xe66. The logs periodically report the cumulative count of ECN Marked packet on interface xe66, providing real-time visibility into the ECN marking activity. The counts align with the increasing values seen in sh int counters ecn.

The configuration given in the ECN Configuration (Bridging and ECN Marking) successfully sets up an L2 path, induces congestion on the egress interface xe66 via shaping, and applies an ECN policy. Validation confirms that packets exceeding the WRED thresholds in queue 3 are being marked with ECN (not dropped), as shown by the dedicated ECN counters, interpreted policy map statistics, and system logs.

Verify the traffic rates on interfaces.

The egress interface xe66 is capped at ~1000.12 Mbps due to the shaper. The ingress interface cd2/1 also shows a receive rate throttled down to ~1032 Mbps. This indicates that PFC pausing is throttling the source to match the egress shaper rate.

 

Verify packet and byte counters for traffic passing through queues defined in applied policy maps.

Interface xe66, queue q3 shows a large number of Total pkts (224865972) and Total bytes (28782844416) successfully transmitted, but critically shows Dropped pkts: 0. This confirms that PFC is working as intended in this L2 scenario, preventing drops despite the 1 Gbps shaper inducing congestion on a lossless queue.

 

Verify the counters for PFC pause frames sent and received per priority per interface.

Interface cd2/1 shows a large and rapidly increasing count of pause sent frames specifically for priority 3 (6323876 -> 6486658 ). No pause frames are received (pause received is 0). This confirms that Switch1 is sending PFC pause frames out of the ingress interface cd2/1 for priority 3. This happens because the downstream path (egress interface xe66) is congested for queue 3 (due to the shaper), and PFC is enabled for this priority.

 

Verify the administrative and operational status of PFC per interface.

Logs periodically report the number of Pause-Tx (transmitted pause frames) for Priority Group 3 (PG[3]) on interface cd2/1, confirming the PFC activity shown in the statistics command. Conversely, if pause frames are received rather than transmitted, equivalent Pause-Rx logs will be displayed.

 

The configuration given in the PFC Configuration (Bridging and PFC Pausing) establishes an L2 path with shaping on egress (xe66) and PFC enabled on both ingress (cd2/1) and egress for relevant priorities. Validation confirms that congestion on the egress interface triggers PFC pause frames to be sent from the ingress interface (cd2/1), successfully throttling the traffic source. The policy map statistics verify that no packets are dropped for the shaped lossless queue (q3) due to PFC being active.

The feature introduces the following configuration commands.

 

 

 

Use this command to enable Explicit-Congestion-Notification (ECN) marked packets monitoring on a physical interface.

Use the no form of this command to disable ECN monitoring on the interface

None

None

Interface mode

This command was introduced beforeOcNOS version 7.0.0

Use this command to enable Priority-based Flow Control (PFC) pause frames monitoring on a physical interface.

Use the no form of this command to disable PFC monitoring on the interface.

None

None

Interface mode

This command was introduced before OcNOS version 7.0.0

 

The following table provides definitions for key terms or abbreviations and their meanings used throughout this document: