The existing ECN functionality is enhanced to support dynamic ECN (D-ECN) that allows more precise ECN marking based on shared buffer usage.

In Dynamic ECN, a dynamic threshold is used to allow more precise ECN marking by taking into account actual shared buffer usage. Typically, a D-ECN threshold value is based on D-ECN-Offset and D-ECN-Floor values.

D-ECN Threshold = (Queue's shared limit - D-ECN-Floor) - D-ECN-Offset, where

While off-offset likely provides hysteresis (a threshold to stop marking). The off-offset, used in conjunction with the on-offset (a threshold for ECN marking), provides hysteresis for D-ECN marking. It determines the point (likely calculated as Queue's shared limit – off-offset) below which the queue's shared buffer usage must fall for the device to stop marking packets with ECN for that queue, after having started marking due to crossing the on-offset threshold.

This mechanism prevents rapid oscillation between marking and not marking when the queue usage hovers near the activation threshold, contributing to more stable congestion signaling.

Both Static ECN (S-ECN) and Dynamic ECN (D-ECN) can be enabled simultaneously, with the final ECN marking determined by a logical OR operation between the two.

Limitation:

This functionality is supported on Broadcom Tomahawk5 platforms only.

More than 3 D-ECN user defined profiles cannot be created.

This enhancement provides the following capabilities for AI/ML networking environment where lossless traffic is crucial:

Ensure baseline QoS is enabled and potentially Static ECN via WRED is configured in an applied policy map.

The following topology shows ECN Marked packets on the egress interfaces xe1 of Switch 1.

The following procedure defines a QoS profile with specific queue offsets, applying it to an interface, and validating its behavior under different traffic conditions.

In this scenario, traffic is sent with Priority 0, which maps to Queue 0 of the DECN profile. The queue parameters for Queue 0 are: on-offset 4000 kbytes and off-offset 1000 kbytes. The on-offset (4 MB) is greater than the off-offset (1 MB).

Repeatedly execute the following show command during congestion. Focus on the Q-Size (current queue depth in bytes) for the relevant egress queues (q0(E)).

The ECN bit starts marking from the 5.3MB.

The ECN marking started at 4.9 MB, this suggests the system's dynamic calculation or effective buffer space led to ECN marking somewhere between the on-offset (4 MB) and the full buffer capacity (which must be greater than 5.3 MB). The drops confirm the queue eventually exceeded its maximum ECN marking threshold and hit the drop threshold (likely the maximum queue depth).

In this scenario, traffic is sent with Priority 3, which maps to Queue 3 of the DECN profile. The queue parameters for Queue 3 are: on-offset 5,000,000 bytes (5 MB) and off-offset 6,000,000 bytes (6 MB). The off-offset is greater than the on-offset.

Verify that the D-ECN profile DECN has been created with the intended on-offset and off-offset values for specific queues.

Repeatedly execute the following show command during congestion. Focus on the Q-Size (current queue depth in bytes) for the relevant egress queues (q3(E)).

Here's a real-time example illustrating how Dynamic ECN (D-ECN) functions:

Traffic enters Switch1 on interface xe0 and is destined out of interface xe11.The D-ECN profile EG1 is applied to the egress interface xe1. Profile EG1 configures queue 3 with an on-offset of 7000 bytes and an off-offset of 8000 bytes.

A burst of traffic arrives at xe0, destined for xe1, and this traffic maps to egress queue 3. The egress interface xe1 starts experiencing congestion, causing packets to build up in queue 3.

The shared buffer usage for queue 3 on interface xe1 begins to increase due to the congestion.

The switch continuously compares the current shared buffer usage for queue 3 against the D-ECN threshold. This threshold is calculated as Queue's shared limit – D-ECN-Offset (where D-ECN-Offset is the on-offset value) . In this case, it's Queue's shared limit – 7000 bytes.

When the shared buffer usage for queue 3 reaches or exceeds (Queue's shared limit – 7000 bytes), the D-ECN mechanism signals the switch to start marking the ECN field (setting the CE codepoint) in the IP header of subsequent packets exiting queue 3 on interface xe1. Packets are marked, not dropped.

ECN-capable receiving endpoints detect the CE marking and signal congestion back to the sender. The sender then reduces its transmission rate, helping to alleviate the congestion on Switch1's egress queue 3.

As the sender reduces its rate, the buffer usage for queue 3 decreases. Once the usage drops below the deactivation threshold (calculated as Queue's shared limit – off-offset, or Queue's shared limit – 8000 bytes), the switch stops marking packets for queue 3. The slightly larger off-offset provides hysteresis, preventing rapid on/off marking if the queue usage fluctuates near the on-offset threshold.

Congestion is signaled dynamically based on shared buffer pressure before potential packet drops occur, allowing endpoints to react proactively. The on-offset and off-offset provide precise control over when marking starts and stops, tailored to buffer conditions.

The feature introduces the following configuration commands.

Use this command to set the queue mapping for egress dynamic ECN profile.

Use the no form of this command delete the profile.

Use qos map-profile to map the profile on interface.

By default, "default" profile is created for all the profile types. These profiles can only be updated by the user neither be created nor be destroyed.

config-eg-dynamic-ecn-map

This command was introduced in OcNOS version 7.0.0.

This command applies only to the Tomahawk 5 (TH5) platforms.

To create a new profile:

To create Dynamic-ECN profile

To delete D-ECN thresholds mapping for each queue in the profile.

To attach a D-ECN profile on an interface

Simultaneously supporting Static and D-ECN profile on an interface. Static-ECN is via WRED ECN marking. The final marking is the logical OR of the two ECN features.

The following two CLIs are modified to include egress-dynamic-ecn attribute.

Use this command to attach (map) a profile to an interface.

Use the no form of this command to remove a profile.

No default D-ECN profile.

Interface modes

The egress-dynamic-ecn option is added in OcNOS version 7.0.0. It applies only for Tomahawk 5 (TH5) platforms.

To create a D-ECN profile EG1 is to the egress interface xe1.

Use this command to show the all configured QoS profiles configurations like type, name, maps configured except for the default maps, attached info (to how many interfaces it is attached) etc,

Privileged exec

Configuration Mode

Interface Mode

The egress-dynamic-ecn option is added in OcNOS version 7.0.0. It applies only for Tomahawk 5 (TH5) platforms.

To display the Dynamic-ECN profile mapping deatils:

Following are some of the commands used for troubleshooting:

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