Enable ECN on lossless queues

Overview

IP based RDMA protocol (e.g. RoCEv2) uses PFC to enable drop-free network. ECN is the important algorithm to support the model. However, ECN is currently enabled for lossy queues for current configuration. Therefore, we need also enable ECN for lossless queues for new configurations.

Requirement

Enabling ECN on lossless queue is considered necessary and it should be enabled in SONiC configuration. For non-ECN capable packet in lossless queue, if threshold is exceeded it should be dropped. Design test cases to ensure ECN works on lossless queue.

ECN shall always go out before PFC packets got generated on the same link.

ECN threshold will be calculated and push down to SAI. It is not SAI's responsibility to auto-calculate ECN threshold.

Example change

QoS configuration for TD2

Change queue 0-1 at the end of the line to 3-4.

Reference

Please reference section 4 of Congestion Control for Large-Scale RDMA Deployments