What Is Lossless Ethernet?

Lossless Ethernet is standard Ethernet configured so that a chosen class of traffic, typically RDMA, is not dropped when the network gets congested. It is achieved with Priority Flow Control (PFC), which pauses a specific priority queue before its buffer overflows, and Explicit Congestion Notification (ECN), which marks packets early so the sender slows down before any queue fills. The goal is a no-drop path for traffic that reacts badly to loss, without turning the whole network no-drop.

They operate at different speeds. ECN is the primary, fine-grained control: a switch marks packets when a queue starts to build, and the receiving NIC signals the sender to reduce its rate. DCQCN is the end-to-end algorithm that ties ECN marking on the switch to rate control on the NIC in a continuous loop; the switch role in DCQCN is the ECN marking. PFC is the fast backstop: when a queue is about to overflow anyway, PFC pauses that priority hop by hop so nothing is dropped while ECN and DCQCN take effect. Well-tuned fabrics lean on ECN and DCQCN and keep PFC as the last resort.

RoCEv2 is RDMA carried over UDP and IP. RDMA was designed for a no-drop transport, so a dropped packet forces recovery that is expensive in tail latency and can stall a GPU collective. Making the Ethernet path lossless with PFC and ECN gives RoCEv2 the no-drop behavior it expects, which is why a production RoCEv2 GPU fabric always runs on a lossless-configured network.

On Broadcom Tomahawk 5 hardware, OcNOS-DC supports PFC (including PFC over L3, with DCBX and LLDP), ECN, Dynamic ECN, Enhanced Transmission Selection (ETS), WRED, and PFC Deadlock Detection and Recovery, along with buffer tuning and RTAG7 hashing. Congestion state and queue counters stream over gNMI and OpenConfig telemetry for closed-loop tuning. See the PFC deadlock and DCQCN pages for the detail behind each.