Size priority flow control headroom per port from link speed, cable length and pause response time.
When a receiver sends a PFC pause frame, the sender keeps transmitting for one round trip plus its own reaction time, and all of those bytes must land in buffer rather than be dropped. Headroom therefore scales with both link speed and cable length: at 100G a 50 m cable already holds over 6 KB in flight. Under-sized headroom breaks the lossless guarantee RoCE depends on, and over-sized headroom consumes the shared buffer that absorbs incast — both show up as unexplained RDMA retransmits.
PFC Buffer
headroom = link speed × (2 × cable metres × 5 ns/m + pause response time) ÷ 8 + 2 × MTU.
headroom = link speed × (2 × cable metres × 5 ns/m + pause response time) ÷ 8 + 2 × MTU. When a receiver sends a PFC pause frame, the sender keeps transmitting for one round trip plus its own reaction time, and all of those bytes must land in buffer rather than be dropped. Headroom therefore scales with both link speed and cable length: at 100G a 50 m cable already holds over 6 KB in flight.
Under-sized headroom breaks the lossless guarantee RoCE depends on, and over-sized headroom consumes the shared buffer that absorbs incast — both show up as unexplained RDMA retransmits.
This calculator takes 6 inputs: Link speed, Cable length, MTU, Pause response time, Lossless priority queues, Ports per switch. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.