PFC Buffer Calculator
Size priority flow control headroom per port from link speed, cable length and pause response time.
Inputs
Headroom per Queue per Port
28,000bytes
Headroom per Queue per Port
27.34KB
Bytes in Flight on the Cable
6,250bytes
Headroom per Port
54.69KB
Switch Buffer for Headroom
1.71MB
Step by step
Values used
Link speed = 100 Gbps; Cable length = 50 m; MTU = 1,500 bytes; Pause response time = 1.50 µs; Lossless priority queues = 2 queues; Ports per switch = 32 ports
PFC Buffer
headroom = link speed × (2 × cable metres × 5 ns/m + pause response time) ÷ 8 + 2 × MTU.
Headroom per Queue per Port
= 28,000 bytes
Headroom per Queue per Port
= 27.34 KB
Bytes in Flight on the Cable
= 6,250 bytes
Headroom per Port
= 54.69 KB
Switch Buffer for Headroom
= 1.71 MB
How it works
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.
Formula
PFC Buffer
headroom = link speed × (2 × cable metres × 5 ns/m + pause response time) ÷ 8 + 2 × MTU.
- 2 × cable metres × 5 ns/m
- Round-trip propagation — data already in flight when the pause is sent
- 2 × MTU
- Allowance for the frame in transmission and the one being processed
Frequently Asked Questions
How is PFC Buffer calculated?
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.
Why does PFC Buffer matter?
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.
What values do I need to enter?
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.