PFC Buffer Calculator
Size priority flow control headroom per port from link speed, cable length and pause response time.
About PFC Buffer Calculator
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
Worked example
With the default values below, PFC Buffer Calculator returns Headroom per Queue per Port of 28,000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Link speedaccepts 1–800 Gbps
- 100 Gbps
- Cable lengthaccepts 1–2,000 m
- 50 m
- MTUaccepts 1,500–9,216 bytes
- 1,500 bytes
- Pause response timeaccepts 0.1–50 µs
- 1.5 µs
- Lossless priority queuesaccepts 1–8 queues
- 2 queues
- Ports per switchaccepts 1–512 ports
- 32 ports
Results
- 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
Step by step
- 1Values used
- 2PFC Buffer
- 3Headroom per Queue per Port= 28,000 bytes
- 4Headroom per Queue per Port= 27.34 KB
- 5Bytes in Flight on the Cable= 6,250 bytes
- 6Headroom per Port= 54.69 KB
- 7Switch Buffer for Headroom= 1.71 MB
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.