Traffic Burst Calculator
Size a token bucket: the committed burst size a policer needs to pass a burst without dropping it.
About Traffic Burst Calculator
A token bucket refills at the CIR and empties at the sending rate, so only the difference between the two drains it. The burst a given bucket tolerates is its depth divided by that drain rate; sizing the bucket the other way round gives the depth a known burst needs. Policers configured with a default CBS drop the first few milliseconds of every TCP transfer, which triggers retransmits and makes a link that is nowhere near capacity feel broken.
Formulas
Traffic Burst
CBS = (peak rate − CIR) × burst duration ÷ 8, the excess bytes the bucket must hold.
- CIR
- Committed information rate the bucket refills at
- peak rate
- Rate the source actually bursts at
- CBS
- Committed burst size in bytes
Maximum conforming burst
burst the bucket allows = CBS × 8 ÷ (peak rate − CIR)
- CBS
- Configured bucket depth in bytes
- peak rate − CIR
- Rate at which tokens are drained faster than they refill
Worked example
With the default values below, Traffic Burst Calculator returns Committed Burst Size Required of 5 MiB. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Committed information rate (CIR)minimum 0.1 Mbit/s
- 100 Mbit/s
- Peak burst rateminimum 0.1 Mbit/s
- 1,000 Mbit/s
- Configured committed burst sizeminimum 1 bytes
- 125,000 bytes
- Burst durationminimum 0.1 ms
- 50 ms
Results
- Committed Burst Size Required
- 5 MiB
- Burst The Configured CBS Allows
- 1.111 ms
- Bytes Sent During The Burst
- 6 MiB
- Bytes Covered By The CIR
- 610 KiB
- Verdict
- Exceeding — tokens run out and packets drop
Step by step
- 1Values used
- 2Traffic Burst
- 3Maximum conforming burst
- 4Committed Burst Size Required= 5,625,000
- 5Burst The Configured CBS Allows= 1.111 ms
- 6Bytes Sent During The Burst= 6,250,000
- 7Bytes Covered By The CIR= 625,000
- 8Verdict= Exceeding — tokens run out and packets drop
Assumptions and limits
- The result is derived only from the 4 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 3 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 Traffic Burst calculated?
CBS = (peak rate − CIR) × burst duration ÷ 8, the excess bytes the bucket must hold. A token bucket refills at the CIR and empties at the sending rate, so only the difference between the two drains it. The burst a given bucket tolerates is its depth divided by that drain rate; sizing the bucket the other way round gives the depth a known burst needs.
Why does Traffic Burst matter?
Policers configured with a default CBS drop the first few milliseconds of every TCP transfer, which triggers retransmits and makes a link that is nowhere near capacity feel broken.
What values do I need to enter?
This calculator takes 4 inputs: Committed information rate (CIR), Peak burst rate, Configured committed burst size, Burst duration. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.