Leaky Bucket Algorithm

A traffic shaping algorithm that turns bursty traffic into a steady, constant-rate flow, like water leaking from a bucket through a small hole at a fixed rate.

Working

1. Packets arriving from the host are placed in a finite-size queue (bucket).
2. Packets leave the bucket at a fixed rate $\rho$, whatever the arrival rate.
3. If the bucket is full, new packets are dropped (or marked).
4. For variable-size packets, a fixed number of bytes per clock tick is released.

Use in traffic shaping

• Smooths bursts so the network sees a predictable, uniform flow, reducing congestion at routers.
• Applied at the network edge so traffic conforms to the agreed profile (SLA).

Use in rate-limiting

• The output rate $\rho$ is the maximum allowed rate; the bucket size limits the burst absorbed.
• Excess traffic beyond bucket capacity is discarded, which enforces the limit (e.g. API/ISP rate limiting, policing in routers and gateways).

Note

The leaky bucket never allows output above $\rho$, even after idle periods. The Token Bucket algorithm allows controlled bursts.