Generic cell rate algorithm

The generic cell rate algorithm, or GCRA, is a leaky-bucket-type scheduling algorithm used in Asynchronous Transfer Mode networks. It measures the timing of cells on a virtual channel or virtual path against the bandwidth and jitter limits written into that connection's traffic contract. Cells that do not conform may be delayed, which is traffic shaping, or dropped or reduced in priority, which is traffic policing. The ATM Forum and ITU-T recommendation I.371 describe it in two equivalent ways, as a continuous-state leaky bucket and as a virtual scheduling algorithm that tracks a theoretical arrival time. Unlike a token bucket, it does not simulate the bucket leaking: each arriving cell triggers a calculation of when the bucket will next empty, and because ATM cells have a fixed length that calculation needs no multiplication or division. It is given only as a reference, so network providers may use any other algorithm that gives the same result, and two copies can be run together as a dual leaky bucket to limit both the sustained cell rate and the peak cell rate.