The token bucket is an algorithm used in packet-switched and telecommunications networks. It can be used to check that data transmissions, in the form of packets, conform to defined limits on bandwidth and burstiness (a measure of the unevenness or variations in the traffic flow). It can also be used as a scheduling algorithm to determine the timing of transmissions that will comply with the limits set for the bandwidth and burstiness: see network scheduler.
Overview The token bucket algorithm is based on an analogy of a fixed capacity bucket into which tokens, normally representing a unit of bytes or a single packet of predetermined size, are added at a fixed rate. When a packet is to be checked for conformance to the defined limits, the bucket is inspected to see if it contains sufficient tokens at that time. If so, the appropriate number of tokens, e.g. equivalent to the length of the packet in bytes, are removed ("cashed in"), and the packet is passed, e.g., for transmission. The packet does not conform if there are insufficient tokens in the bucket, and the contents of the bucket are not changed. Non-conformant packets can be treated in various ways:
They may be dropped. They may be enqueued for subsequent transmission when sufficient tokens have accumulated in the bucket. They may be transmitted, but marked as being non-conformant, possibly to be dropped subsequently if the network is overloaded. A conforming flow can thus contain traffic with an average rate up to the rate at which tokens are added to the bucket, and have a burstiness determined by the depth of the bucket. This burstiness may be expressed in terms of either a jitter tolerance, i.e. how much sooner a packet might conform (e.g. arrive or be transmitted) than would be expected from the limit on the average rate, or a burst tolerance or maximum burst size, i.e. how much more than the average level of traffic might conform in some finite period.
Algorithm The token bucket algorithm can be conceptually understood as follows:
A token is added to the bucket every 1 / r {\displaystyle 1/r} seconds. The bucket can hold at the most b {\displaystyle b} tokens. If a token arrives when the bucket is full, it is discarded. When a packet (network layer PDU) of n bytes arrives, if at least n tokens are in the bucket, n tokens are removed from the bucket, and the packet is sent to the network. if fewer than n tokens are available, no tokens are removed from the bucket, and the packet is considered to be non-conformant.
Variations Implementers of this algorithm on platforms lacking the clock resolution necessary to add a single token to the bucket every 1 / r {\displaystyle 1/r} seconds may want to consider an alternative formulation. Given the ability to update the token bucket every S milliseconds, the number of tokens to add every S milliseconds = ( r ∗ S ) / 1000 {\displaystyle (r*S)/1000} .
Properties
Average rate Over the long run the output of conformant packets is limited by the token rate, r {\displaystyle r} .
Burst size Let M {\displaystyle M} be the maximum possible transmission rate in bytes/second. Then T max = { b / ( M − r ) if r < M ∞ otherwise {\displaystyle T_{\text{max}}={\begin{cases}b/(M-r)&{\text{ if }}r<M\\\infty &{\text{ otherwise }}\end{cases}}} is the maximum burst time, that is the time for which the rate M {\displaystyle M} is fully utilized. The maximum burst size is thus B max = T max ∗ M {\displaystyle B_{\text{max}}=T_{\text{max}}*M}
Uses The token bucket can be used in either traffic shaping or traffic policing. In traffic policing, nonconforming packets may be discarded (dropped) or may be reduced in priority (for downstream traffic management functions to drop if there is congestion). In traffic shaping, packets are delayed until they conform. Traffic policing and traffic shaping are commonly used to protect the network against excess or excessively bursty traffic, see bandwidth management and congestion avoidance. Traffic shaping is commonly used in the network interfaces in hosts to prevent transmissions being discarded by traffic management functions in the network. The token bucket algorithm is also used in controlling database IO flow. In it, limitation applies to neither IOPS nor the bandwidth but rather to a linear combination of both. By defining tokens to be the normalized sum of IO request weight and its length, the algorithm makes sure that the time derivative of the aforementioned function stays below the needed threshold.
Comparison to leaky bucket
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