Leaky bucket

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Although the leaky bucket algorithm has several uses, it is best understood in the context of network traffic shaping or rate limiting. Typically, the algorithm is used to control the rate at which data is injected into a network, smoothing out "burstiness" in the data rate.

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[edit] Traffic shaping algorithms

Two predominant methods for shaping traffic exist : a leaky bucket implementation and a token bucket implementation. Sometimes the leaky bucket and token bucket algorithms are mistakenly lumped together under the same name. Both these schemes have distinct properties and are used for distinct purposes [1]. They differ principally in that the leaky bucket imposes a hard limit on the data transmission rate, whereas the token bucket allows a certain amount of burstiness while imposing a limit on the average data transmission rate.

[edit] What is the Leaky-Bucket algorithm used for?

Figure 1: A simple Leaky Bucket [1]
Figure 1: A simple Leaky Bucket [1]

The leaky-bucket implementation is used to control the rate at which traffic is sent to the network (see Figure 1). A leaky bucket provides a mechanism by which bursty traffic can be shaped to present a steady stream of traffic to the network, as opposed to traffic with erratic bursts of low-volume and high-volume flows.

[edit] Automobile traffic analogy

An appropriate analogy for the leaky bucket is a scenario in which four lanes of automobile traffic converge into a single lane. A regulated admission interval into the single lane of traffic flow helps the traffic move. The benefit of this approach is that traffic flow into the major arteries (the network) is predictable and controlled. The major liability is that when the volume of traffic is vastly greater than the bucket size, in conjunction with the drainage-time interval, inward traffic for the bucket and beyond bucket capacity, is discarded [1]

[edit] The Leaky-bucket algorithm

The algorithm can be conceptually understood as follows:

  • Consider a bucket with a hole in the bottom.
  • The empty space of the bucket represents an amount of credit available measured in bytes.
  • The size of the bucket is b bytes. This means that if the bucket is empty, b bytes of credit is available.
  • If a packet arrives and its size is less than the available credit, the packet can be forwarded. Otherwise, it is discarded or queued depending on the application.
  • The bucket leaks through the hole in its bottom at a constant rate of r bytes per second, this indicates credit accumulation.

The Generic Cell Rate Algorithm, I , L used in traffic shaping of ATM networks is equivalent to the leaky bucket algorithm.

[edit] Inefficiency of the Leaky-bucket implementation

The leaky-bucket implementation does not efficiently use available network resources. Because its leak rate is a fixed parameter, there will be many instances when the traffic volume is very low and large portions of network resources (bandwidth in particular) are not being used. Therefore no mechanism exists in the leaky-bucket implementation to allow individual flows to burst up to port speed, effectively consuming network resources at times when there would not be resource contention in the network. The token bucket implementation does however accommodate traffic flows with bursty characteristics [1]. The leaky-bucket and token bucket implementations can be combined to provide maximum efficiency and control of the traffic flows into a network.

[edit] See also

[edit] References

  • "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-12-370549-5)
  • Ferguson P., Huston G., Quality of Service: Delivering QoS on the Internet and in Corporate Networks, John Wiley & Sons, Inc., 1998.