QoS - Policing and Shaping

4 important questions on QoS - Policing and Shaping

What is the difference between Traffic Policing and Traffic Shaping?

Policers Drop or Re-mark incoming or outgoing traffic that goes beyond a desired traffic rate.

Shapers Buffer and delay outgoing traffic that momentarily peaks above the desired rate until it drops down again. If the rate is below the desired rate the traffic is sent immediatly.

How does the single-rate two-color policers work?

It is based on the single token bucket algorithm. Traffic can either be conforming or exceeding and marking down or dropping can be performed for each of the two states.

How does the single-rate three-color policers work?

Based on RFC 2697 it uses two token buckets and traffic can be classified as Conforming, Exceeding or Violating the CIR.
Marking down or Dropping are performed for either of the 3 states.
The first bucket operates much like the single-rate two-color system, except in period of low traffic the excess Tokens are placed in a second bucket instead of discarded, referred to as the excess burst (Be).
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How does the two-rate Three-Color policers work?

Based on RFC 2698 it is similer to the single-rate three-color marker/policer. However, it does not rely on Excess Tokens to fill the second bucket. Instead as well as a CIR it has a PIR or Peak Information Rate which is used to fill the bucket.
Another difference is that the traffic is checked against the PIR at first and if it conforms it is checked against the CIR. This means a Violate condition is checked first, then an exceed condition and then a conform condition.

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