Policing and Shaping: Types of Policers

Single-Rate Two-Colour Markers and Policers

The first policers implemented use a single rate two colour model based on a single token bucket algorithm.

For this type of policer, traffic can be either conforming to the committed information rate or exceeding it. Marking down traffic or dropping it can be performed to either of these two states.

Single-Rate Three-Colour Markers and Policers

A single rate three colour policer algorithm is based on RFC 2697.

This type of policer uses two token buckets. Traffic can be classed as conforming, exceeded, or violating the committed information rate.

Traffic can be marked down or dropped on any of these three traffic states.

The first token bucket operates in a similar way to a single-rate two colour system. The difference between this system and the earlier two-colour system is that instead of discarding excess tokens in the first token bucket, they are stored in the second bucket to be used later for temporary bursts that exceed the committed information rate.

Tokens that are placed in the second bucket are known as excess burst. Excess burst is the maximum number of bits that can exceed the committed burst size.

The two bucket algorithm works well with TCP causing fewer TCP transmissions and is more efficient in the utilisation of bandwidth. It is an ideal policer to use with Assured Forwarding (AF) classes.

This system defines the three-colour mechanism, with three different classes for traffic.

Confirming

Traffic under the committed burst size is classed as confirming, or ‘green’. This traffic is normally transmitted and left unmarked.

Exceeding

Traffic over the committed burst size but under the excess burst is classed as exceeding, or ‘yellow’. Exceeding traffic can be dropped, or marked down and transmitted.

Violating

Traffic over committed burst size and excess burst size is classed as violating or ‘red’. This type of traffic is usually dropped but can be marked down instead

Using this three colour system makes sense with different actions for each level, e.g. marking down on exceeding and dropping on violating.

Parameters Used

The single-rate three colour marker and policer uses the following parameters to meter traffic:

1. Committed Information Rate
2. Committed Burst Size
3. Excess Burst Size
4. Committed Burst Size Bucket Token Count
5. Excess Burst Size Bucket Token Count
6. Incoming Packet Length

Two-Rate Three-Colour Markers and Policers

The two-rate three colour system is based on RFC 2698, it is similar to the single rate three-colour policer.

The difference between the two is a single-rate three colour policer relay on excess tokens from the committed burst size bucket. This introduces a level of variability and unpredictability in traffic flows. The two-rate three-colour marker address this level of variability and unpredictability by using two distinct rates, the committed information rate and the peak information rate.

The two rate three colour marker and policer allows for a sustained excess rate based on the peak information rate. This allows for different actions to be performed on the traffic that exceeds different burst values. Violating traffic can be dropped at a defined rate; this is not possible with a single rate system.

The two rate three colour system uses two token buckets. Instead of transferring excess unused tokens from the burst committed size to the burst excess bucket, the policer has two totally separate buckets filled with two different token rates.

The burst excess bucket is filled with peak information rate tokens, and the committed burst size bucket is filled with committed information rate tokens. The burst excess represents the peak limit of traffic that can be sent during a sub second interval.

The initial check is to see whether the traffic is within the peak information rate, or in a violation state. If the traffic is not, then the committed information rate, or excess state comparison, is made.

Parameters Used

1. Committed Information Rate
2. Peak Information Rate
3. Committed Burst Size
4. Peak Burst Size
5. Committed Burst Size Token Count
6. Excess Burst Size Token Count
7. Incoming Packet Length