Single rate three color marker

Single-rate traffic contract has there are three parameters, the, CIR, CBS and EBS. associated with this type of contract:

Marking is based on CIR and two associated burst sizes, CBS and EBS. Packets are marked:

Green - if it does not exceed the CBS
Yellow - if it does exceed the CBS, but not the EBS
Red - otherwise

The SrTCM is useful for ingress policing of a service, where only the length, not the peak rate, of the burst determines service eligibility.

Implementation

SrTCM traffic policing is implemented by tracking the current burst size using token-buckets, and discarding packets that exceed the CIR. In SrTCM the three color scheme has any incoming burst classified as either conforming (green, under CBS), exceeding (yellow, over CBS but under EBS), or violating (red, over EBS).

Every arriving packet is first compared to CBS and then to the EBS to determine the next action. There is a single flow of the tokens that fills the CBS bucket first and then continues to filling the EBS bucket. The second bucket is only filled if there was enough idle time to let the first bucket fill up completely.

The drawback of single-rate traffic contracts is that the service provider should be cautions assigning CIR bandwidth, by offering less bandwidth than it can service at any moment. The reason for this is that not all customers send traffic simultaneously, so network links may effectively become underutilized even at weak spots.