Process
RFC 2475 describes traffic policing elements like a meter and a dropper. They may also optionally include a marker. The meter measures the traffic and determines whether or not it exceeds the contract (for example by GCRA). Where it exceeds the contract, some policy determines if any given PDU is dropped, or if marking is implemented, if and how it is to be marked. Marking can comprise setting a congestion flag (such as ECN flag of TCP or CLP bit of ATM) or setting a traffic aggregate indication (such as Differentiated Services Code Point of IP).
In simple implementations, traffic is classified into two categories, or "colors" : compliant (green) and in excess (red). RFC 2697 proposes a more precise classification, with three "colors". In this document, the contract is described through three parameters: Committed Information Rate (CIR), Committed Burst Size (CBS), and Excess Burst Size (EBS). A packet is "green" if it doesn't exceed the CBS, "yellow" if it does exceed the CBS, but not the EBS, and "red" otherwise.
The "single-rate three-color marker" described by RFC 2697 allows for temporary bursts. The bursts are allowed when the line was under-used before they appeared. A more predictable algorithm is described in RFC 2698, which proposes a "double-rate three-color marker". RFC 2968 defines a new parameter, the Peak Information Rate (PIR).
Read more about this topic: Traffic Policing
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