4-2Evaluation is performed for each arriving packet. In each evaluation, if the number of tokens in thebucket is enough, the traffic conforms to the specification and the tokens for forwarding the packet aretaken away; if the number of tokens in the bucket is not enough, the traffic is excessive.Complicated evaluationYou can set two token buckets, the C bucket and the E bucket, to evaluate traffic in a more complicatedenvironment and achieve more policing flexibility. For example, traffic policing uses four parameters:z CIR: Rate at which tokens are put into the C bucket, that is, the average packet transmission orforwarding rate allowed by the C bucket.z CBS: Size of the C bucket, that is, transient burst of traffic that the C bucket can forward.z Peak information rate (PIR): Rate at which tokens are put into the E bucket, that is, the averagepacket transmission or forwarding rate allowed by the E bucket.z Excess burst size (EBS): Size of the E bucket, that is, transient burst of traffic that the E bucket canforward.CBS is implemented with the C bucket and EBS with the E bucket. In each evaluation, packets aremeasured against the buckets:z If the C bucket has enough tokens, packets are colored green.z If the C bucket does not have enough tokens but the E bucket has enough tokens, packets arecolored yellow.z If neither the C bucket nor the E bucket has sufficient tokens, packets are colored red.Traffic PolicingA typical application of traffic policing is to supervise the specification of certain traffic entering a networkand limit it within a reasonable range, or to "discipline" the extra traffic. In this way, the networkresources and the interests of the carrier are protected. For example, you can limit bandwidth for HTTPpackets to less than 50% of the total. If the traffic of a certain session exceeds the limit, traffic policingcan drop the packets or reset the IP precedence of the packets.Figure 4-1 Schematic diagram for traffic policingTokenbucketPackets droppedPacketclassificationPackets to be sentthrough this interfacePackets sentTokens are put into thebucket at the set rateQueueTraffic policing is widely used in policing traffic entering the networks of internet service providers (ISPs).It can classify the policed traffic and take pre-defined policing actions on each packet depending on theevaluation result:
