4-2
One evaluation is performed on each arriving packet. In each evaluation, if the number of tokens in the
bucket is enough, the traffic conforms to the specification and the corresponding tokens for forwarding
the packet are taken away; if the number of tokens in the bucket is not enough, it means that too many
tokens have been used and the traffic is excessive.
Complicated evaluation
You can set two token buckets (referred to as the C bucket and E bucket respectively) in order to
evaluate more complicated conditions and implement more flexible regulation policies. 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 or
forwarding 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 average
packet 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
can forward.
CBS and EBS are carried by two different token buckets. In each evaluation, packets are measured
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 are
colored yellow.
z
If neither the C bucket nor the E bucket has sufficient tokens, packets are colored red.
Traffic Policing
The typical application of traffic policing is to supervise the specification of certain traffic entering a
network and limit it within a reasonable range, or to "discipline" the extra traffic. In this way, the network
resources and the interests of the carrier are protected. For example, you can limit bandwidth
consumption of HTTP packets to less than 50% of the total. If the traffic of a certain session exceeds
the limit, traffic policing can drop the packets or reset the IP precedence of the packets.
Figure 4-1
Schematic diagram for traffic policing
Token
bucket
Packets dropped
Packet
classification
Packets to be sent
through this interface
Packets sent
Tokens are put into the
bucket at the set rate
Queue
