3Com 5500-EI PWR Install Manual page 672

Figure 1-5 Evaluate the traffic with the token bucket
Packets to be sent
through this port
Packet
classification
Evaluating the traffic with the token bucket
When token bucket is used for traffic evaluation, the number of the tokens in the token
bucket determines the amount of the packets that can be forwarded. If the number of
tokens in the bucket is enough to forward the packets, the traffic is conforming to the
specification; otherwise, the traffic is nonconforming or excess.
Parameters concerning token bucket include:
Average rate: The rate at which tokens are put into the bucket, namely, the permitted
average rate of the traffic. It is generally set to committed information rate (CIR).
Burst size: The capacity of the token bucket, namely, the maximum traffic size that is
permitted in each burst. It is generally set to committed burst size (CBS). The set burst
size must be greater than the maximum packet length.
One evaluation is performed on each arriving packet. In each evaluation, if the number of
tokens in the bucket is enough, the traffic is conforming to the specification and you must
take away some tokens whose number is corresponding to the packet forwarding authority;
if the number of tokens in the bucket is not enough, it means that too many tokens have
been used and the traffic is excess.
Traffic policing
The typical application of traffic policing is to supervise specific traffic into the network and
limit it to a reasonable range, or to "discipline" the extra traffic. In this way, the network
resources and the interests of the operators are protected. For example, you can limit
HTTP packets to be within 50% of the network bandwidth. If the traffic of a certain
connection is excess, traffic policing can choose to drop the packets or to reset the priority
of the packets.
Traffic policing is widely used in policing the traffic into the network of internet service
providers (ISPs). Traffic policing can identify the policed traffic and perform pre-defined
policing actions based on different evaluation results. These actions include:
Put tokens in the bucket at the
set rate
Token
bucket
Drop
Continue
to send
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