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Token bucket
A token bucket can be considered as a container holding a certain number of tokens. The
system puts tokens into the bucket at a set rate. When the token bucket is full, the extra
tokens will overflow.
Figure 1-6 Evaluate the traffic with the token bucket
Token bucket
Drop
Packet
classification
Packets to be sent
through this port
Continue to send
Put tokens in the bucket at the set rate
Evaluating the traffic with the token bucket
The evaluation of traffic specification is based on whether the number of tokens in the
bucket can meet the need of packet forwarding. If the number of tokens in the bucket is
enough for forwarding the packets, the traffic conforms to the specification and is called
conforming traffic; otherwise, the traffic does not conform to the specification and is called
exceeding traffic.
A token bucket uses the following parameters:
z Average rate: The rate at which tokens are put into the bucket, namely, the permitted
average rate of the traffic. It is usually set to the committed information rate (CIR).
z Burst size: The capacity of the token bucket, namely, the maximum traffic size that is
permitted in each burst. It is usually set to the committed burst size (CBS). The set
burst size must be greater than the maximum packet size.
Evaluation is performed each time a packet arrives. If the number of tokens in the bucket
is enough, the traffic conforms to the specification and the 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 exceeds the specification.
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