3Com 4200G 12-Port Configuration Manual page 366

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
Packets to be sent
through this port
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:
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).
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.
Traffic policing
A 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 exceeding traffic. In this way, the network
resources and the interests of the carrier are protected. For example, you can limit the bandwidth for
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 to re-mark the priority of the packets.
Put tokens in the bucket at the set rate
Packet
classification
1-9
Continue to send
Token bucket
Drop