H3C Technologies H3C S3100 Series Switches - Manual (Page 588)

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network resources and provide better service for more users. For example, a traffic flow can be limited
to get only its committed resources during a time period to avoid network congestion caused by
excessive bursts.
Traffic policing and traffic shaping is each a kind of traffic control policy used to limit the traffic and the
resource occupied by supervising the traffic. The regulation policy is implemented according to the
evaluation result on the premise of knowing whether the traffic exceeds the specification when traffic
policing or traffic shaping is performed. Normally, token bucket is used for traffic evaluation.
Token bucket
The token bucket can be considered as a container with a certain capacity to hold tokens. The system
puts tokens into the bucket at the set rate. When the token bucket is full, the extra tokens will overflow
and the number of tokens in the bucket stops increasing.
Figure 1-5 Evaluate the traffic with the token bucket
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:
z 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).
z 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.