Configuring traffic policing, traffic shaping,
and line rate
Traffic policing, traffic shaping, and rate limit are QoS technologies that help assign network resources,
such as assign bandwidth. They increase network performance and user satisfaction. For example, you
can configure a flow to use only the resources committed to it in a certain time range. This avoids network
congestion caused by burst traffic.
Traffic policing, generic traffic shaping (GTS), and line rate limit the traffic rate and resource usage
according to traffic specifications. Once a particular flow exceeds its specifications, such as assigned
bandwidth, the flow is shaped or policed to make sure that it is under the specifications. You can use
token buckets for evaluating traffic specifications.
Traffic evaluation and token buckets
A token bucket is analogous to a container that holds a certain number of tokens. Each token represents
a certain forwarding capacity. The system puts tokens into the bucket at a constant rate. When the token
bucket is full, the extra tokens cause the token bucket to overflow.
Evaluating traffic with the token bucket
A token bucket mechanism evaluates traffic by looking at the number of tokens in the bucket. 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 "excess traffic".
A token bucket has the following configurable parameters:
Mean rate at which tokens are put into the bucket, which is the permitted average rate of traffic. It
is usually set to the committed information rate (CIR).
Burst size or the capacity of the token bucket. It is the maximum traffic size 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.
Each arriving packet is evaluated. In each evaluation, 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, the traffic is excessive.
You can set two token buckets, bucket C and bucket E, to evaluate traffic in a more complicated
environment and achieve more policing flexibility. For example, traffic policing uses the following
CIR—Rate at which tokens are put into bucket C. It sets the average packet transmission or
forwarding rate allowed by bucket C.
CBS—Size of bucket C, which specifies the transient burst of traffic that bucket C can forward.