3Com WX3000 Evaluating the traffic with the token bucket, Traffic policing, Traffic shaping

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:

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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:

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Discarding the nonconforming packets.

Forwarding the conforming packets or nonconforming packets.

Marking the conforming packets or nonconforming packets with 802.1p precedence and then forwarding the packets.

Marking the conforming packets or nonconforming packets with DSCP precedence and forwarding the packets.

Traffic shaping

Traffic shaping is a measure to regulate the output rate of traffic actively. Its typical application is to control local traffic output based on the traffic policing indexes of downstream network nodes.

The major difference between traffic shaping and traffic policing is that the packets to be dropped in traffic policing are cached in traffic shaping——usually in buffers or queues, as shown in Figure 1-6. When there are enough tokens in the token bucket, the cached packets are sent out evenly. Another difference between traffic policing and traffic shaping is that traffic shaping may increase the delay while traffic policing hardly increases the delay.

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