3Com 10014299 manual Traffic POLICING, Traffic Shaping and Line Rate

668CHAPTER 48: TRAFFIC POLICING, TRAFFIC SHAPING AND LINE RATE

■Dropping directly — CAR drops the packets that do not "conform to" the traffic specifications.

■Forwarding after modifying the packet priority level — The packets with the estimated result of "partial conformance" are forwarded after they are marked as the lower priority level flows.

■Entering the next level of policing — Traffic policing may be stacked level by level, and each level concerns and polices more specific targets. A downstream network can receive the estimated result from an upstream network, or it can be classified according to its own standard.

Traffic policing uses the Token Bucket algorithm, and each service has tokens which are transmitted at a specified rate. If the reaching speed of the user packets is faster than the speed at which the tokens are transmitted, it is necessary to take measures for the data exceeding the specified rate, for example, they are marked and allowed to pass through the network only when the network is not congested and they are dropped first when the network is congested. These data packets can also be dropped directly, which is completely dependent on the agreement and rules between the operators and users.

Token bucket feature The token bucket may be regarded as a container that stores tokens. The system puts tokens into the bucket at the set speed. When the bucket is full of tokens, the excessive tokens overflow, and the number of the tokens in the bucket does not increase.

Figure 210 Schematic diagram of packet line classification and traffic policing

Tokens enter bucket at the given speed

incoming packets

outgoing packets

classify

Token Bucket

Discarded

packets

Measuring the traffic by the token bucket Evaluating the traffic specification by the token bucket is based on whether the number of the tokens in the token bucket is enough for packet forwarding. If the bucket has sufficient tokens to forward packets, the traffic does not exceed the specification, otherwise, it exceeds the specification. Usually, one token is associated with one bit of forwarding authority.

Three main parameters are used in the evaluation of the traffics:

■Time Interval: Evaluates the traffic in every other period. This parameter is set by the system. For every evaluation, if the bucket has sufficient tokens to be used by one or more packets, it is considered “in conformance”. If the bucket