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User Manual - Configuration Guide (Volume 3)
Versatile Routing Platform
Chapter 2
Traffic Classification and Policing
2-3
II. Traffic Measuring with Token Bucket
Whether the total number of Tokens in Token Bucket meet the packet forwarding
requirement is the basis to evaluate the traffic size. If there are enough Token to
forward the packets (usually one Token is associated with the forwarding authority of
one bit), the traffic does not exceed the specification; otherwise, the traffic exceeds the
specification.
There are main three parameters to evaluate the traffic:
z
Average speed: the speed of Token put into Token Bucket. It is usually set to CIR
(Committed Information Rate), that is, allowed average flow speed.
z
Burst size: the capacity of the Token Bucket. It is usually set to CBS (Committed
Burst Size), that is, the allowed maximum traffic during the evaluation time interval.
Burst size must be greater than the longest packet.
z Time Interval: evaluate the traffic every other cycle. It is set by the system. If there
are enough Tokens for the packets, it is evaluated as “confirming”. If there are not
enough Tokens, it is “not conforming”. “Conforming” indicates that the traffic does
not exceed the specification and corresponding number of Tokens that conform to
packet forwarding authority will be taken out from the Bucket; “not conforming”
indicates that the traffic exceeds the specification and too many Tokens have
been consumed.
III Complicacy Evaluation
If there is only one Token Bucket, the evaluation result falls into two types: conforming
and not conforming.
In order to evaluate more complicated situation and implement more flexible control
policy, two Token Buckets will be set. For example, there are 3 parameters in CAR.
CIR(Committed Information Rate)
CBS(Committed Burst Size)
EBS(Excess Burst Size)
The two Token Buckets used transceives Tokens at the same speed of CIR, but with
different size--CBS and EBS respectively (CBS is less than EBS. The two Buckets are
called C and B.) CBS and EBS are different burst level allowed. The evaluation results
in the three cases (that is, sufficient Tokens in C, insufficient Tokens in C but sufficient in
D, and sufficient Tokens in neither C nor D), the results are “conforming, partly
conforming and not conforming” respectively.
2.1.3 Introduction to CAR
CAR provides classification service and traffic policing by limiting the speed. The user
can set IP precedence of the packets coming into the network with CAR classification in
order to divide the traffic into multiple precedence levels and service types. The other
network equipment can process the data according to the modified IP precedence.
The user can be defined as 6 types of services according to the precedence field of ToS
segment in IP packet header. The rule to define the type of packets can be based on
multiple criterion, including physical port, source IP address, source MAC address,
destination IP address, destination MAC address, application port, IP protocol type or
other criterion which can perform classification by using access list or extended access
list. Packets can also be classified according to the situation outside the network (i.e.,
the client type). The network can accept or ignore the classification or reclassify the
packets according to a certain rule.