3Com Switch 7750 Configuration Guide Guide

618 CHAPTER 58: QOS CONFIGURATION
When the token bucket evaluates the traffic, its parameter configurations include:
■ 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 every burst. It is generally set to committed burst size (CBS).
The set burst size must be bigger 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.
2 Complicated evaluation
You can set two token buckets in order to evaluate more complicated conditions
and implement more flexible regulation policies. For example, TP includes 4
parameters:
■ CIR
■ CBS
■ Peak information rate (PIR)
■ Excess burst size (EBS)
Two token buckets are used in this evaluation. Their rates of putting tokens into
the buckets are CIR and PIR respectively, and their sizes are CBS and EBS
respectively (the two buckets are called C bucket and E bucket respectively for
short), representing different permitted burst levels. In each evaluation, you can
implement different regulation policies in different conditions, including "enough
tokens in C bucket", "insufficient tokens in C bucket but enough tokens in E
bucket" and "insufficient tokens in both C bucket and E bucket".
TP
The typical application of TP is to supervise the specification of certain traffic into
the network and limit it within a reasonable range, or to punish the extra traffic.
Therefore, the network resources and the interests of the operators are protected.
For example, you can limit HTTP packets within 50% of the network bandwidth. If
the traffic of a certain connection is excess, TP can choose to drop the packets or
to reset the priority of the packets.
TP is widely used in policing the traffic into the network of internet service
providers (ISP).TP can classify the policed traffic and perform pre-defined policing
actions according to different evaluation results. These actions include:
■ Forward: Forward the packet whose evaluation result is "conforming" or mark
DSCP precedence for Diff-Serv packets and then forward them.
■ Drop: Drop the packet whose evaluation result is "nonconforming".