ZyXEL Communications GS-2750, User Manual

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Chapter 28 Differentiated Services
GS-2750 User’s Guide
226
28.1.2 DiffServ Network Example
The following figure depicts a DiffServ network consisting of a group of directly connected
DiffServ-compliant network devices. The boundary node (
A in Figure 116 ) in a DiffServ
network classifies (marks with a DSCP value) the incoming packets into different traffic flows
(
Platinum , Gold , Silver , Bronze ) based on the configured marking rules. A network
administrator can then apply various traffic policies to the traffic flows. For example, one
traffic policy would be to give higher drop precedence to one traffic flow over others. In our
example packets in the
Bronze traffic flow are more likely to be dropped when congestion
occurs than the packets in the Platinum traffic flow as they move across the DiffServ network.
Figure 116 DiffServ Network
28.2 Two Rate Three Color Marker Traffic Policing
Traffic policing is the limiting of the input or output transmission rate of a class of traffic on
the basis of user-defined criteria. Traffic policing methods measure traffic flows against user-
defined criteria and identify it as either conforming, exceeding or violating the criteria.
Two Rate Three Color Marker (TRTCM, defined in RFC 2698) is a type of traffic policing
that identifies packets by comparing them to two user-defined rates: the Committed
Information Rate (CIR) and the Peak Information Rate (PIR). The CIR specifies the average
rate at which packets are admitted to the network. The PIR is greater than or equal to the CIR.
CIR and PIR values are based on the guaranteed and maximum bandwidth respectively as
negotiated between a service provider and client.
Two Rate Three Color Marker evaluates incoming packets and marks them with one of three
colors which refer to packet loss priority levels. High packet loss priority level is referred to as
red, medium is referred to as yellow and low is referred to as green. After TRTCM is
configured and DiffServ is enabled the following actions are performed on the colored
packets:
• Red (high loss priority level) packets are dropped.
• Yellow (medium loss priority level) packets are dropped if there is congestion on the
network.
G S B P
S
B
B
G P P S
P - Platinum
G - Gold
S - Silver
B - Bronze
G P P
S
A