Chapter 32 Differentiated Services
32.1.2 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 139) 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. An example traffic policy, is 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 139 DiffServ Network
A
S 
G 
P 
P
| P G S B | S G P P |
| | | | | | |
P - Platinum | | | | | |
| S |
G - Gold | | | B | | |
S - Silver | | | | B | |
B - Bronze | | | | | |
32.2 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.
