Chapter 29 Differentiated Services
can define up to 64 service levels and the remaining 2 bits are defined as currently unused (CU). The following figure illustrates the DS field.
Figure 139 DiffServ: Differentiated Service Field
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ compliant, ToS-enabled network device will not conflict with the DSCP mapping.
The DSCP value determines the PHB (Per-Hop Behavior), that each packet gets as it is forwarded across the DiffServ network. Based on the marking rule different kinds of traffic can be marked for different priorities of forwarding. Resources can then be allocated according to the DSCP values and the configured policies.
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 140) 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 140 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 | | | | | |
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