Chapter 29 Differentiated Services
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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
DSCP (6 bits) CU (2 bits)
G
S
B
P
S
B
B
G
P
P
S
P - Platinum
G - Gold
S - Silver
B - Bronze
G
P
P
S
A
