Chapter 30 Differentiated Services
GS2200-24 User’s Guide
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30.3.1 DSCP and Per-Hop Behavior
DiffServ defines a new DS (Differentiated Services) field to replace the Type of
Service (ToS) field in the IP header. The DS field contains a 6-bit DSCP field which
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 126 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.
30.3.2 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
127) 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
DSCP (6 bits) CU (2 bits)
