ZyXEL Communications ES-3148 Series - Manual (Page 225)

ES-3148 User’s Guide
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H A P T E R
27
Differentiated Services
This chapter shows you how to configure Differentiated Services (DiffServ) on the Switch.
27.1 DiffServ Overview
Quality of Service (QoS) is used to prioritize source-to-destination traffic flows. All packets in
the flow are given the same priority. You can use CoS (class of service) to give different
priorities to different packet types.
DiffServ is a class of service (CoS) model that marks packets so that they receive specific per-
hop treatment at DiffServ-compliant network devices along the route based on the application
types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs) indicating the
level of service desired. This allows the intermediary DiffServ-compliant network devices to
handle the packets differently depending on the code points without the need to negotiate
paths or remember state information for every flow. In addition, applications do not have to
request a particular service or give advanced notice of where the traffic is going.
27.1.1 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 118 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.
DSCP (6 bits) CU (2 bits)