Chapter 10 QoS
GS1500-24P User’s Guide
80
Use the Priority to Queue Mapping screen (Section 10.7 on page 86) to
configure the priority level-to-physical queue mapping.
Use the Packet Scheduling screen (Section 10.8 on page 87) to set priorities for
the Switch queues. This can help distribute bandwidth across the different traffic
queues.
10.3 What You Need to Know
Read on for concepts on QoS that can help you configure the screens in this
chapter.
10.3.1 DiffServ
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.
10.3.2 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 35 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)
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