Chapter 11 Quality of Service (QoS)
P-870HW-51a v2 User’s Guide
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DiffServ
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 (Differentiated Services) 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.
DSCP and Per-Hop Behavior
DiffServ defines a new Differentiated Services (DS) field to replace the Type of Service (TOS)
field in the IP header. The DS field contains a 2-bit unused field and a 6-bit DSCP field which
can define up to 64 service levels. The following figure illustrates the DS 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 forwarding behavior, the PHB (Per-Hop Behavior), that each
packet gets across the DiffServ network. Based on the marking rule, different kinds of traffic
can be marked for different kinds of forwarding. Resources can then be allocated according to
the DSCP values and the configured policies.
Level 4 Typically used for controlled load, latency-sensitive traffic such as SNA (Systems
Network Architecture) transactions.
Level 3 Typically used for “excellent effort” or better than best effort and would include important
business traffic that can tolerate some delay.
Level 2 This is for “spare bandwidth”.
Level 1 This is typically used for non-critical “background” traffic such as bulk transfers that are
allowed but that should not affect other applications and users.
Level 0 Typically used for best-effort traffic.
Table 42 IEEE 802.1p Priority Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
DSCP (6 bits) Unused (2 bits)
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