Cisco Systems 15310-CL, 15310-MA Understanding QoS

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Cisco ONS 15310-CL and Cisco ONS 15310-MA Ethernet Card Software Feature and Configuration Guide R8.5
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Chapter 11 Configuring Quality of Service on the ML-Series Card
Understanding QoS
Understanding QoS
The ML-Series card multiplexes multiple IP/Ethernet services onto the SONET circuit and dynamically
allocates transmission bandwidth to data services based on data service requirements. This allows the
network to operate at a significantly higher level of utilization. To support service-level agreements
(SLAs), this dynamic allocation must accommodate the service elements of bandwidth, including loss
and delay. The characteristics of these service elements make up QoS.
The QoS mechanism has three basic steps. It classifies types of traffic, specifies what action to take
against a type of traffic, and specifies where the action should take place.
Priority Mechanism in IP and Ethernet
For any QoS service to be applied to data, there must be a way to mark or identify an IP packet or an
Ethernet frame. When identified, a specific priority can be assigned to each individual IP packet or
Ethernet frame. The IP Precedence or the IP Differentiated Services Code Point (DSCP) field prioritizes
the IP packets, and the Ethernet class of service (IEEE 802.1p defined class of service [CoS]) is used for
the Ethernet frames. IP precedence and Ethernet CoS are further described in the following sections.
IP Precedence and Differentiated Services Code Point
IP precedence uses the three precedence bits in the IPv4 header’s ToS (type of service) field to specify
class of service for each IP packet (RFC 1122). The most significant three bits of the IPv4 ToS field
provide up to eight distinct classes, of which six are used for classifying services and the remaining two
are reserved. On the edge of the network, the IP precedence is assigned by the client device or the
ML Series, so that each subsequent network element can provide services based on the determined
policy or the SLA.
IP DSCP uses the six bits in the IPv4 header to specify class of service for each IP packet (RFC 2474).
Figure 11-1 illustrates IP precedence and DSCP. The DSCP field classifies packets into any of the
64 possible classes. On the network edge, the IP DSCP is assigned by the cli ent device or the ML Series,
so that each subsequent network element can provide services based on the determined policy or the
SLA.