wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Quality of Service 19-11
Forwarded packets
Flow 1
WFQ + drop
Queue 1
Flow 2
WFQ + drop
Queue 2
WFQ
scheduler
Hardware queuing
system
Hardware Q
Interface
Flow N
WFQ + drop
Queue N
FIGURE 19.6
Class-based weighted fair queuing.
queuing system for Ethernet links. An extension is CBWFQ (Figure 19.6), which provides support for
user-defined traffic classes.
The classes allow specifying the exact amount of bandwidth to be allocated for a specific class of traffic.
Taking into account available bandwidth on the interface, you can configure up to 64 classes and
control distribution among them.
19.4 Special Considerations for Managing the Quality of Service
The way to specify the parameters and rules that control QoS for IP networks is using software tools and
devices. It will depend on the type of network, the magnitude and the application running on it.
Many times it is only necessary to apply the QoS mechanisms in each device, case by case. But, in
many others, for more complex and big networks, the management is done using different kind of network
management tools as HP OpenView or Cisco Works. These tools always use as the preferred management
protocol the well-established simple network management protocol (SNMP) [24], to manage
many different situations in a network (general network management and network monitoring tasks),
some of them being probably QoS tasks.
Although they are not the only devices to implement the QoS techniques, the routers (and any level
3 switch is also a router in this sense) are the most frequently used ones. At the routers, and using the
real-time data from the packets they process, the QoS statistics are calculated. A consequence that must
be taken into account is that we must size correctly the needed resources for the router with its QoS
mechanisms implemented and using them. Also we must be consistent and apply the same mechanisms
for every router in the network.
At the routers, there are other special considerations to do about QoS, the congestion avoidance features
and the high availability solutions for the routers being two of the most important ways it is handled.
19.4.1 Congestion Avoidance
Congestion avoidance techniques monitor network traffic loads, trying to anticipate and avoid congestion
at common network bottleneck points. It is got through packet dropping by using more complex
techniques than simple tail drop.
When an interface on a router cannot transmit a packet immediately, the packet is queued. Packets
are afterward taken out of the queue and eventually transmitted on the interface.
If the arrival rate of packets to the outgoing interface exceeds the router capability to buffer and
forward traffic, the queues increase to their maximum length and the interface becomes congested.
© 2011 by Taylor and Francis Group, LLC