Employing Quality of Service

Employing Quality of Service

The previous section presented two means of examining the network to gather a fundamental understanding of how the network is being used. This information forms the foundation from which you can make decisions about how to align network resources to accommodate the relative priority of applications and hosts that are using the network. This information is also important to ensure that the network is configured in such a way that it provides the appropriate levels of handling and control based on application and business requirements.

Using information gathered through NetFlow or other mechanisms (for instance, network analysis modules or accelerators with monitoring capabilities, both of which generally provide similar reporting capabilities), network administrators can then define what levels of service need to be applied to different types of traffic and how this data should be handled. These same tools also serve as a means of validation after such policies have been implemented within the network to verify that the application requirements and business priority definitions are being met.

Without QoS, best-effort handling is provided to all flows on the network, thereby introducing the possibility that noncritical traffic may delay or block the service of business-critical traffic or traffic from applications that have sensitivities to network characteristics such as latency or loss. With best-effort handling, all traffic is considered equal, as shown in Figure 3-7. In this figure, peer-to-peer sharing consumes all the available network resources, leading to bandwidth starvation for the applications that need precious WAN bandwidth to drive user productivity. Such situations could quickly lead to employee frustration with business applications, loss of revenue, poor morale, and job dissatisfaction.

Figure 3-7. Networks with Best-Effort Delivery—No Quality of Service

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With QoS configured, appropriate handling can be provided to traffic flows based on classification and priority. As shown in Figure 3-8, the result is that the business-critical applications that demand bandwidth and service on the network are unimpeded even when unsanctioned traffic is present. In this example, peer-to-peer sharing is shown as being permitted, and it should be noted that such applications could be blocked altogether.

Figure 3-8. Networks with Quality of Service

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With QoS in place, network administrators are able to better allocate available network capacity to those applications that need it the most. This allocation of network resources with business priority helps to ensure precious network capacity is used by sanctioned applications, thereby encouraging user productivity.

The QoS architecture is built around a behavioral model that comprises the following four key functions, each of which is outlined in the following sections. These functions provide the facilities necessary to align network resources with business priority and application requirements.

• Classification: Identifies application and traffic flows on the network. Once identified, further action and specific handling can be applied to the flow.

• Pre-queuing: Includes operations that are performed against flows prior to consuming network device (router or switch) resources such as queues. The operations include dropping packets (undesirable flows), traffic conditioning (policing), and marking relative priority on the packets themselves.

• Queuing and scheduling: Enforce priority of selected packet streams through the use of configurable queuing mechanisms, such as high-priority handling of delay-sensitive traffic, selective delay of lower-priority traffic during periods of congestion, traffic conditioning (shaping), and enforcement of bandwidth allocation.

• Post-queuing: Improves link throughput using optional operations such as packet compression, header compression, and link fragmentation and interleaving (LFI).

The behavioral model provides the facilities necessary to align network resources to business priority and to optimize user productivity over the network. Figure 3-9 shows an example.

Figure 3-9. QoS Behavioral Model

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