QoS functionality in routers. It is important to understand the characteristics of the applications that need protection. Some applications tend to be sensitive to latency or packet loss, while others are considered "aggressive" because they are bursty or consume a lot of bandwidth. If the application is bursty, determine if there is a constant burst or a small burst. Is the packet size of the application large or small? Is the application TCP or UDP based? The table “Understanding the Characteristics of Applications” provides some-high level guidelines .
Content 3.2 Implementing Cisco IOS QoS 3.2.6 Step 2: Define Traffic Classes After identifying and measuring network traffic, use business requirements to perform the second step: define the traffic classes. A class is a group of network flows that share similar characteristics. For example, an ISP might define classes to represent the different service levels offered to customers. An enterprise might define service level agreements (SLAs) that give different levels of service to various applications. Because of its stringent QoS requirements, voice traffic is usually in a class by itself. Cisco has developed specific QoS mechanisms, such as LLQ, to ensure that voice always receives priority treatment over all other traffic. After you define the applications with the most critical requirements, the remaining traffic classes are defined using business requirements. Example: Define Traffic Classes
A typical enterprise might define the following five traffic classes as shown in Figure based on department requirements or based on the preponderance of a particular application in the network traffic:
Content 3.2 Implementing Cisco IOS QoS 3.2.7 Step 3: Define QoS Policy In the third step, define a QoS policy for each traffic class as shown in Figure . A QoS policy typically defines the following: When packets pass to your network, QoS evaluates the packet headers. The QoS policy determines the action that QoS takes. Defining a QoS policy involves one or more of the following activities: Example: Define QoS Policies
As an example, consider a network that has a finite amount of bandwidth available. Using the traffic classes, previously defined QoS policies can be mandated based on the following priorities (with Priority 5 being the highest and Priority 1 being the lowest):
Content 3.3 Selecting an Appropriate QoS Policy Model 3.3.1 Three QoS Models The following are three models for implementing QoS in a network as illustrated in Figure :
Content 3.3 Selecting an Appropriate QoS Policy Model 3.3.2 Best-Effort Model The basic design of the Internet provides for best-effort packet delivery and provides no guarantees. This approach is still predominant on the Internet today and remains appropriate for most purposes. The best-effort model treats all network packets in the same way, so an emergency voice message is treated the same way a digital photograph attached to an e-mail is treated. Without QoS, the network cannot tell the difference between packets and, as a result, cannot treat packets preferentially. These points are summarized in Figure . When you mail a letter using standard postal mail, you are using a best-effort model. Your letter is treated exactly the same as every other letter. With the best-effort model, the letter may never arrive, and, unless you have a separate notification arrangement with the letter recipient, you may never know that the letter did not arrive. Figure summarizes the benefits and drawbacks of best effort model as follows:
Content 3.3 Selecting an Appropriate QoS Policy Model 3.3.3 IntServ