How to Define the Classes for Your QoS Policy

The first step in defining the QoS policy is organizing traffic flows into classes. You do not need to create classes for every type of traffic on a Diffserv network. Moreover, depending on your network topology, you might have to create a different QoS policy for each IPQoS-enabled system.

For an overview of classes, see IPQoS Classes.

The next procedure assumes that you have determined which systems on your network are to be IPQoS-enabled, as identified in How to Prepare a Network for IPQoS.

1. Create a QoS planning table for organizing the QoS policy information.

   For suggestions, refer to Table 27–1.

2. Perform the remaining steps for every QoS policy that is on your network.

3. Define the classes to be used in the QoS policy.

   The following questions are a guideline for analyzing network traffic for possible class definitions.

   • Does your company offer service-level agreements to customers?

     If yes, then evaluate the relative priority levels of the SLAs that your company offers to customers. The same applications might be offered to customers who are guaranteed different priority levels.

     For example, your company might offer web site hosting to each customer, which indicates that you need to define a class for each customer web site. One SLA might provide a premium web site as one service level. Another SLA might offer a “best-effort” personal web site to discount customers. This factor indicates not only different web site classes but also potentially different per-hop behaviors that are assigned to the web site classes.

   • Does the IPQoS system offer popular applications that might need flow control?

     You can improve network performance by enabling IPQoS on servers offering popular applications that generate excessive traffic. Common examples are electronic mail, network news, and FTP. Consider creating separate classes for incoming and outgoing traffic for each service type, where applicable. For example, you might create a mail-in class and a mail-out class for the QoS policy for a mail server.

   • Does your network run certain applications that require highest-priority forwarding behaviors?

     Any critical applications that require highest-priority forwarding behaviors must receive highest priority in the router's queue. Typical examples are streaming video and streaming audio.

     Define incoming classes and outgoing classes for these high-priority applications. Then, add the classes to the QoS policies of both the IPQoS-enabled system that serves the applications and the Diffserv router.

   • Does your network experience traffic flows that must be controlled because the flows consume large amounts of bandwidth?

     Use netstat, snoop, and other network monitoring utilities to discover the types of traffic that are causing problems on the network. Review the classes that you have created thus far, and then create new classes for any undefined problem traffic category. If you have already defined classes for a category of problem traffic, then define rates for the meter to control the problem traffic.

     Create classes for the problem traffic on every IPQoS-enabled system on the network. Each IPQoS system can then handle any problem traffic by limiting the rate at which the traffic flow is released onto the network. Be sure also to define these problem classes in the QoS policy on the Diffserv router. The router can then queue and schedule the problem flows as configured in its QoS policy.

   • Do you need to obtain statistics on certain types of traffic?

     A quick review of an SLA can indicate which types of customer traffic require accounting. If your site does offer SLAs, you probably have already created classes for traffic that requires accounting. You might also define classes to enable statistics gathering on traffic flows that you are monitoring. You could also create classes for traffic to which you restrict access for security reasons.

4. List the classes that you have defined in the QoS planning table you created in Step 1.

5. Assign a priority level to each class.

   For example, have priority level 1 represent the highest-priority class, and assign descending-level priorities to the remaining classes. The priority level that you assign is for organizational purposes only. Priority levels that you set in the QoS policy template are not actually used by IPQoS. Moreover, you can assign the same priority to more than one class, if appropriate for your QoS policy.

6. When you finish defining classes, you next define filters for each class, as explained in How to Define Filters in the QoS Policy.

Prioritizing the Classes

As you create classes, you quickly realize which classes have highest priority, medium priority, and best-effort priority. A good scheme for prioritizing classes becomes particularly important when you assign per-hop behaviors to outgoing traffic, as explained in How to Plan Forwarding Behavior.

In addition to assigning a PHB to a class, you can also define a priority selector in a filter for the class. The priority selector is active on the IPQoS-enabled host only. Suppose several classes with equal rates and identical DSCPs sometimes compete for bandwidth as they leave the IPQoS system. The priority selector in each class can further order the level of service that is given to the otherwise identically valued classes.