How to Prepare a Network for IPQoS

The following procedure lists general planning tasks to do before you create the QoS policy.

1. Review your network topology. Then, plan a strategy that uses IPQoS systems and Diffserv routers.

   For topology examples, see Planning the Diffserv Network Topology.

2. Identify the hosts in the topology that require IPQoS or that might become good candidates for IPQoS service.

3. Determine which IPQoS-enabled systems could use the same QoS policy.

   For example, if you plan to enable IPQoS on all hosts on the network, identify any hosts that could use the same QoS policy. Each IPQoS-enabled system must have a local QoS policy, which is implemented in its IPQoS configuration file. However, you can create one IPQoS configuration file to be used by a range of systems. You can then copy the configuration file to every system with the same QoS policy requirements.

4. Review and perform any planning tasks that are required by the Diffserv router on your network.

   Refer to the router documentation and the router manufacturer's web site for details.

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.

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Note - For an overview of classes, see IPQoS Classes.

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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 33-1.

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.

More Information
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.

Defining Filters

You create filters to identify packet flows as members of a particular class. Each filter contains selectors, which define the criteria for evaluating a packet flow. The IPQoS-enabled system then uses the criteria in the selectors to extract packets from a traffic flow. The IPQoS system then associates the packets with a class. For an introduction to filters, see IPQoS Filters.

The following table lists the most commonly used selectors. The first five selectors represent the IPQoS 5-tuple, which the IPQoS system uses to identify packets as members of a flow. For a complete list of selectors, see Table 37-1.

Table 33-2 Common IPQoS Selectors

Name	Definition
saddr	Source address.
daddr	Destination address.
sport	Source port number. You can use a well-known port number, as defined in /etc/services, or a user-defined port number.
dport	Destination port number.
protocol	IP protocol number or protocol name that is assigned to the traffic flow type in /etc/protocols.
ip_version	Addressing style to use. Use either IPv4 or IPv6. IPv4 is the default.
dsfield	Contents of the DS field, that is, the DSCP. Use this selector for extracting incoming packets that are already marked with a particular DSCP.
priority	Priority level that is assigned to the class. For more information, see How to Define the Classes for Your QoS Policy.
user	Either the UNIX user ID or user name that is used when the upper-level application is executed.
projid	Project ID that is used when the upper-level application is executed.
direction	Direction of traffic flow. Value is either LOCAL_IN, LOCAL_OUT, FWD_IN, or FWD_OUT.

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Note - Be judicious in your choice of selectors. Use only as many selectors as you need to extract packets for a class. The more selectors that you define, the greater the impact on IPQoS performance.

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