GOCS Class Based QoS

Most QoS tools do not need to differentiate between each flow. In the Figure 2-10, for instance, flows to web Serverl were identified. Most network engineers would want to treat those collective web flows the exact same way with their QoS tools. Therefore, most QoS tools tend to operate on the idea of a category, or class, of flows and packets. Consider Figure 2-11, for example, which has thousands of flows, all of which are classified into four types of traffic.

Figure 2-11 GOCS Approach to QoS with Classes jC

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Class-based QoS tools do not have to identify each flow. However, they do need to identify packets based on something in the packet header—such as TCP destination port 80 for web traffic—and consider that traffic to be in one category or class for QoS treatment. Once again, the reasons and rationale behind deciding what traffic gets what QoS treatment changes from network to network, but the basic process works the same, but per class rather than per flow:

• Identify each packet, determining which class it belongs to.

• Apply some QoS action to the packets in each class.

• The QoS actions on a single router may be different for each class.

• The QoS actions among all routers may be different for each class.

Unlike flow-based QoS tools, class-based QoS tools typically require the engineer to specify exactly what must be seen in the packet header to classify a packet. If this network currently has 4 flows to the web server, or 400, or 4000, if the classification criteria just states "all TCP port 80 traffic," no additional configuration is required as the network scales. Both flow-based and class-based tools need to examine every packet to classify the packet into the appropriate flow or class. Because class-based tools typically only need a small number of classifications, however, the tool can reasonably be configured to specify the types of traffic that get added to each class.

Class-based QoS tools can use more complex rules to classify packets than do flow-based tools. For instance, a class-based tool can examine subsets of the IP addresses (matching a subnet, for example), the incoming interface, the URL for web traffic, and anything that an IP ACL can match. For flow-based tools, the router always look at five fields, all in the IP header—Source and Destination Address, Source and Destination Port, and the Protocol Type field (which identifies the transport layer protocol). In short, classification options for class-based tools tend to be much more varied and functional, but they require more configuration work to take advantage of the different options.

Flow-based and class-based QoS tools both have a useful place in a QoS strategy for a network. Most QoS tools tend to be based on general classes, as opposed to looking at each individual flow.

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