3Com 2928 User Guide - Page 463

End-to-End QoS Figure 2-2 End-to-end QoS model As shown in Figure 2-2, traffic classification, traffic policing, traffic shaping, congestion management, and congestion avoidance are the foundations for a network to provide differentiated services. Mainly they implement the following functions: z Traffic classification uses certain match criteria to organize packets with different characteristics into different classes. Traffic classification is usually applied in the inbound direction of a port. z Traffic policing polices particular flows entering or leaving a device according to configured specifications and can be applied in both inbound and outbound directions of a port. When a flow exceeds the specification, some restriction or punishment measures can be taken to prevent overconsumption of network resources. z Traffic shaping proactively adjusts the output rate of traffic to adapt traffic to the network resources of the downstream device and avoid unnecessary packet drop and congestion. Traffic shaping is usually applied in the outbound direction of a port. z Congestion management provides a resource scheduling policy to arrange the forwarding sequence of packets when congestion occurs. Congestion management is usually applied in the outbound direction of a port. z Congestion avoidance monitors the usage status of network resources and is usually applied in the outbound direction of a port. As congestion becomes worse, it actively reduces the amount of traffic by dropping packets. Among these QoS technologies, traffic classification is the basis for providing differentiated services. Traffic policing, traffic shaping, congestion management, and congestion avoidance manage network traffic and resources in different ways to realize differentiated services. Traffic Classification When defining match criteria for classifying traffic, you can use IP precedence bits in the type of service (ToS) field of the IP packet header, or other header information such as IP addresses, MAC addresses, IP protocol field and port numbers. You can define a class for packets with the same quintuple (source address, source port number, protocol number, destination address and destination port number for example), or for all packets to a certain network segment. When packets are classified on the network boundary, the precedence bits in the ToS field of the IP packet header are generally re-set. In this way, IP precedence can be directly adopted to classify the packets in the network. IP precedence can also be used in queuing to prioritize traffic. The downstream 2-3