Traffic Classification; Packet Precedences - H3C S5120-SI Series Operation Manual

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.
This section is focused on traffic classification, and the subsequent sections will introduce the other
technologies in details.

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
network can either adopt the classification results from its upstream network or classify the packets
again according to its own criteria.
To provide differentiated services, traffic classes must be associated with certain traffic control actions
or resource allocation actions. What traffic control actions to adopt depends on the current phase and
the resources of the network. For example, CAR is adopted to police packets when they enter the
network; GTS is performed on packets when they flow out of the node; queue scheduling is performed
when congestion happens; congestion avoidance measures are taken when the congestion
deteriorates.

Packet Precedences

This section introduces IP precedence, ToS precedence, differentiated services codepoint (DSCP)
values, and 802.1p precedence.
1) IP precedence, ToS precedence, and DSCP values
Figure 1-3 DS field and ToS bytes
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