3Com 2928 User Guide - Page 464

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 2-3 DS field and ToS bytes As shown in Figure 2-3, the ToS field of the IP header contains eight bits: the first three bits (0 to 2) represent IP precedence from 0 to 7; the subsequent four bits (3 to 6) represent a ToS value from 0 to 15. According to RFC 2474, the ToS field of the IP header is redefined as the differentiated services (DS) field, where a DSCP value is represented by the first six bits (0 to 5) and is in the range 0 to 63. The remaining two bits (6 and 7) are reserved. Table 2-1 Description on IP Precedence IP Precedence (decimal) 0 1 2 3 4 5 6 7 IP Precedence (binary) 000 001 010 011 100 101 110 111 Description Routine priority immediate flash flash-override critical internet network In a network in the Diff-Serve model, traffic is grouped into the following four classes, and packets are processed according to their DSCP values. 2-4