2-4network can either adopt the classification results from its upstream network or classify the packetsagain according to its own criteria.To provide differentiated services, traffic classes must be associated with certain traffic control actionsor resource allocation actions. What traffic control actions to adopt depends on the current phase andthe resources of the network. For example, CAR is adopted to police packets when they enter thenetwork; GTS is performed on packets when they flow out of the node; queue scheduling is performedwhen congestion happens; congestion avoidance measures are taken when the congestiondeteriorates.Packet PrecedencesThis section introduces IP precedence, ToS precedence, differentiated services codepoint (DSCP)values, and 802.1p precedence.1) IP precedence, ToS precedence, and DSCP valuesFigure 2-3 DS field and ToS bytesAs 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. Theremaining two bits (6 and 7) are reserved.Table 2-1 Description on IP PrecedenceIP Precedence (decimal) IP Precedence (binary) Description0 000 Routine1 001 priority2 010 immediate3 011 flash4 100 flash-override5 101 critical6 110 internet7 111 networkIn a network in the Diff-Serve model, traffic is grouped into the following four classes, and packets areprocessed according to their DSCP values.
