1-5Traffic classification identifies packets conforming to certain characteristics accordingto certain criteria. It is the foundation for providing differentiated services.In traffic classification, the priority bits in the type of service (ToS) field in the IP headercan be used to identify packets of different priorities. You can also define traffic matchcriteria to classify packets by the combination of source address, destination address,MAC address, IP protocol or the port number of an application. Contents other than theheader information in packets are rarely used for traffic classification. You can define aclass for packets with the same quintuple (source address, source port number,protocol number, destination address and destination port number for example), or forall packets to a certain network segment.1.3.2 Priority Trust ModeI. Introduction to precedence types1) IP precedence, ToS precedence, and DSCPFigure 1-2 DS field and ToS byteAs shown in Figure 1-2, the ToS field of the IP header contains eight bits: the first threebits (0 to 2) represent IP precedence from 0 to 7 and the subsequent four bits (3 to 6)represent a ToS value from 0 to 15. According to RFC 2474, the ToS field of the IPheader is redefined as the DS field, where a DiffServ code point (DSCP) precedence isrepresented by the first six bits (0 to 5) and is in the range 0 to 63. The remaining twobits (6 and 7) are reserved.Table 1-2 Description on IP precedenceIP precedence value(decimal)IP precedence value(binary) Description0 000 Routine1 001 priority2 010 immediate3 011 flash4 100 flash-override
