1-4Priority Trust ModePrecedence types1) IP precedence, ToS precedence, and DSCP precedenceFigure 1-2 DS field and ToS byteThe ToS field in an IP header contains eight bits numbered 0 through 7, among which,z The first three bits indicate IP precedence in the range 0 to 7.z Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.z In RFC2474, the ToS field in IP packet header is also known as DS field. The first six bits (bit 0through bit 5) of the DS field indicate differentiated service codepoint (DSCP) in the range of 0 to 63,and the last two bits (bit 6 and bit 7) are reserved.Table 1-2 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 providing differentiated services, traffics are grouped into the following four classes, andpackets are processed according to their DSCP values.z Expedited Forwarding (EF) class: In this class, packets can be forwarded regardless of link shareof other traffic. The class is suitable for preferential services with low delay, low packet loss ratio,low jitter, and assured bandwidth (such as virtual leased line);z Assured forwarding (AF) class: This class is further divided into four subclasses (AF1/2/3/4) and asubclass is further divided into three drop priorities, so the AF service level can be segmented. TheQoS rank of the AF class is lower than that of the EF class;z Class selector (CS) class: This class comes from the IP ToS field and includes eight subclasses;
