Operation Manual – QoS-QoS ProfileH3C S3100 Series Ethernet Switches Chapter 1 QoS Configuration1-51.3.2 Priority Trust ModeI. Precedence 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 firstsix bits (bit 0 through 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 fourclasses, and packets are processed according to their DSCP values.z Expedited Forwarding (EF) class: In this class, packets can be forwardedregardless of link share of other traffic. The class is suitable for preferentialservices with low delay, low packet loss ratio, low jitter, and assured bandwidth(such as virtual leased line);
