3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 5 Protocol Packet Priority Configuration3Com Corporation5-1Chapter 5 Protocol Packet Priority Configuration5.1 Introduction to IP Packet PriorityProtocol packets carry their priorities themselves. You can however assign them newpriorities. By associating these priorities with QoS actions, you can provide protocolpackets different QoS services.Two types of priorities are available with IP packets: IP precedence and DSCP.I. IP precedenceIP precedence is the first three bits in the ToS field of the IP header. These three bitscan specify eight service classes, as shown in the following table.Table 5-1 IP precedence levelsIP precedence level IP precedence bits Description0 000 Routine1 001 Priority2 010 Immediate3 011 Flash4 100 Flash-override5 101 Critical6 110 Internet7 111 NetworkThe internet and network levels are only intended for use within a network only.On an access network, traffic is usually classified by the combination of IP precedenceand IP quintuple. Different traffic streams are offered appropriate QoS according to thetraffic behaviors configured on each node.II. DSCPDifferentiated Services (DiffServ) model defines the six most significant bits in the IPDS (also known as ToS) field as DSCP. The first three bits of DSCP are for classselector, the fourth and fifth bits are for drop priorities, and the sixth bit is set to 0meaning the current device sets service classes based on DS model.With DiffServ networks, traffic is sorted into the following four per-hop-behavior (PHB)groups:
