Chapter 5 Protocol Packet Priority Configuration; Introduction To Ip Packet Priority - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide

Chapter 5 Protocol Packet Priority Configuration

5.1 Introduction to IP Packet Priority

Protocol packets carry their priorities themselves. You can however assign them new
priorities. By associating these priorities with QoS actions, you can provide protocol
packets different QoS services.
Two types of priorities are available with IP packets: IP precedence and DSCP.
I. IP precedence
IP precedence is the first three bits in the ToS field of the IP header. These three bits
can specify eight service classes, as shown in the following table.
Table 5-1 IP precedence levels
IP precedence level
0
1
2
3
4
5
6
7
The 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 precedence
and IP quintuple. Different traffic streams are offered appropriate QoS according to the
traffic behaviors configured on each node.
II. DSCP
Differentiated Services (DiffServ) model defines the six most significant bits in the IP
DS (also known as ToS) field as DSCP. The first three bits of DSCP are for class
selector, the fourth and fifth bits are for drop priorities, and the sixth bit is set to 0
meaning 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:
Chapter 5 Protocol Packet Priority Configuration
IP precedence bits
000
001
010
011
100
101
110
111
3Com Corporation
5-1
Description
Routine
Priority
Immediate
Flash
Flash-override
Critical
Internet
Network