3com 5500-ei pwr Instrução De Instalação
1-5
Priority Trust Mode
Introduction to precedence types
1) IP precedence, ToS precedence, and DSCP precedence
Figure 1-2 DS field and ToS byte
The 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 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 Precedence
IP Precedence (decimal)
IP Precedence (binary)
Description
0 000
Routine
1 001
priority
2 010
immediate
3 011
flash
4 100
flash-override
5 101
critical
6 110
internet
7 111
network
In a network providing differentiated services, traffics are grouped into the following four
classes, and packets are processed according to their DSCP values.
z
Expedited Forwarding (EF) class: In this class, packets can be forwarded regardless of
link share of 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 a subclass is further divided into three drop priorities, so the AF