3com WX3000 User Manual

 

1-3 

information carried in packet header. Packet payload is rarely adopted for traffic classification. The 

identifying rule is unlimited in range. It can be a quintuplet consisting of source address, source port 

number, protocol number, destination address, and destination port number. It can also be simply a 

network segment.  

Figure 1-1 DS field and ToS byte 

0

7

6

DSCP

M
B
Z

Class Selector 
codepoints

Differentiated Services 

Codepoint ( DSCP)

RFC 2474

Currently
Unused

RFC 1122

IP Type of Service (ToS)

RFC 791

Must

Be

Zero

RFC 1349

DS-Field

(for IPv4,ToS 

octet,and for 

IPv6,Traffic 

Class octet )

IPv4 ToS

 byte

1

5

4

3

2

Bits:

0

7

6

1

5

4

3

2

Bits:

Preced

ence

Type of 
Service

CU

 

 

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 1-1 Description on IP precedence 

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 service level can be segmented. The 

QoS rank of the AF class is lower than that of the EF class;