3com 4210 PWR 9-Port 3CR17341-91-ME User Manual

consisting of source address, source port number, protocol number, destination 
address, and destination port number. It can also be simply a network segment.

Figure 92   DS field and ToS byte

The ToS field in an IP header contains eight bits numbered 0 through 7, among 
which,

■

The first three bits indicate IP precedence in the range 0 to 7.

■

Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.

■

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.

In a network providing differentiated services, traffics are grouped into the 
following four classes, and packets are processed according to their DSCP values.

■

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);

■

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;

Table 220   Description of 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

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