Precedence - 3Com WX3000 Series Operation Manual

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.

Precedence

IP precedence, ToS precedence, and DSCP precedence
Figure 1-1 DS field and ToS byte
Bits: 0
DS-Field
(for IPv4,ToS
octet,and for
IPv6,Traffic
Class octet )
Class Selector
codepoints
Differentiated Services
Codepoint ( DSCP)
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.
Table 1-1 Description on IP precedence
IP Precedence (decimal)
0
1
2
3
4
5
6
7
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;
1 2 3 4 5 6 7
DSCP
CU
Currently
Unused
RFC 2474
IP Precedence (binary)
000
001
010
011
100
101
110
111
Bits: 0 1 2 3 4 5
Preced Type of
IPv4 ToS
ence Service
byte
RFC 1349
RFC 1122
IP Type of Service (ToS)
RFC 791
1-3
6 7
M
B
Z
Must
Be
Zero
Description
Routine
priority
immediate
flash
flash-override
critical
internet
network