3Com 4500 PWR 26-Port Configuration Manual page 538

protocol or the port number of an application. Normally, traffic classification is done by checking the
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 n
network segment.
Prior ity rust Mode T
In troduction to precedence type
1) IP precedence, ToS precedence, and DSCP precedence
Figure 45-2 DS field and ToS byte
Bits: 0 1
DS-Field
(for IPv4,ToS
octet,and for
IPv6,Traffic
Class octet )
Class Selector
codepoints
Differentiated Services
Codepoint (DSCP)
RFC 2474
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
In RFC2474, the ToS field in IP packet head
through bit 5) of the DS field indicate differentiat
and the last two bits (bit 6 a
Table 45-2 Description on IP Prec
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
pack ets 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
umber, destination address, and destination port number. It can also be simply a
s
2 3 4 5 6 7
DSCP
CU
Currently
Unused
nd bit 7) are reserved.
edence
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
of 0 to 15.
er is also known as DS field. The first six bits (bit 0
ed service codepoint (DSCP) in the range of 0 to 63,
);
45-4
6 7
M
B
Z
Must
Be
Zero
Description
Routine
priority
immediate
flash
flash-override
critical
internet
network