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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 number, destination address, and destination port number. It can also be simply a
network segment.
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);