H3C S5500-EI series Operation Manual page 1052

Operation Manual – QoS
H3C S5500-EI Series Ethernet Switches
scheduling is performed on the packets; when congestion get worse, congestion
avoidance is performed on the packets.
2.1.2 Priority
The following describes several types of precedence:
1) IP precedence, ToS precedence, and DSCP precedence
Figure 2-1 DS field and ToS field
The ToS field in an IP header contains eight bits, which are described as follows:
The first three bits indicate IP precedence in the range of 0 to 7.
Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.
RFC2474 re-defines the ToS field in the IP packet header, which is called the DS
field. The first six (bit 0 to bit 5) bits of the DS field indicate DSCP precedence in
the range of 0 to 63. The last two bits (bit 6 and bit 7) are reserved bits.
Table 2-1 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
IP Precedence (binary)
000
001
010
011
100
101
110
111
2-2
Chapter 2 Traffic Classification,
TP, and LR Configuration
Description
Routine
priority
immediate
flash
flash-override
critical
internet
network