Operation Manual – QoS
H3C S3610&S5510 Series Ethernet Switches
Chapter 2 Traffic Classification, TP, and TS
Configuration
2-2
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:
z
The first three bits indicate IP precedence in the range of 0 to 7.
z
Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.
z
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)
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
