H3C Technologies H3C S3100 Series Switches - Manual (Page 395)

Operation Manual – QoS
H3C S3100-52P Ethernet Switch Chapter 1 QoS Configuration
1-2
Figure 1-1
DS field and TOS byte
The TOS field in an IP header contains eight bits:
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 first three bits in DSCP precedence are class selector
codepoints, bit 4 and bit 5 indicate drop precedence, and bit 6 is zero indicating
that the device sets the service class with the DS model. The last two bits (bit 6
and bit 7) are reserved bits.
The precedence values of the IP packet indicate eight different service classes.
Table 1-1
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
The Diff-Serv network defines four traffic classes:
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);
z
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