H3C Technologies H3C SR8800 - Manual (Page 39)

30
Packet precedences
This section introduces IP precedence, ToS precedence, differentiated services codepoint (DSCP) values,
802.1p priority, and EXP values.
1. IP precedence, ToS precedence, and DSCP values
Figure 11 DS field and ToS bytes
As shown in Figure 11 , the ToS field in the IP header contains eight bits: the first three bits (0 to 2)
represent IP precedence from 0 to 7; the subsequent four bits (3 to 6) represent a ToS value from 0 to 15.
According to RFC 2474, the ToS field is redefined as the differentiated services (DS) field, where a DSCP
value is represented by the first six bits (0 to 5) and ranges from 0 to 63. The remaining two bits (6 and
7) are reserved.
Table 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 in the Diff-Serve model, traffic is grouped into the following classes, and packets are
processed according to their DSCP values:
• Expedited forwarding (EF) class—In this class, packets are forwarded regardless of link share of
other traffic. The class is suitable for preferential services requiring low delay, low packet loss, low
jitter, and high bandwidth.
• Assured forwarding (AF) class—This class is divided into four subclasses (AF 1 to AF 4), each
containing three drop priorities for more granular classification. The QoS level of the AF class is
lower than that of the EF class.
• Class selector (CS) class—This class is derived from the IP ToS field and includes eight subclasses.
• Best effort (BE) class—This class is a special CS class that does not provide any assurance. AF traffic
exceeding the limit is degraded to the BE class. All IP network traffic belongs to this class by default.