3Com 2928 - Baseline Plus Switch PWR, Руководство пользователя

Страница: 464 / 505

2-4
network can either adopt the classification results from its upstream network or classify the packets
again according to its own criteria.
To provide differentiated services, traffic classes must be associated with certain traffic control actions
or resource allocation actions. What traffic control actions to adopt depends on the current phase and
the resources of the network. For example, CAR is adopted to police packets when they enter the
network; GTS is performed on packets when they flow out of the node; queue scheduling is performed
when congestion happens; congestion avoidance measures are taken when the congestion
deteriorates.
Packet Precedences
This section introduces IP precedence, ToS precedence, differentiated services codepoint (DSCP)
values, and 802.1p precedence.
1) IP precedence, ToS precedence, and DSCP values
Figure 2-3
DS field and ToS bytes
As shown in
Figure 2-3 , the ToS field of 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 of the IP header is redefined as the differentiated services (DS)
field, where a DSCP value is represented by the first six bits (0 to 5) and is in the range 0 to 63. The
remaining two bits (6 and 7) are reserved.
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 in the Diff-Serve model, traffic is grouped into the following four classes, and packets are
processed according to their DSCP values.