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.

Table 150 IP precedence, ToS precedence, and DSCP values

b.DS field and ToS bytes

As shown in b, 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.

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 four 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.

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