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Operation Manual – QoS-QoS Profile

H3C S3600 Series Ethernet Switches-Release 1510 Chapter 1

QoS Configuration

1-2

1.1.3 Precedence

1) IP precedence, ToS precedence, and DSCP precedence

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

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