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Appendix b introduction to packet precedences, Ip precedence and dscp values – H3C Technologies H3C S6300 Series Switches User Manual

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Appendix B Introduction to packet precedences

IP precedence and DSCP values

Figure 23 ToS and DS fields

As shown in

Figure 23

, the ToS field in the IP header contains eight bits. The first three bits (0 to 2)

represent IP precedence from 0 to 7. 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 is in

the range 0 to 63. The remaining two bits (6 and 7) are reserved.

Table 9 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

Table 10 DSCP values

DSCP value (decimal)

DSCP value (binary)

Description

46 101110

ef

10 001010

af11

12 001100

af12

14 001110

af13

18 010010

af21

20 010100

af22

22 010110

af23

26 011010

af31

M
B
Z

RFC 1122

IP Type of Service (ToS)

RFC 791

Must

Be

Zero

RFC 1349

IPv4 ToS

byte

0

7

6

1

5

4

3

2

Bits:

Preced

ence

Type of
Service

0

7

6

DSCP

Class Selector
codepoints

Differentiated Services

Codepoint (DSCP)

RFC 2474

Currently
Unused

DS-Field

(for IPv4,ToS

octet,and for

IPv6,Traffic

Class octet )

1

5

4

3

2

Bits:

CU