IP Header QoS Fields Precedence and DSCP

The two most popular marking fields for QoS are the IP Precedence and IP DSCP fields that were introduced in Chapter 2, "QoS Tools and Architectures." QoS tools frequently use these two fields in part because the IP packet header exists from endpoint to endpoint in a network, as shown in Figure 3-1.

Figure 3-1 Headers Used During Typical Packet Flow

Figure 3-1 Headers Used During Typical Packet Flow

168.1.1.1

As seen in Figure 3-1, the IP packet en route to host PC2 stays intact throughout the network, whereas the data-link headers only exist while a frame is crossing between a host and a router, or between routers.

Figure 3-2 outlines the two marking fields and their positions inside an IP header.

Figure 3-2 IP Precedence and IP DSCP Fields

8 bits

Figure 3-2 IP Precedence and IP DSCP Fields

8 bits

8 bits

You can mark the Precedence and DSCP fields with any valid binary value of either 3 or 6 bits, respectively. Chapter 2 contains detailed discussion of the recommended values used in these two fields. Briefly, Precedence field values should grow in importance, and in QoS behavior, as the number gets higher. DSCP differs in that several per-hop behavior (PHB) RFCs define suggested DSCP values for which the larger number does not always get a better QoS treatment.

Table 3-4 lists the IP precedence and DSCP values, and their names, for review. Note that not all DSCP values are listed; only the DSCP values suggested by the DiffServ RFCs are listed in the table. QoS tools that are capable of setting DSCP can set any of the actual 64 values.

Table 3-4 IP Precedence and DSCP—Popular Values and Names

Table 3-4 lists the IP precedence and DSCP values, and their names, for review. Note that not all DSCP values are listed; only the DSCP values suggested by the DiffServ RFCs are listed in the table. QoS tools that are capable of setting DSCP can set any of the actual 64 values.

Table 3-4 IP Precedence and DSCP—Popular Values and Names

Field and Value (Decimal)

Binary Value

Name

Defined by This RFC

Precedence 0

000

routine

791

Precedence 1

001

priority

791

Precedence 2

010

immediate

791

Precedence 3

011

flash

791

Precedence 4

100

flash override

791

Precedence 5

101

critic

791

Precedence 6

110

internetwork control

791

Precedence 7

111

network control

791

DSCP 0

000000

best effort or default

2475

Table 3-4 IP Precedence and DSCP—Popular Values and Names (Continued)

Field and Value (Decimal)

Binary Value

Name

Defined by This RFC

DSCP 8

001000

CS1

2475

DSCP 16

010000

CS2

2475

DSCP 24

011000

CS3

2475

DSCP 32

100000

CS4

2475

DSCP 40

101000

CS5

2475

DSCP 48

110000

CS6

2475

DSCP 56

111000

CS7

2475

DSCP 10

001010

AF11

2597

DSCP 12

001100

AF12

2597

DSCP 14

001110

AF13

2597

DSCP 18

010010

AF21

2597

DSCP 20

010100

AF22

2597

DSCP 22

010110

AF23

2597

DSCP 26

011010

AF31

2597

DSCP 28

011100

AF32

2597

DSCP 30

011110

AF33

2597

DSCP 34

100010

AF41

2597

DSCP 36

100100

AF42

2597

DSCP 38

100110

AF43

2597

DSCP 46

101110

EF

2598

CS = Class Selector AF = Assured Forwarding EF = Expedited Forwarding

CS = Class Selector AF = Assured Forwarding EF = Expedited Forwarding

The two IP header QoS marking fields do not provide all the QoS marking fields needed today. One day, all other Layer 3 protocols besides IP may no longer be used. One day, all LAN switches will be capable of looking at IP headers, including IP DSCP and Precedence, and perform QoS based on those fields. Likewise, one day, all WAN services, including Frame Relay and ATM switches, will be able to perform QoS based on these same fields. However, today's reality is that even as more and more devices become capable of marking and reacting to IP precedence and DSCP, it will be a long time before all networking devices are both capable and configured to use these fields for QoS purposes. So, other QoS marking fields are needed.

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