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DSCP Marking

DSCP (Differentiated Services Code Point) marking uses 6 bits of the 8 bit ToS field in the IP Header to provide up to 64 classes (or code points) for traffic. Since DSCP is a layer 3 marking method, there is no concern about compatibility as there is with 802.1p marking. Devices that do not support DSCP will simply ignore the tags, or at worst, they will reset the tag value to 0.

The above diagram depicts an IP packet, with a close-up on the ToS portion of the header. The ToS bits were originally used for Precedence and ToS (delay, throughput, reliability, and cost) settings, but were later repurposed by RFC2474 for the more versatile DSCP settings.

The following table shows the commonly used code points, as well as their mapping to the legacy Precedence and ToS settings.

DSCP
	
DSCP Description
	
Legacy IP Precedence
	
Legacy IP ToS (D, T, R)
0
	
Best effort
	
0 (Routine - 000)
	
-
8
	
Class 1
	
1 (Priority - 001)
	
-
10
	
Class 1, gold (AF11)
	
1 (Priority - 001)
	
T
12
	
Class 1, silver (AF12)
	
1 (Priority - 001)
	
D
14
	
Class 1, bronze (AF13)
	
1 (Priority - 001)
	
D, T
16
	
Class 2
	
2 (Immediate - 010)
	
-
18
	
Class 2, gold (AF21)
	
2 (Immediate - 010)
	
T
20
	
Class 2, silver (AF22)
	
2 (Immediate - 010)
	
D
22
	
Class 2, bronze (AF23)
	
2 (Immediate - 010)
	
D, T
24
	
Class 3
	
3 (Flash - 011)
	
-
26
	
Class 3, gold (AF31)
	
3 (Flash - 011)
	
T
27
	
Class 3, silver (AF32)
	
3 (Flash - 011)
	
D
30
	
Class 3, bronze (AF33)
	
3 (Flash - 011)
	
D, T
32
	
Class 4
	
4 (Flash Override - 100)
	
-
34
	
Class 4, gold (AF41)
	
4 (Flash Override - 100)
	
T
36
	
Class 4, silver (AF42)
	
4 (Flash Override - 100)
	
D
38
	
Class 4, bronze (AF43)
	
4 (Flash Override - 100)
	
D, T
40
	
Express forwarding
	
5 (CRITIC/ECP - 101)
	
-
46
	
Expedited forwarding (EF)
	
5 (CRITIC/ECP - 101)
	
D, T
48
	
Control
	
6 (Internet Control - 110)
	
-
56
	
Control
	
7 (Network Control - 111)
	
-