Differentiated Services Code Point. An IP-header field that specifies how a packet should be classified. Originally, specific bits within the DiffServ field were intended to mark traffic for a specific type of service. Typically, however, vendors and network administrators specify how networking equipment grants adequate QoS to traffic that is marked with a specific DSCP . A method for using differentiated services in VPNs is specified in RFC 2983.

The six DS bits are used to classify a packet, and there are three basic types of DS designators:
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CS , as defined in RFC 2474, provide backwards compatibility for the old ToS field from RFC 791. The figure below shows how the ToS byte is translated into DSCP.

The first three most significant bits of the DSCP field encode the numerical designation of the ToS precedence (digits 0-7), and the last three bits are always zero.
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The AF PHB designations, defined in RFC 2597, specify four classes (which the user can assign as desired) and three drop precedences (low, medium, high).

The first three most significant bits define the class and the next two encode the drop precedence. The last bit is always zero, as per RFC 2474.
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EF PHB, which is defined by RFC 3246.
The table below shows DSCP values in the IANA registry.
Registry Name
|
Value
|
Definition
|
CS0
|
000 000 (0)
|
Best effort
|
CS1
|
001 000 (8)
|
Class 1
|
CS2
|
010 000 (16)
|
Class 2
|
CS3
|
011 000 (24)
|
Class 3
|
CS4
|
100 000 (32)
|
Class 4
|
CS5
|
101 000 (40)
|
Express Forwarding
|
CS6
|
110 000 (48)
|
Stays the same (used for IP routing protocols)
|
CS7
|
111 000 (56)
|
Stays the same (link-layer and routing protocol keepalive)
|
AF11
|
001 010 (10)
|
Class 1, low drop precedence
|
AF12
|
001 100 (12)
|
Class 1, medium drop precedence
|
AF13
|
001 110 (14)
|
Class 1, high drop precedence
|
AF21
|
010 010 (18)
|
Class 2, low drop precedence
|
AF22
|
010 100 (20)
|
Class 2, medium drop precedence
|
AF23
|
010 110 (22)
|
Class 2, high drop precedence
|
AF31
|
011 010 (26)
|
Class 3, low drop precedence
|
AF32
|
011 100 (28)
|
Class 3, medium drop precedence
|
AF33
|
011 110 (30)
|
Class 3, high drop precedence
|
AF41
|
100 010 (34)
|
Class 4, low drop precedence
|
AF42
|
100 100 (36)
|
Class 4, medium drop precedence
|
AF43
|
100 110 (38)
|
Class 4, high drop precedence
|
EF PHB
|
101 110 (46)
|
Expedited forwarding per-hop behavior
|

|