About DSCP

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 Differentiated Services Code Point. 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:

  1. CS Class Selectors, as defined in RFC 2474, provide backwards compatibility for the old ToS Type of Service 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.

  1. The AF Assured Forwarding Assured ForwardingPHB Per-Hop Behavior 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.

  1. EF Expedited Forwarding 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