Diff Serv Code Points – Assured Forwarding explained.
Looking at the 6 bits used for DSCP:-
The first 3 bits define the PHB (per Hop Behaviour) and the remaining 3 bits define the Drop Precedence.
PHB is then segregated into the following categories:-
  • Default PHB (Per hop behavior)—which is typically best-effort traffic [000000]
  • Expedited Forwarding (EF) PHB—dedicated to low-loss, low-latency traffic
  • Assured Forwarding (AF) PHB—gives assurance of delivery under prescribed conditions
  • Class Selector PHBs—which maintain backward compatibility with the IP Precedence field.
The most commonly used PHB is AF (Assured Forwarding). Assured Forwarding is made up of 4 Classes from 1-4 –where 1 is the highest priority. These classes are then further segmented by 3 (low, medium and high) Drop Precedence markings.
Drop Precedence
This is defined as the likelihood of packets getting dropped when congestion occurs on multiple streams within the same class. There are 3 drop precedence level:-
Low Drop – Low likelihood of packets being dropped
Medium Drop – Medium likelihood of packets being dropped
High Drop – Highest likelihood of packets being dropped
Thus looking at the class and drop precedence combination a complete understanding of DSCP is possible as indicated in the table below.
Forwarding Type
Class
Bits
(012)
Bits
(345)
 AF Class
Label
Drop
Precedence
Use
Assured Forwarding
Class 1
001
010
AF11
Low Drop
Voice
Payload
100
AF12
Medium Drop
110
AF13
High Drop
Class 2
010
010
AF21
Low Drop
Video
100
AF22
Medium Drop
110
AF23
High Drop
Class 3
011
010
AF31
Low Drop
Voice
Signalling
100
AF32
Medium Drop
110
AF33
High Drop
Class 4
100
010
AF41
Low Drop
General
Data
100
AF42
Medium Drop
110
AF43
High Drop
How the AF Class Label is calculated
The first 3 bits make up the first digit, the next 2 bits make up the second digit, the last bit is ignored.

PB

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