Diff Serv Code Points – 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
Expedited Forwarding
101
111
EF46
N\A
Voice
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 digit is ignored by the AF classes.
How the EF Class Label is calculated
All 6 bits are directly converted to decimal (common sense! What a surprise) thus 101111 = 46

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