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Basic QoS Concepts

E-NIT-CTC-20041213-0013 v0.5

3.1 Precedence and TOS

Introduction

There are two generations of quality of service architectures in the Internet Protocol. 
The interpretation of the 

 in the Internet Protocol header varies 

between these two generations.
The figure below shows the Internet Protocol header.
The Type of Service Octet is the second 8-bit octet of the Internet Protocol header.

First generation

Precedence and Type of Service bits.
The initial definition of the 

looked like this:

Most 

 descriptions are obscure: they relate to message handling priorities 

of US military communications in the 1960s. The essence is that higher values of 
Precedence lead to higher levels of network service.
To prevent high link utilisation causing routing traffic to be lost, it is traditional to use 
Precedence = 7 for interior routing protocols, such as OSPF and RIP and to use 
Precedence = 6 for exterior routing protocols such as BGP.
The 

 type of service bit can be a value of 0 to request normal delay, a value of 1 to 

request a low delay service.
The 

 type of service bit can be a value of 0 to request normal throughput, a value of 

1 to request a high throughput service.
The 

 type of service bit can be a value of 0 to request normal reliability, a value of 1 

to request a high reliability service.
The 

 type of service bit can be a value of 0 to request normal costs, a value of 1 to 

request a low cost service.

0

4

8

16

31

Version

Header 

Length

Type of Service

Total Length

Identification

DM

OFF

Time to Live

Protocol

Header Chuckles

Source Address

Destination Address

0

1

2

3

4

5

6

7

Precedence

D

T

R

C

The D,T,R and C type of service bit is defined in 

 (Internet Protocol)