Traffic control
Basic - L3P
Release
3.1
06/07
8.4
Prioritization
139
8.4.3 IP ToS / DiffServ
U TYPE of Service
The Type of Service field (ToS) in the IP header (see Fig. 39) was
already part of the IP protocol from the start, and is used to differentiate
different services in IP networks. Even back then, there were ideas about
differentiated treatment of IP packets, due to the limited bandwidth
available and the unreliable connection paths. Because of the
continuous increase in the available bandwidth, there was no need to
use the ToS field. Only with the real-time requirements of today's
networks has the ToS field become significant again. Selecting the ToS
byte of the IP header enables you to differentiate between different
services. However, this field is not widely used in practice.
Fig. 39: ToS field in IP header
Bits (0-2): IP Precedence Defined Bits (3-6): Type of Service Defined Bit (7)
111 - Network Control 0000 - [all normal] 0 - Must be zero
110 - Internetwork Control 1000 - [minimize delay]
101 - CRITIC / ECP 0100 - [maximize throughput]
100 - Flash Override 0010 - [maximize reliability]
011 - Flash 0001 - [minimize monetary cost]
010 - Immediate
001 - Priority
000 - Routine
Bits
0
1324567
MBZPrecedence Type of Service
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