Traffic control
140
8.4
Prioritization
Basic - L3P
Release
3.1
06/07
U Differentiated Services
The newly defined Differentiated Services field in the IP header in RFC
2474
(see Fig. 40) - often known as the DiffServ code point or DSCP,
replaces the ToS field and is used to mark the individual packets with a
DSCP. Here the packets are divided into different quality classes. The
first three bits of the DSCP are used to divide the packets into classes.
The next three bits are used to further divide the classes on the basis of
different criteria. In contrast to the ToS byte, DiffServ uses six bits for the
division into classes. This results in up to 64 different service classes.
Fig. 40: Differentiated Services field in the IP header
The different DSCP values get the switch to use a different forwarding
behavior, the Per-Hop Behavior (PHB). PHB classes:
D Class Selector (CS0-CS7): For reasons of compatibility to TOS/IP
precedence
D Expedited Forwarding (EF): Premium service.
Reduced delay, jitter + packet loss (RFC 2598)
D Assured Forwarding (AF): Provides a differentiated schema for
handling different data traffic (RFC 2597).
D Default Forwarding/Best Effort: No particular prioritizing.
Bits
0
1324567
Currently
Unused
(CU)
Differentiated Services Codepoint
(DSCP) RFC 2474
Class Selector
Codepoints
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