Network Load Control
Basic Configuration
Release
6.0
07/2010
8.4
QoS/Priority
145
Differentiated Services
The newly defined Differentiated Services field in the IP header (see
fig. 39) - 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 3 bits of the
DSCP are used to divide the packets into classes. The next 3 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.
Figure 39: Differentiated Services field in the IP header
The different DSCP values get the device to employ a different forwarding
behavior, namely Per-Hop Behavior (PHB). PHB classes:
X Class Selector (CS0-CS7): For reasons of compatibility to TOS/IP
Precedence
X Expedited Forwarding (EF): Premium service.
Reduced delay, jitter + packet loss (RFC2598)
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
Table 13: ToS field in the IP header
Bits
0
1324567
Currently
Unused
(CU)
Differentiated Services Codepoint
(DSCP) RFC 2474
Class Selector
Codepoints
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