Differentiated Services - Hirschmann RS20 User Manual

Network Load Control
Bits (0-2): IP Precedence Defined Bits (3-6): Type of Service Defined Bit (7)
010 - Immediate
001 - Priority
000 - Routine
Table 9: ToS field in the IP header

Differentiated Services


The Differentiated Services field in the IP header
defined in RFC 2474 - 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 6 bits for the division
into classes. This results in up to 64 different service classes.
Bits 0 1
Differentiated Services Codepoint
Class Selector
Codepoints
Figure 30: Differentiated Services field in the IP header
The different DSCP values get the device to employ a different
forwardingbehavior, namely Per-Hop Behavior (PHB). PHB classes:
 Class Selector (CS0-CS7): For reasons of compatibility to TOS/IP
Precedence
 Expedited Forwarding (EF): Premium service.
Reduced delay, jitter + packet loss (RFC 2598)

Assured Forwarding (AF): Provides a differentiated schema for
handling different data traffic (RFC 2597).

Default Forwarding/Best Effort: No particular prioritizing.
132
2 3
(DSCP) RFC 2474
4 5 6
Explicit
Congestion
Notification
(ECN)
8.4 QoS/Priority
(see figure 30) newly
7
UM BasicConfig L2E
Release 8.0 05/2013