
Quality of Service (QoS)
©2008 Allied Telesis Inc. All rights reserved.
39.4
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
Application with VLAN double tagging
Note that if you are using VLAN double tagging, you could use the
“match cos”
command to
set the individual QoS requirements
within
each client network and also separately within the
provider network. You can then use the
“match inner-cos” on page 40.10
command to apply
particular “
client
” QoS requirements that you want to apply within the provider network. This
process applies two levels of QoS within the provider network; one that operates specifically
for the network provider, and another that is specific for traffic belonging to selected clients.
See
“VLAN Double Tagging (VLAN Stacking)” on page 14.18
.
At the network layer IPv4 packets contain an 8 bit field field specifically to carry QoS
information. This field, defined in RFC 1349, was originally named the Type of Service (ToS)
field and contained a
ToS
component and a
Precedence
component. The ToS field however, has
since been replaced by the Differentiated Services field.
Differentiated Services Architecture
Whilst a full description of the differential services model is outside the scope of this software
reference, a brief introduction is provided. For further information, RFC 2475 provides an in
depth definition of the architecture.
The basic differential services model envisages a multi router network within which common
service qualities are applied. At the network boundary,
QoS Edge Routers
inspect the traffic and
classify it into common service quality groups called Per Hop Behaviors (PHBs). A specific
marker value called a Differential Services Code Point (DSCP) is added to the IP header of
each packet, which allocates it to a PHB.
QoS Core Routers
within the network can then use the
DSCP to decide on an appropriate service quality level to apply. When a network contains a
consistently applied differential services code points DSCP it is referred as a Differential
Services Domain (often shortened to DiffServe Domain).
Figure 39-2
shows a simple
Differential Services Domain.
Figure 39-2: Differentiated Services Domain
DiffServe-Domain
QoS
Boundary
Switch
Unmarked
Packets
QoS
Boundary
Switch
Classify by DSCP=40
Limit bandwidth
QoS
Core
Switches
QoS
Core
Switches
Differential Services
Domain
Classify by source IP
address
Mark with DSCP=40
Limit bandwidth
Classify by DSCP=40
Limit bandwidth
Classify by DSCP=40
Limit bandwidth
Remark to a new DSCP value
Classify by DSCP=40
Limit bandwidth