User`s guide

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QoS is a complex subject and can have a significant impact on the performance of the
router. For detailed background information on QoS, refer to RFC2472 (Definition of the
Differentiated Services Field).
In Digi TransPort routers, the classification of incoming IP packets for the purposes of QoS
takes place within the firewall. The firewall allows the system administrator to assign a
DSCP code to a packet with any combination of source/destination IP address/port and
protocol. Details of how this is done are given in the section on firewall scripts.
When the routing code within the unit receives an incoming packet, it directs it to the
interface applicable to that packet at that time (this is the case whether or not QoS is being
applied). Just before the packet is sent to the interface, the QoS code intercepts the packet
and assigns it to one of the available priority queues (currently 10 per PPP instance) based
on its DSCP value.
Each priority queue has a profile assigned to it. This profile specifies parameters such as the
minimum transmit rate to attempt, maximum queue length and WRED parameters.
The packet is then processed by the queue management code and either dropped or placed
in the queue for later transmission.
There are a couple of configuration web pages associated with QoS functionality:
• DSCP Mappings
• Queue Profiles
The Configuration – Network > QoS > DSCP Mappings page which contains parameters
to configure DSCP operation and Configuration – Network > Queue Profiles page which
contains parameters to manage the queue “profiles”.
Each Configuration – Interfaces > Ethernet and Configuration – Interfaces > PPP
instance page contains a QoS sub-page which control how QoS behaves on that particular
interface.
When configuring QoS, be aware that the router supports ten queues, numbered from 0 to
9 and that DSCP codes range from 0 to 64.
DSCP Mappings
Configuration – Network > QoS > DSCP Mappings
Each DSCP value must be mapped to a queue. These mappings are set up using this page.
Default
This drop-down list selects the default queue. When this is changed, any DSCP codes that
are set to use the default will have their queue number changed.
DSCP
This column is simply a list of valid DSCP codes with an associated drop-down list box to the
right.
Queue
Each of the DSCP codes in the left-hand column has a queue associated with it. To change
the value from what is shown, select the desired value from the drop-down list.
Related CLI Commands
Entity
Instance
Parameter
Values
Equivalent Web Parameter
dscp n q
0 – 63
Queue