User`s guide

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Configuration – Network > QoS
The Quality of Service (QoS) functionality provides the means of prioritising different types
of IP traffic. It is generally used to ensure that low priority applications do not “hog” the
available bandwidth to the detriment of those having a higher priority. For example, this
might mean that EPOS transactions carried out over XOT will be prioritised over HTTP-type
traffic used for Internet access. Without some form of QoS, all IP packets are treated as
being equal, i.e. there is no discrimination between applications.
The IP packet Type of Service (TOS) field is used to indicate how a packet should be
prioritised. Using the top 6 bits of the TOS field, a router that supports QoS will assign a
Differentiated Services Code Point (DSCP) code to the packet. This may take place within
the router when it receives the packet or another router closer to the packet source may
have already assigned it. Based on the DSCP code, the router will assign the packet to a
priority queue. There are currently four such queues for each PPP instance within the router
and each queue can be configured to behave a particular way so that packets in that queue
are prioritised for routing according to predefined rules.
There are two principal ways in which prioritisation may be effected:
A priority queue can be configured to allow packets to be routed at a specific data rate
(providing that queues of a higher priority are not already using the available bandwidth)
Weighted Random Early Dropping (WRED) of packets may be used as queues become busy,
in an attempt to get the TCP socket generating the packets to “back off” its transmit timers,
thus preventing the queue overflow (which would result in all subsequent packets being
dropped).
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:
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.