Westermo MR Series - Manual (Page 218)

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Web Interface and Command Line Reference Guide www.westermo.com
Confi gure > Quality of Service 4.70
Introduction 4.70.1
QOS (Quality of Service) provides the means to prioritise 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 with a higher priority. For example, this might mean that EPOS transactions carried over
XOT will be prioritised at a higher level than HTTP type traffic used for Internet access. Without
some form of QOS all IP packets are treated as being equal so 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 DSCP (Differentiated
Services Code Point) 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 routers and each queue can be configured to behave in
a particular way so that packets in that queue are prioritised for routing according to predefined
rules.
There are two principle ways in which prioritisation may be effected:
A priority queue can be configured to allow packets to be routed at a specified data rate (pro viding
that queues of 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 pre-
venting 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 your router.
For detailed background information on QOS refer to RFC2474 (Definition of the Differentiated
Services Field).
Basic Operation 4.70.2
In Westermo 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 on 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
appli cable to that packet at the 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 avail able 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 mini-
mum 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 number of configuration pages associated with QOS operation:
The Configure > Quality of Service folder contains pages for setting up the basic Quality of
Service parameters including the DSCP mappings and the Q “profiles”
Each Configure > ETH and Configure > PPP instance page then contains a QOS sub-page which
allows you to set up the specific QOS parameters to be used for those interfaces