Quality of service
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UALITY OF SERVICE
I
NTRODUCTION
Quality of Service is a generic term given to the queuing systems mechanisms through which
packets are received or transmitted on a router.
It includes rules about: deciding which packets to accept, if accept them or not, at which speed they
should be routed on an interface; on which packets transmit and in which order transmit them from
the output interface.
The solution of Imola offers the following functions:
it limits the total bandwidth available on an interface to a specified value (shaping)
it reserves the bandwidth for particular types of service by using mechanisms of
classification (scheduling)
it offers preferential routes to types of traffic sensitive to latency times (classes)
it redistributes excess bandwidth
it assures dropping of excess packet (policing)
It is possible to define any number of classifications of traffic with any number of classes with
different characteristics.
The classification of traffic is made according to HTB (Hierarchical Token Bucket), while the default
queuing behavior is FIFO (First In, First Out) kind.
T
RAFFIC POLICY CONFIGURATION
It is necessary to configure a traffic policy first:
set qos-ext policy <name>
The policy must be associated with an interface:
set qos-ext policy <name> interface <interface>
It is possible to define the maximum bandwidth allowed for this policy. It is optional to specify the
bandwidth only with ADSL interface; in this case if the bandwidth is not specified, it is set to the
value of bandwidth to which the modem is synchronized. The unit of measurement is Kbits/sec:
set qos-ext policy <name> bandwidth <value>
It is possible to specify a percentage of available bandwidth to be reserved for non- classified
traffic:
set qos-ext policy <name> reserved-bandwidth <value>
It is possible to specify the burst, i.e. the maximum number of Kbit that the root class, which is the
owner of the all classes, can transmit, when it is authorized to do it:
set qos-ext policy <name> bandwidth burst <value>
It is also possible to specify the burst when the root class has exceeded the guaranteed bandwidth
and therefore the entire system is blocked:
set qos-ext policy <name> bandwidth burst-max <value>
